The design of the next generation of multifunctional and structural materials requires an understanding of their complex physics. Materials such as soft active polymers, additively manufactured metals, batteries, and biomaterials are crucial for applications including soft robotics, protection, energy storage, and biomechanical implants. Yet for most emerging functional materials, the governing physics remain only partially understood. We present two approaches to learn and develop high-fidelity models for these complex materials.
\r\n\r\nWe first present a more traditional approach that combines experiments and continuum modeling to study a class of multifunctional materials called Liquid Crystal Elastomers (LCEs). Their behavior at large strains and high strain rate regime was characterized, for the first time, using a novel tensile drop-tower experimental setup. This new insight leads to the development of a complete, high-fidelity engineering model that captures their behavior across multiple orders of strain rates while remaining consistent with their underlying multiscale physics.
\r\n\r\nWe then present a second approach that addresses model discovery for new, emerging materials, where physics knowledge is often sparse or incomplete. Advances in imaging now provide rich, full-field experimental data, but traditional inversion techniques fail to fully exploit this raw information. We introduce an image-to-constitutive model (I2C) method that directly infers models from raw experimental images and data, leveraging all the underlying information in the raw data to obtain high-fidelity models. This method is well-posed (the laws of physics regularize the ill-posed image correlation problem without ad hoc filters) and is amenable to any general form of constitutive relations under large deformations, non-pristine test conditions, and complex loadings. This method has been demonstrated to successfully recover the constitutive relation in rubber materials under large deformations.
\r\n\r\nFinally, this thesis presents an approach that uses the current model and physics knowledge of the material to design optimal experiments that maximize information gain. This approach couples a Bayesian framework with topology optimization to design sample geometries that produce maximally informative data within a single experiment. Together, these contributions create a closed-loop framework between modeling and experimentation for learning and developing next-generation materials.
", "doi": "10.7907/dx5f-qy46", "publication_date": "2027", "thesis_type": "phd", "thesis_year": "2027" }, { "id": "thesis:18514", "collection": "thesis", "collection_id": "18514", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04272026-182608505", "primary_object_url": { "basename": "Yingjin Wang_2026_Thesis.pdf", "content": "final", "filesize": 5924868, "license": "other", "mime_type": "application/pdf", "url": "/18514/5/Yingjin Wang_2026_Thesis.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Additive Manufacturing and Characterization of Micro-Architected Lithium-ion Battery Electrodes", "author": [ { "family_name": "Wang", "given_name": "Yingjin", "orcid": "0009-0002-1239-3422", "clpid": "Wang-Yingjin" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "See", "given_name": "Kimberly", "orcid": "0000-0002-0133-9693", "clpid": "See-Kimberly" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Electrode structure is closely coupled with mechanical behavior, ion transport, and reaction uniformity during battery operation. In addition to conventional slurry-cast electrodes, emerging fabrication approaches provide new opportunities to study and design electrode architectures. This thesis investigates lithium-ion battery electrodes through two complementary perspectives related to the electrode structure: micro-scale mechanical characterization to elucidate degradation mechanisms and additive manufacturing of three-dimensional (3D) micro-architected electrodes to investigate structure-transport relationships.
\r\n\r\nIn Chapters 2 and 3, the mechanical behavior of lithium-ion battery electrodes was investigated using nanoindentation and micro-pillar compression experiments. State-of-charge-dependent mechanical properties of electroplated LiCoO2 (LCO) cathodes were quantified, revealing a decreasing tendency in elastic modulus and hardness during delithiation, which is attributed to the expansion of LCO layered structure. Fracture toughness distribution across the electrode thickness was analyzed to understand how structural heterogeneity contributes to the mechanical property landscape. In addition, we studied the deformation of lithium-based composite anodes, confirming that the Li/Na composite anode exhibits higher deformability at the electrode-electrolyte interface, which enhances interfacial contact.
\r\n\r\nThe interconnected pore structure and large surface-to-volume ratio of 3D architected battery electrodes render them promising for enhancing electrochemical performance via more efficient ionic transport. In Chapters 4 and 5, we develop a hydrogel infusion additive manufacturing (HIAM)-based approach to fabricate micro-architected LiFePO4 (LFP)/C composite electrodes with feature sizes down to 18 \u00b5m. The concomitant formation of carbon within the lattice enhances the mechanical strength, which preserves shape integrity of the 3D structure during cell assembly and function. We designed electrodes with different geometries, including tilted cubes, honeycombs, and triply periodic minimal surfaces (TPMS), to probe the influence of geometric factors on electrochemical performance under various charge-discharge rates. We propose an experimentally informed electrochemical model that demonstrates the roles of Li+ transport in the electrolyte and Li+ diffusion in the electrode in determining the utilization of active materials. This work introduces a versatile manufacturing platform for printing 3D battery components and provides insights into structure optimization for high-performance rechargeable batteries.
", "doi": "10.7907/tkgq-7c28", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18544", "collection": "thesis", "collection_id": "18544", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05072026-220136945", "primary_object_url": { "basename": "Alex_Acosta_Thesis.pdf", "content": "final", "filesize": 82595669, "license": "other", "mime_type": "application/pdf", "url": "/18544/1/Alex_Acosta_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Experiments on Fluid-Structure Coupling Under Impinging Shock Wave Loading", "author": [ { "family_name": "Acosta", "given_name": "Alexander Ryan", "orcid": "0009-0004-4721-1638", "clpid": "Acosta-Alexander-Ryan" } ], "thesis_advisor": [ { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "An impinging shock wave/turbulent boundary layer interaction over a compliant panel was experimentally examined in a Mach 4 Ludwieg Tube through simultaneous measurements of both the fluid and structure response. Synchronized measurements were obtained using colinear focused laser differential interferometry and high-speed schlieren together with single-camera stereo-photogrammetry. The static and dynamic behavior of the compliant panel itself was also characterized through response to pressurization, roving hammer, and panel flutter experiments.
\r\n\r\nWhen the compliant panel was subjected to shock impingement at the mid-chord and leading-edge locations, the initial panel response was dominated by oscillations at the fundamental vibration mode and was primarily governed by one-way uncoupled fluid-structure behavior. However, under leading-edge impingement, oscillations at the fundamental vibration mode eventually dampened, and a transition to a two-way coupled fluid-structure response was observed. Under two-way coupling, the interaction was characterized by a reduction in oscillation amplitudes and an increase in relative strength of high-order vibration modes.
\r\n\r\nPhase analysis at the fundamental vibration mode between the separation and reattachment shock motion revealed that the one-way uncoupled mid-chord impingement case exhibited a repeatable phase alignment, where the phase of the separation shock tended to lead that of the reattachment shock by approximately one quarter of a panel oscillation. However, consistent with the behavior in rigid wall shock wave/boundary layer interactions, the instantaneous reattachment motion was anti-correlated and led that of the separation shock in all compliant cases. The transition to a two-way coupled response strengthened the synchronization between the separation shock and panel motion compared to that of the one-way uncoupled response.
\r\n\r\nSpectral analysis of the mid- and high-frequency content of density fluctuations in the separation bubble and boundary layer showed that the spectral distributions were not significantly modified due to surface compliance. Instead, the magnitude of the density fluctuations inside the separation bubble were attenuated under mid-chord impingement compared to that of leading edge impingement. Moreover, in all compliant cases, surface compliance amplified the spectral energy in the boundary layer downstream of flow reattachment by over a factor two compared to that of the rigid case.
", "doi": "10.7907/qt3f-ss69", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17783", "collection": "thesis", "collection_id": "17783", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12042025-001904943", "primary_object_url": { "basename": "Thesis_v3.pdf", "content": "final", "filesize": 39055323, "license": "other", "mime_type": "application/pdf", "url": "/17783/1/Thesis_v3.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Degradation Mechanisms of Oxide Ceramics Under Molten Regolith Electrolysis Conditions", "author": [ { "family_name": "Yu", "given_name": "Kevin", "orcid": "0000-0003-3130-4309", "clpid": "Yu-Kevin" } ], "thesis_advisor": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "See", "given_name": "Kimberly", "orcid": "0000-0002-0133-9693", "clpid": "See-Kimberly" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Molten regolith electrolysis (MRE) is a promising in situ resource utilization process that produces both metals and O2 through the direct electrolysis of molten lunar regolith (dirt), in support of a permanent human presence on the Moon. However, MRE requires an operational temperature of 1600\u00b0C, involves contact with corrosive molten regolith, and causes an oxidizing atmosphere during electrolysis. These conditions prevent the usage of many refractory materials due to rapid degradation. Moreover, there are additional challenges associated with MRE, such as bubble detachment and O2 collection at the anode.
\r\n\r\nIn this thesis, a hollow anode design, composed of an oxygen-conducting yttria-stabilized zirconia (YSZ) shell and a platinum current collector, is presented to address the challenges of MRE. Research is performed to evaluate the performance of YSZ electrolytes and containment materials in the extreme MRE environment and identify the mechanisms governing their degradation. From these experiments, a laboratory-scale MRE cell is designed and fabricated to support hollow anode testing. Electrolysis experiments with lunar regolith simulants successfully demonstrate sustained oxygen production for up to 12 hours. Extended testing with a degradation mitigation strategy further increases O2 production efficiencies and enables cumulative operation of 40 hours, establishing design life estimates for YSZ hollow anodes and guidelines for integration into industrial-scale MRE systems.
\r\n\r\nA previously unreported Sc2O3-rich phase, silicon aluminum scandate (SAS), is discovered while performing materials compatibility testing. The crystal structure of SAS is solved using microcrystal electron diffraction, and its material properties are characterized. These results indicate that SAS is an entropy-stabilized oxide with potential applications as a thermally insulating, refractory oxide material.
\r\n\r\nUltimately, the work presented demonstrates the feasibility of YSZ hollow anodes for MRE and expands the understanding of ceramic behavior in molten oxide environments. The successful production of O2 with a hollow anode provides a foundation for scaling MRE toward industrial operation on the lunar surface, while the discovery of SAS highlights the potential for uncovering new oxide materials in extreme environments.
", "doi": "10.7907/41bv-7696", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18655", "collection": "thesis", "collection_id": "18655", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272026-222421613", "primary_object_url": { "basename": "Nakahara_Caltech_PhD_Thesis_v5.pdf", "content": "final", "filesize": 39924467, "license": "other", "mime_type": "application/pdf", "url": "/18655/2/Nakahara_Caltech_PhD_Thesis_v5.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Fabrication and Mechanical Characterization of Nano-Architected Composites Across Scales and Strain Rates", "author": [ { "family_name": "Nakahara", "given_name": "Kevin Hiroshi Keesun", "clpid": "Nakahara-Kevin-Hiroshi-Keesun" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Shaikeea", "given_name": "Angkur", "orcid": "0000-0002-6706-0492", "clpid": "Shaikeea-Angkur-J" }, { "family_name": "Kagias", "given_name": "Matias", "orcid": "0000-0003-0435-6672", "clpid": "Kagias-Matias" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "div_eng" } ], "abstract": "Transportation, infrastructure, personnel protection, and all other applications requiring dynamic impact resistance drive the demand to develop advanced manufacturing for structural materials that are simultaneously lightweight and superior at energy absorption. Polymer matrix composites (PMCs) are widely used in mechanical applications due to their high strength-to-weight ratios, high stiffness-to-weight ratios, corrosion resistance, and design flexibility. However, these composites often suffer from matrix and interfacially driven failure mechanisms under dynamic compression. Nano-architected materials are an emergent class of metamaterials capable of achieving high-stiffnesses and strengths while also exhibiting high specific energy absorptions under micro particle impacts. Most studies of nano-architected materials focus on periodic lattices geometries or other cellular solids while their use as reinforcements in composites, especially at large-scales, remains limited. Combining the works of nano-architected metamaterials and PMCs, we create nano-architected composites possessing high mechanical energy absorptions under dynamic compression without the need for dense constituent reinforcement materials.
\r\n\r\nWe first explore how nano-architected materials are fabricated at large-scales, and we demonstrate how they can be incorporated with epoxy polymer matrices to create nano-architected composites using molding methods. Subsequently, we characterize these nano-architected composites, showing how changing fabrication parameters can produce various configurations of nano-architecture within samples, and how fabrication limitations can result in defects at multiple scales. Through various quasi-static and dynamic testing methods, we study how nano-architectures deform, fail, and contribute to composite performance --- decoupling their effects from defects. Our study shows that while composite performance can be mitigated by defects, increasing the amount of nano-architectures present in composites leads to higher strengths and delayed catastrophic failure. In-situ observations of these tests allow us to directly connect deformation and failure mechanisms to enhanced stress-strain performance. We then use phenomenological modeling to relate mechanical performances across loading rates showing high nano-architected rate sensitivities and verifying the role of nano-architectures in delaying catastrophic failure. Nano-architected performance is compared against classical PMCs reported in literature demonstrating their high energy absorptions and unique capability to address shortcomings of fiber- and particle-reinforced composites.
\r\n\r\nOur nano-architected composites offer a new way to toughen polymer matrix composites, utilizing small scale architectures rather than changes to constituent material composition to delay catastrophic failures. These nano-architected composites are a unique demonstration of the capabilities of nano-architectures to perform under high rates, loads, and energies, paving the route to new possibilities for composite design. Our modeling work of nano-architected composite performance shows the potential of these materials to absorb energies through the introduction and control of nano-architected structures.
", "doi": "10.7907/gvm7-qr90", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18647", "collection": "thesis", "collection_id": "18647", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272026-045028254", "primary_object_url": { "basename": "Carmi_Meital_2026.pdf", "content": "final", "filesize": 16519350, "license": "other", "mime_type": "application/pdf", "url": "/18647/1/Carmi_Meital_2026.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Buckling of Open Cross-Section Deployable Composite Thin Shells with Manufacturing Imperfections", "author": [ { "family_name": "Carmi", "given_name": "Meital Oshrit", "orcid": "0009-0000-7837-2910", "clpid": "Carmi-Meital-Oshrit" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Shaikeea", "given_name": "Angkur", "orcid": "0000-0002-6706-0492", "clpid": "Shaikeea-Angkur-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Thin composite shells are increasingly being used to support large area space systems due to their high strength to mass ratio and ability to withstand tight packaging for launch and then deploy once in space. This thesis specifically focuses on long, slender composite shells (longerons) with an open cross-section consisting of two circular arc flanges bonded along one edge. They are useful components of lightweight space structures, including the Caltech Space Solar Power Project spacecraft. The flanges of the longerons are less than 100 \u00b5m thick, and these extremely thin composite shells are prone to manufacturing imperfections and local buckling. Therefore, the primary objective of this thesis is to better understand the buckling behavior of open cross-section, ultra-thin composite shells that contain manufacturing imperfections, with the goal of informing the design of future space structures that are more resistant to buckling.
\r\n\r\nThe first step towards understanding a structure\u2019s imperfection sensitivity is to measure its imperfections. Thus, the thesis begins by characterizing the geometric imperfections present in experimental composite longerons. A method to measure and quantify the parameters of both local and global imperfections in thin shells is developed and applied. Results show that the longerons contain local imperfections as large as five to ten times the shell thickness, which can have serious implications for local buckling.
\r\n\r\nOnce the imperfections were measured, both numerical and experimental studies are performed to study the effects of these imperfections on the buckling behavior and knockdown factor of longerons loaded in pure bending. In the numerical study, a finite element analysis is used to characterize the effect of a single imperfection, created using a simplified model based on the shape of the experimentally measured imperfections, with a wide range of geometric parameters. Then, experiments are performed to measure the buckling behavior of actual longerons, whose random manufacturing imperfections were characterized. The results of these studies show that imperfections, especially ones with large amplitudes, significantly reduce the longeron\u2019s critical buckling load and bending stiffness. Good agreement between the experimental and numerical results was achieved, particularly for higher quality longerons with a single, dominant imperfection.
\r\n\r\nMotivated by the imperfections and their detrimental effects found in the earlier parts of the thesis, key parameters of the longeron's cross-section are varied with the goal of increasing its stability. The subtended angle of the longeron's flanges is varied in both experiments and numerical simulations of longerons loaded in bending. The results show good agreement between the experiments and simulations, with both showing a trend of increasing critical buckling load and bending stiffness with increasing flange subtended angle. Then, based on these promising results, the radius at the edge of the flange is decreased, which is shown to significant improve the longeron's stability and imperfection sensitivity without increasing its mass.
\r\n\r\nFinally, the effect of length on the buckling behavior of longerons loaded in bending is studied numerically with the goal of extending the current work to longer longerons. For lengths varying from 0.5 m to 5 m, both perfect and imperfect longerons with realistic geometric imperfections are studied. It is shown that for longer longerons, the critical buckling moment and imperfection sensitivity remain almost constant with increasing length, which is promising for future large space structures.
", "doi": "10.7907/ffpp-gr08", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18747", "collection": "thesis", "collection_id": "18747", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012026-192750566", "primary_object_url": { "basename": "soni_divesh_2026.pdf", "content": "final", "filesize": 45342987, "license": "other", "mime_type": "application/pdf", "url": "/18747/1/soni_divesh_2026.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Agile Slew Maneuvers of Ultralight Space Structures with Momentum Actuators", "author": [ { "family_name": "Soni", "given_name": "Divesh", "orcid": "0000-0002-5239-756X", "clpid": "Soni-Divesh" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Watkins", "given_name": "Michael M.", "orcid": "0000-0001-7524-4833", "clpid": "Watkins-M-M" }, { "family_name": "Manchester", "given_name": "Zachary", "orcid": "0000-0002-3071-7091", "clpid": "Manchester-Zachary" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Ultralight deployable spacecraft structures are at least an order of magnitude lighter and more volume efficient for launch than state-of-the art spacecraft structures of the same size, thanks to advancements in thin lightweight composites and new packaging schemes. As a result, they are suitable for a wide range of applications requiring extremely large apertures. For example, one of the application that we are currently interested is in-space computer servers. These ultralight spacecraft could not only support photovoltaics that generate large amounts of power, but at the same time act as extremely large radiators for dissipating heat. Similarly, for space-based solar power missions, they can hold functional layers of photovoltaics and phased array antennas to provide uninterrupted energy transmission to any location on Earth.
\r\n\r\nHowever, this improvement in mass/volume efficiency comes at a price. Ultralight spacecraft exhibit high structural compliance posing new challenges in spacecraft attitude control, which includes residual vibrations in the flexible structure and decreased pointing accuracy, leading to new requirements in structure/control design and testing of these architectures. The present work demonstrates new maneuver techniques and testing methodologies for Caltech\u2019s Space Solar Power Project (SSPP) architecture.
\r\n\r\nReal-world structures are imperfect and require ground tests to characterize their in-space behavior. We present an experimental methodology to perform slew maneuvers to help study the simultaneous behavior of the actuator, control scheme, and the flexible structure response. We present the design of a momentum device prototype and a small-scale, lightweight, flexible structure built for performing slew maneuvers. The flexible structure is designed to be highly compliant along the maneuver direction, allowing the study of its elastic deformation. Smooth polynomial maneuvers are performed with the actuator-structure system mounted on an air bearing; the central hub orientation and flexible structure response is measured. Assuming deformations to be comprised of a single dominant mode, input shaping is applied to the smooth polynomial maneuver, using Zero Velocity (ZV) shaper to suppress vibrations post maneuver. Input-shaping experiments show a remarkable reduction in structural vibrations for high-agility rotations, which directly contributes to an increased power transmission capability in full-scale spacecraft.
\r\n\r\nAlthough small-scale prototypes are useful for demonstrating control techniques and system-level performance, its quite complex to predict results for full-sized space structures given the lack of detailed similitude models for a combination of actuator-structure behavior and real-world defects. Moreover, full-scale structures are difficult to test on the ground in their deployed configuration, due to limitations in the current gravity compensation techniques. With increases in computational capability, data-centric system identification is the solution to robust modeling of real-world structures. These techniques are powerful not only in decomposing spatial-temporal variations but also in finding complex nonlinear behaviors. In the present work, reduced-order models are created from actuator-flexible structure experiments with equation-based nonlinear extensions to existing data-driven techniques. These models are then used to explore vibration-optimal agile maneuvers without making any assumptions about structural properties. These maneuvers, when tested in the experiments, result in fast rotations with a significant reduction of residual vibrations. This work directly contributes to improving the robustness of existing control schemes, enabling scalability to larger spacecraft and decreasing sensitivity to variations in the deployed structure over time.
\r\n\r\nTogether, these results demonstrate that large ultralight spacecraft can be designed and robustly controlled while addressing unwanted effects due to their low stiffness. This development enables their application to next-generation space missions such as space-based solar power, in-space servers, solar sail demonstrations, and much more.
", "doi": "10.7907/4f89-0w03", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18555", "collection": "thesis", "collection_id": "18555", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05122026-171140300", "primary_object_url": { "basename": "ToledoBarrios_Amanda_2026_Thesis.pdf", "content": "final", "filesize": 56526838, "license": "other", "mime_type": "application/pdf", "url": "/18555/2/ToledoBarrios_Amanda_2026_Thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Multiscale Response of Granular Materials under Cyclic Shear: Packing State, Force Chains, and Stress Transmission", "author": [ { "family_name": "Toledo Barrios", "given_name": "Patricia Amanda", "orcid": "0009-0000-2018-7616", "clpid": "Toledo-Barrios-Patricia-Amanda" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Shaikeea", "given_name": "Angkur", "orcid": "0000-0002-6706-0492", "clpid": "Shaikeea-Angkur-J" }, { "family_name": "Andrade", "given_name": "Jose E.", "orcid": "0000-0003-3741-0364", "clpid": "Andrade-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Granular materials are widely encountered in natural and engineered systems, yet their behavior under cyclic loading remains difficult to predict because bulk response emerges from complex grain-scale interactions. Repeated loading can produce irreversible deformation, history dependence, and evolving internal load-transfer mechanisms that are not fully captured by macroscopic measurements alone. Although cyclic behavior has been studied extensively at the continuum scale, direct experimental characterization of the grain-scale processes governing cyclic response remains limited.
\r\n\r\nThis thesis investigates the multiscale behavior of dry, cohesionless granular materials subjected to quasi-static cyclic simple shear loading through laboratory experiments coupled with the Granular Element Method (GEM), a mechanics-based force-inference framework for estimating interparticle contact forces from experimentally measurable grain-scale data. An experimental framework was developed by upgrading an existing simple shear apparatus to enable controlled cyclic loading and by integrating imaging, tracking, and post-processing methods for multiscale measurements.
\r\n\r\nAt the macroscale, the experiments show that cyclic simple shear produces direction-dependent and history-dependent behavior. Repeated loading generates asymmetric stress and deformation responses, incomplete recovery of the initial state, and progressive changes that depend on packing condition and confinement level. Densely packed and loosely packed assemblies exhibit qualitatively different cyclic responses, while normal confinement primarily influences the strength and persistence of those responses.
\r\n\r\nAt the grain-scale, the results show that cyclic behavior is governed largely by the reorganization of the internal force network rather than by large changes in overall contact connectivity. Force chains evolve continuously with loading direction and cycle history, while anisotropy plays a central role in linking internal structure to bulk shear resistance. Dense systems initially develop stronger, more coherent load-bearing structures that weaken with repeated cycling, whereas loose systems deform via more distributed force transmission and progressive compaction.
\r\n\r\nOverall, this work provides a mechanistic framework for understanding how granular materials accumulate history, weaken, and reorganize under repeated shear. The combined experimental and GEM results also provide benchmark data to calibrate and validate physics-based particle models for broader granular systems and relevant engineering applications.
", "doi": "10.7907/eydy-1z48", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17120", "collection": "thesis", "collection_id": "17120", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03312025-221040469", "primary_object_url": { "basename": "PhD Thesis_SLee.pdf", "content": "final", "filesize": 40115968, "license": "other", "mime_type": "application/pdf", "url": "/17120/7/PhD Thesis_SLee.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Multiscale Design, Fabrication, and Mechanical Analysis of Structural Hierarchies in Functional Materials", "author": [ { "family_name": "Lee", "given_name": "Seola", "orcid": "0000-0002-4538-0890", "clpid": "Lee-Seola" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Wang", "given_name": "Zhen-Gang", "orcid": "0000-0002-3361-6114", "clpid": "Wang-Zhen-Gang" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Hierarchical structuring has emerged as a powerful strategy in functional material design to enhance mechanical performance and impart functional properties across multiple scales. In architected materials, leveraging tessellated multiscale geometrical features enables unconventional properties such as ultra-low density and high energy absorption. Similarly, in functional polymers, rational design of molecular chemistries and polymer microstructures allows for tunable mechanical properties and stimuli-responsive behaviors. However, a substantial knowledge gap persists in understanding how multiscale interactions connect to determine the macroscale performance of these materials. This gap arises from challenges in scalable fabrication, multiscale characterization, and limited mechanistic insight from theory and simulations. To address these challenges, this thesis presents a comprehensive approach that integrates scalable fabrication, multiscale characterization, and theoretical modeling to develop hierarchical materials with tunable functionalities. Specifically, we (1) demonstrate scalable fabrication of hierarchical materials using additive manufacturing, (2) investigate the bulk mechanical responses by tuning the smallest level in hierarchical design, and (3) perform multiscale studies to bridge the gap between unit-level interactions and macroscale performance.
\r\n\r\nIn the first study, we explore the role of structural hierarchies in architected polymeric materials for enhanced energy dissipation. Using metasurface-based holographic lithography, we fabricate nano-architected polymeric sheets and demonstrate how geometrical parameters for unit cell design such as relative density and beam aspect ratio influence stiffness, energy dissipation, and deformation modes. These findings highlight the significance of hierarchical structuring in enhancing mechanical performance and establish design principles for scalable manufacturing. In the second study, we focus on dynamic polymers and examine how dynamic crosslinking at the molecular level influences macroscale material responses. A single-step stereolithography approach is developed to tune molecular-level controls in the material. Through multiscale modeling and experimental characterizations, we reveal how dynamic bonding mechanisms govern stiffness, stretchability, and fracture energy. The results underscore the significance of multiscale interactions in tuning mechanical behavior and suggest a pathway for designing materials with programmable responses. In the final study, we build on these insights by integrating molecular-level controls with nonlinear structural responses such as buckling and shape transformations. The central premise is that molecular interactions dictate local responsiveness, while structural geometries can amplify or suppress these responses through localized deformation or stress redistribution. As a demonstration, we explore how tailored viscoelasticity and controlled instabilities can determine the buckling mode of a structural beam. This synergistic interplay highlights the potential of the materials requiring programmed reconfigurability, shape morphing, and stimuli-responsive properties.
\r\n\r\nThe findings presented in this thesis offer a robust framework for bridging molecular-level design with macro-scale performance through scalable fabrication and characterization strategies. By expanding the design space of material-level behavior, this work lays the groundwork for developing next-generation materials with enhanced functionality, adaptability, and intelligence for applications such as soft robotics, healthcare, and sustainable materials.
", "doi": "10.7907/1ee0-bc98", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16934", "collection": "thesis", "collection_id": "16934", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12172024-163805389", "type": "thesis", "title": "Thermoelastic Deflections of Thin-Shell Composite Space Structures", "author": [ { "family_name": "Pederson", "given_name": "John Monroe Jr.", "orcid": "0009-0002-8500-2285", "clpid": "Pederson-John-Monroe-Jr" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "3MT Competition (Caltech)" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "As space structures become larger, lighter, and deployable, thermal deflections induced by sunlight become a significant source of structural inaccuracy and even spacecraft vibration. Studying these deflections is notoriously difficult: analytical solutions rapidly become intractable, experiments under vacuum and cooling are low-visibility and expensive, and multiphysics finite-element simulations are computationally demanding and usually don\u2019t account for coupled thermo-structural analyses and/or changing radiation view factors.
\r\n\r\nThis work demonstrates key improvements in experimental methods and thermo-structural simulation of these thermal deflections. First, simultaneous full-field measurements of structural temperatures and deflections are achieved by constructing and using a custom vacuum chamber and heating setup; significant thermal gradients and repeatable thermal deformations are measured and analyzed, forming a ground truth for succeeding simulations. Second, multiphysics models of the experimental chamber are created in COMSOL Multiphysics and characterized, even accounting for residual convection, and used to inform prototype improvements and more advanced simulations. Third, based off such predictions, the unit structure prototype composite is improved by adding a layer of graphitized polymer film, with further experimentation showing a dramatic reduction in deflections.
\r\n\r\nFinally, the accumulated knowledge is used to simulate a satellite slew maneuver with realistic orbital heating; a custom technique to couple thermal (Thermal Desktop) and structural (Abaqus) finite-element software via a MATLAB script allows for the recalculation of radiation view factors during simulations, a feat necessary for accurate heating calculations on deployable structures. These results have immediate applicability in predicting structural temperatures and deflections during the satellite maneuvers proposed for the Caltech Space Solar Power Project, as well as suggesting critical improvements to ensure reliability and mission success.
", "doi": "10.7907/xyry-w852", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17128", "collection": "thesis", "collection_id": "17128", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04022025-010011484", "primary_object_url": { "basename": "Caltech_Thesis_Tracy_Lu_Final.pdf", "content": "final", "filesize": 46306862, "license": "other", "mime_type": "application/pdf", "url": "/17128/1/Caltech_Thesis_Tracy_Lu_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Variable-Stiffness and Shape-Morphing Structured Media", "author": [ { "family_name": "Lu", "given_name": "Tracy", "orcid": "0009-0004-4919-814X", "clpid": "Lu-Tracy" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "orcid": "0000-0003-3741-0364", "clpid": "Andrade-J-E" }, { "family_name": "Bordeenithikasem", "given_name": "Punnathat", "orcid": "0000-0002-0590-2447", "clpid": "Bordeenithikasem-Punnathat" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Advancements in additive manufacturing and material synthesis with highly controlled geometries have enabled the creation of structured media, engineered materials with patterned micro- and meso-scale geometries that impart unique mechanical properties. By fine-tuning these architectures, structured materials can achieve properties beyond those of their base materials. A subcategory, structured fabrics, consists of discrete granular particles rather than continuous fibers. Their mechanical behavior is governed by jamming, a transition driven by geometric constraints, allowing them to switch between flexible and rigid states. By leveraging the interactions of the building blocks, structured fabrics enable tunable stiffness, global shape change, and adaptive functionalities, making them ideal for wearable, deployable, and morphing structures.
\r\n \r\nThe first structured fabric study explores a topologically interlocking material (TIM) system with adjustable bending stiffness controlled by external pre-stress. The system consists of truncated tetrahedral particles connected by tensioned nylon wires, allowing stiffness to be tuned by varying wire tension. Experiments examine the effects of surface friction and interlocking angle on bending response, guided by Level Set Discrete Element Method (LS-DEM) simulations. The second design presents deployable 3D structures that fold without rigid mechanisms, offering compact storage and stable deployment. The design consists of computationally generated rigid tiles adhered to a pre-stretched elastic sheet, which transforms from a flat state and jams into a predetermined 3D shape when released. Although the designs exhibited unique mechanical properties, experimentally understanding their internal mechanics was challenging due to limited visibility of the concealed membrane upon jamming. To optimize future designs, simulations were conducted to analyze the effects of various pattern designs and folding on membrane behavior.
", "doi": "10.7907/7wvv-cs52", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16636", "collection": "thesis", "collection_id": "16636", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08142024-070224435", "primary_object_url": { "basename": "Jagannadh_Boddapati_2025_Thesis_Redacted.pdf", "content": "final", "filesize": 79546983, "license": "other", "mime_type": "application/pdf", "url": "/16636/8/Jagannadh_Boddapati_2025_Thesis_Redacted.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Shear-Normal Coupled Deformations in Anisotropic Structured\r\nMaterials", "author": [ { "family_name": "Boddapati", "given_name": "Purna Chandra Jagannadh Kumar", "orcid": "0000-0001-8706-5963", "clpid": "Boddapati-Purna-Chandra-Jagannadh-Kumar" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The advent of additive manufacturing has allowed the design and engineering of a new class of materials known as metamaterials, or structured/architected materials. These metamaterials exhibit unique functionalities, such as ultrahigh strength-to-density ratios, which their base materials cannot achieve. Often designed to exhibit near-isotropic behavior, metamaterials derive their special properties from the distinctive deformation, dynamic motion, and elastic energy distribution of their micro- and meso-architectures. However, designing metamaterials for anisotropy, despite their ability to attain unique properties, is challenging. Fully characterizing anisotropic stiffness in planar loading requires six independent elastic tensor moduli. This high number of independent elastic stiffness parameters also expands the design space of structured materials and leads to unusual phenomena, such as materials that can shear under uniaxial compression. This direction-dependent shear-axial coupling is crucial for many applications such as shape-morphing, elastic wave manipulation devices and impact redirection.
\r\n\r\nThis thesis aims to understand the fundamental limits of shear-normal coupled deformations in anisotropic structured materials. Currently, there are no established upper and lower bounds on anisotropic moduli achieving extreme elastic anisotropy, similar to the Hashin-Shtrikman bounds in isotropic composites. This range is known as G-closure and provides limits for the achievable tensors. To date, there are no experimental methods that can measure the stiffness parameters of fully anisotropic structured materials from a single experiment. To address these challenges, we first introduce a method to generate two-phase periodic anisotropic unit cell geometries and construct a database of unit cells with a diverse range of effective elasticity tensors. The constructed database is compared with the properties achieved by hierarchical laminates and identify the regions where hierarchical designs are necessary to reach a specific extreme elasticity tensor.
\r\n\r\nWe then propose an experimental methodology to evaluate the anisotropic material properties. Our technique, which utilizes the virtual fields method, allows for the determination of six separate stiffness tensor parameters of two-dimensional structured materials using just one tension test. This method thus eliminates the need for multiple experiments as is typical in traditional methods. We show the accuracy of our method using synthetic data generated from finite element simulations as well as by conducting experiments on four additively manufactured specimens. The approach requires no stress data and uses the full-field displacement data measured using digital image correlation and global force data.
\r\n\r\nWe present a method for creating functionally graded anisotropic structures that smoothly transition between unit cells with distinct patterns. Isotropic materials with spatially varying density gradients have been shown to exhibit unique characteristics such as superior energy absorption. However, achieving smooth spatial gradients in the anisotropic mechanical properties while ensuring the connectivity of adjacent meso-architectures is non-trivial. This method allows for independent control of several functional gradients, such as porosity, anisotropic moduli, and symmetry. We show that certain nonlinearly graded structures when designed with unit cells positioned at distinct corners of the property space boundary exhibit novel mechanical behaviors. We conclude by designing specific functionally graded structures that demonstrate peculiar behaviors such as selective strain energy localization, localized rotations, compressive strains under tension, and longitudinal-shear wave mode conversion.
", "doi": "10.7907/k1t8-4t34", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17290", "collection": "thesis", "collection_id": "17290", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292025-003646523", "primary_object_url": { "basename": "tran_thomas_2025.pdf", "content": "final", "filesize": 20060046, "license": "other", "mime_type": "application/pdf", "url": "/17290/1/tran_thomas_2025.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Microstructural and Mechanical Characterization of Additively Manufactured Binary Metallic Alloys", "author": [ { "family_name": "Tran", "given_name": "Thomas Tuan", "orcid": "0009-0003-7034-9486", "clpid": "Tran-Thomas-Tuan" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Nelson", "given_name": "Hosea M.", "orcid": "0000-0002-4666-2793", "clpid": "Nelson-H-M" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Hydrogel infusion-based additive manufacturing (HIAM) is a chemically versatile solid-state processing pathway that allows 3D structuring of ceramics and metals with micro-scale precision. Using controlled thermal treatments of 3D-printed metal ion-infused gels, this process generates intricate microstructures which are heavily influenced by the kinetics of gas-solid reactions and their subsequent phase evolution. This work seeks to refine our understanding of the process-structure-property relationships in HIAM-produced alloys and provide general insights for AM-enabled alloy development and microstructure design using metal oxide reduction.
\r\n\r\nThrough HIAM, we demonstrate the arbitrary alloying of CuxNi1-x binary alloys, where systematic characterization of microstructures down to the atomic scale revealed that reduction, or the lack thereof, drove the formation of chemically homogeneous alloy grains with numerous annealing twins and entrapped unreduced oxide nano-inclusions, resulting in a hierarchical two-phase composite. These features appear to elevate the average nanoindentation hardnesses by up to four times that of bulk annealed CuxNi1-x and lead to a composition dependence on the scaling of the \u201csmaller is stronger\u201d size effect in uniaxial micropillar compressions. This compositional dependence of hardness and deformation mechanisms arises from changes in reduction kinetics which influence the density of inclusions and voids developed by HIAM processing. As a result, HIAM demonstrates the capability to fabricate heterogeneous alloy systems as a result of their oxide reduction pathways, which are revealed by thermogravimetry experiments and kinetic analysis.
", "doi": "10.7907/ej4t-7e95", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17059", "collection": "thesis", "collection_id": "17059", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03132025-055626664", "primary_object_url": { "basename": "thesis_WenxinZhang.pdf", "content": "final", "filesize": 13336993, "license": "other", "mime_type": "application/pdf", "url": "/17059/18/thesis_WenxinZhang.pdf", "version": "v10.0.0" }, "type": "thesis", "title": "Advanced Nano Manufacturing Enables Probing Fundamental Mechanical Behaviors of Materials", "author": [ { "family_name": "Zhang", "given_name": "Wenxin", "orcid": "0000-0002-6318-0622", "clpid": "Zhang-Wenxin" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The trend of miniaturization has revolutionized modern technologies, with micro- and nanoscale materials driving transformative advancements in high-tech industries and scientific discovery. Among the various properties and applications enabled at these small scales, nanomechanical properties play a fundamental role, underpinning the integrity and functionality of any structures or systems. However, despite advancements in both conventional and emerging micro- and nano-manufacturing strategies, there has remained a lack of direct \u201cbottom-up\u201d experimental pathways to fabricate and probe the mechanical responses of submicron-sized monolithic nano-specimens with unconventional microstructures and/or 3D nano-architectures with submicron-sized features, particularly for non-carbon materials.
\r\n\r\nIn this work, I will present novel nano-fabrication and manufacturing strategies and their applications in addressing these nanomechanical challenges through three key studies. In Chapter 2, the deformation characteristic of organic ice is studied via cryogenic micro-compression and molecular dynamics simulations, providing insights into a benzene-ring re-orientation-mediated densification deformation route and offering new insights into planetary geology for celestial bodies such as Titan. In Chapter 3, we experimentally unveiled unprecedented two-regime size effects in additively manufactured metallic nanopillars with hierarchical microstructures, revealing a nanocrystallinity-, nanoporosity-mediated plasticity mechanism through atomistic insights. In Chapter 4, we extended this nano-manufacturing approach to explore nanoporosity-driven deformation behaviors in nano-architected metals with in situ experiments and finite element analysis. Together, these studies not only elucidate previously unprobed fundamental small-scale mechanical behaviors but also lay the groundwork for developing an advanced micro-to-nanoscale manufacturing platform, enabling complex systems and functional applications such as energy storage, biomedical microrobots, nanophotonics, and beyond, which I will briefly discuss in Chapter 5 as an outlook with a few examples from metal/oxide nanocomposites to interpenetrated pyrolytic carbon microarchitectures.
", "doi": "10.7907/fxq3-7817", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17018", "collection": "thesis", "collection_id": "17018", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02242025-045741904", "primary_object_url": { "basename": "Zhou_Ziran_2025.pdf", "content": "final", "filesize": 65604453, "license": "other", "mime_type": "application/pdf", "url": "/17018/1/Zhou_Ziran_2025.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Predictive Modeling of Architected Solids across Scales", "author": [ { "family_name": "Zhou", "given_name": "Ziran", "orcid": "0009-0008-9327-3505", "clpid": "Zhou-Ziran" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "orcid": "0000-0003-3741-0364", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "orcid": "0000-0003-3741-0364", "clpid": "Andrade-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Architected solids, comprising discrete or continuous materials and structures, are purposefully designed to achieve specific functional objectives, such as tailored mechanical properties or enhanced performance. The integration of architectural features and material science has revolutionized design and functionality across multiple length scales. However, experimental exploration of architected solids is often constrained by physical, financial, or technological limitations. To address these challenges, this study leverages computational models as powerful tools for validating and probing the behaviors of architected solids through three distinct case studies spanning different length scales.
\r\n\r\nThe first case study focuses on capturing the seismic performance of multiblock concrete structures at CERN for radiation shielding. The Level Set Discrete Element Method (LS-DEM), combined with Monte Carlo sampling of material properties, is employed to benchmark the displacement profiles of four concrete configurations against experimental data. In the second case study, a bonded LS-DEM model is utilized to investigate the bending response of a woven topological interlocking material (TIM). After validation against experimental results, the model is employed to explore how friction and contact area influence the bending resistance of the TIM system. The third case study introduces a 3D translational tensegrity structure modeled using the Finite Element Method (FEM). This model captures the deformation responses of single cells, monolayers, and multicellular spheroids under various loading conditions. Additionally, a data-driven (DD) framework with multiscale analysis is implemented, offering accurate results with enhanced computational efficiency. Through these three case studies, this research illustrates the evolution of computational models from tools for validating known behaviors to frameworks for exploring new phenomena.
", "doi": "10.7907/cn82-z804", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17190", "collection": "thesis", "collection_id": "17190", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05012025-194857973", "primary_object_url": { "basename": "Brayden_Aller_Thesis.pdf", "content": "final", "filesize": 383178046, "license": "other", "mime_type": "application/pdf", "url": "/17190/1/Brayden_Aller_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Strain Sensing in Thin Composite Laminates with Embedded Fiber Bragg Grating Sensors", "author": [ { "family_name": "Aller", "given_name": "Brayden Gieschen", "orcid": "0000-0002-5258-4154", "clpid": "Aller-Brayden-Gieschen" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Watkins", "given_name": "Michael M.", "clpid": "Watkins-M-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Deployable structures are popular for space applications as they enable large, complex spacecraft structures to overcome the size constraints of launch vehicle fairings. Such structures are increasingly manufactured out of thin (< 200 \u03bcm thick) composite laminates as they have a high stiffness-to-weight ratio, the ability to withstand high curvatures during stowage, and the potential for self-deployment using stored strain energy. To ensure the reliability of these thin composite spacecraft structures in operation, it is of interest to be able to continuously monitor their internal strain state to detect potential changes or damage that may compromise their integrity.
\r\n \r\nAlthough there are a number of potential sensors that could be used for this, fiber Bragg grating (FBG) sensors are especially well suited for this task and have a track record of successfully monitoring both composite materials and large aerospace structures. However standard size FBG sensors, which have a cladding diameter of 125 \u03bcm, are too large to be integrated into the thin composite structures of interest. To overcome this, we worked with several suppliers to develop and manufacture ultra-thin FBG sensors (< 30 \u03bcm cladding diameter) for this work that are able to be successfully embedded into thin composite laminates.
\r\n\r\nThe primary objective of this thesis was to investigate the suitability of ultra-thin FBG sensors for the monitoring of strain changes in thin composite spacecraft structures. To this end, the work in this thesis first investigated how to best embed ultra-thin FBG sensors to be able to measure the internal strain changes of interest while minimizing their disruptions to the surrounding laminates. Second, mechanical testing was performed to assess the effect that the embedded ultra-thin FBG sensors have on the mechanical properties of thin laminates. Third, the ability of these sensors to detect and monitor for strain changes in thin composite laminates was assessed through further mechanical testing. Finally, the effects of temperature on ultra-thin FBG sensors were studied experimentally.
\r\n\r\nThrough this work, which was done at the coupon level, we sought to demonstrate the ability of these ultra-thin FBG sensors to monitor for strain changes in thin composite laminates and their potential for the health monitoring of thin composite spacecraft structures. It is our hope that our findings in this thesis help lay the groundwork for the future implementation of these sensors in not only thin composite spacecraft structures, but to many other composite materials and aerospace structures as well.
", "doi": "10.7907/zj2k-h305", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16838", "collection": "thesis", "collection_id": "16838", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11072024-033128289", "primary_object_url": { "basename": "Dassanayake_Sahangi_2025a.pdf", "content": "final", "filesize": 122088212, "license": "other", "mime_type": "application/pdf", "url": "/16838/27/Dassanayake_Sahangi_2025a.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Space Legos: A Concept for In-Space Assembly of Large Structures with a Stationary Robot", "author": [ { "family_name": "Dassanayake", "given_name": "D. M. Sahangi Pulsarani", "orcid": "0000-0002-1363-5764", "clpid": "Dassanayake-D-M-Sahangi-Pulsarani" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "3MT Competition (Caltech)" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Human nature is inherently driven by the desire to build; advancing from primitive shelters to skyscrapers, and extending this relentless pursuit of progress to space through technological innovations. As space missions require larger and more complex structures, traditional deployable systems face challenges due to constraints on launch mass, volume, and complex deployment mechanisms. In-space assembly (ISA) offers a promising solution for constructing large structures, such as telescopes and satellites, directly in space.
\r\n\r\nThis thesis introduces a novel ISA concept with a centralized `truss builder' for autonomous assembly of polygonal-ring structures, using simple, repetitive operations and focusing on scalable mesh reflectors for communication and imaging. Utilizing the standard AstroMesh architecture, a rapid generalized design method is developed. Through the analysis of reflector geometry, optimized cable prestress, structural design, and a high-fidelity finite element model, analytical scaling laws are derived for mass, stowed envelope, and natural frequency based on aperture diameter. A semi-analytical homogenization model is introduced to efficiently predict fundamental natural frequencies. Stowed volume is a key limitation for large deployable reflectors, approaching current and future launch capacity limits, while the proposed ISA reflectors face no such constraints for apertures up to 200 meters.
\r\n\r\nA two-dimensional finite element model simulates the assembly kinematics of large ring-like structures with the proposed ISA concept, enhancing understanding of the process and evaluating key design aspects of a stationary robot assembling scalable ring-like trusses. The model provides insights for optimizing autonomous assembly systems and underscores the need for advanced numerical simulations to ensure smooth assembly and stability during ISA, especially as structures scale.
\r\n\r\nLab-scale prototype testing validates the ISA concept, with results aligning qualitatively with simulations. Both experiments and simulations reveal a range of viable solutions, demonstrating flexibility for future mission designs. This research offers crucial insights into the design and scaling of mesh reflectors, setting the stage for comparing ISA with traditional deployable systems. The proposed ISA concept presents a practical solution for building high-precision, large-scale structures in space, advancing the field of space construction and supporting future extended space missions.
", "doi": "10.7907/0pn5-3w42", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17276", "collection": "thesis", "collection_id": "17276", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272025-224848756", "type": "thesis", "title": "Mechanical Characterization of Irregular Architected Two-Phase Materials", "author": [ { "family_name": "Fox", "given_name": "Chelsea Brae", "orcid": "0009-0002-6612-8309", "clpid": "Fox-Chelsea-Brae" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Architected materials offer a wide range of mechanical properties through the choice of their constitutive materials and the design of their structure. Periodic architected materials are the most widely studied and used in practical applications, as their repeating unit cells are easy to design, fabricate, and analytically model, but these materials are only a small subset of the possible design space. Irregular architected materials, which are aperiodic but not necessarily stochastic, offer a way to achieve a wider design space of mechanical properties.
\r\n\r\nIn this thesis, we explore the design space of irregular architected materials and relate structural irregularity to the mechanical properties using measures of topology and geometry. We show that these measures of irregularity can be used to spatially and temporally control the mechanical response across linear and non-linear regimes, including fracture and dynamic impact, and we show that irregularity leads to improved mechanical properties when compared with periodic equivalents. To generate the irregular architected materials, we use a virtual growth algorithm, which imitates the stochastic growth process of biological structures by assembling a finite set of building blocks according to local connectivity rules. By varying the building blocks and connectivity rules, we show how to achieve a wide range of structures with varying degrees of irregularity all the way up to fully periodic structures. This thesis primarily focuses on the fabrication and characterization of additively manufactured two-phase polymer composites, but the design methods and irregular structure characterizations are material-agnostic, opening up a wide design space for future architected materials which use irregularity to achieve excellent mechanical performances.
", "doi": "10.7907/58vg-9217", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17272", "collection": "thesis", "collection_id": "17272", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262025-195518377", "primary_object_url": { "basename": "Aakila_Thesis-FinalProofread.pdf", "content": "final", "filesize": 26063218, "license": "other", "mime_type": "application/pdf", "url": "/17272/1/Aakila_Thesis-FinalProofread.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Methods for Learning Mechanics: Inverse Problems, Constitutive Modeling, and Design", "author": [ { "family_name": "Rajan", "given_name": "Aakila", "orcid": "0000-0001-9218-5062", "clpid": "Rajan-Aakila" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Buskohl", "given_name": "Philip", "orcid": "0000-0001-5517-1956", "clpid": "Buskohl-Philip" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Predicting the behavior of materials and structures under complex loading is a fundamental challenge in solid mechanics. Traditional techniques rely on idealized experiments, a priori information and are computationally intensive. This work introduces a unified framework for constitutive identification, multiscale modeling, and design optimization, grounded in physical laws and enhanced by machine learning.
\r\n\r\nWe first focus on the inverse problem of identifying constitutive behavior from experimental data. We formulate this task as an optimization problem with the governing equations of the experiment as a constraint. This allows us to infer material parameters from full-field or contact-based measurements. This approach enforces physical laws and accommodates complex loading, noise, and limited data. We apply it to recover properties for a history-dependent material using both synthetic and experimental datasets. We further extend the framework using recurrent neural operators to learn constitutive responses directly from data, bypassing the need for an explicit model form.
\r\n\r\nIn the second part of the thesis, we extend our focus to multiscale modeling and structural design. Neural operators are used to learn homogenized solutions of linear elliptic PDEs with discontinuous coefficients, eliminating the need to resolve fine-scale features during inference. For topology optimization, we develop reduced-order neural surrogates embedded within the design loop, achieving efficient yet accurate updates. Together, these contributions offer a cohesive, data-driven strategy for advancing modeling and design in solid mechanics.
", "doi": "10.7907/caht-w798", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16637", "collection": "thesis", "collection_id": "16637", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08142024-141724425", "primary_object_url": { "basename": "Shengduo_Liu_Thesis_Final.pdf", "content": "final", "filesize": 68545231, "license": "other", "mime_type": "application/pdf", "url": "/16637/1/Shengduo_Liu_Thesis_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Numerical Analyses of Frictional Sliding on Rate-and-State Interfaces: Fluid Effects, Dynamic Weakening, and Potential-Based Formulation Through Machine Learning", "author": [ { "family_name": "Liu", "given_name": "Shengduo", "orcid": "0009-0009-5259-0966", "clpid": "Liu-Shengduo" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Rate-and-state friction formulations have been widely used to reproduce a number of observations on faulting in the earth's crust, including earthquake nucleation, creeping fault segments, dynamic earthquake rupture, aftershock sequences, and episodic slow slip events. The formulations have also been used to explain the motion of landslides and glaciers. In this thesis, we use numerical simulations to study various factors that can affect the stability of fault slip with rate-and-state friction, including poroelastic bulk properties and dilatation/compaction of the fault material in the presence of fluids, fault healing, injection rate when there is fluid injected into the fault, as well as dynamic weakening of the fault gouge. We also seek to optimize simulations with rate-and-state friction by developing a potential-based formulation using machine learning.
\r\n\r\nFirst, we study the stability of frictional fault slip in the presence of fluids, with a focus on fault loading due to fluid injection into the fault as done in many field and laboratory experiments. In Chapter 2, we present a boundary-integral approach on simulating frictional fault slip in a permeable shear layer surrounded by poroelastic bulk. The approach is then used to explore the effects of poroelasticity and inelastic dilatancy on the stability of frictional fault slip in a fluid-injection problem. We find that the diffusion into and poroelastic properties of the bulk can significantly stabilize fault slip, with the stabilization by bulk diffusion and poroelastic properties comparable to the well-known stabilizing effects of the dilatancy mechanism.
\r\n \r\nIn Chapter 3, we further develop the boundary integral code to allow for purely elastic bulk with the same fluid transport properties as the poroelastic bulk material and consider the effect of fault healing and fluid injection rate on fault slip. We show that the poroelastic bulk effects can be very closely captured by using the undrained value of Poisson\u2019s ratio in an elastic bulk model with the same fluid mass diffusivity of the bulk. We find that fault healing significantly delays the onset of dynamic slip events and restricts their spatial extent, making the initial response of the fault to fluid injection much different than its longer-term response. While this is an expected conclusion, fault healing is not typically accounted for in fluid injection modeling which often uses simpler slip-dependent friction laws. We also find that faster or intermittent injection rates lead to more frequent but more spatially constrained dynamic slip events, for the same injected fluid mass, motivating further investigations into injection strategies that would optimize fault stability.
\r\n \r\nSecond, in Chapter 4, we numerically simulate a laboratory experiment of spontaneous dynamic rupture by developing a 3D finite-element model of the experiment with rate-and-state friction. In the experiment, a dynamic rupture is initiated on a Homalite-100 interface and then produces an intermittent slip in the rock gouge embedded into a part of the interface. Our simulations show that the laboratory findings are consistent with rock gouge which is rate-strengthening at low slip rates but dynamically weakening at high slip rates through the mechanism similar to flash heating. However, to fit the experimental results, the traditional flash-heating formulation needs to be substantially modified, potentially due to effects of localization and delocalization of slip in the rock gouge.
\r\n\r\nThe third part of the thesis focuses on identifying a potential-based formulation for the rate-and-state friction laws. Due to their empirical derivation, the rate-and-state friction laws cannot be written as the gradients of a potential, which leads to difficulties in implicit solution of dynamic frictional problems. In Chapter 5, we present a potential-based formulation for the rate-and-state friction law through Neural Network approximation and training on datasets generated by a one-degree of-freedom spring-slider system with the rate-and-state friction law. The learnt potential is able to reproduce the results with rate-and-state friction law, and indeed facilitates an implicit solution of dynamic problems. However, the training of the potential requires a much larger dataset than fitting the original rate-and-state friction law.
\r\n\r\nOverall, our modeling significantly advances our understanding of the factors that control stability of frictional sliding on natural faults and suggests promising machine-learning directions in replacing the empirical rate-and-state formulations with the ones based on thermodynamic potentials.
", "doi": "10.7907/1tzb-pn69", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17306", "collection": "thesis", "collection_id": "17306", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302025-081050551", "primary_object_url": { "basename": "Lawlor_Barry_2025.pdf", "content": "final", "filesize": 172840356, "license": "other", "mime_type": "application/pdf", "url": "/17306/1/Lawlor_Barry_2025.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Full-Field Quantitative Visualization of Shock-Driven Pore Collapse in Solids: Mechanics of Deformation, Failure, and Interaction", "author": [ { "family_name": "Lawlor", "given_name": "Barry Patrick", "orcid": "0000-0003-4487-2129", "clpid": "Lawlor-Barry-Patrick" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Mello", "given_name": "Michael", "orcid": "0000-0003-2129-9235", "clpid": "Mello-Michael" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Porosity in solids is ubiquitous throughout engineering applications: inherent in energetic materials and exaggerated upon degradation, incorporated into shock-absorbing structures via materials such as metallic foams and metamaterials, and arising through manufacturing defects---especially in metal additive manufacturing methods. In these applications, many phenomena at both the macro- and meso-scale are critical to the operation under dynamic compression. Macroscopic shock wave structure, including shock attenuation and disruption are important for engineered structures like metallic foams, while mesoscopic localized shear deformation near porous defects can be a cause of failure in structures and is thought to be a mechanism for mechanically-induced hot spots in energetic materials which can dictate their ignition behavior. While the macroscopic shock response of porous materials has been well studied, the mesoscopic response has received less attention. Recent studies have improved the understanding through sophisticated numerical simulations and pore collapse experiments leveraging innovative high-speed imaging technologies, but many details of the mesoscopic response remain unclear.
\r\n\t\r\nThis thesis is focused on the mesoscopic domain, with an overarching goal of characterizing local details of pore collapse, such as the rate of collapse, pore geometry (asymmetry) evolution, deformation induced in the material surrounding the pore, localization/failure mechanisms, and interactions between pores. Fundamental understanding of these mesoscopic phenomena is a critical step toward unraveling the physics which couple the mesoscale and macroscale responses, enabling predictive modeling for the dynamic response of porous materials/structures, and developing innovative engineering designs with porous materials.
\r\n\t\r\nThe first part of this thesis develops a novel internal digital image correlation (DIC) technique for use in full-scale dynamic laboratory experiments, which enables investigation of phenomena which occur under confinement or are sensitive to boundary effects. The technique consists of manufacturing transparent specimens with an internally embedded speckle pattern, which is then dynamically deformed via the experiment of choice. During dynamic loading, the internal speckle pattern is visualized with a high-speed camera, after which DIC software is used to process the images and compute the displacement, velocity, and strain fields. The technique is implemented and validated using polymethyl methacrylate (PMMA) specimens under compression with split-Hopkinson (Kolsky) pressure bar and plate impact experiments---providing validation under both uniaxial stress and uniaxial strain conditions, at strain rates of 10\u00b3-10\u2076 s\u207b\u00b9 and impact stresses up to 0.65 GPa.
\r\n\t\r\nThe second part of the thesis implements the internal DIC technique to investigate the mechanics of a single spherical pore during collapse induced by weak shock loading up to 1 GPa impact stress in PMMA. The first of its kind internal strain measurements reveal concentrations around the collapsing pore, which are approximately consistent with elastostatic theory. Equivalent shear strain measurements uncover a transition from classical strain concentrations to the development of shear bands at 0.6 GPa, and raw deformation images show the development of fracture at 0.8 GPa---representing two distinct failure mechanisms arising within a small range of impact stresses. The shear bands arise due to large stress concentrations near the pore, which leads to plastic deformation and heating. Thermal softening generates local material instabilities, which can grow into regions of large, localized deformation. These bands are captured via explicit finite element analysis through a thermo-viscoplastic material model. The numerical simulations further indicate the crack to be a shear crack propagating through the weakened material of an adiabatic shear band. Finally, theoretical approaches elucidate the mechanics which govern the initiation of, spacing between, and preferred paths for these failure modes.
\r\n\t\r\nThe third part of the thesis follows a natural extension toward real porous media, investigating the collapse of pore arrays in PMMA with a focus on the role of interactions between pores on the localization and failure response. Experiments are conducted on pairs of pores in vertical and horizontal configurations. By utilizing internal DIC and shadowgraphy, the evolution of shear bands and cracks is visualized and measured. Further, apparent interactions between pores are identified through shifts in impact stress thresholds for failure initiation and through delayed crack growth. Baroclinicity, and accompanying baroclinic torque, is identified as the driving mechanism for crack propagation in these experiments. Finally, shear diffraction waves initiate upon plane wave interaction with pores and propagate toward neighboring pores. This is considered as a possible interaction mechanism between pores which alters the failure response.
\r\n\t\r\nThe work presented in this thesis enabled the first in-situ observation of adiabatic shear banding during pore collapse in addition to a much-improved spatiotemporal characterization of crack propagation compared to previous works. Analysis of the experimental results revealed the ability of theoretical and numerical (FEA) models to capture many details of shear localization in pore collapse. Further analysis unraveled mechanisms governing pore collapse and associated failure modes, including the importance of pore asymmetry during collapse as well as planar shock interaction with the pore and the resultant baroclinicity and diffracted shear waves.
", "doi": "10.7907/p8hs-da41", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:15030", "collection": "thesis", "collection_id": "15030", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09152022-195715025", "type": "thesis", "title": "Mechanical Response of Lattice Structures under High Strain-Rate and Shock Loading", "author": [ { "family_name": "Weeks", "given_name": "John Stephen IV", "orcid": "0000-0002-7971-5919", "clpid": "Weeks-John-Stephen" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Lattice structures are a class of architected cellular materials composed of similar unit cells with structural components of rods, plates, or sheets. Current additive manufacturing (AM) techniques allow control and tunability of unit cell geometries, which enable lattice structures to demonstrate exceptional mechanical properties such as high stiffness- and strength-to-mass ratios and energy absorption. Lattice structures exist on two length scales corresponding to the unit cell and continuum material, and therefore demonstrate mechanical behavior dependent on structural geometry and base material. These effects extend to the dynamic regime where lattice structures demonstrate distinct deformation modes under varying strain-rate loading. Experimental investigation of the dynamic and shock compression behavior of lattice structures remains largely unstudied and is the central focus of this thesis where the high strain-rate, transient dynamic, and shock compression behaviors of different topologies of lattice materials are explored.
\r\n\r\nThe first part of this thesis investigates the high strain-rate behavior of lattice structures via polymeric Kelvin lattices with rod- and plate-based geometries and relative densities of 15-30%. High strain-rate behavior is characterized by deformation modes similar to that of low strain-rate behavior. High strain-rate experiments (1000/s) are performed and validated using a viscoelastic polycarbonate split-Hopkinson (Kolsky) pressure bar system coupled with high-speed imaging. Both low and high strain-rate experiments show the formation of a localized deformation band which initiates in the middle of the specimen. Strain-rate effects of lattice specimens are observed to correlate with effects of the base polymer material and mechanical properties depend strongly on the relative density of the lattice specimen and exhibit distinct scaling with geometry type (rod, plate) and loading rate despite a similar unit cell shape. Explicit finite element simulations with a tensile failure material model are then used to validate deformation modes and scaling/property trends, and match those observed in experiments.
\r\n\r\nThe second part of this thesis explores the transient dynamic and transition to shock compression behavior of lattice structures using polymeric lattices with cubic, Kelvin, and octet-truss topologies with relative densities of about 8%. Transient dynamic behavior is characterized by a compaction wave initiating at an impact surface and additional deformation bands with modes similar to low strain-rate modes of deformation. Dynamic testing is conducted through gas gun direct impact experiments (25 - 70 m/s) with high-speed imaging coupled with digital image correlation (DIC) and a polycarbonate Hopkinson pressure bar. Full-field DIC measurements are used to characterize distinct mechanical behaviors induced by topology such as elastic wave speeds, deformation modes, and particle velocities. At lower impact velocities, a transient dynamic response is observed. At higher impact velocities, shock compression behavior occurs and is characterized by a sole compaction wave initiating and propagating from the impact surface of the lattice. One-dimensional continuum shock theory with Eulerian forms of the Rankine-Hugoniot jump conditions is used with full-field measurements to quantify a non-steady shock response and the varied effect of topology on material behaviors.
\r\n\r\nThe final part of this thesis examines the steady-state shock compression behavior of lattice structures through stainless steel 316L (SS316L) octet-truss lattices with relative densities of 10-30%. Powder gun plate impact experiments (270 - 390 m/s) with high-speed imaging and DIC are conducted and reveal a two-wave structure consisting of an elastic precursor wave and a planar compaction (shock) wave. Local shock parameters of lattice structures are defined using full-field DIC measurements and a linear shock velocity (us) versus particle velocity (up) relation is found to approximate measurements with a unit slope and linear fit constant equal to the crushing speed. One-dimensional continuum shock analysis is again performed using Eulerian forms of the Rankine-Hugoniot jump conditions to extract relevant mechanical quantities. Explicit finite element simulations of the lattice specimens using the Johnson-Cook constitutive model exhibit similar shock behavior to experiments. The simulations reveal a linear us-up relation and corresponding Hugoniot calculations agree with experimental trends. Notably, 1D shock theory is applied to simulations without resorting to a us-up relation for the base material, which characterizes this deformation regime and compaction wave as a `structural shock.'
\r\n\r\nMajor contributions of this thesis include experimental demonstration of ranged strain-rate behaviors for lattice structures of various base materials and topologies including low strain-rate, high strain-rate, transient dynamic, and shock compression regimes; use of full-field quantitative visualization techniques for local mechanical behavior and shock analysis; and finally, characterization of a 'structural' shock compression regime in lattice structures.
", "doi": "10.7907/9v5k-1157", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15038", "collection": "thesis", "collection_id": "15038", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10032022-224522966", "type": "thesis", "title": "The Hemodynamics of Native and Surgical Aortic Valves with Regards to Wall Shear Stress and Residence Time", "author": [ { "family_name": "Rosakis", "given_name": "Alexandros Yiannis", "orcid": "0000-0001-9170-1002", "clpid": "Rosakis-Alexandros-Yiannis" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Shapiro", "given_name": "Mikhail G.", "orcid": "0000-0002-0291-4215", "clpid": "Shapiro-M-G" }, { "family_name": "Koochesfahani", "given_name": "Manoochehr", "orcid": "0000-0002-7001-8455", "clpid": "Koochesfahani-Manoochehr" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Cardiovascular diseases are the leading causes of illness and death all around the world. The third most common cardiovascular disease is aortic stenosis (AS). AS is most commonly characterized as a stiffening of the native trileaflet aortic valve, which impedes blood flow into the aorta and puts extra stress on the heart. The aorta is the main artery that supplies oxygenated blood to the body. AS has been widely studied in the past. However, there has been little work in understanding the complex effects that non uniform stiffening of the aortic valve can have on the hemodynamics inside the aorta.\r\n\r\nThe most effective treatment for AS is to replace the stiffened valve with a prosthetic valve. Care must be taken to ensure that the replacement actually performs better hemodynamically. A major metric for prosthetic valve performance is the transvalvular pressure drop which is a measure of how much pressure, and energy, is lost as the heart pumps blood through the valve. Generally speaking, larger valves exhibit a smaller pressure drop because they restrict the flow to a lesser degree. This phenomenon has led to a trend for surgeons to implant the largest prosthetic valve possible, and in some cases, to expanding the aorta to fit even larger valves. However, there has been relatively little work done on determining the effects of valve oversizing on the blood flow inside the Aorta. \r\n\r\nThe aims of this study were two-fold. First, a model of AS was tested inside an in vitro aortic simulator in order to identify how different individual leaflet stiffnesses would affect blood flow. Digital particle image velocimetry (DPIV) was used to measure velocity profiles inside a model aorta. The DPIV results were used to estimate the wall shear stress and blood residence time. Our analysis suggests that leaflet asymmetry greatly affects the amount of WSS by vectoring the systolic jet and that stiffened leaflets have an increased residence time. This study indicates that valve leaflets with different stiffness conditions can have a more significant impact on wall shear stress than stenosis caused by the uniform increase in all three leaflets (and the subsequent increased systolic velocity) alone. Second, the experimental apparatus was used to test different prosthetic valve sizes and valve mounting methods in order to identify how they affected residence time inside the sinus bulge. Dye residence experiments and DPIV were used to measure fluid stasis in several different combinations of prosthetic valve sizes, sinus sizes, and valve mounting methods. Our results indicate that valve to sinus sizing and mounting method is very important and can lead to greatly increased residence time and thrombosis risk. We have also identified a metric that can predict the threshold at which valves become oversized.", "doi": "10.7907/exsa-nm30", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15152", "collection": "thesis", "collection_id": "15152", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05052023-185856720", "primary_object_url": { "basename": "Vatsa_Thesis_Final.pdf", "content": "final", "filesize": 51493548, "license": "other", "mime_type": "application/pdf", "url": "/15152/1/Vatsa_Thesis_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Shock Compression of Body-Centered Cubic Metals from the Atomistic to Continuum Scale: Iron and Molybdenum", "author": [ { "family_name": "Gandhi", "given_name": "Vatsa Bhupeshkumar", "orcid": "0000-0002-6752-113X", "clpid": "Gandhi-Vatsa-Bhupeshkumar" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Mello", "given_name": "Michael", "orcid": "0000-0003-2129-9235", "clpid": "Mello-Michael" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Fundamental understanding of material behavior under extreme conditions is crucial for designing high strength, light weight, and high temperature resistance materials, and for modeling planetary physics problems such as behavior of the core and impact phenomena. Under extreme conditions, materials not only exhibit a different mechanical, thermal, and failure response but can also undergo structural changes, such as phase transformations, which significantly alters their material properties. This motivates studying their dynamic response and developing constitutive models for applications such as hypersonics, high speed manufacturing, impact and blast of structures, aircraft and spacecraft shielding, meteorite impact, and collision of planets. Despite the importance, experimental investigations of shock induced phase transitions, inelastic material behavior, and elastic-plastic anisotropy under multi-axial stress states and at microscopic length scales of metals still remains largely unexplored. Thus, the focus of this thesis is on the shock compression behavior of body-centered cubic (BCC) metals, specifically iron and molybdenum, under compression-shear loading and at the atomistic-continuum spatial scales. In particular, the role of solid-solid phase transformation of body-centered cubic (BCC) iron on material strength and the orientation dependence of single crystal molybdenum on its elastic-plastic transition is investigated.
\r\n\t \r\nIron in its high pressure hexagonal close-packed (HCP) \u03f5-phase is critical in geological and planetary applications such as inner cores of rocky planets and hypervelocity impacts of asteroids, and meteorites. Thus, understanding plasticity behavior of iron under these condensed matter states is important to develop more accurate models for such applications and to understand deformation mechanisms of inner planetary cores. Because the \u03f5-phase is unstable, iron reverts to its ambient \u03b1-phase (BCC) upon release making it difficult to probe the strength behavior using conventional methods. Additionally, solid-solid phase transformations provide a unique opportunity to study material strength as they are crucial for expanding the design space for various load-bearing applications. In the first part of the thesis, the pressure dependent dynamic strength behavior of both the ambient BCC \u03b1-phase and high-pressure HCP \u03f5-phase of iron at strain rates on the order of 1 X 10\u2075 s\u207b\u00b9 and pressures up to 42 GPa is investigated. Pressure shear plate impact experiments are conducted using a sandwich configuration to decouple the effect of pressure and shear thereby allowing to probe shear strength once the sample reaches an equilibrated state of pressure but prior to release. The strength of the \u03f5-phase is observed to be more than double the strength of \u03b1-phase possibly due to microstructural evolution during phase transformation. Additionally, the evolution of yield properties with pressure, temperature, and strain is presented for the first time, enabling more accurate modeling of extreme deformation phenomena associated with iron-rich celestial bodies such as planetary collisions.
\r\n\t\r\nMolybdenum, its alloys, and other body-centered cubic (BCC) refractory metals are critical in geological and planetary applications such as structural properties of terrestrial planetary composition, formation of the earth-moon system, and hypervelocity impacts of rocky planets. Additionally, the high temperature specific strength, creep resistance, and ductility of BCC refractory metals make them ideal for aerospace and armor/anti-armor applications. Under high strain-rate inelastic loadings, the macroscopic response of these metals is often influenced by the atomistic mechanisms including dislocation motion and deformation twinning. Current material models rely on investigations that involve continuum measurements followed by postmortem microstructural analysis of recovered samples. However, these may not reflect the material behavior during the passage of the shock wave and, thus, requires real-time in-situ atomistic characterization to link the microstructure to macroscopic response. In the second part of the thesis, plate impact experiments coupled with both laser interferometry continuum measurements and in-situ dynamic Laue x-ray diffraction (XRD), at the Advanced Photon Source (APS), are conducted on single crystal molybdenum. Here, the role of crystal orientation, either [100] or [111], on deformation mechanisms during the elastic-plastic transition and the steady state response is explored at pressures ranging from 9-19 GPa. Complementary simulation methodology is developed to analyze the evolution of the Laue diffraction spots captured during impact. By extracting the lattice strain and stresses from XRD images, dislocation slip along [110]\u2329111\u232a and [112]\u2329111\u232a is found to be the probable deformation mechanism during compression with negligible anisotropy observed at the Hugoniot state. For the first time, real-time evidence of molybdenum undergoing deformation twinning along [112\u0305]\u2329111\u232a during shock release beyond a critical pressure of 16 GPa irrespective of the loading orientation is presented.
", "doi": "10.7907/kwf1-7y79", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15202", "collection": "thesis", "collection_id": "15202", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232023-164041915", "primary_object_url": { "basename": "Zichen_Gu_PhD_Thesis.pdf", "content": "final", "filesize": 14373092, "license": "other", "mime_type": "application/pdf", "url": "/15202/1/Zichen_Gu_PhD_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Interparticle Forces and Stress Transfer in Saturated and Unsaturated Granular Systems", "author": [ { "family_name": "Gu", "given_name": "Zichen", "orcid": "0000-0002-6345-0178", "clpid": "Gu-Zichen" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "orcid": "0000-0003-3741-0364", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Andrade", "given_name": "Jose E.", "orcid": "0000-0003-3741-0364", "clpid": "Andrade-J-E" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Rittel", "given_name": "Daniel", "clpid": "Rittel-D" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Granular systems are ubiquitous in nature and engineering applications. The macroscopic behavior of such systems is governed by the behavior at the grain-scale, including force transfer between adjacent grains. The correlation between continuum behavior and interparticle forces in granular systems is yet to be fully understood. For a saturated or unsaturated granular system under external load, it is important to decode stress partition and transfer in the solid, fluid, and gas phases. In the meantime, the presence of the fluid phase and gas phase greatly increases the difficulty of measuring interparticle forces in opaque granular systems. This thesis describes the theoretical and experimental works on interparticle forces and effective stresses in two types of granular systems: i) fully saturated granular media, and ii) unsaturated granular media.
\r\n\r\nThe first part of the thesis focuses on the direct measurement of interparticle forces and the experimental validation of the concept of effective stress introduced by Karl Terzaghi. The grain-scale expression of Terzaghi's effective stress for saturated granular media under small deformation and quasi-static state is derived using stress decomposition and balance of forces and moments. For the experimental validation of the analytical solution, an experimental setup was designed to study 2D saturated rubber rod packing under classic 1D consolidation. A hybrid optical-mechanical method based on the Granular element method (GEM) and Digital image correlation (DIC) is applied. The interparticle forces are directly computed from 2D strain distribution of the grains, and the effective stress is calculated using the grain-scale forces. With pore water pressure measured by a pressure sensor, the summation of the effective stress and the pore water pressure is then compared with the external load applied in the 1D consolidation experiment, which is the core of Terzaghi's principle. The 1D consolidation experiment is also compared with the 1D consolidation model and matches the results from Discrete element simulations (DEM).
\r\n\r\nThe second part of the thesis investigates the measurement of interparticle forces in more complex unsaturated granular systems consisting of solid, pore fluid, and pore air phases. In the case of quasi-static, point contact, and low saturation, an expression for the partition of stress is derived as a function of interparticle forces. To simplify the expression of the stress partition equation, capillary bridges, which are integral parts of unsaturated systems under low saturation condition, are simulated numerically using 2D finite element method (FEM) to further understand the influence of gravity on pore fluid clusters. As the original GEM for fully saturated systems focuses on interparticle interactions, the GEM is further developed for unsaturated systems based on the original GEM and considering capillary forces. Finally, a hybrid optical-mechanical approach combined with the granular element method (GEM) is developed to extract interparticle forces in a classic 1D consolidation experiment. The partition of stresses is determined by experimental results and compared with the analytical results.
\r\n\r\nThe major contributions of this thesis are the theoretical derivation and experimental validation of the link between the grain-scale properties (interparticle forces, branch vectors, etc.) and the stress transfer in fully saturated and unsaturated systems. The theoretical and experimental methodology employed in the thesis could pave the way for exploring the mechanics and physics behind the constitutive behaviors of a variety of poromechanical systems.
", "doi": "10.7907/rgys-kh14", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15245", "collection": "thesis", "collection_id": "15245", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312023-210300139", "primary_object_url": { "basename": "Thesis_Uba_K_Ubamanyu.pdf", "content": "final", "filesize": 291588500, "license": "other", "mime_type": "application/pdf", "url": "/15245/1/Thesis_Uba_K_Ubamanyu.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Time-Dependent Failure of Thin-Ply Composite Laminates", "author": [ { "family_name": "Ubamanyu", "given_name": "Kanthasamy (Uba)", "orcid": "0000-0002-3679-6173", "clpid": "Ubamanyu-Kanthasamy-Uba" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The demand for larger and lighter structures for next-generation space designs necessitates the use of deployable structures. Among the materials that hold promise for such applications, thin-laminate fiber composites with thicknesses less than 200 μm stand out due to their strength-to-weight ratio, packaging efficiency, and ability to deploy using stored strain energy. However, designing deployable structures with thin-laminate composites is challenging as they need to be stiff enough to withstand loads during deployment while also having a small volume in the packaged configuration. Complicating matters further, stress relaxation of the polymer matrix within the composite during long-term stowage in response to an imposed curvature can drastically impact both the deployment process and the performance of the structure in its deployed state, even leading to catastrophic failure in the stowed configuration.
\r\n\r\nThis thesis presents a comprehensive study of the time-dependent failure behavior of thin-laminate fiber composites under bending, with a focus on a fundamental material-level understanding. The work is divided into three main parts. First, a novel test method called Flattening to Rupture (FTR) test was developed to effectively load composite coupons under long-term bending, enabling the measurement of time-dependent rupture and identification of the underlying failure mechanisms. Second, numerical simulations using the Abaqus/Standard finite element software were developed to understand the sequence of rupture events and the influence of several parameters that affect time-dependent rupture. Finally, a statistical approach was proposed to model the stochastic nature of the failure of thin composite laminates.
\r\n\r\nThe contributions of this thesis extend the understanding of the microscale failure mechanisms involved in the time-dependent failure of fiber composites. These new insights pave the way for the efficient design of tightly and safely packaged deployable structures under long-term loading. The findings of this research can be utilized to optimize the design and performance of deployable space structures made of fiber composites, leading to new technologies that can advance space exploration.
", "doi": "10.7907/x286-g488", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15246", "collection": "thesis", "collection_id": "15246", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312023-212652388", "primary_object_url": { "basename": "EricOcegueda_2023Thesis.pdf", "content": "final", "filesize": 66084671, "license": "other", "mime_type": "application/pdf", "url": "/15246/2/EricOcegueda_2023Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Physics-Based and Data-Driven Computational Models of Inelastic Deformations", "author": [ { "family_name": "Ocegueda", "given_name": "Eric", "orcid": "0000-0001-7845-6890", "clpid": "Ocegueda-Eric" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Stuart", "given_name": "Andrew M.", "orcid": "0000-0001-9091-7266", "clpid": "Stuart-A-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Crystalline materials inevitably exhibit inelastic deformation when applied to large enough loads. The behavior in this inelastic regime is a coupling of physics across several length scales: from initiating as defects at the atomic scale, interacting with crystal defects, and finally spanning multiple grains and influencing macroscopic stress behavior. These length-scale interactions make predicting material response an open challenge and an avenue for leveraging microscale physics for material design. This thesis examines developing physics-based and data-driven computational models to capture complex inelastic behavior at appropriate length scales.
\r\n\r\nFirst, we present a mesoscale model for capturing deformation twinning physics at the polycrystal scale. Mechanical twinning is a form of inelastic deformation observed in low-symmetry crystals, such as magnesium and other hexagonal close-packed (hcp) metals. Twinning, unlike slip, forms as bands collectively across grains with complex local morphology propagating into bulk behavior, drastically affecting strength and ductility. We, thus, propose a model where twinning is treated using a phase-field approach, while dislocation slip is considered using crystal plasticity. Lattice reorientation, length-scale effects, interactions between dislocations and twin boundaries, and twin and slip interactions with grain boundaries are all carefully considered. We first outline the model and its implementation using a novel approach of accelerated computational micromechanics in a two-dimensional, single twin-slip system, polycrystal case to demonstrate its capabilities. Finally, we consider multiple twin-slip systems and conduct three-dimensional simulations of polycrystalline magnesium. We summarize the insights gained from these studies and the implications on the macroscale behavior of hcp materials.
\r\n\r\nThe second part of the thesis focuses on data-driven models for capturing microscopic history-dependent phenomena for multiscale modeling applications. The multiscale modeling framework has seen increased usage over the last few decades for its ability to capture complex material behavior over a range of time/length scales by solving a macroscale problem directly with a constitutive relation defined implicitly by the solution of a microscale problem. However, this implementation is computationally expensive -- needing to solve a microscale problem at each point and time of the macroscopic calculation. In this study, we examine the use of machine learning by utilizing data generated through repeated solutions of a microscale problem to: (i) gain insights into the history dependent macroscopic internal variables that govern the response and (ii) create a computationally efficient surrogate. We do so by introducing a recurrent neural operator, which can provide accurate approximations of the stress response and insights into the physics of the macroscopic problem. We illustrate these capabilities on a laminate composite and polycrystal made of elasto-viscoplastic materials, summarize insights on the learned internal variables, and accuracy of stress predictions.
", "doi": "10.7907/3gqd-zp93", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15010", "collection": "thesis", "collection_id": "15010", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08282022-230140735", "primary_object_url": { "basename": "Chittur Priya_final thesis.pdf", "content": "final", "filesize": 22914581, "license": "other", "mime_type": "application/pdf", "url": "/15010/1/Chittur Priya_final thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "A Millifluidic Bulge Test for Multiscale Properties of Engineered Biofilms", "author": [ { "family_name": "Chittur", "given_name": "Priya K.", "orcid": "0000-0002-9409-4671", "clpid": "Chittur-Priya-K" } ], "thesis_advisor": [ { "family_name": "Kornfield", "given_name": "Julia A.", "orcid": "0000-0001-6746-8634", "clpid": "Kornfield-J-A" } ], "thesis_committee": [ { "family_name": "Tirrell", "given_name": "David A.", "orcid": "0000-0003-3175-4596", "clpid": "Tirrell-D-A" }, { "family_name": "Kornfield", "given_name": "Julia A.", "orcid": "0000-0001-6746-8634", "clpid": "Kornfield-J-A" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Okumura", "given_name": "Mitchio", "orcid": "0000-0001-6874-1137", "clpid": "Okumura-M" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "Biofilms \u2013 communities of bacterial cells associated with their extracellular polymeric matrices \u2013 are complex materials whose features span many length scales, ranging from bulk cohesive material properties, to mesoscale structural and compositional heterogeneity, down to the microscopic cellular morphology and cell-cell interaction chemistry.
\r\n\r\nHere, we demonstrate a tool to study the mechanical properties of biofilms across length scales from the mesoscale (0.2 mm) to the bulk (1 mm) using a simplified model system based on genetically engineered E. coli. Using a custom millifluidic device that suspends a 3 mm dia. biofilm across a support, we impose tunable hydrostatic pressure drops in the Pa-kPa range across the biofilm. The resulting deformation of the film through an aperture is visualized with optical coherence tomography and used to estimate bulk and mesoscale mechanical properties of the film. Our method requires only microliters of material, causes minimal disruption to the film structure, and allows for estimates of both average properties as well as local heterogeneity as a function of cell-cell interaction chemistry and biofilm damage and healing. In the final chapter we introduce other model biofilm systems for their unique optical and mechanical properties.
", "doi": "10.7907/e1yv-mr74", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15017", "collection": "thesis", "collection_id": "15017", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08312022-141029875", "primary_object_url": { "basename": "CChari_Thesis_CompleteVersion.pdf", "content": "final", "filesize": 13690059, "license": "other", "mime_type": "application/pdf", "url": "/15017/2/CChari_Thesis_CompleteVersion.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Degradation of Ceramic Surfaces and its Mitigation: From Electric Propulsion to Cultural Heritage", "author": [ { "family_name": "Chari", "given_name": "Celia S.", "orcid": "0000-0001-6404-4203", "clpid": "Chari-Celia-S" } ], "thesis_advisor": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Rossman", "given_name": "George Robert", "orcid": "0000-0002-4571-6884", "clpid": "Rossman-G-R" }, { "family_name": "McEnerney", "given_name": "Bryan W.", "orcid": "0000-0003-0869-0686", "clpid": "McEnerney-B-W" }, { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Ceramics have played an evolving role throughout human history, with the earliest known fired clay figurines dating back to 29,000-25,000 BCE in what is today the Czech Republic. Such examples of porous, low-fired potteries transitioned into vitreous and heat-resistant porcelain with the advancements of kiln technologies, which steadily grew to reach temperatures up to 1270 \u00b0C by the time of the Song Dynasty (960-1279 CE). In the modern-age, furnaces technologies have become exceedingly more sophisticated, reaching ultra-high temperatures of up to 3000 \u00b0C with dedicated calibration systems. With the advancement of firing technologies came the advancement of materials processing methods, which have transformed the role of ceramics in everyday applications: from ceramic fibers used in tennis racquets, to ceramics used in space shuttle tiles and even in artificial joints. Indeed, ceramics are candidate solutions to the most stringent material problems faced today, including high-temperature applications like electric propulsion and turbine engine systems. However, in order to improve the long-term use and sustainability of ceramics, it is essential to evaluate both their performance and their eventual degradation from mechanical wear and chemical erosion.
\r\n\r\nThis work explores the processing-microstructure-performance relationship of ceramics to better understand the performance and degradation mechanisms of ceramic surfaces. This relationship is investigated using a series of material case studies, including (i) advanced high-temperature ceramics composed of h-BN rich composites, and (ii) historic ceramics, ranging from low-fired pottery to porcelain. Details are provided for the design and manufacturing of novel high-performing ceramics, while simultaneously referring to ceramics of the past to understand how their surfaces have altered over their lifetime. Highlights of this work include the innovative use of carbothermic reactions to create h-BN surface layers for electric propulsion; self-healing strategies from AlN/BN composites; DFT-supported analysis of pottery corrosion in acidic soils; and nanoscale processing of historic porcelain glazes. These analyses provide us with an opportunity to learn from materials of the past to create more sustainable materials for the future, with an emphasis on ways of mitigating degradation by controlling processing conditions and environmental exposures.
", "doi": "10.7907/22st-q436", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14376", "collection": "thesis", "collection_id": "14376", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09282021-231505066", "type": "thesis", "title": "Modeling and Programming Shape-Morphing Structured Media", "author": [ { "family_name": "McMahan", "given_name": "Connor Glenn", "orcid": "0000-0001-5024-6138", "clpid": "McMahan-Connor-Glenn" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Shape-morphing and self-propelled locomotion are examples of mechanical behaviors that can be \"programmed\" in structured media by designing geometric features at micro- and mesostructural length scales. This programmability is possible because the small-scale geometry often imposes local kinematic modes that are strongly favored over other deformations. In turn, global behaviors are influenced by local kinematic preferences over the extent of the structured medium and by the kinematic compatibility (or incompatibility) between neighboring regions of the domain. This considerably expands the design space for effective mechanical properties, since objects made of the same bulk material but with different internal geometry will generally display very different behaviors. This motivates pursuing a mechanistic understanding of the connection between small-scale geometry and global kinematic behaviors. This thesis addresses challenges pertaining to the modeling and design of structured media that undergo large deformations.
\r\n\r\nThe first part of the thesis focuses on the relation between micro- or mesoscale patterning and energetically favored modes of deformation. This is first discussed within the context of twisted bulk metallic glass ribbons whose edges display periodic undulations. The undulations cause twist concentrations in the narrower regions of the structural element, delaying the onset of material failure and permitting the design of structures whose deployment and compaction emerge from the ribbons' chirality. Following this discussion of a periodic system, we study sheets with non-uniform cut patterns that buckle out-of-plane. Motivated by computational challenges associated with the presence of geometric features at disparate length scales, we construct an effective continuum model for these non-periodic systems, allowing us to simulate their post-buckling behavior efficiently and with good accuracy.
\r\n\r\nThe second part of the thesis discusses ways to leverage the connection between micro/mesoscale geometry and energetically favorable local kinematics to create \"programmable matter\" that undergo prescribed shape changes or self-propelled locomotion when exposed to an environmental stimulus. We first demonstrate the capabilities of an inverse design method that automates the design of structured plates that morph into target 3D geometries over time-dependent actuation paths. Finally, we present devices made of 3D-printed liquid crystal elastomer (LCE) hinges that change shape and self-propel when heated.
", "doi": "10.7907/rcw1-r139", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14513", "collection": "thesis", "collection_id": "14513", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03112022-002649428", "primary_object_url": { "basename": "HaoZhou_thesis.pdf", "content": "final", "filesize": 22892896, "license": "other", "mime_type": "application/pdf", "url": "/14513/4/HaoZhou_thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Accelerated Computational Micromechanics", "author": [ { "family_name": "Zhou", "given_name": "Hao", "orcid": "0000-0002-6011-6422", "clpid": "Zhou-Hao" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Lebensohn", "given_name": "Ricardo A.", "orcid": "0000-0002-3152-9105", "clpid": "Lebensohn-R-A" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The development of new materials is an important component of many cutting edge technologies such as space technology, electronics and medical devices. The properties of advanced materials involve phenomena across multiple scales. The material may be heterogeneous on a scale that is small compared to that of applications, or may spontaneously develop fine-scale structure. Numerical simulation of such phenomena can be an effective tool in understanding the complex physics underlying these materials, thereby assisting the development and refinement of such materials, but can also be challenging.
\r\n\r\nThis thesis develops a new method to exploit the use of graphical processing units and other accelerators for the computational study of complex phenomena in heterogeneous materials. The governing equations are nonlinear partial differential equations, typically second order in space and first order in time. We propose an operator-splitting scheme to solve these equations by observing that these equations come about by a composition of linear differential constraints like kinematic compatibility and balance laws, and nonlinear but local constitutive equations. We formulate the governing equation as an incremental variational principle. We treat both the deformation and the deformation gradient as independent variables, but enforce kinematic compatibility between them as a constraint using an augmented Lagrangian. The resulting local-global problem is solved using the alternating direction method of multipliers. This enables efficient implementation on massively parallel graphical processing units and other accelerators. We use the study of elastic composites in finite elasticity to verify the method, and to demonstrate its numerical performance. We also compare the performance of the proposed method with that of other emerging approaches.
\r\n\r\nWe apply the method to understand the mechanisms responsible for a remarkable in-plane liquid-like property of liquid crystal elastomers (LCEs). LCEs are rubber-like solids where rod-like nematic molecules are incorporated into the main or a side polymer chain. They undergo isotropic to nematic phase transition accompanied by spontaneous deformation which can be exploited for actuation. Further, they display a soft behavior at low temperatures due to the reorientation of the nematic directors. Recent experiments show that LCEs exhibit an in-plane liquid-like behavior under multiaxial loading, where there is shear strain with no shear stress. Our numerical studies of LCEs provides insights into the director distribution and reorientation in polydomain specimens, and how these lead to the observed liquid-like behavior. The results show good agreement with experimental observations. In addition to providing insight, this demonstrates the ability of our computational approach to study multiple coupled fields.
\r\n\r\nThe core ideas behind the method developed in this thesis are then applied elsewhere. First, we use it to study multi-stable deployable engineering structures motivated by origami. The approach uses two descriptions of origami kinematics, angle/face based approach and vertex/truss based approach independently, and enforces the relationship between them as a constraint. This is analogous to the treatment of kinematic compatibility above where both the deformation and deformation gradient are used as independent variables. The constraint is treated using a penalty. Stable and rigid-foldable configurations are identified by minimizing the penalty using alternate directions, and pathways between stable states are found using the nudged elastic band method. The approach is demonstrated using various examples.
\r\n\r\nSecond, we use a balance law or equilibrium to the problem of determining the stress field from high resolution x-ray diffraction. This experimental approach determines the stress field locally, and errors lead to non-equilibriated fields. It is hypothesized that imposing equilibrium leads to a more accurate stress reconstruction. We use Hodge decomposition to project a non-equilibriated stress field onto the divergence-free (equilibriated) subspace. This projection is numerically implemented using fast Fourier transforms. This method is first verified using synthetic data, and then applied to experimental data obtained from a beta-Ti alloy. It results in large corrections near grain boundaries.
", "doi": "10.7907/r4jb-4e98", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14367", "collection": "thesis", "collection_id": "14367", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09182021-045958776", "type": "thesis", "title": "Geometry Synthesis and Multi-Configuration Rigidity of Reconfigurable Structures", "author": [ { "family_name": "Dorn", "given_name": "Charles Jacob", "orcid": "0000-0001-6516-2586", "clpid": "Dorn-Charles-Jacob" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Reconfigurable structures are structures that can change their shapes to change their functionalities. Origami-inspired folding offers a path to achieving shape changes that enables multi-functional structures in electronics, robotics, architecture and beyond. Folding structures with many kinematic degrees of freedom are appealing because they are capable of achieving drastic shape changes, but are consequently highly flexible and therefore challenging to implement as load-bearing engineering structures. This thesis presents two contributions with the aim of enabling folding structures with many degrees of freedom to be load-bearing engineering structures.
\r\n\r\nThe first contribution is the synthesis of kirigami patterns capable of achieving multiple target surfaces. The inverse design problem of generating origami or kirigami patterns to achieve a single target shape has been extensively studied. However, the problem of designing a single fold pattern capable of achieving multiple target surfaces has received little attention. In this work, a constrained optimization framework is presented to generate kirigami fold patterns that can transform between several target surfaces with varying Gaussian curvature. The resulting fold patterns have many kinematic degrees of freedom to achieve these drastic geometric changes, complicating their use in the design of practical load-bearing structures.
\r\n\r\nTo address this challenge, the second part of this thesis introduces the concept of multi-configuration rigidity as a means of achieving load-bearing capabilities in structures with multiple degrees of freedom. By embedding springs and unilateral constraints, multiple configurations are rigidly held due to the prestress between the springs and unilateral constraints. This results in a structure capable of rigidly supporting finite loads in multiple configurations so long as the loads do not exceed some threshold magnitude. A theoretical framework for rigidity due to embedded springs and unilateral constraints is developed, followed by a systematic method for designing springs to maximize the load-bearing capacity in a set of target configurations. An experimental study then validates theoretical predictions for a linkage structure. Together, the application of geometry synthesis and multi-configuration rigidity constitute a path towards engineering reconfigurable load-bearing structures.
", "doi": "10.7907/ph2w-9a34", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14932", "collection": "thesis", "collection_id": "14932", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012022-214247822", "type": "thesis", "title": "Oxidative and Internal Stress Effects of Dopants in Multilayer Yb\u2082Si\u2082O\u2087 Environmental Barrier Coatings", "author": [ { "family_name": "Herren", "given_name": "Benjamin Riley", "orcid": "0000-0002-6011-710", "clpid": "Herren-Benjamin-Riley" } ], "thesis_advisor": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Almer", "given_name": "Jonathan D.", "orcid": "0000-0002-1900-2271", "clpid": "Almer-Jonathan-D" }, { "family_name": "Lee", "given_name": "Kang N.", "orcid": "0000-0003-1215-0228", "clpid": "Lee-Kang-N" }, { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "One of the best approaches to improving the efficiency of turbine engines is to increase their operating temperatures. A revolutionary improvement toward this goal will be the replacement of structural metallic components with silicon-based ceramic-matrix composites (CMCs). However, corrosive chemistries in combustion engines necessitate chemical protection of the structural material. Environmental Barrier Coatings (EBCs) are designed for this purpose through thermomechanical and chemical compatibility with the system. High-temperature, humid oxidation of the EBC bondcoat, a silicon layer which adheres the topcoat to the substrate, remains a critical failure mode EBC systems.
\r\n\r\nThis work studies the effects of chemical modifications on bondcoat oxidation during steam cycling in a current state-of-the-art EBC (Yb\u2082Si\u2082O\u2087/Si), and the implications for the durability of the system. Chemically modified EBCs have been shown to decrease oxide growth by more than 85 percent after 1000 hours of steam cycling. Post-exposure analyses are used to investigate the effects of chemistry on thermal oxide thickness and microstructure. Synchrotron X-ray scattering at the Advanced Photon Source, Argonne National Laboratory, is used to observe internal strains, connected to oxidation and chemistry through microstructure. Compared internal stresses, oxidation, and microstructure between baseline and modified EBCs promote the effectiveness of topcoat chemical modifications as they may apply to EBC durability. Microindentation, nanoindentation, and ongoing beam-bending experiments are also used to assess EBC interface toughness in a baseline EBC system. Additionally, a custom induction furnace has been designed and tested for in-situ steam cycling at the synchrotron. Traits facilitating the use of the custom furnace at the synchrotron make for convenient steam-cycling and other exposures in conventional laboratory settings, as well, with enhanced customizability and flexibility.
", "doi": "10.7907/4cm0-8e79", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14398", "collection": "thesis", "collection_id": "14398", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10152021-195617065", "primary_object_url": { "basename": "Harmon_John_2021.pdf", "content": "final", "filesize": 9262762, "license": "other", "mime_type": "application/pdf", "url": "/14398/1/Harmon_John_2021.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Predicting the Strength of Planetary Surfaces", "author": [ { "family_name": "Harmon", "given_name": "John Michael", "orcid": "0000-0002-8323-3488", "clpid": "Harmon-John-Michael" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Our curiosity and spirit for exploration has fueled advancements towards visiting Earth's neighbors in the solar system. Environments outside of Earth are extreme, however, and it is far from guaranteed that landing and operating on the surface of these bodies is an easy task. Conditions such as reduced gravity, extreme temperatures and sparse atmospheres play a role in the compressive, shear, and tensile strength of a surface. These environmental factors make experiments that work to inform design decisions for spacecraft-surface interaction difficult and expensive. In order to better ensure successful mission operations in the future, this thesis focuses on the development of a platform of numerical modeling for planetary surface interaction.
\r\n \r\nDry regolith and water ice are two surface materials that are pervasive in the solar system. For each, the mechanical properties are heavily reliant on features at the microscale that are insufficiently modeled. The first part of the thesis will focus on crushable dry regolith. There will be two chapters on this topic, the first of which discusses the development of the modeling capability to capture both the highly irregular particle shapes and the brittle nature of regolith. The second chapter on regolith will focus on the validation of this method on a crushable sand sample experiment. This model demonstrates excellent predictive capability for the constituitive relationship, the evolution of particle sizes, and the evolution of particle shape in the sample. Further, evidence from the forces between the particles shows that despite larger particles being weaker on average, many survive due to two reasons. One, the surviving particles are generally on the stronger side of the particle strength distribution, and second, that larger particles have a higher coordination number producing a more isotropic stress state in the particle.
\r\n \r\nUnlike dry regolith, distinct neighboring water ice particles will sinter together over time at varying rates depending on their environment. This leads to a large amount of the water ice surfaces that are of interest to future missions, having a highly varied and many times unknown levels of strength. The contact interaction between water ice particles at the microscale will be handled the same as regolith, however a modification was added to account for sintering. The strong cohesiveness sintering generates is modeled by placing massless bonds where sinters would form. The cross-sectional area of the bond represents the amount of sintering that has taken place and can be thought of as a representation of the neck geometry that early stage sintering is described as. The bonds used are linear elastic and breakable in order to capture the crushable nature of porous ice. Three chapters are dedicated to ice modeling. First, the model development will be shown with verification examples for its use. Second, the model will be used to predict cone penetration tests on ice that were previously conducted in experiment. Comparisons show that the model can produce similar stresses and qualitative features observed in the experiment. A sensitivity analysis is conducted and shows that the most important controlling parameters are the ice\u2019s critical strength and the sinter's neck thickness. The relation of the bond characteristics to the sintering process is discussed. In the third chapter on water ice, the landing of a footpad on the surface of Enceladus is modeled. The model predicts that a lack of sintering could result in catastrophic sinkage, however even moderate sintering provides enough strength to support a lander. Also the model predicts landing on inclined surfaces and shows that landing could be possible at angles as high as 20 degrees.
", "doi": "10.7907/r9zm-kp19", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14198", "collection": "thesis", "collection_id": "14198", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282021-233441075", "type": "thesis", "title": "A Shock Compression Investigation of Failure Waves and Phase Transition in Soda-Lime Glass", "author": [ { "family_name": "Joshi", "given_name": "Akshay", "orcid": "0000-0001-8347-8357", "clpid": "Joshi-Akshay" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Mello", "given_name": "Michael", "orcid": "0000-0003-2129-9235", "clpid": "Mello-Michael" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Soda-lime glass (SLG) and other silica glasses find use in many technological applications involving high pressures and strain rates, such as systems with laser-matter interactions, transparent armor, etc. An experimentally validated constitutive model for these glasses is required for modeling their mechanical behavior at high pressures and strain rates. Also, due to the abundance of silica in the earth's crust, understanding the behavior of these glasses at high pressures can provide significant insights into many geophysical processes. To this end, shock compression experiments are carried out on SLG to study the material's behavior under impact stresses of 5-10 GPa. These experiments are accompanied by numerical simulations and constitutive modeling of SLG to gain further insights into the reported failure-wave phenomenon and phase transitions associated with the material.
\r\n\r\nThe significant findings of this study in relation to the failure-wave phenomenon were the sudden densification/compaction of SLG associated with the failure-wave and the disappearance of the failure-wave phenomenon for impact stresses above 10 GPa. When viewed in the context of the findings from past experiments, these results seem to suggest that localized densification/compaction of SLG causes nucleation of cracks and subsequent comminution in the material under shock compression. These results and observations offer a potential explanation of the mechanism underlying the failure-wave phenomenon.
\r\n\r\nFurther, the shock compression and release experiments performed in this work provided significant insights into the onset of possible phase-transition in SLG under shock compression. A loading-unloading hysteresis is observed in the material\u2019s stress-strain curve for impact stresses higher than 5.8 GPa, with the permanent/residual strain increasing with impact stress. Further analysis of these results strongly indicates that the hysteresis is more likely due to a gradual, irreversible phase transition of SLG than due to regular inelastic behavior. Thus, the results suggest that the SLG undergoes a gradual phase transition to a stiffer phase, although other properties of this phase remain unclear. It can also be noted that this phase transition is postulated to start occurring under shock compression of SLG to stresses above 5 GPa, which is also the threshold stress for the onset of the failure-wave phenomenon. It is, therefore, possible that the two phenomena are interrelated. The experimental results from this study are further used to construct a constitutive model to capture the unloading behavior of SLG.
", "doi": "10.7907/b8xs-8r91", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14125", "collection": "thesis", "collection_id": "14125", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04202021-184720643", "primary_object_url": { "basename": "YingShiTeh2021Thesis.pdf", "content": "final", "filesize": 17497003, "license": "other", "mime_type": "application/pdf", "url": "/14125/5/YingShiTeh2021Thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Understanding Imperfections and Instabilities in Crystals via Physics-Based and Data-Driven Models", "author": [ { "family_name": "Teh", "given_name": "Ying Shi", "orcid": "0000-0003-1743-4158", "clpid": "Ying-Shi-Teh" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "div_eng" } ], "abstract": "In crystals, atoms are arranged in a periodic manner in space. However in reality, imperfections and instabilities exist and this repeated arrangement is never perfect. The coupling between crystal defects, lattice instabilities, other defects like domain walls and domain patterns, and material properties generates interesting phenomena that can be leveraged on for future materials design. Nevertheless, the coupling of different scales and processes also makes the modeling and understanding of these materials an open challenge. This thesis examines these various aspects of crystalline solids through the development of both physics-based and data-driven computational models at the appropriate length scales.
\r\n\r\nAbove-bandgap photovoltaic (PV) effect has been observed experimentally in multi-domain ferroelectric perovskites, but the underlying working mechanisms are not well understood. The first part of the thesis presents a device model to study the role of ferroelectric domain walls in the observed PV effect. The model accounts for the intricate interplay between ferroelectric polarization, space charges, photo-generation, and electronic transport. When applied to bismuth ferrite, results show a significant electric potential step across both 71\u00b0 and 109\u00b0 domain walls, which in turn contributes to the PV effect. The domain-wall-driven PV effect is further shown to be additive in nature, allowing for the possibility of generating the above-bandgap voltage.
\r\n\r\nIn the second part, we present a lattice model incorporating random fields and long-range interactions where a frustrated state emerges at a specific composition, but is suppressed elsewhere. The model is motivated by perovskite solid solutions, and explains the phase diagram in such materials including the morphotropic phase boundary (MPB) that plays a critical role in applications for its enhanced dielectric, piezoelectric, and optical properties. Further, the model also suggests the possibility of entirely new phenomena by exploiting MPBs.
\r\n\r\nThe final part of the thesis focuses on constructing data-driven models from first principles calculations, particularly density functional theory (DFT) for studying crystalline materials. Specifically we propose an approach that exploits machine learning to approximate electronic fields in crystalline solids subjected to deformation. When demonstrated on magnesium---a promising light weight structural material---our model predicts the energy and electronic fields to the level of chemical accuracy, and it even captures lattice instabilities. This DFT-based machine learning approach can be very useful in methods that require repeated DFT calculations of unit cell subjected to strain, especially multi-resolution studies of crystal defects and strain engineering that is emerging as a widely used method for tuning material properties.
", "doi": "10.7907/kd3n-eq78", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14112", "collection": "thesis", "collection_id": "14112", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03262021-224805539", "primary_object_url": { "basename": "Ginsberg_Caltech_thesis.pdf", "content": "final", "filesize": 14421172, "license": "other", "mime_type": "application/pdf", "url": "/14112/1/Ginsberg_Caltech_thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Multiscale Mechanical Characterization of Subcellular Structures in Living Walled Cells", "author": [ { "family_name": "Ginsberg", "given_name": "Leah Morgan", "orcid": "0000-0001-9685-7014", "clpid": "Ginsberg-Leah-Morgan" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Roumeli", "given_name": "Eleftheria", "orcid": "0000-0002-2828-1428", "clpid": "Roumeli-Eleftheria" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The physiology of walled cells is dramatically different from that of human cells, but the biomechanics of walled cells are far less studied. Most bacterial, fungal, and plant cells have a strong cell wall (CW), which allows them to withstand large hydrostatic pressures in the cytoplasm, called turgor. Turgor pressure conflates the mechanics of subcellular components and complicates the characterization of the cell. In this dissertation, new models are introduced and explored for single cells to investigate the multiscale mechanics of plant and bacterial cells using micro- and nano-indentation experiments.
\r\n\r\nA multi-scale biomechanical assay is used to study the mechanical properties of plant cells. The plant CW is typically around 5% of the width of the entire cell, and is thought to carry most of the mechanical load. Large-scale indentations using a micro-indentation system probe the behavior of the overall cell structure, and atomic-force microscopy (AFM) nano-scale indentations are used to isolate the CW response. To determine the effect of external osmotic pressure, indentations are performed on cells in different osmotic conditions: hypotonic, isotonic, and hypertonic. The cell is idealized as two springs acting in series, one to represent the CW and one to represent the cytoplasm. The model uses the experimentally determined initial stiffnesses as input to the model to determine the relative stiffness contributions of the CW and the cytoplasm.
\r\n\r\nThe first type of walled cells investigated is the xylem vessel element of Arabidopsis thaliana. The xylem is responsible for transporting water through the stem of any vascular plant (more commonly known as a land plant), and hence it must maintain structural integrity against high internal pressures while transporting water from the roots to the leaves. For extra structural support, xylem vessel elements develop secondary cell walls (SCWs), which are known to be a key component for mediating mechanical strength and stiffness in vascular plants. The structure and biomechanics of cultured plant cells are investigated during the cellular developmental stages associated with SCW formation using the multi-scale biomechanical assay described above. To determine the effect of morphological changes during differentiation, micro- and nano-indentations are performed on cells in different observed stages of the differentiation process.Prior to triggering differentiation, cells in hypotonic pressure conditions are significantly stiffer than cells in isotonic or hypertonic conditions, highlighting the dominant role of turgor pressure. Plasmolyzed cells with a SCW reach similar levels of stiffness as cells with maximum turgor pressure. Analysis using the two-spring model shows that the stiffness of the primary CW in all of these conditions is lower than the stiffness of the fully-formed SCW. These results provide the first experimental characterization of the mechanics of SCW formation at the single-cell level in plant cells.
\r\n\r\nNext, the mechanical response of individual Nicotiana tabacum cells from a suspension culture is studied using the same multi-scale biomechanical assay. The role played by the microtubules (MTs) and actin filaments (AFs) is determined through the use of drug treatments which selectively remove MTs and AFs. A generative statistical model is added to the two-spring model to quantify the stiffnesses of the CW, cytoplasm, turgor pressure, MTs, and AFs. Analysis of the initial stiffness and energy dissipation calculated from micro-indentation experiments indicates that the MTs and AFs contribute significantly to the mechanical response of a cell under compression. Micro- and nano-indentation tests confirm that turgor pressure is the most significant contributor to the stiffness response of turgid cells in compression. Finally, the results reveal that turgor pressure exerts stress on the CW, which leads to a measurable stiffening of the CW.
\r\n\r\nThe studies described above focused on developing a discrete model to describe the mechanics of a cell in indentation experiments. However, the most common type of model used to evaluate the mechanics of a cell are continuum models. Continuum models are also necessary to decouple the material properties of subcellular components from their structure. In the final section, AFM indentations are simulated on a gram-negative bacterium, Escherichia coli, and a sensitivity study and inverse analysis are performed to solve for the CW elastic modulus and turgor pressure simultaneously. Sensitivity study results reveal that uncertainty in turgor pressure and CW elasticity indeed contribute the most to variability in force spectra from AFM measurements. The parameter space of possible values for CW elastic modulus and turgor pressure is discretized using triangular elements. \"Simulated experiments\" are tested throughout the parameter space, and correlations between the CW elastic modulus and turgor pressure, which depend on the type of objective function, are investigated. Two unique objective functions are tested in the inverse analysis, and a third objective function, which is a weighted sum of the first two, is found to reduce errors in estimated CW elastic modulus and turgor pressure by 20% and 11%, respectively. The use of this type of inverse analysis has the potential to elucidate the material properties of CWs using a single indentation measurement and reliably decouple these properties from the high turgor pressures inside walled cells.
", "doi": "10.7907/avj4-ve78", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14219", "collection": "thesis", "collection_id": "14219", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012021-150444627", "primary_object_url": { "basename": "Edwards_Bryce_2021_final.pdf", "content": "final", "filesize": 21680369, "license": "other", "mime_type": "application/pdf", "url": "/14219/1/Edwards_Bryce_2021_final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Mechanical Investigations: Experimental Fracture Techniques and Frozen Small-Molecule Organics", "author": [ { "family_name": "Edwards", "given_name": "Bryce Walker", "orcid": "0000-0002-2393-5488", "clpid": "Edwards-Bryce-Walker" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Fracture of architected lattices: Three-dimensional diamond, kagome, and octet lattices were prepared for validation of a standard compact tension fracture experiment at two different length scales. Solid polymer lattices were written via two-photon lithography at the microscale, and solid polymer lattices are printed via digital light processing at the macroscale. Several of the macrolattices were pyrolyzed into carbon lattices to yield a brittle material for testing. The scaling laws of fracture toughness with relative density are explored, and this offers one of the first experimental studies of a fully 3D kagome lattice.
\r\n\r\nMechanical properties of solid benzene: We explore the mechanical properties and deformation of 10 um-sized cuboid-shaped solid benzene crystals made by freezing directly onto a liquid-nitrogen-cooled sample stage and compressed quasi-statically to 10% strain at 125 K with an in-situ nanomechanical instrument inside a Scanning Electron Microscope (SEM). Cryo-Transmission Electron Microscopy (cryo-TEM) and diffraction of frozen benzene confirms the orthorhombic crystal structure of benzene. Compressive contact pressure-strain response generated from load-displacement data suggests the deformation mechanism to occur via densification, with a loading modulus of 9 GPa, slightly larger than that of other small molecules composed of aromatic rings, such as naphthalene and biphenyl. Molecular dynamics (MD) simulations of experimentally equivalent compressions of 10-30 nm benzene samples of the same crystal structure and geometry along the principal lattice directions at 10-30 K suggest densification could, initially, occur by local amorphization of the compressed region. The discovered deformation mechanism, stiffness, and strength of benzene at 125 K can inform our understanding of geological processes on cold planetary bodies. For example, the surface of Saturn\u2019s moon Titan is teeming with solid organics at an ambient temperature of 95 K; this work will have significant impact on designing in-situ sampling tools for future missions to Titan and to substantiate speculative surface compositions.
", "doi": "10.7907/8f8y-5h58", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14209", "collection": "thesis", "collection_id": "14209", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312021-185024653", "type": "thesis", "title": "Probing the Buckling of Thin-Shell Space Structures", "author": [ { "family_name": "Royer", "given_name": "Fabien A.", "orcid": "0000-0003-2452-2893", "clpid": "Royer-Fabien-A" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The overarching goal of the research presented in this dissertation is to apply and extend a newly developed methodology to understand the buckling of complex thin shell structures. This methodology enables the determination of tighter buckling criteria and paves the way to the development of more efficient structures, used closer than ever to their buckling load and even beyond. It would result in dramatically lighter structures to be built and has the potential to enable new applications, such as extremely large aperture satellites.
\r\n\r\nWe first analyze the stability of open section thin shell structures under a pure bending moment, through simulations. These structures are composed of longitudinal thin-shell elements connected transversely by thin rods, and inspired by real spacecraft structures. The present study applies and extends recent work on the stability of cylindrical and spherical shells. The role of localization in the buckling of these structures is investigated and early transitions into the post-buckling regime are unveiled using a probe that locally displaces the structure. The probing method enables the computation of the energy input needed to transition early into a post-buckling state, which is central to determining the critical buckling mechanism for the structure. We show that the structure follows stability landscapes also found in cylindrical and spherical shell buckling problems. This initial computational study is the basis for the first ever probing experiment on a complex structure.
\r\n\r\nIn order to test these new structures under bending, a new bending apparatus is designed and implemented. The boundary conditions are chosen such that the apparatus is statically determinate (isostatic), and no state of self stress can develop in the sample during its mounting and testing. This feature is especially desirable in the study of thin shell structures and their elastic instabilities, for which imperfection sensitivity plays a crucial role in the buckling transition and the post-buckling regime. The accuracy of the isostatic bending machine is first assessed through the testing of rods, and its imperfection insensitive behavior is then highlighted in experiments on tape springs, and through numerical studies of the same structures.
\r\n\r\nThe new bending machine is complemented by a probing apparatus, and the stability of the open section thin-shell structures subjected to a pure bending moment is studied experimentally. The experiment confirms that localization of deformations plays a paramount role in the structure's nonlinear post-buckling regime and is extremely sensitive to imperfections. This characteristic is investigated through probing experiments. The range of moments for which the early buckling of the structure can be triggered using this probe perturbation is determined, as well as the energy barrier separating the pre-buckling and post-buckling states. The stability of the local buckling mode is then illustrated by an experimental stability landscape of shell buckling, and probing is then extended to the entire structure to reveal alternate buckling modes disconnected from the structure's fundamental path. These results can be used to elaborate efficient buckling criteria for this type of structures, through the use of transition diagrams determined experimentally.
\r\n\r\nFinally, the buckling and post-buckling behavior of ultralight ladder-type coilable structures is investigated. These specific structures are used in the Space Solar Power Project at Caltech and are referred to as strips. Similarly to the previous studies, the stability of strip structures loaded by normal pressure is computationally studied by applying controlled perturbations through localized probing. The probing technique is generalized to higher-order bifurcations along the post-buckling path, and low-energy escape paths into buckling that cannot be predicted by a classical eigenvalue formulation are identified. It is shown that the stability landscape for a pressure-loaded strip is similar to the landscape for classical shells, and the open section thin shell structure studied initially in this thesis. While classical shell structures buckle catastrophically, strip structures feature a large stable post-buckling range. Probing enables the full characterization of the structure's unstable behavior, which paves the way to extend its operation closer than ever to the buckling load, and even in the post-buckling regime. It would enable the design of more efficient structures by dramatically reducing their mass, therefore enabling new large spacecraft to be built.
", "doi": "10.7907/ksn2-t598", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14243", "collection": "thesis", "collection_id": "14243", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06042021-213808812", "primary_object_url": { "basename": "lee_victoria_2021.pdf", "content": "final", "filesize": 33148421, "license": "other", "mime_type": "application/pdf", "url": "/14243/1/lee_victoria_2021.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Theoretical, Computational, and Experimental Characterization of Nematic Elastomers", "author": [ { "family_name": "Lee", "given_name": "Victoria Jin-Young", "orcid": "0000-0002-2748-0089", "clpid": "Lee-Victoria-Jin-Young" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Nematic elastomers are programmable soft materials that display large, reversible, and predictable deformation under an external stimulus such as a change in temperature or light. They are composed of a lightly crosslinked polymer network with stiff, rod-like liquid crystal molecules incorporated within the polymer chains. In thermotropic nematic elastomers, the liquid crystals undergo a continuous and reversible phase transition between the randomly oriented isotropic state and the highly oriented nematic state. Further, there is a direct thermo-mechanical coupling between the underlying temperature-responsive orientational order of the liquid crystal molecules and the macroscopic shape change of the surrounding elastomer chains. Finally, these materials display an unusually soft behavior. These remarkable properties make them promising materials for applications in aerospace as deployable structures and skins, in biomedical engineering as a soft pump, and in communications as the actuation mechanism in a reconfigurable antenna. Motivated by these applications, this thesis discusses the theoretical, computational, and experimental characterization of nematic elastomers.
\r\n\r\nWe begin by investigating an example of actuation that takes advantage of the programmable, soft nature of these materials as well as instabilities associated with large deformation. We outline the multi-stable equilibrium solutions to a cylindrical balloon subjected to internal inflation, the material's microstructure formation due to this deformation, and its use as a soft pump with large ejection fraction, which involves a snap-through instability. Then we extend the Agostiniani-DeSimone-Dolzmann relaxed energy to a generalized Mooney-Rivlin constitutive relation and study four examples of Ericksen's universal deformations -- the inflation of cylindrical and spherical balloons, the cavitation of a disk, and the bending of a block.
\r\n\r\nWe then move beyond the modeling of ideal materials and present a new constitutive relation for isotropic-genesis polydomain nematic elastomers. It is based on internal variables that describe the fine-scale domain patterns and evolve according to a kinetic process with dissipation. We discuss the model's implementation in the commercial finite-element software, ABAQUS, and study the problem of torsion of a cylinder. We identify an interesting instability at large torsional strains as a result of the Poynting effect. Finally, we present the design of a thermo-mechanical tensile setup and the experimental results for strain-rate dependence and temperature-dependence of samples that we synthesize in-house.
", "doi": "10.7907/f2hp-qe09", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14213", "collection": "thesis", "collection_id": "14213", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012021-002457442", "primary_object_url": { "basename": "Antonio_Pedivellano-Thesis_2020.pdf", "content": "final", "filesize": 45771850, "license": "other", "mime_type": "application/pdf", "url": "/14213/1/Antonio_Pedivellano-Thesis_2020.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Deployment Dynamics of Thin-Shell Space Structures", "author": [ { "family_name": "Pedivellano", "given_name": "Antonio", "orcid": "0000-0003-2321-7301", "clpid": "Pedivellano-Antonio" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Thin-shell structures provide a lightweight solution for deployable structure applications. Despite being only few tens of microns thick, these structures provide excellent bending stiffness, thanks to their curved cross-section. Their thinness also allows them to be elastically packaged into small volumes to fit into a launch vehicle; once in space, they can be self-deployed by releasing their stored elastic energy.
\r\n\r\nMost space applications use thin-shell structures to deploy and tension thin membranes, such as solar sails, drag sails, and solar arrays. Recently, a novel space solar power architecture has been developed at Caltech, and it relies on distributed thin-shell components, connected in a space frame, to create large-area deployable structures. Thanks to the unique properties of thin shells, these structure provide superior stiffness-to-mass ratio and self-deployment capabilities. However, to demonstrate their reliability and enable their use on space missions, their deployment dynamics must be understood and predicted.
\r\n\r\nGround testing is the established approach to verify a structure throughout its design and qualification process. However, replicating the space environment in a laboratory setting is generally not possible, especially for lightweight structures, which are very sensitive to the effects of gravity and air. Numerical models are therefore the only tool to predict the behavior of a structure in space. However, validation with ground experiments is necessary to build confidence in the models, which must be able to capture the complexity of the interaction with air, gravity, and the suspension system that supports the weight of the structure.
\r\n\r\nThe goal of this thesis is to develop high-fidelity models for large space structures, where multiple thin-shell components are folded together and deploy by releasing their strain energy. This overall objective is achieved in 3 steps. First, a ladder-type rectangular strip is introduced, as a building block for more complex architectures. The strip is composed by two thin-shell longerons, symmetrically folded at two locations. The deployment dynamics of this structure is investigated through experiments on 1 m-scale prototypes, both in air and in vacuum. A detailed analysis of its elastic folds is performed using full-field displacement measurements from Digital Image Correlation. A finite element model of this strip is presented, and it is shown to accurately capture the dynamics of the strip for all tested conditions. Then, the implementation of the packaging and deployment scheme of a space solar power spacecraft, composed of multiple strips, is discussed. A kinematic model of the structure is proposed as a design tool to achieve systematic folding. A novel concept of a deployment mechanism to coil the structure in a robust and reliable way is proposed. Also, a staged deployment scheme is demonstrated, to reduce the uncertainty of strain-energy deployment for large space structures. Finally, the deployment dynamics of a 2 m-scale space structural prototype, based on the space solar power architecture, is investigated. A full-scale finite element model of the structure is implemented to replicate its complex folding scheme and capture the deployment process, including the interaction with the deployment mechanism and the suspension system. The simulations predict well the behavior of the structure observed in experiments through motion capture techniques.
\r\n\r\nThe work presented in this thesis advances previous studies on the deployment dynamics of simple thin-shell components, and demonstrates that even complex thin-shell architectures can be packaged and deployed in a controlled and predictable way. The solutions proposed in this thesis have guided the packaging process and the design of the deployment mechanism for DOLCE, an upcoming flight demonstration of the space solar power architecture described in this work. However, this research has much broader implications, as the experimental and numerical framework presented herein can be generalized to different shell-based architectures, and contributes to enabling a new generation of lightweight deployable structures for future space applications.
", "doi": "10.7907/4zbq-g037", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:13980", "collection": "thesis", "collection_id": "13980", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10182020-133113205", "type": "thesis", "title": "Therapeutic Microparticles and Biolistic Drug-Delivery to the Cornea", "author": [ { "family_name": "Laccetti", "given_name": "Benjamin Joseph", "orcid": "0000-0002-0399-1204", "clpid": "Laccetti-Benjamin-Joseph" } ], "thesis_advisor": [ { "family_name": "Kornfield", "given_name": "Julia A.", "orcid": "0000-0001-6746-8634", "clpid": "Kornfield-J-A" } ], "thesis_committee": [ { "family_name": "Flagan", "given_name": "Richard C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Kornfield", "given_name": "Julia A.", "orcid": "0000-0001-6746-8634", "clpid": "Kornfield-J-A" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Gao", "given_name": "Wei", "orcid": "0000-0002-8503-4562", "clpid": "Gao-Wei" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "The ability to deliver therapeutic compounds to the cornea using high-velocity microparticles is assessed and a method to synthesize therapeutic particles suitable for the cornea is demonstrated. Using a commercial gene gun (BioRad; PDS1000), a pneumatic capillary gun, and custom biolistic technology, microparticles were accelerated and made to embed in target materials: either homogeneous gels or corneal tissue. In homogeneous gels, penetration was shown to be directly proportional to particle size and density. In contrast, penetration of microparticles into the cornea is insensitive to particle size and density: varying the sectional density by 680% failed to penetrate beyond the epithelium (ca. 50 microns). The corneal epithelium exhibits two distinct kinetic energy thresholds that must be exceeded to first embed particles in the epithelium (rather than stopping on its anterior surface) and second to embed particles in the stroma (rather than stopping at the posterior surface of the epithelium). Penetration profiles show that the stroma is a highly effective stopping medium for high velocity microparticles. Despite the high water content of corneal tissue (76 w%) compared to the stratum corneum of skin (40 w%), the resistance to penetration of the cornea is comparable to literature values for skin. Ideal particles for drug delivery to the cornea would dissolve away completely, leaving no residue that might scatter light. With a vibrating orifice aerosol generator and a temperature-controlled column, 30-50 \u00b5m particles were composed of 1% Eosin Y with poly(ethylene glycol). Using low density polymer particles with a therapeutic agent payload, it was demonstrated that bulk material can be ballistically delivered to the central 1 cm\u00b2 of the corneal epithelium rapidly, in an even, quantifiable layer.", "doi": "10.7907/nf29-gy82", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14134", "collection": "thesis", "collection_id": "14134", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05062021-023201965", "type": "thesis", "title": "Multi-Functional Metamaterials", "author": [ { "family_name": "Injeti", "given_name": "Sai Sharan", "orcid": "0000-0003-1941-9752", "clpid": "Injeti-Sai-Sharan" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Optimally designing interdependent mechanical properties in a structure allows for it to be used in application where an arbitrary combination of properties is desired. Architected materials have proven to be an effective way of attaining mechanical behaviors that are unattainable using their constituent materials alone, such as unusual static mechanical properties, unusual wave propagation behavior, and shape morphing. The advent of 3-D printing has allowed for fabricating metamaterials with complex topologies that display engineered mechanics. However, much of the current efforts have focused on optimally designing simple mechanical behaviors such as designing for stiffness and weight, particular frequency bandgaps, or bi-stability. In this work, we study two metamaterial systems where we control and optimize a wide set of static and dynamic properties, and one complex multi-stable structure.
\r\n\r\nMost studies on the optimal design of static properties have focused on engineering stiffness and weight, and much remains unknown about ways to decouple the critical load to failure from stiffness and weight. This is the focus of the first part of our work. We show that the addition of local internal pre-stress in selected regions of architected materials enables the design of materials where the critical load to failure can be optimized independently from the density and/or quasistatic stiffness. We propose a method to optimize the specific load to failure and specific stiffness using sensitivity analysis, and derive the maximum bounds on the attainable properties. We demonstrate the method in a 2-D triangular lattice and a 3-D octahedral truss, showing excellent agreement between experimental and theoretical results. The method can be used to design materials with predetermined fracture load, failure location and fracture paths.
\r\n\r\nFor the second part of our work, we focus on designing acoustically transparent structures, by engineering the acoustic impedance -- a combination of wave speed and density, to match that of the surroundings. Owing to the strong correlation between acoustic wave speed and static stiffness, it is challenging to design acoustically transparent materials in a fluid, while maintaining their high structural rigidity. We provide a sensitivity analysis to optimize these properties with respect to design parameters of the structure, that include localized masses at specific positions. We demonstrate the method on five different periodic, three dimensional lattices, to calculate bounds on the longitudinal wave speed as a function of their density and stiffness. We then perform experiments on 3-D printed structures, to validate our numerical simulations. Further, using the sensitivity analysis together with a data-driven approach, we design and demonstrate a mode demultiplexer, that is capable of splitting arbitrarily mixed modes. The tools developed in this work allow for designing structures in a plethora of applications, including ultrasound imaging, wave filtering, and waveguiding.
\r\n\r\nFinally, most multi-stable structures are limited by bi-stability either at the macroscopic or the unit cell level owing to the difficulty in engineering a highly non-linear energy landscape using just elements that display convex energy landscapes. We demonstrate a method to design arbitrarily complex multi-stable shape morphing structures, by introducing rigid kinematic constraints together with disengaging energy storing elements. We present the idea on a kagome lattice configuration, producing a quadri-stable unit cell and complex stable topologies with larger tessellations, validated by demonstrations on 3-D printed structures. Most designs that use passive actuation address one-way shape morphing along the direction of least resistance. We demonstrate reversible, thermally actuated shape morphing between stable open and closed topologies using shape memory springs. The designs can be extended to non-planar structures and fabricated at vastly different length scales.
", "doi": "10.7907/3rfm-1x78", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:13632", "collection": "thesis", "collection_id": "13632", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02032020-164711057", "primary_object_url": { "basename": "Serena_Ferraro_2020.pdf", "content": "final", "filesize": 39530460, "license": "other", "mime_type": "application/pdf", "url": "/13632/1/Serena_Ferraro_2020.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Topology Optimization and Failure Analysis of Deployable Thin Shells with Cutouts", "author": [ { "family_name": "Ferraro", "given_name": "Serena", "orcid": "0000-0002-6038-7863", "clpid": "Ferraro-Serena" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Shell structures with cutouts are widely used in architectural and engineering applications. For thin, lightweight, and deployable space structures, cutouts are cleverly positioned to fold and store the structure in a small volume. To maintain shape accuracy, these structures must fold without becoming damaged and must be stiff in their deployed configurations. Intuitive designs often fail to satisfy these two requirements. This research proposes solutions to the topology optimization of composite, thin shell structures with cutouts.
\r\n\r\nA novel optimization algorithm was developed that makes no assumptions on the initial number, shape, and location of cutouts on deployable thin shells. The algorithm uses a density-based approach, which distributes the material within the structure by assigning a density parameter to discretized locations. This parametrization of the design domain allows for the finding of new features and the connectivity of the domain, thus providing a completely general formulation to the optimization problem. The goal is to study the effects of volume and stress constraints imposed in a deformed configuration of thin shell structures. While classical topology optimization studies focus on finding solutions to linear problems, this method is applicable to geometrically nonlinear problems and implements stress constraints in the deformed, and hence most stressed, configuration of these shells. A mathematical formulation of the optimization problem and interpolation schemes for stiffness tensor, volume, and stress are presented. A sensitivity analysis of objective function, volume, and stress constraints is provided. Finally, solutions for a thin plate and a tape spring are proposed.
\r\n\r\nDensity-based methods are computationally expensive when applied to large structures and complex shapes because of the large number of design variables. To address these challenges, two optimization methods that provide more specific solutions to the problem of composite, deployable shells are proposed. The first method uses level sets to parametrize the cutouts, thereby restricting the design space and simultaneously limiting the number of design variables. This greatly reduces the computational cost. Using this approach, successful solutions are found for stiff, composite, thin shells with complex shapes that can fold without becoming damaged. The second method uses a spline representation of the contour of a single cutout on the shell, thus performing fine tuning of the shape of the cutout. Modeling techniques that simulate localized strain and experimental methods for studying the quasi-static folding of these composite shells are developed. A laminate failure criterion suitable for thin, plain-weave composites is used in simulations to predict the onset of failure in folded shells. Numerical results are validated with folding experiments that demonstrated good agreement with numerical solutions.
\r\n\r\nLastly, it was discovered that many of the best performing solutions have multiple closely spaced cutouts, as opposed to current designs for deployable space structures that have fewer large cutouts. This leads to the formation of small strips of material between cutouts. Hence, the behavior of thin, plain-weave composite material was characterized and the first study on size-scaling effects at small length scales (\u2264 15 mm) in this type of material was performed. Size-scaling effects on stiffness and strength shown in this study were introduced in numerical simulations of deployable thin shells. The study demonstrates that the prediction of the onset of failure in folded shells strongly depends on these size effects. Numerical predictions are corroborated by an experimental investigation of localized damage in thin strips of material forming between cutouts. Deployable shells resulting from the optimization studies are built and tested and localized damage is measured via digital volume correlation techniques.
", "doi": "10.7907/9VZ4-3E71", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13628", "collection": "thesis", "collection_id": "13628", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01202020-210729635", "type": "thesis", "title": "High-Cycle Dynamic Cell Fatigue with Applications on Oncotripsy", "author": [ { "family_name": "Figueroa-Schibber", "given_name": "Erika", "orcid": "0000-00002-6629-297X", "clpid": "Figueroa-Schibber-Erika" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The method of oncotripsy (from Greek, onco- meaning \"tumor\" and \u2013tripsy \"to break\") exploits aberrations in the material properties and morphology of cancerous cells to target them selectively using tuned low-intensity pulsed ultrasound. Compared to other noninvasive ultrasound treatments that ablate malignant tissue, oncotripsy has the capability of targeting unhealthy tissue with minimal damage to healthy cells in the ablation process.
\r\n\r\nWe propose a model of oncotripsy that follows as an application of cell dynamics, statistical mechanical theory of network elasticity and 'birth-death' kinetics to describe processes of damage and repair of the cytoskeleton. We also develop a reduced dynamical model that approximates the three-dimensional dynamics of the cell and facilitates parameter studies, including sensitivity analysis and process optimization.
\r\n\r\nThe dynamical system encompasses the relative motion of the nucleus to the cell membrane and a state variable measuring the extent of damage to the cytoskeleton. The dynamical system evolves in time as a result of structural dynamics and kinetics of cytoskeletal damage and repair. The resulting dynamics are complex and exhibits behavior on multiple time scales, including the period of vibration and attenuation, the characteristic time of cytoskeletal healing, the pulsing period and the time of exposure to the ultrasound. Damage on the cells develops in the order of millions of ultrasound cycles, and the failure mechanism is explained as a fatigue process. We also account for cell variability and estimate the attendant variance of the time-to-death of a cell population. We show that the dynamical model predicts \u2014 and provides a conceptual basis for understanding \u2014 the oncotripsy effect and other trends observed in experiments.
", "doi": "10.7907/0425-SN62", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13641", "collection": "thesis", "collection_id": "13641", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02192020-135417079", "primary_object_url": { "basename": "Thesis_Tomoyuki Oniyama.pdf", "content": "final", "filesize": 24263439, "license": "other", "mime_type": "application/pdf", "url": "/13641/1/Thesis_Tomoyuki Oniyama.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Shock Compression of Molybdenum Single Crystals to High Stresses", "author": [ { "family_name": "Oniyama", "given_name": "Tomoyuki", "orcid": "0000-0001-6097-9917", "clpid": "Oniyama-Tomoyuki" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "To investigate the role of crystal anisotropy and the impact stress on the shock induced elastic-plastic deformation of BCC single crystals at high stresses, molybdenum single crystals were shock compressed along [100], [111], and [110] orientations. A series of plate impact experiments were conducted with various impact stresses (23 - 190 GPa) along each orientation. Along the [100] and [111] orientations, two-wave structure - an elastic shock wave trailed by a plastic shock wave - was observed to 110 GPa. Along the [110] orientation, the two-wave structure was observed only up to 90 GPa.
\r\n\r\nBased on the measured quantities, in-material quantities at the elastic limit and at the peak state were calculated. The elastic wave amplitudes were analyzed to determine the crystal anisotropy effects, the impact stress dependence, and the activated slip systems on the elastic limit. The elastic wave amplitude increased linearly with\r\nincreasing impact stress, and that was significantly larger along the [111] orientation compared to the other orientations. The difference between calculated maximum resolved shear stresses at the elastic limit and corresponding Peierls stress suggested the activation of {110}<111> slip systems.
\r\n\r\nAt the peak state, the Hugoniot relations were calculated along each orientation and compared with polycrystalline molybdenum Hugoniot relations. The Hugoniot relations along three orientations were in agreement within experimental uncertainties, even though the elastic limit showed considerable anisotropy. Also, they agreed reasonably well with the polycrystalline molybdenum data. This implied that the in-material quantities at the peak state do not depend on crystal orientation or the presence of grain boundaries.
\r\n\r\nIn addition to the plate impact experiments, finite element simulations of shock compressed molybdenum single crystals were conducted using Abaqus Explicit in order to gain insight into deformation mechanisms activated during the elasticplastic\r\ndeformation. Shear strains on slip systems were explicitly considered by the crystal plasticity model implemented using Abaqus VUMAT subroutine. The results of FEM simulations indicated that {110}<111> systems were likely to be operating at the elastic limit. This observation was consistent with the experimental results from the present study.
", "doi": "10.7907/YWPJ-5379", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13587", "collection": "thesis", "collection_id": "13587", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11212019-100323260", "type": "thesis", "title": "Application of Path-Independent Integrals to Soil-Structure Interaction", "author": [ { "family_name": "Garc\u00eda Su\u00e1rez", "given_name": "Antonio Joaqu\u00edn", "orcid": "0000-0001-8830-4348", "clpid": "Garc\u00eda-Su\u00e1rez-Antonio-Joaqu\u00edn" } ], "thesis_advisor": [ { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Assessing seismic pressure increment on buried structures is a critical step in the design of infrastructure in earthquake-prone areas. Due to intrinsic complexities derived from the need to match the solution in the far-field to the localized solution around the structure, the near-field, researchers have aimed at finding simplified models focused on engineering variables as the seismic earth thrust. One such model is the so-called Younan-Veletsos model, which pivots on a stringent assumption on the stress tensor.
\r\n\r\nAt the same time, the might of the path-independent integrals of solid mechanics to deal with problems in Geotechnical Engineering at large, and Soil-Structure Interaction in particular, has remained unexplored, despite of a rich landscape of potential applications. The unbridled success of these path-independent integrals in Fracture Mechanics, a discipline which cannot be understood without them currently, may be mirrored in problems in Geotechnical Engineering, since the two fields, despite appearing very detached from each other at first glance, share deep traits: in both cases, the system under consideration can be conceptualized as a domain with simple, easy-to-assess regions (the areas where remote loading is applied and the far-field, respectively) and also with other complex, hard-to-understand regions (the crack tip, the near-field).
\r\n\r\nWe present the first derivation of the exact solution of the Younan-Veletsos problem, which is later analyzed to reveal phenomena not captured by previous approximate solutions. Then, we introduce a novel model which relies on the path-independent Rice\u2019s J-integral, a customary tool in Fracture Mechanics, which is applied here in the Soil-structure Interaction context for the first time. This novel model captures those features of the exact solution that were missed by prior approximations. The capabilities of the J-integral to, first, find an upper bound of the force induced by earthquakes over the walls of underground structures, under some conditions, and, second, to understand the soil-structure kinematic interaction phenomenon are also assessed.
\r\n\r\nAdditionally, the intermediate step of analyzing of the far-field yielded some results concerning Site Response Analysis which are also included in the text.
", "doi": "10.7907/MMWW-B046", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:11759", "collection": "thesis", "collection_id": "11759", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08092019-151803660", "primary_object_url": { "basename": "MacDonaldKimberley2020thesis.pdf", "content": "final", "filesize": 78530454, "license": "other", "mime_type": "application/pdf", "url": "/11759/1/MacDonaldKimberley2020thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Three-Dimensional Quantitative Visualization for Mechanics of Discontinuous Materials", "author": [ { "family_name": "Mac Donald", "given_name": "Kimberley Ann", "orcid": "0000-0003-4512-9740", "clpid": "Mac-Donald-Kimberley-Kimberley-Ann" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "clpid": "Hunt-M-L" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The complexity and multiscale nature of material microstructures introduces significant intricacies to many mechanics problems for which we do not have a full theoretical understanding. Under loading, these microstructures can introduce significant nonlinearities that cannot be described sufficiently by current theories and models. This leads us to consider experiments we could perform to improve our understanding of such effects. This thesis describes the design of experiments exploring two aspects of material microstructure effects: (i) crack propagation and renucleation in soft brittle polymers and (ii) interparticle forces in granular materials.
\r\n\r\nFirst, experimental and analysis methods are developed to study fracture mechanics in soft brittle polymers with the goal of developing a more detailed understanding of the effects of microstructural heterogeneities on crack propagation and renucleation in three-dimensions. To better understand these processes, experiments on crack propagation in thin soft polymers using confocal microscopy images are conducted. Traditional metrics associated with crack propagation including stress intensity factor (SIF, K) and energy release rate (ERR, G) are calculated by direct measurement of the crack tip opening displacement (CTOD, \u03b4t) on the sub-millimeter scale. Errors in these calculations are comparable to those reported in the literature for more traditional fracture experiment geometries. Fluorescent speckle images are captured using confocal microscopy imaging, a fast and low cost 3D optical imaging technique, to study crack geometry during propagation. Images of renucleation events are also captured allowing investigation of factors contributing to slow crack roughening observed by earlier researchers. The goal of this study is to provide an experimental method to enhance understanding of crack interactions with microstructural heterogeneities and of renucleation events, which can significantly improve our ability to design material toughness.
\r\n\r\nTo begin to understand the effects of engineered microstructural heterogeneities such as inclusions in materials, we must be able to produce such engineered systems and understand the interparticle interactions. To this end, a method to manufacture volumetrically speckled spheres in-house with controlled diameters was developed. Additionally, an experimental method combining confocal microscopy with digital volume correlation (DVC) was also used to study interparticle force transmission in 3D. Analysis of an in-plane 2D projection of volumetric surface data shows that three-dimensional effects play a significant role in the deformation of granular assemblies. Study of a single grain in 3D demonstrates progress in experimental capabilities and highlights the need for more studies to validate existing numerical models and theories for granular matter. Analysis of particle scale deformations and strains with the Granular Element Method (GEM) allows us to determine interparticle forces and understand the development and evolution of force chains in a granular assembly under a wide variety of loading conditions. These experiments can also lead to development of new understanding of the effects of inclusions on material properties, processes, and damage evolution.
", "doi": "10.7907/0M4F-FG13", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13754", "collection": "thesis", "collection_id": "13754", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012020-003455628", "primary_object_url": { "basename": "Thesis_Kirsti_Pajunen_6-2-2020_Full.pdf", "content": "final", "filesize": 31055307, "license": "other", "mime_type": "application/pdf", "url": "/13754/9/Thesis_Kirsti_Pajunen_6-2-2020_Full.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Dynamics of Lightweight Tensegrity-Inspired Metamaterials Fabricated with 3D-Printing", "author": [ { "family_name": "Pajunen", "given_name": "Kirsti Mari", "orcid": "0000-0002-5366-1190", "clpid": "Pajunen-Kirsti-Mari" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Hofmann", "given_name": "Douglas C.", "clpid": "Hofmann-D-C" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Tensegrity structures and lattices have been of interest in engineering applications for decades, with their dynamics becoming a thriving field of study. Tensegrities consist of structural members under purely axial loading, either tension or compression, and obtain their stability from prestress. They possess unique characteristics such as high strength-to-weight ratio, nonlinear behavior, and elastic response under severe deformation. Tensegrity lattices (or metamaterials) have been shown to exhibit appealing dynamic attributes such as continuous tunability with prestress, impact mitigation, energy trapping and lensing, and nonlinear wave propagation, to name a few. However, their pin-jointed and prestressed nature presents significant manufacturing limitations, especially in the formation of lattices with large numbers of tessellated unit cells. Therefore, experimental validation of the dynamics of tensegrity metamaterials has remained elusive. For lattices with tensegrity-like characteristics to be manifested for real-world applications, a method for producing tensegrity-like metamaterials at multiple length scales is needed.
\r\n\r\nIn this thesis, we present a design for a 3D-printable tensegrity-inspired structure with the equivalent strain energy capacity and stress-strain response as a pin-jointed tensegrity. Using this structure as a building block for multidimensional lattices, we subject them to a range of dynamic loading conditions to study their response. First, we perform experiments and simulations to obtain the dispersion relations for 1D and 3D lattices. We demonstrate the lattices\u2019 ability to continuously tune the dispersion characteristics (e.g., band gap and wave speed) under precompression. This trait shows potential for acoustic lensing and dispersive wave propagation. In 3D, we show that the lattice shows the same type of unique properties, such as faster shear speed than longitudinal speed, as pin-jointed tensegrity lattices. Next, we study the lattices under impact loading. Long-duration impact experiments on baseline unit cells and 1D lattices show their resilience to repeated deformation, elasticity, and load limitation behaviors. Short-duration impulse experiments and simulations exhibit a wealth of desirable properties, such as high force transmission reduction, highly dispersive wave propagation, tunable wave speeds, energy trapping, and redirection of energy. We demonstrate that these tensegrity-inspired metamaterials not only exhibit and experimentally demonstrate tensegrity-like characteristics, but open a new range of lightweight metamaterials with unprecedented dynamic properties.
", "doi": "10.7907/wm2f-4013", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13574", "collection": "thesis", "collection_id": "13574", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11122019-171331857", "type": "thesis", "title": "Fracture and Toughening of Brittle Structures with Designed Anisotropy", "author": [ { "family_name": "Brodnik", "given_name": "Neal Ryan", "orcid": "0000-0002-4426-5997", "clpid": "Brodnik-Neal-Ryan" } ], "thesis_advisor": [ { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Despite good thermal and chemical properties, the use of ceramic materials in structural applications is limited by their inherently brittle nature. Efforts have been made to improve the toughness of ceramics through composite design, but recent developments in net shape processing such as additive manufacturing have significantly expanded this design space. Where composite topologies and morphologies were previously limited by material composition and thermodynamics, tools like 3D printing now allow for the design of composite structures of nearly any shape or arrangement.
\r\n\r\nThis work seeks to understand how these processing advances might be utilized to improve the toughness of brittle composites by exploring how previously inaccessible anisotropic inclusion structures might influence fracture behavior. The study begins with the evaluation of printed photopolymer structures as model brittle materials. First, printed structures are used to explore how elastic contrast between inclusions and matrix can affect crack propagation and improve toughness. Here, anisotropy presents an opportunity to achieve similar toughness to isotropic structures at smaller volume fractions by virtue of topologies that only exhibit toughening only in a singular direction, but require significantly less material to do so. Next, the effect of anisotropic voids is explored as a means of controlling crack nucleation and growth. With consideration of both compliance and directional propagation, a \"fracture diode\" that exhibits controlled, predictable fracture 100% of the time can be realized.
\r\n\r\nAfter exploring brittle polymers, ceramics systems with similar toughness and higher stiffness are considered. First, a model layered system of mica is explored, where wedge splitting can be used achieve stable crack growth. This allows for the evaluation of how changes in compliance can improve the interlayer toughness without directly interacting with the crack. Finally, this study extends further into ceramics by exploring silicon oxycarbide (SiOC) truss structures and truss elements produced from 3D printed preceramic polymers. In addition to considering the material itself, changes in truss structure are explored as a means of changing deformation mode, and by consequence, failure strength. These model experiments suggest that if trusses are compatible, they can be interchanged to control failure of the bulk structure.
\r\n\r\nThis study demonstrates how designed heterogeneities with anisotropic structure can be used to both enhance the toughness of brittle composites as well achieve a greater degree of control over both crack nucleation and propagation in brittle systems where predicting failure is otherwise difficult. Looking forward, new processing tools like additive manufacturing present major opportunities for expanding the design space of brittle composites to achieve higher toughness and better fracture control than previously available. These new techniques may be able to expand the mechanical viability of ceramics, and make them better suited to mechanically demanding applications in the future.
", "doi": "10.7907/ET0C-MK61", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:11207", "collection": "thesis", "collection_id": "11207", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09272018-211547049", "primary_object_url": { "basename": "thesis_Paul_Mazur.pdf", "content": "final", "filesize": 21905514, "license": "other", "mime_type": "application/pdf", "url": "/11207/1/thesis_Paul_Mazur.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Controlling the Buckling Behavior of Bilayered Systems", "author": [ { "family_name": "Mazur", "given_name": "Paul Antoine Benoit", "orcid": "0000-0002-2837-9716", "clpid": "Mazur-Paul-Antoine-Benoit" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Audoly", "given_name": "Basile", "clpid": "Audoly-Basile" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A bilayered system is an assembly of two different materials and has the form of flat and thin layers. The two materials are attached to each other at the surface. The attachment method varies depending on the materials properties. Bilayered systems made of materials with different dimensions and stiffness have been widely studied and used for different applications. The characteristic scale of this kind of system can go from hundreds of km in the case of geological layers on the Earth surface to some \u00b5m in the case of very small electronic systems or microlenses.
\r\n\r\nThe behavior of a bilayered system, when submitted to a stimulus, is characterized by the conflict between the preferred response of each material and the constraint that one imposes on the other. As a result, the deformation of the bilayered system will be different from that which could be obtained when the materials are taken separately. Of particular interest is the buckling of such systems: when submitted to a particular stress distribution, one material will expand significantly more than the other, but as the two materials are attached at the interface surface, the material displacements must be continuous through this interface. The conflict between the continuity of displacement and the need to expand differently may result in nonlinear patterns at this interface. Those unstable situations can be used to define a limit of constraint for the materials or can be used as actuators for a desired surface pattern. Many studies have focused on characterizing homogeneous buckling within an entire surface due to homogeneous strain distribution within the top surface. This characterization was performed theoretically, numerically, and experimentally. But, some studies have shown different possibilities of evolution of the buckling patterns known today. As a consequence, we can pose two questions: 1) Is there a possibility to modify non-linear patterns regardless of what is imposed by mechanical properties and dimensions? 2) What happens in the case of a non-uniform state of constraints within the bilayered system?
\r\n\r\nThis thesis explores those questions for the case of a thin stiff film attached to a compliant thick substrate. The first part of this thesis serves to describe the initial buckling theory in the case of uniform strain and explains how to define the loading threshold resulting in uniform buckling at the surface characterized by a finite number of spatial frequencies. The second part of the thesis studies the consequences of a non-uniform loading within the surface. A numerical method based on the theory of the first part is implemented to show the emergence of new frequencies due to the discontinuous loading distribution. The third part focuses on the possibility of tuning a uniform buckling by including an electromechanical coupling into the bilayered system. This coupling makes the materials sensitive to electric fields, thus creating a new energy term to interfere with the mechanical energy of deformation, thereby modifying the resulting spatial frequency of the buckling. This study is done theoretically and numerically by finite element modeling.
", "doi": "10.7907/C70B-K221", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11142", "collection": "thesis", "collection_id": "11142", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08042018-083338014", "type": "thesis", "title": "A Line-Free Method of Monopoles for 3D Dislocation Dynamics", "author": [ { "family_name": "Deffo Nde", "given_name": "Arnold Durel", "orcid": "0000-0001-9077-8315", "clpid": "Deffo-Nde-Arnold-Durel" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ariza", "given_name": "Pilar", "clpid": "Ariza-Pilar" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Despite the emergence of architected materials for various applications, metals still play a key role in engineering in general and aeronautics in particular. Turbine blades in jets engines for instance are made from single-crystal Nickel superalloys. As a result, studying the failure mechanism of these crystalline materials would help understand the limits of their applications. At the core of this mechanism are line defects called dislocations. Indeed, the plastic deformation of metals is governed by the motion of dislocation ensembles inside the crystal. In this thesis, we propose a novel approach to dislocation dynamics through the method of monopoles. In this approach, we discretize the dislocation line as a collection of points (or monopoles), each of which carries a Burgers \"charge\" and an element of line. The fundamental difference between our method and current methods for dislocation dynamics lies in the fact that the latter discretize the dislocation as a collection of line segments from which spans a need to keep track of the connectivity of the nodes. In our approach, we propose a \"line-free\" discretization where a linear connectivity or sequence between monopoles need not be defined. This attribute of the formulation offers significant computational advantages in terms of simplicity and efficiency. Through verification examples, we show that our method is consistent with existing results for simple configurations. We then build on this success to investigate increasingly complex examples, this with the ultimate goal of simulating the plastic deformation of a BCC grain in an elastic matrix.
", "doi": "10.7907/23YV-3312", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11395", "collection": "thesis", "collection_id": "11395", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02172019-174051312", "primary_object_url": { "basename": "Pressure_and_Stress_Transients_in_Autoinjector_Devices.pdf", "content": "final", "filesize": 29005814, "license": "other", "mime_type": "application/pdf", "url": "/11395/1/Pressure_and_Stress_Transients_in_Autoinjector_Devices.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Pressure and Stress Transients in Autoinjector Devices", "author": [ { "family_name": "Veilleux", "given_name": "Jean-Christophe", "orcid": "0000-0002-5420-9411", "clpid": "Veilleux-Jean-Christophe" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "Explosion Dynamics Laboratory" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The viscosity of drug solutions delivered parenterally has been increasing over the years. Injecting viscous drug solutions using spring-actuated autoinjector devices is challenging due to a number of technical and human factor constraints. Some of the related challenges are investigated in this thesis.
\r\n\r\nActuation of autoinjector devices powered using stiff springs can create deleterious pressure and stress transients which are not needed to achieve the normal functions of the device. Experimental measurements have shown that peak pressures and stresses substantially larger than what is needed to achieve the normal device function can occur during the actuation phase, creating unnecessary potential for device failure.
\r\n\r\nThe acceleration of the syringe during actuation can be very large, often creating transient cavitation in the cone region. The occurrence or absence of cavitation is determined by the relative timing of syringe pressurization and syringe acceleration, which is affected by several factors such as the presence, location, and size of an air gap inside the syringe, and the friction between the plunger-stopper and the syringe.
\r\n\r\nExperiments and numerical simulations have shown that sharp pressure waves traveling inside the syringe can be amplified within the cone terminating the syringe. Despite the potential for shock focusing, the impulsive pressurization and the rapid deceleration of pre-filled syringes create a potential for failure which is localized in the syringe shoulder and at the junction between the flange and the barrel, not inside the cone. The cavitation events, on the other hand, create a potential for failure which is limited to a region in close proximity of the bubble upon collapse. The collapse of cavitation bubbles located within the syringe cone can be enhanced due to geometrical effects, and the resulting stresses can be large enough to cause syringe failure.
\r\n\r\nThis thesis demonstrates that static and quasi-static analyses do not provide accurate estimates of the peak pressures and stresses occurring within the device. The pressure and stresses created by the highly dynamic events occurring during actuation need to be accounted for during device design in order to improve device reliability, the user's experience, and patient's adherence to prescribed treatments. The findings discussed in this work provide insights and guidance as to how the transient events can be mitigated.
", "doi": "10.7907/VJSH-TF65", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11233", "collection": "thesis", "collection_id": "11233", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10162018-093212227", "primary_object_url": { "basename": "Yang_Jin_2019.pdf", "content": "final", "filesize": 20790536, "license": "other", "mime_type": "application/pdf", "url": "/11233/1/Yang_Jin_2019.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Fast Adaptive Augmented Lagrangian Digital Image Correlation", "author": [ { "family_name": "Yang", "given_name": "Jin", "orcid": "0000-0002-5967-980X", "clpid": "Yang-Jin" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Digital image correlation (DIC) is a powerful experimental technique for measuring full-field displacement and strain. The basic idea of the method is to compare images of an object decorated with a speckle pattern before and after deformation in order to compute the displacement and strain fields. Local Subset DIC and finite element-based Global DIC are two widely used image matching methods; however there are some drawbacks to these methods. In Local Subset DIC, the computed displacement field may not satisfy compatibility, and the deformation gradient may be noisy, especially when the subset size is small. Global DIC incorporates displacement compatibility, but can be computationally expensive. In this thesis, we propose a new method, the augmented-Lagrangian digital image correlation (ALDIC), that combines the advantages of both the local (fast and in parallel) and global (compatible) methods. We demonstrate that ALDIC has higher accuracy and behaves more robustly compared to both Local Subset DIC and Global DIC.
\r\n\r\nDIC requires a large number of high resolution images, which imposes significant needs on data storage and transmission. We combined DIC algorithms with image compression techniques and show that it is possible to obtain accurate displace- ment and strain fields with only 5 % of the original image size. We studied two compression techniques \u2013 discrete cosine transform (DCT) and wavelet transform, and three DIC algorithms \u2013 Local Subset DIC, Global DIC and our newly proposed augmented Lagrangian DIC (ALDIC). We found the Local Subset DIC leads to the largest errors and ALDIC to the smallest when compressed images are used. We also found wavelet-based image compression introduces less error compared to DCT image compression.
\r\n\r\nTo further speed up and improve the accuracy of DIC algorithms, especially in the study of complex heterogeneous strain fields at various length scales, we apply an adaptive finite element mesh to DIC methods. We develop a new h-adaptive technique and apply it to ALDIC. We show that this adaptive mesh ALDIC algorithm significantly decreases computation time with no loss (and some gain) in accuracy.
", "doi": "10.7907/MZ5G-PS98", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11235", "collection": "thesis", "collection_id": "11235", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10162018-124906600", "type": "thesis", "title": "Small-Scale Deformation and Fracture of Hard Biomaterials", "author": [ { "family_name": "Tertuliano", "given_name": "Ottman Aeman", "orcid": "0000-0003-0524-3944", "clpid": "Tertuliano-Ottman-Aeman" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "div_eng" } ], "abstract": "Structural materials engineering often aims to realize materials that are simultaneously strong, tough, and lightweight \u2014 a combination classically considered mutually exclusive. Natural composite materials such as bone exhibit a combination of these properties far exceeding that of their constituents, a feat generally credited to their hierarchical structure \u2014 all the way down the nanoscale. To date, a quantitative description of how this property combination arises in such microstructurally complex materials has remained elusive due to challenges in experimentally isolating and probing the salient deformation and toughening mechanisms at the micro and nanometer scales \u2014 length scales on the order the constituents of many natural composites.
\r\n\r\nIn this thesis, we first investigate the site-specific nanoscale structure of human bone using transmission electron microscopy. We show the presence of previously undiscovered disordered arrangement of collagen and mineral \u2014 alongside a well known ordered structure \u2014 within the trabecular architecture of bone. We perform micro- and nano-mechanical compression experiments to probe strength and deformation of each of these microstructures, revealing a size-dependent strength of bone attributed to the limited number of failure-initiating critical defects (e.g pores) in the small-scale samples relative to macro-scale tissue.
\r\n\r\nUnlike experiments for investigating strength at small-scales, fracture experiments are standardized for the macroscale. To address this, we developed an in situ SEM/nanoindenter methodology that enables 3-point bending fracture experiments with observation and measurement of crack growth and toughening behavior at nano and micrometer scales. Using this technique, we discuss the crack initiation and growth toughness arising primarily from the underlying fibril microstructure in bone. In the context of a crack growth resistance, we describe a transition in the toughening behavior of bone originating from different levels of hierarchy. Given its versatility, this experimental technique establishes a platform for understanding the coupling between structure and fracture behavior of micron-sized materials.
", "doi": "10.7907/CAPE-5661", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11404", "collection": "thesis", "collection_id": "11404", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02202019-104738145", "type": "thesis", "title": "Dynamic Strength of Silica Glasses at High Pressures and Strain Rates", "author": [ { "family_name": "Kettenbeil", "given_name": "Christian", "orcid": "0000-0003-0301-3678", "clpid": "Kettenbeil-Christian" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Mello", "given_name": "Michael", "orcid": "0000-0003-2129-9235", "clpid": "Mello-Michael" }, { "family_name": "Clifton", "given_name": "Rodney J.", "clpid": "Clifton-Rodney-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Understanding the behavior of silica glasses at high pressures and strain rates is of great importance for geological processes and highly relevant to many technological applications including high-powered laser-matter interactions in optical elements and impact/blast damage in defense systems. Materials typically experience large inelastic deformations at high pressures, which are strongly affected by strength-related phenomena such as work hardening, damage and thermal softening. The pressure-shear plate impact experiment (PSPI) provides detailed information on the pressure and strain rate dependent strength properties of materials subjected to uniaxial compression. However, its range of attainable pressures has so far been limited and the assumptions required for its analysis become invalid at pressures beyond the Hugoniot elastic limit of the anvil materials. In this dissertation, a high-pressure PSPI (HP-PSPI) technique is developed that greatly extends the range of attainable experimental conditions by achieving higher terminal projectile velocities in a powder gun setup. A novel fiber-optic heterodyne transverse velocimeter (HTV) is developed to enable the use of robust frequency-based data reduction techniques, which reduce the effect of signal noise and light coupling losses. A forward analysis method, based on finite element simulations, is employed to match the experimentally observed material response during HP-PSPI experiments on soda-lime glass samples while considering the inelastic deformation of the utilized tungsten carbide anvils. Symmetric HP-PSPI experiments on tungsten carbide revealed a loss of strength at normal stresses exceeding 25 GPa, which hint at active damage or softening mechanisms under nominally uniaxial strain compression. A pressure-dependent strain softening model transitions soda-lime glass from an intact strength of 2.8 GPa, below strains of 10-30%, to a failed granular state following extensive inelastic shear deformation, which accurately predicts the measured response over a wide range of stresses (9-21 GPa) and strain rates (3\u2022105-2\u2022107s-1). Extending the range of previously attainable pressures and strain rates in PSPI experiments, combined with more robust diagnostics and analysis tools, will greatly benefit our understanding of material strength in extreme environments and enables the investigation of material behavior in a currently unexplored range of pressures and strain rates.
", "doi": "10.7907/RZJW-MX30", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11432", "collection": "thesis", "collection_id": "11432", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03272019-170619076", "type": "thesis", "title": "Predicting Microstructural Pattern Formation Using Stabilized Spectral Homogenization", "author": [ { "family_name": "Vidyasagar", "given_name": "A.", "orcid": "0000-0003-0262-5429", "clpid": "Vidyasagar-A" } ], "thesis_advisor": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Instability-induced patterns are ubiquitous in nature, from phase transformations and ferroelectric switching to spinodal decomposition and cellular organization. While the mathematical basis for pattern formation has been well-established, autonomous numerical prediction of complex pattern formation has remained an open challenge. This work aims to simulate realistic pattern evolution in material systems exhibiting non-(quasi)convex energy landscapes. These simulations are performed using fast Fourier spectral techniques, developed for high-resolution numerical homogenization. In a departure from previous efforts, compositions of standard FFT-based spectral techniques with finite-difference schemes are used to overcome ringing artifacts while adding grid-dependent implicit regularization.
\r\n\r\nThe resulting spectral homogenization strategies are first validated using benchmark energy minimization examples involving non-convex energy landscapes. The first investigation involves the St. Venant-Kirchhoff model, and is followed by a novel phase transformation model and finally a finite-strain single-slip crystal plasticity model. In all these examples, numerical approximations of energy envelopes, computed through homogenization, are compared to laminate constructions and, where available, analytical quasiconvex hulls.
\r\n\r\nSubsequently, as an extension of single-slip plasticity, a finite-strain viscoplastic formulation for hexagonal-closed-packed magnesium is presented. Microscale intragranular inelastic behavior is captured through high-fidelity simulations, providing insight into the micromechanical deformation and failure mechanisms in magnesium. Studies of numerical homogenization in polycrystals, with varying numbers of grains and textures, are also performed to quantify convergence statistics for the macroscopic viscoplastic response.
\r\n\r\nIn order to simulate the kinetics of pattern evolution, stabilized spectral techniques are utilized to solve phase-field equations. As an example of conservative gradient-flow kinetics, phase separation by anisotropic spinodal decomposition is shown to result in cellular structures with tunable elastic properties and promise for metamaterial design. Finally, as an example of nonconservative kinetics, the study of domain wall motion in polycrystalline ferroelectric ceramics predicts electromechanical hysteresis behavior under large bias fields. A first-principles approach using DFT-informed model constants is outlined for lead zirconate titanate, producing results showing convincing qualitative agreement with in-house experiments. Overall, these examples demonstrate the promise of the stabilized spectral scheme in predicting pattern evolution as well as effective homogenized response in systems with non-quasiconvex energy landscapes.
", "doi": "10.7907/F1VN-1X80", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11484", "collection": "thesis", "collection_id": "11484", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04232019-162807004", "primary_object_url": { "basename": "DavidHuynh_thesis.pdf", "content": "final", "filesize": 22001730, "license": "other", "mime_type": "application/pdf", "url": "/11484/1/DavidHuynh_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Spatio-Temporal Response of a Compliant-Wall, Turbulent Boundary Layer System to Dynamic Roughness Forcing", "author": [ { "family_name": "Huynh", "given_name": "David Pham", "orcid": "0000-0002-8430-6255", "clpid": "Huynh-David-Pham" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis investigates the interaction between an elastic compliant surface and a turbulent boundary layer exposed to dynamic roughness forcing. The goals are to explore a unique perspective of this fluid-structural problem through narrow-band forcing, and to further develop the understanding of dynamic roughness. Water tunnel experiments are designed with flow and surface measurements, both phase-locked to the roughness actuation. This enables a phase-averaged analysis, which leverages the deterministic input to isolate the temporally correlated components of the flow and surface response. Identifying the directly interacting velocity and deformation modes allows the complex, fluid-structural system to be studied in a more tractable, input-output manner.
\r\n\r\nThe first experiment is conducted with a smooth-wall turbulent boundary layer forced by dynamic roughness, and contributes to the knowledge of this type of forcing through structure-resolved particle image velocimetry. This allows for the streamwise-spatial nature and the wall-normal velocity component (v) of the roughness-forced flow to be explored, which had not been previously studied. A spatial amplitude modulation is observed in the synthetic structure and investigated directly through the spatial spectra. Through a parametric study and an empirical fit, the forcing frequency may now be selected to target a particular streamwise length scale.
\r\n\r\nThe second experiment implements a gelatin sample subject to an unforced turbulent boundary layer. The surface response is characterized and serves as a base case with which to identify the roughness-forced component of the deformations. This naturally leads to the third experiment, where the full compliant-wall, dynamic-roughness-forced turbulent boundary layer system is considered. The surface response to the synthetic flow structure is confirmed, which sets the stage for a comparison between the smooth-wall and compliant-wall data to study the effect of the compliant surface.
\r\n\r\nThe smooth/compliant comparison is guided by a resolvent analysis, which predicts a virtual wall feature in the v velocity mode for the elastic material under consideration. Using this prediction to inform a conditional average, the virtual wall is revealed in the experimental data. Thus, the action of the elastic surface is interpreted as opposing the v velocity near the wall, in a manner similar to wall-jet opposition control.\r\nPrevious experimental studies of viscoelastic compliant surfaces have demonstrated the potential for turbulent drag reduction, though either indirectly via the turbulence intensities or with relatively high skin friction measurement error. A common observation in these studies was the importance of the interaction between the surface and the coherent structures in the flow. To that end, this study has isolated and modeled the behavior of the fluid-structural system with a single spatio-temporal scale generated by dynamic roughness forcing. The results provide a physical interpretation of the effect of an elastic surface on turbulent boundary layer flow structures and informs the ongoing development of a reduced-order modeling tool in the resolvent analysis.
", "doi": "10.7907/A5PS-GT54", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11514", "collection": "thesis", "collection_id": "11514", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05132019-143045769", "primary_object_url": { "basename": "Constraining_the_Mantle_s_Rheology_using_Methods_in_Uncertainty_Quantification.pdf", "content": "final", "filesize": 36432455, "license": "other", "mime_type": "application/pdf", "url": "/11514/1/Constraining_the_Mantle_s_Rheology_using_Methods_in_Uncertainty_Quantification.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Constraining the Mantle's Rheology Using Methods in Uncertainty Quantification", "author": [ { "family_name": "Ratnaswamy", "given_name": "Vishagan", "orcid": "0000-0002-2371-807X", "clpid": "Ratnaswamy-Vishagan" } ], "thesis_advisor": [ { "family_name": "Gurnis", "given_name": "Michael C.", "clpid": "Gurnis-M-C" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Simons", "given_name": "Mark", "clpid": "Simons-M" }, { "family_name": "Gurnis", "given_name": "Michael C.", "clpid": "Gurnis-M-C" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "An accurate estimation of the large-scale forces in the mantle has been difficult to obtain as numerical models either do not use an accurate rheology nor reproduce surface observations. While much work has been done in developing high-fidelity forward models that capture the salient physics of shear-thinning and dynamic weakening, they fail to reproduce observations such as plate motions and topography. In this thesis, we develop an optimization methodology that minimizes the misfit in surface observations such as plate motions and average effective viscosity for certain regions of the mantle. We utilize adjoints to calculate the gradient, while using second-order adjoints to construct the Hessian so as to infer the rheological parameters of the mantle's rheology. Furthermore, we build on this optimization scheme by constructing the Gaussian approximation of the posterior distribution for the inferred rheological parameters using the Hessian and establish the trade-offs between each parameter through their conditional distributions. We further extend this Gaussian approximation to infer extrinsic quantities such as the stresses in the fault zones and the average effective viscosity in the hinge zones to not only quantify the uncertainty, but also to see partitioning of the coupling of each subduction zone. ", "doi": "10.7907/F6FW-T648", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11530", "collection": "thesis", "collection_id": "11530", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05222019-132217207", "type": "thesis", "title": "Towards High Performance Robotic Actuation", "author": [ { "family_name": "Georgiev", "given_name": "Nikola-Zlatkov", "orcid": "0000-0002-7997-5577", "clpid": "Georgiev-Nikola-Zlatkov" } ], "thesis_advisor": [ { "family_name": "Burdick", "given_name": "Joel Wakeman", "clpid": "Burdick-J-W" } ], "thesis_committee": [ { "family_name": "Ames", "given_name": "Aaron D.", "clpid": "Ames-A-D" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Mello", "given_name": "Michael", "clpid": "Mello-Michael" }, { "family_name": "Burdick", "given_name": "Joel Wakeman", "clpid": "Burdick-J-W" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The main objective of this thesis is to enable development of high performance actuation for legged, limbed and mobile robots. Due to the fact that such robots need to support their own weight, their actuators need to be light weight, compact and efficient. Furthermore, a dynamics analysis, shows that the actuators' design may have significant impact on a robot's dynamics sensitivity. These consideration motivate improvements in all actuator design aspects.
\r\n\r\nFirst, the application-specific design of outer rotor motors with concentrated windings is considered. It is shown that an intrinsic design trade-off exists between a motor's copper loss, core loss and mass, which allows development of motors with superior performance for a particular application. The three main application categories of interest are: electric vehicles, drones and robotic joints. Due to their outstanding torque density, high pole count outer rotor motors are analysed in terms of their design and optimization for robotic applications. Motor design scaling modes are also described in order to outline the main challenges in the implementation of high torque motors.
\r\n\r\nNext, the design of gearboxes for robotic actuation is discussed. A novel type of high reduction Bearingless Planetary Gearbox is introduced which allows large range of reduction ratios to be achieved in a compound planetary stage. In this concept, all gear components float in an unconstrained manner as the planet carrier is substituted with a secondary sun gear. The advantages of the Bearingless Planetary Gearbox over current approaches in terms of improved robustness, load distribution, manufacturability, and assembly are outlined.
\r\n\r\nFinally, analysis, design, and prototyping of rotary planar springs for rotary series elastic actuators is described. A mathematical model, based on curved beam theory, that allows rapid design, analysis, and comparison of rotary springs is developed. Mass reduction techniques based on composite arm structures are introduced and internal arm contact modeling is presented. Motivated by strain energy density analysis, an optimization based spring design approach is developed that allows significant increase in the torque and torque density.
\r\n", "doi": "10.7907/W64Q-1R69", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11690", "collection": "thesis", "collection_id": "11690", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06052019-161606954", "type": "thesis", "title": "Fabrication, Mechanical Characterization, and Modeling of 3D Architected Materials upon Static and Dynamic Loading", "author": [ { "family_name": "Portela G.", "given_name": "Carlos Mauricio", "orcid": "0000-0002-2649-4235", "clpid": "Portela-G-Carlos-M" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Architected materials have been ubiquitous in nature, enabling unique properties that are unachievable by monolithic, homogeneous materials. Inspired by natural processes, man-made three-dimensional (3D) architected materials have been reported to enable novel mechanical properties such as high stiffness- and strength-to-density ratios, extreme resilience, or high energy absorption. Furthermore, advanced fabrication techniques have enabled architected materials with feature sizes at the nanometer-scale, which exploit material size effects to approach theoretical bounds. However, most architected materials have relied on symmetry, periodicity, and lack of defects to achieve the desired mechanical response, resulting in sub-optimal mechanical response under the presence of inevitable defects. Additionally, most of these nano- and micro-architected materials have only been studied in the static regime, leaving the dynamic parameter space unexplored.
\r\n\r\nIn this work, we address these issues by: (i) proposing numerical and theoretical tools that predict the behavior of architected materials with non-ideal geometries, (ii) presenting a pathway for scalable fabrication of tunable nano-architected materials, and (iii) exploring the response of nano- and micro-architected materials under three types of dynamic loading. We first explore lattice architectures with features at the micro- and millimeter scales and provide an extension to the classical stiffness scaling laws, enabled by reduced-order numerical models and experiments at both scales. After discussing the effect of nodes (i.e., junctions) on the mechanical response of lattice architectures, we propose alternative node-less geometries that eliminate the stress concentrations associated with nodes to provide extreme resilience. Using natural processes such as spinodal decomposition, we present pathways to fabricate a version of these materials with samples sizes on the order of cubic centimeters while achieving feature sizes on the order of tens of nanometers. In the dynamic regime, we design, fabricate, and test micro-architected materials with tunable vibrational band gaps through the use of architectural reconfiguration and local resonance. Lastly, we present methods to fabricate carbon-based materials at the nano- and centimeter scales and test them under supersonic impact and blast conditions, respectively. Our work provides explorations into pathways that could enable the use of nano- and micro-architected materials for applications that go beyond small-volume, quasi-static mechanical regimes.
", "doi": "10.7907/75S6-SB32", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11709", "collection": "thesis", "collection_id": "11709", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-114129662", "primary_object_url": { "basename": "Caltech_Thesis_Final_Yuchen_June_2019.pdf", "content": "final", "filesize": 155575352, "license": "other", "mime_type": "application/pdf", "url": "/11709/1/Caltech_Thesis_Final_Yuchen_June_2019.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Deployable Piezoelectric Thin Shell Structures: Concepts, Characterization and Vibration Control", "author": [ { "family_name": "Wei", "given_name": "Yuchen", "orcid": "0000-0002-7669-020X", "clpid": "Wei-Yuchen" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Chung", "given_name": "Soon-Jo", "orcid": "0000-0002-6657-3907", "clpid": "Chung-Soon-Jo" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The thesis presents three interconnected technology paths to the design and realization of novel deployable active thin shell structures. The baseline concept envisioned is built upon a deployable ultra-thin piezoelectric active thin shell architecture, with segmented tessellations. This vision is motivated by the need to deploy and control large, curved and precise surfaces for a variety of applications including future space telescopes, and is made possible by recent progress in ultra-thin high-performance composites and active material technologies. The thesis uses a combination of heuristic design, theoretical analysis, numerical modeling and novel experimental techniques to construct and validate proposed concepts for deployable piezoelectric thin shells.
\r\n\r\nSpecifically, the thesis answers the following questions: i) How to design and manufacture precise, foldable and curved piezoelectric shells. ii) How to deploy these shells reliably and maintain shape correctability in the deployed state. iii) How to synthesize large, curved deployable surfaces with the aforementioned advantages. iv) How to characterize and predict the nonlinear behavior of piezoelectric materials and thin structures under high electric field actuation and large bending deformations. v) How to improve the shape stability of piezoelectric active thin shells under dynamic disturbances without introducing external sensors.
\r\n\r\nFirst, the thesis proposes new methodologies and design criteria to synthesize deployable, modular edge-supported thin shells based on a combination of origami-inspired folding patterns and spatial mechanisms. In contrast to traditional deployable surface designs, which attach rigid shells to deployable trusses, the proposed methodology enables concurrent folding of flat or curved shells along with the support structures. Starting from a basic module, a variety of deployable surface concepts are proposed through tessellations of the module.
\r\n\r\nA piezoelectric material unimorph architecture is further introduced, providing global curvature and shape correction capabilities. All components of the basic concept are validated through model prototyping and material folding tests, and it is discovered that both the ultra thin carbon fiber composites and piezoelectric ceramic materials can achieve a small folding radius without failure. A composite, doubly-curved foldable shell is also designed and manufactured while still maintaining low shape error. These efforts have led to a new family of deployable piezoelectric thin shell structures that integrate low areal density, high shape accuracy, and structural foldability to an unprecedented degree.
\r\n\r\nThe thesis then tackles the challenge of estimating the actuation response and residual structural deformation of unimorph active thin shells under high electric field and large bending motion. A rate-independent, full field phenomenological constitutive model for a polycrystalline piezoelectric material is characterized experimentally. It successfully captures both the observed ferroelectric and ferroelastic domain switching effects. To overcome the difficulty of testing ultra thin piezoelectric plates, a set of novel characterization techniques is developed and implemented to measure the dielectric and mechanical responses of this material. The characterized material constitutive relation is implemented in an efficient model for estimating the structural response of unimorph thin shells under general electric and mechanical loading. The complete set of governing equations is integrated with a Backward-Euler algorithm, reproducing the measured responses of both the material and the structure under complex loading sequences.
\r\n\r\nActive vibration damping based on self-sensing piezoelectric thin shells is then analyzed and demonstrated on testbed. The self-sensing architecture removes redundant external sensors by making dual use of the piezoelectric layer of the active shell. An adaptive identification method with the associated hardware to track the evolution of field dependent piezoelectric capacitance is implemented, and a new identification strategy is proposed. Closed loop damping with in-situ capacitance adaptation is conducted in bench tests on self-sensing cantilever beams and achieves -12~dB attenuation at the resonance frequency. A highly efficient modeling technique for general self-sensing piezoelectric thin shell structures is proposed which is able to construct closed loop dynamic models based on the vibration eigenmodes and actuation responses obtained from commercial finite element software. These validated modeling techniques are extended to a multi-electrode doubly curved thin shell, where the improvements of shape stability under closed loop damping are evaluated through simulations. It is discovered that the electrode pattern of the self-sensing piezoelectric layer determines the damping performance under the specific boundary conditions of the shell.
", "doi": "10.7907/4DRX-2X87", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:10952", "collection": "thesis", "collection_id": "10952", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282018-024934056", "primary_object_url": { "basename": "Avellar_Louisa_2018.pdf", "content": "final", "filesize": 12288291, "license": "other", "mime_type": "application/pdf", "url": "/10952/14/Avellar_Louisa_2018.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Observations of Failure Phenomena in Periodic Media", "author": [ { "family_name": "Avellar", "given_name": "Louisa Taylor", "orcid": "0000-0003-1299-5343", "clpid": "Avellar-Louisa-Taylor" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "New manufacturing techniques, such as 3D printing, allow for greater control over material properties and can be used to create custom heterogeneous materials. Heterogeneities can be leveraged to increase fracture toughness by redistributing the stresses, such as due to an elastic heterogeneity, or by impeding crack propagation, such as the renucleation at a material interface or edge of a void. The goal of this research is to study the mechanisms by which heterogeneities work to make composite materials more resistant to fracture than either of the individual base materials.
\r\n\r\nThe influence of heterogeneities on the deformation and fracture of 3D printed fracture specimens is investigated. Brick-like heterogeneities are studied in compact tension and plate specimens with soft, stiff, and void heterogeneities. Horizontally layered heterogeneities are studied in compact tension specimens. The specimens are manufactured using a printer capable of printing multiple materials. The specimens are loaded until failure, and full-field displacement and strain data are collected using digital image correlation. The evolution of resistance to fracture is quantified by the energy release rate and fracture toughness values calculated using load, load-point displacement measurements, and crack extension data determined from images of the specimen. Both in soft specimens with stiff heterogeneities and in stiff specimens with soft heterogeneities, stresses are observed to be higher in the stiffer material. Fracture toughness is observed to increase in the presence of stiff inclusions and voids, although in the case of voids this is due to the crack terminating at the edge of the void and renucleating at the other edge.
\r\n\r\nThe effects of interfaces on crack propagation in periodic media are experimentally studied. Comparative experiments on two proposed heterogeneity architectures aim to separate the effects of elastic deformation caused by heterogeneous inclusions in a composite from the effects of passing through an interface during crack propagation. The first, 'stripe' specimens, alternate equal width stripes perpendicular to the plane of the crack. The second, 'cross' specimens, have the same stripe pattern but with a narrow strip of one of the constituent materials in the plane of crack propagation. The 'cross' is wide enough to contain the crack to an area without material interfaces but thin enough that its overall effect on elastic deformation is minimal. Specimens are manufactured from two polymers using polyjet 3D printing. Energy release rate for fracture is calculated from load and displacement measurements. Digital image correlation is used to study strain and stress fields during crack propagation. While the stress fields during crack propagation appear similar, the fracture toughness in the 'stripe' specimens was found to be higher than that of the 'cross' specimens, indicating that fracture toughness is enhanced by renucleation at the interfaces. Additionally, the amount of enhancement was observed to depend on the width of the heterogeneous layers.
\r\n\r\nThe interaction between the cohesive zone and elastic stiffness heterogeneity in the peeling of an adhesive tape from a rigid substrate is examined experimentally and with finite element simulations. It is understood that elastic stiffness heterogeneities can greatly enhance the adhesion of a tape without changing the properties of the interface. However, in peeling experiments performed on pressure sensitive adhesive tapes with both an elastic stiffness heterogeneity and a substantial cohesive zone, muted adhesion enhancement was observed. It is proposed that the cohesive zone acts to smooth out the effect of the discontinuity at the edge of the elastic stiffness heterogeneities, suppressing their effect on peel force enhancement. The results of numerical simulations show that the peel force enhancement depends on the strength of the adhesive and the size of the cohesive zone.
", "doi": "10.7907/8N81-MV74", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10951", "collection": "thesis", "collection_id": "10951", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272018-175543580", "primary_object_url": { "basename": "Schaal_Natalie_thesis_2018.pdf", "content": "final", "filesize": 26842737, "license": "other", "mime_type": "application/pdf", "url": "/10951/1/Schaal_Natalie_thesis_2018.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Modeling of Nucleation and Dynamic Rupture on Heterogeneous Frictional Interfaces with Applications to Foreshocks", "author": [ { "family_name": "Schaal", "given_name": "Natalie Sarah Ann", "orcid": "0000-0003-4825-4344", "clpid": "Schaal-Natalie-Sarah-Ann" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "While many large earthquakes are preceded by observable foreshocks, the mechanisms responsible for the occurrence of these smaller-scale seismic events remain uncertain. One physical explanation of foreshocks with growing support is that they are produced by the interaction of slow slip, due to the nucleation of the upcoming mainshock, with fault patches of different properties. Having a better understanding of how earthquakes nucleate on heterogeneous faults would increase our capacity to forecast potentially hazardous events.\r\n
\r\n\r\nWith this motivation in mind, we seek to understand what conditions produce isolated microseismicity within the nucleating region of the mainshock and to study the mechanics of the resulting events. Inspired by the suggestion from laboratory experiments that foreshocks occur on asperities, i.e., local deviations from planarity that are flattened by the overall compression, we explore the behavior of asperity-type patches of higher compressive stress embedded in the larger seismogenic region of a rate-and-state fault model by conducting 3D numerical simulations of their slip over long-term sequences of aseismic and seismic slip. Our models do produce smaller-scale seismicity during the aseismic nucleation of much larger seismic events, and we explore their properties as well as the separation in length scales needed to produce them. These foreshock-like events have stress drops that are consistent with laboratory and field observations and approximately constant, despite the highly elevated compression assigned to the source patches. Two main factors contributing to the reasonable stress drops are the significant extent of the rupture into the region surrounding the patch and the aseismic stress release just prior to the seismic event.
\r\n\r\n\r\nWe also investigate the seismologically-derived properties of the asperity-type events using the spectral analysis commonly applied to natural microseismic events. We find that the seismological methods cannot adequately capture the properties of the simulated events. In part, the seismological estimates of their stress drops are significantly different from the actual stress drops determined from the on-fault stress changes. This is because our sources have more complex features than the standard models from which the current seismological methods have been built, including heterogeneous stress change over the rupture area with much larger initial stress change, and heterogeneous rupture speed. We identify features in the far-field seismograms of the asperity-type sources that differ from the standard models and can be potentially characteristic of the asperity-type sources.\r\n
\r\n\r\n\r\nOur asperity-type models of microseismicity sources provide insight into the conditions conducive for generating foreshocks on both natural and laboratory faults and the properties of the resulting events. The conclusions provided jointly by the two perspectives in this study -- dynamically simulating the behavior of seismic sources within heterogeneous fault models and seismologically analyzing their far-field source spectra -- have important implications that warrant further study. Topics for future research include the interaction among smaller-scale seismic events and their role in the mainshock nucleation process, the effect of timing on their source properties, and relation to the so-called seismic nucleation phase of the subsequent mainshock.\r\n
\r\n", "doi": "10.7907/YCVJ-PM21", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10879", "collection": "thesis", "collection_id": "10879", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05142018-133453405", "primary_object_url": { "basename": "Thesis_Marteau.pdf", "content": "final", "filesize": 11662230, "license": "other", "mime_type": "application/pdf", "url": "/10879/1/Thesis_Marteau.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Laboratory Studies of Granular Materials Under Shear: From Avalanches to Force Chains", "author": [ { "family_name": "Marteau", "given_name": "Elo\u00efse Sophie H\u00e9l\u00e8ne", "orcid": "0000-0001-7696-6264", "clpid": "Marteau-Elo\u00efse-Sophie-H\u00e9l\u00e8ne" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Granular materials reveal their complexity and some of their unique features when subjected to shear deformation. They can dilate, behave like a solid or a fluid, and are known to carry external forces preferentially as force chains. In this dissertation, we employ laboratory experiments to study the complex behavior of granular materials under shear. We introduce a multiscale approach in which the underlying grain-scale mechanics are experimentally measured and homogenized to obtain enriched macroscopic quantities. First, we investigate granular avalanches spontaneously generated by a rotating drum. Measurements of grain kinematics are directly incorporated into a rate-dependent plasticity model that explains and reproduces the life cycle of laboratory avalanches. The results presented here feature dilatancy as the key material parameter governing the triggering of an avalanche. Second, we report a set of experiments performed on a custom-built mechanical device that allows a specimen composed of a two-dimensional analogue granular assembly to be subjected to quasi-static shear conditions. A numerical force inference technique, the Granular Element Method (GEM), provides direct observation and quantitative characterization of force chain structures in assemblies made of realistic grains. Equipped with a complete description of the grain-scale mechanics, we show that shear deformation creates geometrical (fabric) and mechanical (force) anisotropy. Finally, the influence of grain shape on grain-scale processes is studied. We find that grain interlocking is a prominent deformation mechanism for non-circular grains that ultimately promotes a significant increase in macroscopic shear strength. By seamlessly connecting grain-scale information to continuum scale experiments, this dissertation sheds light on the multiscale mechanical behavior of granular assemblies under shear.
", "doi": "10.7907/FKM0-P754", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:11076", "collection": "thesis", "collection_id": "11076", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06122018-094549478", "primary_object_url": { "basename": "Mateos_Arturo_2018.pdf", "content": "final", "filesize": 128167927, "license": "other", "mime_type": "application/pdf", "url": "/11076/15/Mateos_Arturo_2018.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Tensile Failure and Fracture of Three-Dimensional Brittle Nanolattices", "author": [ { "family_name": "Mateos Arrieta", "given_name": "Arturo Jos\u00e9", "orcid": "0000-0002-9306-3531", "clpid": "Mateos-Arrieta-Arturo-Jos\u00e9" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The emergence of a new class of cellular solids, i.e., nano- and micro-architected materials, poses the question of whether they can be characterized as a continuum solid. Extensive research has shown that these ultralight and strong structural metamaterials are particularly attractive for mechanically-demanding applications; yet their susceptibility to flaws, fracture behavior, and discrete-continuum duality remains relatively unexplored. In the course of this work, we report the fabrication and tensile-to-failure response of three-dimensional ceramic nanolattices, comprised of 50nm-thick alumina tubes that are arranged into periodic 5um-wide octet-truss unit cells, with and without pre-fabricated through-thickness center notches oriented at different angles to the loading direction. In-situ uniaxial tensile experiments revealed that for all notch orientations, failure always initiated at the notch root, as would be in a monolithic material, with the tube walls at nodal junctions fracturing first, followed by instantaneous crack propagation through the discrete lattice architecture along nodal planes orthogonal to the loading direction. Measured tensile strength of 27.4 MPa was highest for the unnotched samples and decreased systematically with the increase of notch orientation to its minimum of 7.2 MPa in the orthogonally-notched samples. We found the specific tensile strength of hollow-tube octet alumina nanolattices to be 4 times higher than what has been reported for architected and bulk materials at similar low densities. Three-dimensional finite element simulations closely reproduce the observed failure mechanism and trends in failure strength. A direct comparison is made between the experimental measurements, finite element simulations, and predictions of linear elastic fracture mechanics for a self-similar monolithic tensile samples made out of an ideally-brittle solid. Results are in good agreement with the scaling of failure strengths from classical mode I fracture criteria and suggest that trajectory of crack propagation can be adequately explained by considering the connectivity of the lattice architecture. These findings imply that the continuum nature of nano-architected materials offers predictability of failure stresses, which helps enable the development of advanced materials through informed architectural design.", "doi": "10.7907/AZXG-NB17", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10393", "collection": "thesis", "collection_id": "10393", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08282017-105838669", "primary_object_url": { "basename": "Newman_Matthew_2018.pdf", "content": "final", "filesize": 15217379, "license": "other", "mime_type": "application/pdf", "url": "/10393/1/Newman_Matthew_2018.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "On the Kinetics of Materials of Geophysical Interest", "author": [ { "family_name": "Newman", "given_name": "Matthew Gregory", "orcid": "0000-0003-2752-0121", "clpid": "Newman-Matthew-Gregory" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Kraus", "given_name": "Richard", "clpid": "Kraus-Richard" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Knowledge of the equation of state and phase diagram of magnesium silicates and light iron alloys is important for understanding the thermal evolution and interior structure of terrestrial planets. Dynamic compression techniques are the primary viable methods to create the temperature and pressure conditions that are relevant to Earth and super-Earth (1-10 Earth mass) sized planets. However, due to the kinetic constraints imposed by the timescale of dynamic compression experiments, the nature of the state within the dynamically compressed sample (whether equilibrium or metastable) is uncertain. Here, we present the results of a series of dynamic compression experiments performed on both laser driven compression and plate impact facilities to study the nanosecond to microsecond response of forsterite and iron silicide. In situ x-ray diffraction measurements are used to probe the crystal structure of solid phases and test for the presence of melt, from which we investigate the decomposition of forsterite and iron silicide into compositionally distinct phases at high pressure. For forsterite, we do not observe chemical segregation in the solid phase, however the presence of melt speeds up the kinetics and allows chemical segregation to occur on nanosecond timescales. For iron silicide, our results show a textured solid phase upon shock compression to pressures ranging from 166(14) to 282(24) GPa consistent with cubic and hcp structures in coexistence. Above 313(29) GPa, the intense and textured solid diffraction peaks give way to a diffuse scattering feature and loss of texture, consistent with melting along the Hugoniot.", "doi": "10.7907/Z9319T35", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10381", "collection": "thesis", "collection_id": "10381", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08182017-103752052", "primary_object_url": { "basename": "MGrivel_Thesis2018.pdf", "content": "final", "filesize": 136293795, "license": "other", "mime_type": "application/pdf", "url": "/10381/1/MGrivel_Thesis2018.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "On the Effect of Large-Scale Patterned Wettability on Contact Line Hydrodynamics", "author": [ { "family_name": "Grivel", "given_name": "Morgane Anne Marie", "orcid": "0000-0002-4391-799X", "clpid": "Grivel-Morgane-Anne-Marie" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Numerous studies have investigated how liquid water behaves on solid surfaces with uniformly hydrophilic or uniformly hydrophobic wetting properties. In particular, uniformly hydrophobic surfaces have been widely studied for modifying flow behavior of rivulets and drops at smaller scales, as well as for drag reduction on ships or other free-surface-piercing bodies at larger scales. Despite the extensive body of work on surfaces with uniform wetting properties, minimal work has been done to investigate how combining hydrophilic and hydrophobic regions onto a single surface to create macroscopic non-uniform wetting properties affects flows. Research in this vein has predominantly focused on low Reynolds number flows, such as in microfluidic channels or droplet impacts.
\r\n\r\nThis thesis expands on the current literature by investigating contact line dynamics and global flow behavior on surfaces with larger-scale non-uniform wetting properties. Experiments were first carried out to study thin sheet flow down an inclined plate at Re ~ 50 - 1200. The plate's wetting condition was changed by introducing alternating hydrophilic and hydrophobic bands 2-25 mm wide oriented at different angles with respect to the flow direction. Results show that the contact line of such flows is heavily modified compared to the uniform cases. At low Reynolds numbers, large-scale wettability heterogeneities are observed to tune the fingering instability wavelength if the bands are parallel to the flow direction and to dampen finger oscillations if the bands are perpendicular to the flow direction. At higher Reynolds numbers, roller structures are introduced at every hydrophilic-to-hydrophobic junction, modifying the global flow morphology. Entrained air bubbles are also captured and observed to coalesce if the bands are perpendicular to the flow direction.
\r\n\r\nThese experiments were then extended to a surface-piercing hydrofoil coated with alternating hydrophilic and hydrophobic bands. Experiments were run in Caltech's Free Surface Laboratory water tunnel for Re on the order of 104 to 105. The experiments demonstrate that the contact line is modulated in this context, alternating from concave to convex over the different wettability regions. The modulation of the contact line propagates to the rest of the water free-surface via the generation of standing waves and further modifies the free-surface separation point's location and steadiness. In addition, changes in wettability are observed to generate side force, which is of interest for vessel maneuvers in naval applications.
", "doi": "10.7907/Z9736P2V", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10365", "collection": "thesis", "collection_id": "10365", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08042017-190200194", "type": "thesis", "title": "Proliferation of Twinning in Metals: Application to Magnesium Alloys", "author": [ { "family_name": "Sun", "given_name": "Dingyi", "orcid": "0000-0003-2109-7123", "clpid": "Sun-Dingyi" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Ponga", "given_name": "Mauricio", "clpid": "Ponga-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In the search for new alloys with a great strength-to-weight ratio, magnesium has emerged at the forefront. With a strength rivaling that of steel and aluminum alloys --- materials which are deployed widely in real world applications today --- but only a fraction of the density, magnesium holds great promise in a variety of next-generation applications. Unfortunately, the widespread adoption of magnesium is hindered by the fact that it fails in a brittle fashion, which is undesirable when it comes to plastic deformation mechanisms. Consequently, one must design magnesium alloys to navigate around this shortcoming and fail in a more ductile fashion.
\r\n\r\nHowever, such designs are not possible without a thorough understanding of the underlying mechanisms of deformation in magnesium, which is somewhat contested at the moment. In addition to slip, which is one of the dominant mechanisms in metallic alloys, a mechanism known as twinning is also present, especially in hexagonal close-packed (HCP) materials such as magnesium. Twinning involves the reorientation of the material lattice about a planar discontinuity and has been shown as one of the preferred mechanisms by which magnesium accommodates out-of-plane deformation. Unfortunately, twinning is not particularly well-understood in magnesium, and needs to be addressed before progress can be made in materials design. In particular, though two specific modes of twinning have been acknowledged, various works in the literature have identified a host of additional modes, many of which have been cast aside as \"anomalous\" observations.
\r\n\r\nTo this end, we introduce a new framework for predicting the modes by which a material can twin, for any given material. Focusing on magnesium, we begin our investigation by introducing a kinematic framework that predicts novel twin configurations, cataloging these twins modes by their planar normal and twinning shear. We then subject the predicted twin modes to a series of atomistic simulations, primarily in molecular statics but with supplementary calculations using density functional theory, giving us insight on both the energy of the twin interface and barriers to formation. We then perform a stress analysis and identify the twin modes which are most likely to be activated, thus finding the ones most likely to affect the yield surface of magnesium.
\r\n\r\nOver the course of our investigation, we show that many different modes actually participate on the yield surface of magnesium; the two classical modes which are accepted by the community are confirmed, but many additional modes --- some of which are close to modes which have been previously regarded as anomalies --- are also observed. We also perform some extensional work, showing the flexibility of our framework in predicting twins in other materials and in other environments and highlighting the complicated nature of twinning, especially in HCP materials.
", "doi": "10.7907/Z93B5XB4", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10988", "collection": "thesis", "collection_id": "10988", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312018-150338327", "type": "thesis", "title": "Laboratory Investigation of Shear Ruptures: Supersonic Propagation and Nucleation by Fluid Injection", "author": [ { "family_name": "Gori", "given_name": "Marcello", "orcid": "0000-0002-7380-3723", "clpid": "Gori-Marcello" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Shear rupture nucleation and dynamic propagation is a challenging, non- linear, highly interactive process with important practical implications. Here we focus on two aspects of this problem: propagation speeds and shock front radiation from the dynamic crack tip as well as nucleation of dynamic rupture due to fluid injection.
\r\n\r\nSpontaneously propagating cracks in solids emit pressure and shear waves and are, in part, driven by energy transfer due to them. When a shear crack propagates faster than the shear wave speed of the material, the coalescence of the shear wavelets emitted by the near-crack-tip region forms a shock front that significantly concentrates particle motion. The equivalent scenario involving a pressure shock front should not be possible, since cracks should not be able to exceed the pressure wave speed, at least in an isotropic linear-elastic solid. Here we present full-field experimental evidence of dynamic shear cracks in viscoelastic polymers that result in the formation of a pressure shock front, in addition to the shear one. In that sense, the crack appears to be supersonic. The apparent violation of classic theories is explained by the strain-rate-dependent material behavior of polymers: the increased wave speeds within the highly- strained region around the crack tip allow for supersonic crack propagation with respect to the (lower) wave speeds at short distances away from the interface, resulting in the formation of the pressure shock front. The crack speed remains below the pressure wave speed prevailing locally, about its tip, in agreement with basic physics and energy considerations of linear-elastic theories.
\r\n\r\nWe find that the shock fronts emitted by the shear cracks in the viscoelastic materials are curved and propose a novel method to quantify the viscoelastic wave speeds of the solids in the dynamic range of strain rates based on the curvature. Only kinematic relationships are used in the method, without the need for the constitutive relationship of the material. Measuring or inferring the material properties at elevated strain rates in viscoelastic solids is a difficult task, because of practical limitations of obtaining accurate measurements in that regime. Under the quasi-elastic solid approximation, in which the strain-rate history is neglected, we use the pressure-wave speed measurements to infer the associated value of the Young\u2019s modulus, estimated by assuming a constant value of the Poisson\u2019s ratio. We complement these results with the characterization of the Young\u2019s modulus at lower strain rates via canonical compressive tests. Our results not only confirm previous findings that the Young\u2019s modulus dependence on the strain rate in PMMA is significant but also demonstrate that its variation is more pronounced in the dynamic strain-rate range, with important consequences for the design of structures employing viscoelastic materials that are required to withstand elevated strain rates.
\r\n\r\nThe second part of the study concentrates on the nucleation of shear dynamic rupture due to fluid injection or, more broadly, on the interaction of frictional faulting with fluids. Fluid overpressure is recognized to play a fundamental role in promoting fault motion. A large number of observations has shed light on the interplay between fluids and faulting, both in natural events and in earth-quakes induced by human activities, such as wastewater disposal associated with oil and gas extraction. Fluids can induce a variety of earthquake source behaviors ranging from unstable, dynamic motions to stable, quasi-static ones, which a number of field studies suggests that can coexist on the same fault areas at different times, depending on the local conditions. In fact, a higher pore pres-sure plays the dual role of reducing the frictional strength of the fault and of increasing the nucleation size, e.g., the critical length for a shear crack to transition from quasi-static to dynamic motions. However, due to the complexity of the frictional problem at the fault interface, the understanding of which of these two effects prevails remains elusive. The assumption of a critical nucleation length represents a powerful, yet simplified concept, which currently does not include the dependence on the rate of the pore pressure increase.
\r\n\r\nHere, we explore the effect of the rate of the pore pressure increase on the rupture nucleation. We find that elevated injection rates induce triggering of the rupture at lower pressure values and minimal volumes of the injected fluid, if compared to slow injection rates. For the slow injection rates, we experimentally observe a much larger portion of interface wetted by the fluid and a phase of accelerated slip prior to the dynamic event (quasi-dynamic nucleation process). In some cases, we record much smaller foreshock-like events at the injection site. These findings suggest the presence of a prominent quasi-static nucleation process over the interface. In cases of rapid pore pressure increase, the nucleation process is much shorter in time and much more compact in space, being highly concentrated around the injection location. The dynamic events, once initiated, are qualitatively similar across different injection rates, but quantitatively different, with the slow-injection ones experiencing higher stress drops and higher slips, perhaps due to the effect of fluids on the friction properties.\r\nThese findings suggest the need to develop nucleation size estimates that include the rate of the pore pressure increase and motivate further investigation of how friction properties depend on the presence of fluids. The details of the obtained experimental findings, once analyzed through numerical modeling, will place important constrains on the forms of the acceptable friction laws, including the effects of pore fluid pressure and its rate of change.
", "doi": "10.7907/AH9X-V905", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10826", "collection": "thesis", "collection_id": "10826", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04222018-122438253", "primary_object_url": { "basename": "IshanTembhekarThesis_Final.pdf", "content": "final", "filesize": 51716351, "license": "other", "mime_type": "application/pdf", "url": "/10826/1/IshanTembhekarThesis_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "The Fully Nonlocal, Finite-Temperature, Adaptive 3D Quasicontinuum Method for Bridging Across Scales", "author": [ { "family_name": "Tembhekar", "given_name": "Ishan", "orcid": "0000-0001-5123-1958", "clpid": "Tembhekar-Ishan" } ], "thesis_advisor": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Computational modeling of metallic materials across various length and time scales has been on the rise since the advent of efficient, fast computing machines. From atomistic methods like molecular statics and dynamics at the nanoscale to continuum mechanics modeled by finite element methods at the macroscale, various techniques have been established that describe and predict the mechanics of materials. Many recent technologies, however, fall into a gap between length scales (referred to as mesoscales), with microstructural features on the order of nanometers (thereby requiring full atomistic resolution) but large representative volumes on the order of micrometers (beyond the scope of molecular dynamics). There is an urgent need to predict material behavior using scale-bridging techniques that build up from the atomic level and reach larger length and time scales. To this end, there is extensive ongoing research in building hierarchical and concurrent scale-bridging techniques to master the gap between atomistics and the continuum, but robust, adaptive schemes with finite-temperature modeling at realistic length and time scales are still missing.
\r\n\r\nIn this thesis, we use the quasicontinuum (QC) method, a concurrent scale-bridging technique that extends atomistic accuracy to significantly larger length scales by reducing the full atomic ensemble to a small set of representative atoms, and using interpolation to recover the motion of all lattice sites where full atomistic resolution is not necessary. We develop automatic model adaptivity by adding mesh refinement and adaptive neighborhood updates to the new fully nonlocal energy-based 3D QC framework, which allows for automatic resolution to full atomistics around regions of interest such as nanovoids and moving lattice defects. By comparison to molecular dynamics (MD), we show that these additions allow for a successful and computationally efficient coarse graining of atomistic ensembles while maintaining the same atomistic accuracy.
\r\n\r\nWe further extend the fully nonlocal QC formulation to finite temperature (termed hotQC) using the principle of maximum entropy in statistical mechanics and averaging the thermal motion of atoms to obtain a temperature-dependent free energy using numerical quadrature. This hotQC formulation implements recently developed optimal summation rules and successfully captures temperature-dependent elastic constants and thermal expansion. We report for the first time the influence of temperature on force artifacts and conclude that our novel finite-temperature adaptive nonlocal QC shows minimal force artifacts and outperforms existing formulations. We also highlight the influence of quadrature in phase space on simulation outcomes.
\r\n\r\nWe study 3D grain boundaries in the nonlocal hotQC framework (previously limited to single-crystals) by modeling coarse-grained symmetric-tilt grain boundaries in coincidence site lattice (CSL) based bicrystals. We predict relaxed energy states of various \u03a3-boundaries with reasonable accuracy by comparing grain boundary energies to MD simulations and outline a framework to model polycrystalline materials that surpasses both spatial and temporal limitations of traditional MD.
", "doi": "10.7907/RF27-NA04", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10617", "collection": "thesis", "collection_id": "10617", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12242017-060345135", "primary_object_url": { "basename": "Maggi-2018.pdf", "content": "final", "filesize": 9300158, "license": "other", "mime_type": "application/pdf", "url": "/10617/1/Maggi-2018.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Three-Dimensional Nano-Architected Materials as Platforms for Designing Effective Bone Implants", "author": [ { "family_name": "Maggi", "given_name": "Alessandro", "orcid": "0000-0001-9438-7258", "clpid": "Maggi-Alessandro" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Burdick", "given_name": "Joel Wakeman", "clpid": "Burdick-J-W" }, { "family_name": "Shapiro", "given_name": "Mikhail G.", "clpid": "Shapiro-M-G" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The growing world population coupled with longer human life expectancy warrants the need for better medical implant development. Recent advances in lithographic techniques have opened the door to a variety of approaches to tackle the aforementioned issue. However, several scientific hurdles must be overcome before patients can use fully synthetic and effective implants.
\r\n\r\nIdentifying the optimal material, porosity, and mechanical properties of the scaffold to induce cell functionality are key obstacles. Limitations in established fabrication techniques have hindered the ability to fully understand cell behavior on 3D substrates. 3D printing is limited to feature sizes that are at least one order of magnitude larger than a single cell (~10\u03bcm); electrospinning is able to yield features that are on the same scale as cells, but its stochastic nature leads to scaffolds with poor mechanical properties; salt leeching doesn\u2019t allow for control of pore size and distribution which have detrimental effects on nutrient diffusion and cell ingrowth, thereby thwarting the formation of functional tissue.
\r\n\r\nMuch effort has been made to create a suitable platform for regenerating a relatively less complex organ, such as bone, yet the inability to fully understand cell mechanics on 3D scaffolds has curbed the fabrication of effective bone implants.
\r\n\r\nThe first part of this thesis focuses on the suitability of nanoarchitected materials as 3D platforms for bone-tissue growth. We employed two-photon lithography to create polymeric and hydroxyapatite-coated 3D nanolattices to explore scaffold biocompatibility and material effects on osteoblast attachment and growth. Our experiments showed that the unit cell geometry, tetrakaidekahedron, and size, 25\u03bcm, were adequate for cell attachment and infiltration, which are hallmark signs of biocompatibility. Our study also corroborated previous findings that mammalian cells respond differently to different materials that they come in contact with. To isolate structural effects, we fabricated nanolattices coated with a uniform 20nm-thick outermost layer of TiO2. These nanolattices, which had fixed porosity and unit cell size (25\u03bcm) while they varied in structural stiffness (~2-9MPa) were used to explore the influence of scaffold properties on the viability of osteoblasts in a microenvironment similar to that of natural bone. Upon growing osteogenic cells on the nanolattices, significant cell attachment and presence of various calcium phosphate species, which are commonly found in natural bone, were observed. These findings suggest that 3-dimensional nano-architected materials can be used as effective scaffolds for bone cell growth and proliferation.
\r\n\r\nThe second part of the thesis investigates the effects of nanolattice structural stiffness and loading conditions on osteoblast behavior. We fabricated nanolattices with stiffness ranging from ~0.7MPa to 100MPa. Experiments done by seeding osteoblast-like cells on these nanolattices revealed that both stress fiber concentration and bioapatite deposition were higher on the most compliant nanolattice, (0.7 MPa) by ~20% and ~40% respectively. These results provide insights into cell behavior in 3D microenvironments which can lead to a better understanding of stress shielding at the cellular level. Preventing stress shielding by creating scaffolds with structural stiffness and porosity that enhances osteoblasts activity could lead to the creation of effective implants with improved mechanical stability which ultimately improves osteointegration.
\r\n\r\nIn addition to investigating static cell-scaffold interactions we took advantage of the nanolattices tunability to study the effects of dynamic loading on cell behavior. Bone adaptation is driven by dynamic, rather than static loading, however there is still wide controversy on whether stress, strain or loading frequency plays the most significant role in bone remodeling, which drives bone healing.
\r\n\r\nIn order to understand cell sensitivity to varying loads, displacements and frequencies, we fabricated hollow TiO2 nanolattices with stiffness ranging from ~0.7-35MPa which were populated with osteoblast-like cells and subjected to cyclic compression to either a constant stress or strain. After seeding SAOS-2 cells on the nanolattices for 12 days different dynamic loading conditions were tested: (1) cyclic uniaxial compressions to strains ranging from ~0.3-2% strain were carried out to investigate the effects of strain magnitude on cell behavior. (2) Cyclic uniaxial compressions to stresses spanning from ~0.02-1MPa were performed to explore the role of stress magnitude on the cells\u2019 stress fibers formation. (3) The nanolattices were cyclically loaded at different frequencies, ~0.1-3Hz, while maintaining stress and strain constant, which provided insights into how loading frequency affects osteoblasts behavior.
\r\n\r\nCell activity, which was measured by monitoring f-actin and vinculin fluorescence intensity, revealed increased fluorescence in those cells that were mechanically stimulated as opposed to those that were statically grown on the nanolattices regardless of loading condition. Cell response was most drastically affected by varying the loading frequency. A ~30% increase in f-actin fluorescence was observed in the cells grown on the nanolattices that were loaded at ~3Hz compared to those that were grown on the nanolattices that were cyclically compressed at ~0.1Hz.
\r\n\r\nThe last part of this thesis is focused on developing a three-dimensional architected capacitor that could be used as a strain gauge to further our understanding of cell mechanics in 3D. We took advantage of the mechanical tunability of the nanolattices to fabricate a 3D parallel-plate capacitor with a basal capacitance of ~280fF and able to sense forces as low as ~30\u03bcN. This work points to nano-architected materials as promising candidates for ideal platforms to investigate more realistic cellular conditions which can immensely benefit the field of tissue engineering.
", "doi": "10.7907/Z947482K", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10975", "collection": "thesis", "collection_id": "10975", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302018-165415595", "type": "thesis", "title": "Design and Characterization of Dual-Matrix Composite Deployable Space Structures", "author": [ { "family_name": "Sakovsky", "given_name": "Maria", "orcid": "0000-0002-3683-8505", "clpid": "Sakovsky-Maria" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Dual-matrix composites are a promising approach to deployable high performance antennas for small satellites. Several techniques exist for packaging large antenna apertures. Assemblies of rigid bars and hinges obtain high deployed precision but are heavy and mechanically complex. Thin shell structures deployed using stored strain energy are a lightweight alternative offering efficient packaging but reduced surface precision. Moreover, elastomer composites shells attain even smaller fold radii upon packaging but are limited by the deployed structure's stiffness. Dual-matrix composites combine the advantages of several of these approaches to enable larger antenna apertures. They consist of a continuous woven fiber reinforcement with an elastomer matrix embedded in localized hinge regions and a stiff epoxy resin elsewhere. Such structures can achieve small fold radii, are strain energy deployable, and promise high deployed stiffness.
\r\n\r\nThis research demonstrates the capabilities of the proposed dual-matrix structures through direct comparison to existing antenna designs. Analytic scaling relations between structural and electromagnetic performance of various deployable antenna designs are developed. These are used to rapidly predict achievable antenna performance as a function of a common set of antenna geometric parameters. Plotting of this data on a coordinated set of 2D design plots enables the direct comparison of antenna concepts and the selection of specific designs meeting all requirements. This methodology was used to design a deployable dual-matrix composite conical log spiral (CLS) antenna for use on CubeSats which outperformed existing off-the-shelf designs through higher gain, higher bandwidth, and more efficient packaging.
\r\n\r\nStarting from this initial design, the antenna is tuned to maximize performance and an assembly including the CubeSat, dual-matrix antenna, dual-matrix hinge for antenna deployment, and a flexible feeding network is developed. All portions of the assembly are prototyped and tested. The antenna electromagnetic performance is predicted using ANSYS HFSS and verified by testing in an anaechoic chamber with antenna gains predicted within 4% of measured values. Structural stiffness is characterized through the antenna's fundamental frequency with simulated performance in the Abaqus finite element software within 6% of measured values. Comparison of antenna performance before and after packaging and deployment shows the structural frequency, antenna gain, and antenna bandwidth are unaffected by folding, demonstrating that dual-matrix composites are appropriate for use as deployable structures.
\r\n\r\nTechniques for the quasi-static deployment of dual-matrix composites are presented. An analytic minimum energy method, which accounts for fiber microbuckling in regions of high curvature, is used to predict the folded shape and deployment moments of a dual-matrix hinge. The model shows excellent agreement with LS-Dyna finite element simulations for a variety of material properties. Comparison with experimental characterization demonstrates the capability of the models to predict folded radii and deployment moment of a prototype hinge withing 5% of measured values. The developed analysis tool-set enables a design of deployment restraints and mechanisms.
\r\n\r\nThe woven elastomer composites forming the fold regions in dual-matrix composites have been the subject of very few studies. Existing methods for predicting the stiffness of woven epoxy composites are applied to elastomer composites here and show poor agreement with measurements. A novel approach is presented for the prediction of tow stiffness in elastomer composites using a semi-empirical approach. The reinforcing efficiency parameter in the well-established Halpin-Tsai model for tow homogenization is estimated using experimental measurements of stiffnesses of several laminates. It is shown that for elastomer composites, the parameter values are orders of magnitude higher than the heuristic values used for epoxy composites. The method is used to predict the stiffness of woven epoxy and elastomer composites making up the dual-matrix structures studied in this work showing agreement withing 15% of experimental measurements for arbitrary layups. The method is extended to the prediction of viscoelastic behavior of dual-matrix structures to enable investigation of deployment reliability after long storage times.
", "doi": "10.7907/CNPY-A883", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10010", "collection": "thesis", "collection_id": "10010", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01132017-035026894", "primary_object_url": { "basename": "nadkarni_thesis.pdf", "content": "final", "filesize": 23136605, "license": "other", "mime_type": "application/pdf", "url": "/10010/1/nadkarni_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Nonlinear Dynamics of Transition Waves in Multi-Stable Discrete and Continuous Media", "author": [ { "family_name": "Nadkarni", "given_name": "Neel P.", "orcid": "0000-0002-4311-2817", "clpid": "Nadkarni-Neel-P" } ], "thesis_advisor": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Cross", "given_name": "Michael Clifford", "clpid": "Cross-M-C" }, { "family_name": "Abeyaratne", "given_name": "Rohan", "clpid": "Abeyaratne-R" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The concept of phase transitions, i.e., switching between two or more different equilibrium states of a system, is commonly encountered in many physical, chemical and biological phenomena. The exact mechanism of this switching is a highly nonlinear dynamical process that is accommodated by the propagation of a localized wave. The characteristics of the nonlinear wave such as its profile, velocity, energy, and width of transition are governed by the type and specifics of the system that it is propagating through which may be conservative, dissipative, or diffusive in nature. The goal of this thesis is to develop a fundamental understanding of the dynamics of such processes in general nonlinear systems capable of undergoing phase transitions and the application of new theories to elucidate the kinetic and energetic properties of transition waves in different scenarios. In conservative systems, we show that there are three different modes of stable wave propagation that we analytically solve for and validate computationally. In contrast, dissipative and diffusive systems allow the stable propagation of only the strongly nonlinear kink mode whose kinetic energy and propagation velocity are linked through a linear relation. We further validate our results in dissipative systems experimentally by fabricating and testing a strongly nonlinear lattice and show that transition waves are unidirectional in nature, as predicted by theory. Finally, as an application, we devise a strategy of using the physics of dissipative phase transitions to propagate stable mechanical signals in highly dissipative media such as soft polymers which effectively damp out small-amplitude linear waves.", "doi": "10.7907/Z9DR2SG2", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10182", "collection": "thesis", "collection_id": "10182", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05182017-095600418", "type": "thesis", "title": "Determining Strength of Materials Under Dynamic Loading Conditions Using Hydrodynamic Instabilities", "author": [ { "family_name": "Sternberger", "given_name": "Zachary Martin Murphy", "orcid": "0000-0002-7612-673X", "clpid": "Sternberger-Zachary-Martin-Murphy" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Hydrodynamic instability experiments allow access to material properties at extreme conditions where the pressure exceeds 100 GPa and the strain rate exceeds 106 1/s. Laser ablation dynamically loads a sample, causing a manufactured initial perturbation to grow due to hydrodynamic instability. The instability growth rate depends on the strength of the sample. Material strength can then be inferred from a measurement of the instability growth. Past experiments relied on in-flight diagnostics to measure the amplitude growth, which are not available at all facilities.
\r\n\r\nRecovery instability experiments, where the initial and final amplitude of the instability are measured before and after the sample is dynamically loaded, obviate the need for in-flight diagnostics. Recovery targets containing copper and tantalum samples coined with 2D (hill and valley) and 3D (eggcrate) initial perturbations were dynamically loaded using the Janus laser at the Jupiter Laser Facility, Lawrence Livermore National Laboratory. The energy of the laser pulse was varied to cover a range of conditions in the dynamically compressed sample with pressures in the range 10 GPa to 150 GPa and strain rates in the range 105 1/s to 108 1/s.
\r\n\r\nThe coupling of laser energy into a loading wave was studied with a combination of laser-matter interaction simulations (Hyades) and velocity interferometry data (VISAR). Laser ablation of the recovery targets generated a blast wave, loading the coined initial perturbations with a shock wave followed by a release wave. Different ablator materials and variations in the amount of laser energy deposited in the ablator lead to variations in the loading wave and consequently variations in instability growth.
\r\n\r\nThe growth of the initial perturbation amplitude from initial to final conditions was studied with hydrocode simulations (CTH). During dynamic loading of the sample, the shock wave caused amplitude growth due to hydrodynamic instability. The release wave accelerated the perturbed interface and slowed amplitude growth, in some cases reversing growth.
\r\n\r\nThe sensitivity of the instability growth to coarse changes in the strength model was demonstrated. However, uncertainty in modeling the laser ablation loading prevented a definitive comparison between simulation and experiment.
", "doi": "10.7907/Z9N877T5", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10336", "collection": "thesis", "collection_id": "10336", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06132017-164608976", "type": "thesis", "title": "Optimal Design of Materials for Energy Conversion\r ", "author": [ { "family_name": "Collins", "given_name": "Lincoln Nash", "clpid": "Collins-Lincoln-Nash" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Minnich", "given_name": "Austin J.", "clpid": "Minnich-A-J" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The efficiency of fuel cells, batteries and thermochemical energy conversion devices depends on inherent material characteristics that govern the complex chemistry and transport of multiple species as well as the spatial arrangement of the various materials. Therefore, optimization of the spatial arrangement is a recurrent theme in energy conversion devices. Traditional methods of synthesis offer limited control of the microstructure and there has been much work in advanced imaging for these uncontrolled microstructures and optimizing gross features. However, the growing ability for directed synthesis allows us to ask the question of what microgeometries are optimal for particular applications. Through this work, we study problems motivated by metal oxides used in solar-driven thermochemical conversion devices designed to split water or carbon dioxide into fuels. We seek to understand the arrangement of the solid and porous regions to maximize the transport given sources and sinks for the gaseous oxygen and vacancies. Three related problems are investigated with the common theme of understanding the role of microstructure design.
\r\n\r\nWe derive the transport equations for electrons and oxygen vacancies through ceria under an externally-applied electric potential in an oxygen environment using various balance laws and constitutive equations. From this, we obtain various thermodynamic potentials that take into consideration the thermal, chemical, and mechanical state of the material. Accordingly, we obtain a system of partial differential equations describing ambipolar diffusion. We present the applicability of strain-engineering as a way to design systems to improve the behavior of thermochemical conversion devices. We look at an idealized thin film of mixed conductor attached to an inert substrate with a thermal mismatch as a way to induce strain into the film. The resulting impact on equilibrium non-stoichiometry is analyzed using data describing non-stoichiometry in ceria as a function of oxygen pressure and temperature.
\r\n\r\nThe optimal design of material microstructure for thermochemical conversion is addressed from two standpoints: the mathematical homogenization of associated transport models, and from topology optimization. We present the homogenization of coupled transport through porous media consisting of linearized Stokes flow, convective diffusion, and diffusion in the solid phase with interface reaction. Depending on the strength of the interface chemistry, different forms of effective behavior are described at the macroscale, and we gain insight into the impact cell-design and pore shape has on the behavior.
\r\n\r\nThe topology optimization of a model energy-conversion reactor is then presented. We express the problem of optimal design of the material arrangement as a saddle point problem and obtain an effective functional which shows that regions with very fine phase mixtures of the material arise naturally. To explore this further, we introduce a phase-field formulation of the optimal design problem, and numerically study selected examples. We find that zig-zag interfaces develop to balance mass transport and interface exchange.
", "doi": "10.7907/Z9X928B7", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10268", "collection": "thesis", "collection_id": "10268", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06042017-210215990", "primary_object_url": { "basename": "SubmittedThesis.pdf", "content": "final", "filesize": 46055271, "license": "other", "mime_type": "application/pdf", "url": "/10268/1/SubmittedThesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Computational Modeling of the Mechanics of Elastic Structural Lattices: Effects of Lattice Architecture and Hierarchy", "author": [ { "family_name": "Zelhofer", "given_name": "Alex James", "orcid": "0000-0002-8064-2876", "clpid": "Zelhofer-Alex-James" } ], "thesis_advisor": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis establishes advanced theoretical-computational techniques to understand and predict the mechanical properties of structural lattice metamaterials with a focus on the effective elastic properties. First, attention is devoted to the effective stiffness of hierarchical nanolattices, which depends on lattice topology, architecture, and inherent geometric imperfections. A computational substructuring technique is applied to predict the mechanics of hierarchical truss networks containing thousands to millions of truss members, with each solid, hollow-tube, or composite truss member requiring full-detail 3D resolution. By applying this methodology to hierarchical nanolattices structural hierarchy is shown to span several decades of relative density and effective stiffness with near-ideal effective stiffness scaling. Comparisons between experimental data and model predictions show convincing agreement and highlight the lattice sensitivity to fabrication-induced geometric imperfection. Second, elastic stress wave propagation in structural lattices is investigated with a focus on wave beaming (i.e., directional energy flow) under harmonic mechanical excitation. A new technique is introduced to obtain pseudo-continuous maps of group velocity magnitude vs. propagation direction vs. frequency to predict directional wave propagation, demonstrating traditional beaming prediction techniques are insufficient for many scenarios. The method is applied to two-dimensional structural lattices to predict directional energy flow. Predictions are verified by comparison to explicit dynamic simulations showing the limitations of the classical dispersion relation method. Overall, improved computational techniques are presented to better described, understand, predict and optimize the elastic behavior of truss lattices.\r\n", "doi": "10.7907/Z91G0J90", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10266", "collection": "thesis", "collection_id": "10266", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06042017-165228124", "type": "thesis", "title": "Effective Toughness of Heterogeneous Materials", "author": [ { "family_name": "Hsueh", "given_name": "Chun-Jen", "orcid": "0000-0001-6522-4505", "clpid": "Hsueh-Chun-Jen" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Bourdin", "given_name": "Blaise", "clpid": "Bourdin-B" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Composite materials are widely used because of their extraordinary performance. It is understood that the heterogeneity / microstructure can dramatically affect the effective behavior of materials. Although there is a well-developed theory for this relation in elasticity, there is no similar theory in fracture mechanics. Therefore, we use theoretical, numerical, and experimental approaches to study the relationship between heterogeneity / microstructure and the effective fracture behavior in this thesis.
\r\n\r\nWe use the surfing boundary condition, a boundary condition that ensures the macroscopic steady crack growth, and then define the effective toughness of heterogeneous materials as the peak energy release rate during crack propagation. We also use the homogenization theory to prove that the effective J-integral in heterogeneous materials is well defined, and that it can be calculated by the homogenized stress and strain field.
\r\n\r\nIn order to study the relationship between heterogeneities and effective toughness, we first use the semi-analytical method under the assumption of small elastic contrast to study selected examples. For strong heterogeneities, we use the phase field fracture method to study the crack propagation numerically. We then optimize the microstructure with respect to effective stiffness and effective toughness in a certain class of microgeometries. We show that it is possible to significantly enhance toughness without significant loss of stiffness. We also design materials with asymmetric toughness.
\r\n\r\nWe develop a new experimental configuration that can measure the effective toughness of specimens with arbitrary heterogeneities. We confirm through preliminary tests that the heterogeneities can enhance the effective toughness.
\r\n\r\nBesides study the effective toughness of heterogeneous materials, we also study a model problem of peeling a thin sheet from a heterogeneous substrate. We develop a methodology to systematically optimize microstructure.
", "doi": "10.7907/Z9HH6H49", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10320", "collection": "thesis", "collection_id": "10320", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06082017-183218154", "type": "thesis", "title": "Hydrodynamics of Insects. Part 1. Jetting of the Dragonfly Larvae. Part 2. Honeybee at the Air-water Interface: Surfing with the Capillary Wave", "author": [ { "family_name": "Roh", "given_name": "Chris", "orcid": "0000-0002-5681-0040", "clpid": "Roh-Chris" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Dickinson", "given_name": "Michael H.", "orcid": "0000-0002-8587-9936", "clpid": "Dickinson-M-H" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents the study on the hydrodynamics of two insects commonly known for their aerial adaptation: the dragonfly and the honeybee.
\r\n\r\nPart 1: Anisopteran dragonflies live underwater in their larval stages. The key factor for their aquatic adaptation is the modified hindgut chamber that is used as a pump. The two main functions of this biological pump are jet propulsion and respiration. Both functions involve jetting and refilling of the chamber through an orifice guard by a tri-leaflet anal valve. Despite it being a unique machinery among insects, associated hydrodynamic studies are limited thus far. In the first part of this thesis, various aspects of the hydrodynamics of the dragonfly larvae\u2019s ventilatory flow are studied. The flow visualization showed that the respiratory flow is laminar but the propulsion flow is turbulent. The hydrodynamic force analysis showed that jetting and refilling phase forces are dominated by quasi-steady momentum flux and unsteady acceleration, respectively. Finally, simultaneous measurement of the anal valve kinematics and jet flow showed that the larvae could influence the direction and magnitude of the jet by controlling the anal valve leaflets.
\r\n\r\nPart 2: Water-collecting honeybees often fall onto water surfaces. However, bees trapped by the \u201cstickiness\u201d of the water can propel by vibrating their wings, often making it to shore. In the second part of this thesis, the honeybee\u2019s propulsion mechanisms at the air\u2013water interface is studied. The result shows that the bees can achieve three body-lengths per second propulsion speed. High-speed video of their wing motion shows that honeybee\u2019s propulsion involves pulling blobs of water with the underside of the wing, while pushing on a surface wave with its trailing edge. This propulsion mechanism resembles surfing on a self-generated capillary wave. Moreover, their wing vibration generates complicated surface waves and flows, below which the deeper water flow shows a single jet stream. From the wave and flow field measurements, the average force imparted to the surrounding fluid is estimated and compared to the average force calculated from the bee\u2019s body motion. The resulting average forces are of the same order of magnitude, which means that generating wave and flow are both important for the bee\u2019s propulsion.
", "doi": "10.7907/Z97P8WFW", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:9890", "collection": "thesis", "collection_id": "9890", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07182016-100030713", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 24333163, "license": "other", "mime_type": "application/pdf", "url": "/9890/1/thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Robotically Assembled Space Telescopes with Deployable Modules: Concepts and Design Methodologies", "author": [ { "family_name": "Hogstrom", "given_name": "Kristina", "clpid": "Hogstrom-Kristina" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Burdick", "given_name": "Joel Wakeman", "orcid": "0000-0002-3091-540X", "clpid": "Burdick-J-W" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis first presents a novel architecture for robotically assembled optical telescopes with apertures between 20 m and 100 m, that utilizes only currently available technology. In this architecture, the primary mirror consists of two layers: a reflective layer and a truss backplane layer. The reflective layer is divided into mirror modules, or groups of mirror segments and actuators. The truss backplane layer is divided into truss modules that fold compactly for launch and are deployed in space by the robot. In this thesis, the design methodology of the mirror modules and truss modules is detailed. The ability of the designed truss layer to maintain precision requirements in the presence of typical space environment loads is demonstrated.
\r\n \r\nThis architecture requires the deployment of many truss modules, and thus the deployment must be reliable despite errors introduced during manufacturing. In this thesis, a new simulation-based toolkit for estimating deployment reliability is described, including the experimental validation of the deployment simulation and the Monte Carlo-style method for repeating deployment simulations with different distributions of random fabrication errors to statistically estimate reliability. Using the toolkit, a set of reliability trade studies are then presented, revealing how different types of errors and design parameters affect reliability. Finally, the manufacturing tolerances and design modifications required to ensure high reliability are proposed.
\r\n \r\nEven if all modules deploy successfully, fabrication errors will still be present and may affect the assembly process. In this thesis, a new simulation method is presented that can model the step-by-step assembly of flexible modules with errors. The method is used to reveal that overall shape errors grow with the number of connections, resulting in significantly decreased surface precision and large-scale deformations from the nominal backplane shape as the size of the backplane increases. The misalignment at each individual connection does not increase as the backplane increases, but can still be much larger than the applied manufacturing tolerances simply due to random combinations. A simple design for the interconnects between modules is then tested, with simulation results demonstrating that it is unlikely to fully engage when the expected errors are present. With this information, a requirement on the complexity of the interconnect design is inferred, and potential modifications that may increase its efficacy are suggested.
", "doi": "10.7907/Z9T151NT", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10250", "collection": "thesis", "collection_id": "10250", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022017-081925673", "primary_object_url": { "basename": "Plucinsky_Paul_2017.pdf", "content": "final", "filesize": 15041133, "license": "other", "mime_type": "application/pdf", "url": "/10250/1/Plucinsky_Paul_2017.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "The Deformations of Thin Nematic Elastomer Sheets", "author": [ { "family_name": "Plucinsky", "given_name": "Paul P.", "orcid": "0000-0003-2060-8657", "clpid": "Plucinsky-Paul-P" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Thin structures exhibit a broad range of mechanical responses as the competition between stretching and bending in these structures can result in buckling and localized deformations like folding and tension wrinkling. Active materials also exhibit a broad range of mechanical responses as features that manifest themselves at the microscale in these materials result in mechanical couplings at the engineering scale (thermal/electrical/dissipative) and novel function (e.g., the shape memory effect and piezoelectricity in select metal alloys and the immense fracture toughness of hydrogels). Given this richness in behaviors, my research broadly aims to address the following questions: What happens when active materials are incorporated into thin structures? Do phenomena inherent to these materials compete with or enhance those inherent to thin structures? Does this interplay result in entirely new and unexpected phenomena? And can all this be exploited to design new functions in engineering systems?
\r\n\r\nIn this thesis, we explore these questions in the context of a theoretical study of thin sheets of nematic liquid crystal elastomer. These materials are active rubbery solids made of cross-linked polymer chains that have liquid crystals either incorporated into the main chain or pendent from them. Their structure enables a coupling between the mechanical elasticity of the polymer network and the ordering of the liquid crystals, and this in turn results in fairly complex mechanical behavior including large spontaneous distortion due to temperature change, soft-elasticity and fine-scale microstructure.
\r\n\r\nWe study thin sheets of nematic elastomer. First, we show that thin of sheets of a particular class of nematic elastomer can resist wrinkling when stretched. Second, we show that thin sheets of another class of nematic elastomer can be actuated into a multitude of complex shapes. In order to obtain these results, we systematically develop two dimensional theories for thin sheets starting from a well-accepted first principles theory for nematic elastomers. These characterize (i) the mechanical response due to instabilities such as structural wrinkling and fine-scale material microstructure, and (ii) thermal actuation of heterogeneously patterned sheets. For the latter, we show that the theory, which comes in the form of a two dimensional metric constraint, admits two broad classes of designable actuation in nonisometric origami and lifted surface. For the former, we show that taut and appreciably stressed sheets of nematic elastomer are capable of suppressing wrinkling by modifying the expected state of stress through the formation of microstructure.
", "doi": "10.7907/Z9765CCT", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10206", "collection": "thesis", "collection_id": "10206", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05242017-230338904", "primary_object_url": { "basename": "Lee_Wilson_Thesis_2017.pdf", "content": "final", "filesize": 35910971, "license": "other", "mime_type": "application/pdf", "url": "/10206/1/Lee_Wilson_Thesis_2017.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Analysis of Packaging and Deployment of Ultralight Space Structures", "author": [ { "family_name": "Wilson", "given_name": "Lee L.", "orcid": "0000-0002-5865-9903", "clpid": "Wilson-Lee-L" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents a new approach to modeling in finite element analysis (FEA) creased thin-film sheets such as those used for drag sails, as well as modeling the packaging behavior of coilable deployable booms. This is highly advantageous because these deployable space structures are challenging to test on the ground due to their lightweight nature and the effects of gravity and air resistance. Such structures are utilized in the space industry due to their low mass and ability to be packaged into a small volume during their launch into space.
\r\n\r\nIt is shown that removing the crease bending stiffness in creased sheets still allows the deployment behavior of a benchmark problem to be captured, including deployment forces and equilibrium configurations. In addition, folding creased sheets from a flat state into a packaged configuration can result in crease crumpling and excessive wrinkling. To avoid this the Momentless Crease Force Folding (MCFF) technique is developed.
\r\n\r\nFurther presented is the behavior of tape springs and Tubular Rollable and Coilable (TRAC) booms when coiled to radii greater than their natural bend radius. Under these conditions the booms can form multiple localized folds which may jam during boom deployment. Understanding this behavior is important for extending the use of these booms to large scale space structures such as orbital solar power stations.
\r\n\r\nA useful analytical model is developed determining when the localized folds in a tape spring will bifurcate and is verified against simulation results. Additionally, a numerical model of the wrapping an isotropic tape spring around a hub with a radius greater than the localized fold radii is validated against physical experiments. This model is used to predict trends in the force required to fully wrap a tape spring around a given hub radii.
\r\n\r\nFinally, when examining the coiling and uncoiling behavior of TRAC booms it was found that the tension force required to keep a TRAC boom tightly coiled is significantly less than the force required to initially coil the boom.
", "doi": "10.7907/Z9B27S96", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10288", "collection": "thesis", "collection_id": "10288", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06052017-162306037", "type": "thesis", "title": "An Investigation of Nonstoichiometric Oxides for Solar-Driven Thermochemical Fuel Production", "author": [ { "family_name": "Ignatowich", "given_name": "Michael Joseph", "orcid": "0000-0002-0097-664X", "clpid": "Ignatowich-Michael-Joseph" } ], "thesis_advisor": [ { "family_name": "Haile", "given_name": "Sossina M.", "clpid": "Haile-S-M" } ], "thesis_committee": [ { "family_name": "Wang", "given_name": "Zhen-Gang", "clpid": "Wang-Zhen-Gang" }, { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Haile", "given_name": "Sossina M.", "clpid": "Haile-S-M" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "In order to realize energy independence and substantially combat global climate change, renewable and sustainable energy technologies must be developed. Solar energy is the most readily abundant, and if converted into a chemical fuel, could be stored and transported easily. Solar-driven thermochemical cycling is a method of chemical fuel production that shows great promise, but current state-of-the-art systems have very low efficiencies. This work discusses new reactor designs and cycling techniques using nonstoichiometric oxides that will enable more efficient solar to fuel energy conversion. Practical aspects of the reactor design are explored \u2013 specifically, thermochemical expansion of the reactive oxide, and morphologies aimed at enhancing the reaction kinetics. Additionally, doped fluorite- and perovskite-structured materials are evaluated for thermodynamic behavior and in-situ thermochemical cycling performance. Oxide morphology and new doped compounds show little improvement over previously established neat ceria due to thermodynamic limitations. The thermodynamic limit is explored in new reactor geometries and is shown to demonstrate significantly more efficient fuel production. Finally, different nonstoichiometry thermodynamics are explored to provide guidance for further material exploration, as well as applicable methodologies.
", "doi": "10.7907/Z92805PM", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:9155", "collection": "thesis", "collection_id": "9155", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09152015-212147583", "primary_object_url": { "basename": "Thesis_JeffAmelang_2015_ver1_withEmbeddedFonts.pdf", "content": "final", "filesize": 301443408, "license": "other", "mime_type": "application/pdf", "url": "/9155/1/Thesis_JeffAmelang_2015_ver1_withEmbeddedFonts.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A Fully-Nonlocal Energy-based Formulation and High-performance Realization of the Quasicontinuum Method", "author": [ { "family_name": "Amelang", "given_name": "Jeffrey Scott", "clpid": "Amelang-Jeffrey-Scott" } ], "thesis_advisor": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Desbrun", "given_name": "Mathieu", "clpid": "Desbrun-M" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The quasicontinuum (QC) method was introduced to coarse-grain crystalline atomic ensembles in order to bridge the scales from individual atoms to the micro- and mesoscales. Though many QC formulations have been proposed with varying characteristics and capabilities, a crucial cornerstone of all QC techniques is the concept of summation rules, which attempt to efficiently approximate the total Hamiltonian of a crystalline atomic ensemble by a weighted sum over a small subset of atoms. In this work we propose a novel, fully-nonlocal, energy-based formulation of the QC method with support for legacy and new summation rules through a general energy-sampling scheme. Our formulation does not conceptually differentiate between atomistic and coarse-grained regions and thus allows for seamless bridging without domain-coupling interfaces. Within this structure, we introduce a new class of summation rules which leverage the affine kinematics of this QC formulation to most accurately integrate thermodynamic quantities of interest. By comparing this new class of summation rules to commonly-employed rules through analysis of energy and spurious force errors, we find that the new rules produce no residual or spurious force artifacts in the large-element limit under arbitrary affine deformation, while allowing us to seamlessly bridge to full atomistics. We verify that the new summation rules exhibit significantly smaller force artifacts and energy approximation errors than all comparable previous summation rules through a comprehensive suite of examples with spatially non-uniform QC discretizations in two and three dimensions. Due to the unique structure of these summation rules, we also use the new formulation to study scenarios with large regions of free surface, a class of problems previously out of reach of the QC method. Lastly, we present the key components of a high-performance, distributed-memory realization of the new method, including a novel algorithm for supporting unparalleled levels of deformation. Overall, this new formulation and implementation allows us to efficiently perform simulations containing an unprecedented number of degrees of freedom with low approximation error.", "doi": "10.7907/Z9SB43PH", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9822", "collection": "thesis", "collection_id": "9822", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022016-152749956", "primary_object_url": { "basename": "Complete Thesis.pdf", "content": "final", "filesize": 5766095, "license": "other", "mime_type": "application/pdf", "url": "/9822/73/Complete Thesis.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Programming Molecular Association and Viscoelastic Behavior in Protein Hydrogels", "author": [ { "family_name": "Dooling", "given_name": "Lawrence Joseph", "orcid": "0000-0002-1688-2066", "clpid": "Dooling-Lawrence-Joseph" } ], "thesis_advisor": [ { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" } ], "thesis_committee": [ { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" }, { "family_name": "Kornfield", "given_name": "Julia A.", "clpid": "Kornfield-J-A" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Wang", "given_name": "Zhen-Gang", "clpid": "Wang-Zhen-Gang" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "Recombinant artificial proteins contain genetically encoded information that specifies their assembly into higher order structures by physical or chemical cross-linking as well as elastic behavior and biological or chemical function. This thesis describes the use of artificial proteins to construct molecular networks containing covalent cross-links involving the thiol side chain of cysteine residues and physical cross-links involving the association of helical domains as coiled coils. The goal of this work was to demonstrate how the viscoelastic properties of protein hydrogels could be encoded within an artificial protein sequence.
\r\n\r\nUsing genetic engineering methods, a telechelic protein denoted ERE was designed from elastin- and fibronectin-derived repeating units and expressed in Escherichia coli. ERE was end-linked by the reaction of terminal cysteine residues with tetrakis-vinyl sulfone-functionalized 4-arm star PEG to form hydrogel networks. The effects of varying the precursor concentration and cross-linker stoichiometry on the swelling ratio and mechanical properties of the hydrogels were studied in detail in Chapter 2. The capacity for ERE hydrogels to serve as an artificial extracellular matrix was also assessed by the encapsulation of mouse fibroblasts, which survived the cross-linking reaction and exhibited a spread morphology within the gel.
\r\n\r\nChapter 3 describes a set of recombinant artificial proteins that can be cross-linked by covalent bonds, by association of helical domains, or by both mechanisms. These proteins were used to construct chemical, physical, and chemical-physical hydrogel networks in which the mechanism of cross-linking determines whether the material response to mechanical deformation is elastic or viscoelastic. In viscoelastic networks, stress relaxation and energy dissipation could be tuned by controlling the ratio of physical cross-linking to chemical cross-linking, and the physical cross-links could be disrupted either by protein denaturation or by mutation of the primary sequence.
\r\n\r\nNetwork dynamics control the viscoelasticity and erosion rate of materials and influence biological processes at multiple length scales.In Chapter 4, variation of the protein sequence was explored as a strategy to tune the characteristic relaxation timescale of protein networks. Single point mutations to coiled-coil physical cross-linking domains in chemical-physical hydrogels altered the characteristic relaxation time over five orders of magnitude. Using a pair of orthogonal coiled-coil physical cross-linking domains, networks with two distinct relaxation timescales were also engineered.
\r\n\r\nThe dynamic properties of protein hydrogels can also be controlled by interactions between protein domains and small molecule ligands. In Chapter 5, the viscoelastic behavior of chemical-physical protein gels was tuned by swelling the gels with small hydrophobic molecules including vitamin D3 and fatty acids. The proposed mechanism for this effect involves binding of the ligands within the hydrophobic pore or channel created by a coiled-coil physical cross-link. Exploiting natural and designed protein-ligand interactions represents a new approach to developing hydrogel \u201cformulations\u201d in which the viscoelastic properties of the material can be engineered to meet specific design criteria.
\r\n\r\nIn addition to exhibiting interesting dynamic properties, polymeric hydrogels containing permanent covalent cross-links and reversible physical cross-links often display enhanced toughness and extensibility. Protein hydrogels cross-linked by covalent thioether bonds and physical coiled coils could be extended further than control covalent hydrogels and exhibited a greater work of extension, which is considered a measure of material toughness. These results demonstrate progress toward engineering tougher, more extensible protein-based materials by the incorporation of coiled-coil physical cross-links within a covalent hydrogel network.
", "doi": "10.7907/Z9PK0D41", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9735", "collection": "thesis", "collection_id": "9735", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232016-115645913", "primary_object_url": { "basename": "LucasMeza_Thesis_Final.pdf", "content": "final", "filesize": 22363609, "license": "other", "mime_type": "application/pdf", "url": "/9735/48/LucasMeza_Thesis_Final.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Design, Fabrication, and Mechanical Property Analysis of 3D Nanoarchitected Materials", "author": [ { "family_name": "Meza", "given_name": "Lucas Rosendo", "orcid": "0000-0003-0250-2621", "clpid": "Meza-Lucas-Rosendo" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "div_eng" } ], "abstract": "Recent developments in micro- and nanoscale 3D fabrication techniques have enabled the creation of materials with a controllable nanoarchitecture that can have structural features spanning 5 orders of magnitude from tens of nanometers to millimeters. These fabrication methods in conjunction with nanomaterial processing techniques permit a nearly unbounded design space through which new combinations of nanomaterials and architecture can be realized. In the course of this work, we designed, fabricated, and mechanically analyzed a wide range of nanoarchitected materials in the form of nanolattices made from polymer, composite, and hollow ceramic beams. Using a combination of two-photon lithography and atomic layer deposition, we fabricated samples with periodic and hierarchical architectures spanning densities over 4 orders of magnitude from \u03c1=0.3-300kg/m3 and with features as small as 5nm. Uniaxial compression and cyclic loading tests performed on different nanolattice topologies revealed a range of novel mechanical properties: the constituent nanoceramics used here have size-enhanced strengths that approach the theoretical limit of materials strength; hollow aluminum oxide (Al2O3) nanolattices exhibited ductile-like deformation and recovered nearly completely after compression to 50% strain when their wall thicknesses were reduced below 20nm due to the activation of shell buckling; hierarchical nanolattices exhibited enhanced recoverability and a near linear scaling of strength and stiffness with relative density, with E\u221d\u03c11.04 and \u03c3y\u221d\u03c11.17 for hollow Al2O3 samples; periodic rigid and non-rigid nanolattice topologies were tested and showed a nearly uniform scaling of strength and stiffness with relative density, marking a significant deviation from traditional theories on \u201cbending\u201d and \u201cstretching\u201d dominated cellular solids; and the mechanical behavior across all topologies was highly tunable and was observed to strongly correlate with the slenderness \u03bb and the wall thickness-to-radius ratio t/a of the beams. These results demonstrate the potential of nanoarchitected materials to create new highly tunable mechanical metamaterials with previously unattainable properties.", "doi": "10.7907/Z9154F1K", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9734", "collection": "thesis", "collection_id": "9734", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232016-115519723", "primary_object_url": { "basename": "arya_manan_2016.pdf", "content": "final", "filesize": 9277785, "license": "other", "mime_type": "application/pdf", "url": "/9734/1/arya_manan_2016.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Packaging and Deployment of Large Planar Spacecraft Structures", "author": [ { "family_name": "Arya", "given_name": "Manan", "orcid": "0000-0003-3522-6010", "clpid": "Arya-Manan" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents a set of novel methods to biaxially package planar structures by folding and wrapping. The structure is divided into strips connected by folds that can slip during wrapping to accommodate material thickness. These packaging schemes are highly efficient, with theoretical packaging efficiencies approaching 100%. Packaging tests on meter-scale physical models have demonstrated packaging efficiencies of up to 83%. These methods avoid permanent deformation of the structure, allowing an initially flat structure to be deployed to a flat state.
\r\n\r\nAlso presented are structural architectures and deployment schemes that are compatible with these packaging methods. These structural architectures use either in-plane pretension -- suitable for membrane structures -- or out-of-plane bending stiffness to resist loading. Physical models are constructed to realize these structural architectures. The deployment of these types of structures is shown to be controllable and repeatable by conducting experiments on lab-scale models.
\r\n\r\nThese packaging methods, structural architectures, and deployment schemes are applicable to a variety of spacecraft structures such as solar power arrays, solar sails, antenna arrays, and drag sails; they have the potential to enable larger variants of these structures while reducing the packaging volume required. In this thesis, these methods are applied to the preliminary structural design of a space solar power satellite. This deployable spacecraft, measuring 60 m x 60 m, can be packaged into a cylinder measuring 1.5 m in height and 1 m in diameter. It can be deployed to a flat configuration, where it acts as a stiff lightweight support framework for multifunctional tiles that collect sunlight, generate electric power, and transmit it to a ground station on Earth.
", "doi": "10.7907/Z9Z60M0D", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9729", "collection": "thesis", "collection_id": "9729", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05182016-164150213", "type": "thesis", "title": "Dynamics of Granular Crystals with Elastic-Plastic Contacts", "author": [ { "family_name": "Burgoyne", "given_name": "Hayden Andrew", "orcid": "0000-0003-0891-6411", "clpid": "Burgoyne-Hayden-Andrew" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Newman", "given_name": "John A.", "clpid": "Newman-J-A" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "We study the behavior of granular crystals subjected to impact loading that creates plastic deformation at the contacts between constituent particles. Granular crystals are highly periodic arrangements of spherical particles, arranged into densely packed structures resembling crystals. This special class of granular materials has been shown to have unique dynamics with suggested applications in impact protection. However, previous work has focused on very low amplitude impacts where every contact point can be described using the Hertzian contact law, valid only for purely elastic deformation. In this thesis, we extend previous investigation of the dynamics of granular crystals to significantly higher impact energies more suitable for the majority of applications. Additionally, we demonstrate new properties specific to elastic-plastic granular crystals and discuss their potential applications as well. We first develop a new contact law to describe the interaction between particles for large amplitude compression of elastic-plastic spherical particles including a formulation for strain-rate dependent plasticity. We numerically and experimentally demonstrate the applicability of this contact law to a variety of materials typically used in granular crystals. We then extend our investigation to one-dimensional chains of elastic-plastic particles, including chains of alternating dissimilar materials. We show that, using the new elastic-plastic contact law, we can predict the speed at which impact waves with plastic dissipation propagate based on the material properties of the constituent particles. Finally, we experimentally and numerically investigate the dynamics of two-dimensional and three-dimensional granular crystals with elastic-plastic contacts. We first show that the predicted wave speeds for 1D granular crystals can be extended to 2D and 3D materials. We then investigate the behavior of waves propagating across oblique interfaces of dissimilar particles. We show that the character of the refracted wave can be predicted using an analog to Snell's law for elastic-plastic granular crystals and ultimately show how it can be used to design impact guiding \"lenses\" for mitigation applications.", "doi": "10.7907/Z9J38QG6", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9282", "collection": "thesis", "collection_id": "9282", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11222015-173649284", "primary_object_url": { "basename": "Panagiotis_Natsiavas_2016_Thesis.pdf", "content": "final", "filesize": 12728171, "license": "other", "mime_type": "application/pdf", "url": "/9282/1/Panagiotis_Natsiavas_2016_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Stability of Electrode-Electrolyte Interfaces During Charging in Lithium Batteries", "author": [ { "family_name": "Natsiavas", "given_name": "Panagiotis Philippos", "clpid": "Natsiavas-Panagiotis-Philippos" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Weinberg", "given_name": "Kerstin", "clpid": "Weinberg-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this thesis we study the growth of a Li electrode-electrolyte interface in the presence of an elastic prestress. In particular, we focus our interest on Li-air batteries with a solid electrolyte, LIPON, which is a new type of secondary or rechargeable battery. Theoretical studies and experimental evidence show that during the process of charging the battery the replated lithium adds unevenly to the electrode surface. This phenomenon eventually leads to dendrite formation as the battery is charged and discharged numerous times. In order to suppress or alleviate this deleterious effect of dendrite growth, we put forth a study based on a linear stability analysis. Taking into account all the mechanisms of mass transport and interfacial kinetics, we model the evolution of the interface. We find that, in the absence of stress, the stability of a planar interface depends on interfacial diffusion properties and interfacial energy. Specifically, if Herring-Mullins capillarity-driven interfacial diffusion is accounted for, interfaces are unstable against all perturbations of wavenumber larger than a critical value. We find that the effect of an elastic prestress is always to stabilize planar interfacial growth by increasing the critical wavenumber for instability. A parametric study results in quantifying the extent of the prestress stabilization in a manner that can potentially be used in the design of Li-air batteries. Moreover, employing the theory of finite differences we numerically solve the equation that describes the evolution of the surface profile and present visualization results of the surface evolution by time. Lastly, numerical simulations performed in a commercial finite element software validate the theoretical formulation of the interfacial elastic energy change with respect to the planar interface.", "doi": "10.7907/Z93R0QR8", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9783", "collection": "thesis", "collection_id": "9783", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272016-104209268", "primary_object_url": { "basename": "Thesis2016_VinamraAgrawal - 2.pdf", "content": "final", "filesize": 7388241, "license": "other", "mime_type": "application/pdf", "url": "/9783/1/Thesis2016_VinamraAgrawal - 2.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Shock Wave Propagation in Composites and Electro-Thermomechanical Coupling of Ferroelectric Materials", "author": [ { "family_name": "Agrawal", "given_name": "Vinamra", "orcid": "0000-0002-1698-1371", "clpid": "Agrawal-Vinamra" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "How is material behavior at the macro scale influenced by its properties and structure at the micro and meso-scales? How do heterogeneities influence the properties and the response of a material? How does nonlinear coupling of electro-thermo-mechanical properties influence the behavior of a ferroelectric material? How can design at the micro-scale be exploited to obtain selective response? These questions have been topics of significant interest in the materials and mechanics community. Recently, new materials like multifunctional composites and metamaterials have been developed, targeted at selective applications. These materials find applications in areas like energy harvesting, damage mitigation, biomedical devices, and various aerospace applications. The current thesis explores these questions with two major thrusts: (i) internal reflects of shocks in composite media and (ii) shocks in ferroelectric media.
\r\n\r\nUnder the application of high-pressure, high strain rate loading, such as during high velocity impact, shock waves are generated in the material. They can cause the material to achieve very high stress states, and if transmitted without mitigation, can lead to failure of key components. An important question here is 'Can we design materials which can successfully mitigate damage due to shocks?' In a heterogeneous material, like a layered composite, the traveling waves undergo scattering due to internal reflections. In order to understand internal reflections, an idealized problem that focuses on nonlinear shocks and ignores less important elastic waves was formulated and studied in detail. The problem is studied by classifying all possible interactions in the material and then solving corresponding Riemann problems. Using dynamic programming tools, a new algorithm is designed that uses these solutions to generate a complete picture of the impact process. Different laminate designs are explored to study optimal design, by varying individual layer properties and their arrangement. Phenomena like spallation and delamination are also investigated.
\r\n\r\nUpon high strain rate loading, ferroelectric materials like lead zirconate titanate (PZT) undergo ferroelectric to anti-ferroelectric phase transition leading to large pulsed current output. These materials have thus found applications as pulsed power generators. The problem of shock induced depolarization and the associated electro-thermo-mechanical coupling of ferroelectric materials is studied in this thesis using theoretical and numerical methods. A large deformation dynamic analysis of such materials is conducted to study phase boundary propagation in the medium. The presence of high electrical fields can lead to formation of charges in the material, such as surface charge on the phase boundary. Using conservation laws and the second law of thermodynamics, a set of governing equations are formulated that dictate the phase boundary propagation in isothermal and adiabatic environments. Due to the possibility of surface charges on the phase boundary, the curvature of the phase boundary starts to play a role in the driving force acting on the phase boundary. The equations of motion and driving force see the contribution of nonlinear electro-thermomechanical coupling in the material. Using the equations derived, a canonical problem of impact on a ferroelectric material is studied. A new finite-volume, front-tracking method is developed to solve these equations. Finally, results from numerical simulations are compared to the experimental results.
", "doi": "10.7907/Z98G8HN8", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9052", "collection": "thesis", "collection_id": "9052", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07072015-124133131", "primary_object_url": { "basename": "Thesis_Stephanie_Mitchell_2016.pdf", "content": "final", "filesize": 13515900, "license": "other", "mime_type": "application/pdf", "url": "/9052/1/Thesis_Stephanie_Mitchell_2016.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Metaconcrete: Engineered Aggregates for Enhanced Dynamic Performance", "author": [ { "family_name": "Mitchell", "given_name": "Stephanie Jane", "orcid": "0000-0002-7303-8216", "clpid": "Mitchell-Stephanie-Jane" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Pandolfi", "given_name": "Anna", "clpid": "Pandolfi-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This work presents the development and investigation of a new type of concrete for the attenuation of waves induced by dynamic excitation. Recent progress in the field of metamaterials science has led to a range of novel composites which display unusual properties when interacting with electromagnetic, acoustic, and elastic waves. A new structural metamaterial with enhanced properties for dynamic loading applications is presented, which is named metaconcrete. In this new composite material the standard stone and gravel aggregates of regular concrete are replaced with spherical engineered inclusions. Each metaconcrete aggregate has a layered structure, consisting of a heavy core and a thin compliant outer coating. This structure allows for resonance at or near the eigenfrequencies of the inclusions, and the aggregates can be tuned so that resonant oscillations will be activated by particular frequencies of an applied dynamic loading. The activation of resonance within the aggregates causes the overall system to exhibit negative effective mass, which leads to attenuation of the applied wave motion. To investigate the behavior of metaconcrete slabs under a variety of different loading conditions a finite element slab model containing a periodic array of aggregates is utilized. The frequency dependent nature of metaconcrete is investigated by considering the transmission of wave energy through a slab, which indicates the presence of large attenuation bands near the resonant frequencies of the aggregates. Applying a blast wave loading to both an elastic slab and a slab model that incorporates the fracture characteristics of the mortar matrix reveals that a significant portion of the supplied energy can be absorbed by aggregates which are activated by the chosen blast wave profile. The transfer of energy from the mortar matrix to the metaconcrete aggregates leads to a significant reduction in the maximum longitudinal stress, greatly improving the ability of the material to resist damage induced by a propagating shock wave. The various analyses presented in this work provide the theoretical and numerical background necessary for the informed design and development of metaconcrete aggregates for dynamic loading applications, such as blast shielding, impact protection, and seismic mitigation.", "doi": "10.7907/Z9H12ZXN", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9058", "collection": "thesis", "collection_id": "9058", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07132015-150843708", "primary_object_url": { "basename": "MontemayorThesis.pdf", "content": "", "filesize": 88804608, "license": "other", "mime_type": "application/pdf", "url": "/9058/1/MontemayorThesis.pdf", "version": "v1.0.0" }, "type": "thesis", "title": "Fabrication, Characterization, And Deformation of 3D Structural Meta-Materials ", "author": [ { "family_name": "Montemayor", "given_name": "Lauren Christine", "clpid": "Montemayor-Lauren-Christine" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Current technological advances in fabrication methods have provided pathways to creating architected structural meta-materials similar to those found in natural organisms that are structurally robust and lightweight, such as diatoms. Structural meta-materials are materials with mechanical properties that are determined by material properties at various length scales, which range from the material microstructure (nm) to the macro-scale architecture (\u03bcm \u2013 mm). It is now possible to exploit material size effect, which emerge at the nanometer length scale, as well as structural effects to tune the material properties and failure mechanisms of small-scale cellular solids, such as nanolattices. \r\nThis work demonstrates the fabrication and mechanical properties of 3-dimensional hollow nanolattices in both tension and compression. Hollow gold nanolattices loaded in uniaxial compression demonstrate that strength and stiffness vary as a function of geometry and tube wall thickness. Structural effects were explored by increasing the unit cell angle from 30\u00b0 to 60\u00b0 while keeping all other parameters constant; material size effects were probed by varying the tube wall thickness, t, from 200nm to 635nm, at a constant relative density and grain size. In-situ uniaxial compression experiments reveal an order-of-magnitude increase in yield stress and modulus in nanolattices with greater lattice angles, and a 150% increase in the yield strength without a concomitant change in modulus in thicker-walled nanolattices for fixed lattice angles. These results imply that independent control of structural and material size effects enables tunability of mechanical properties of 3-dimensional architected meta-materials and highlight the importance of material, geometric, and microstructural effects in small-scale mechanics. \r\nThis work also explores the flaw tolerance of 3D hollow-tube alumina kagome nanolattices with and without pre-fabricated notches, both in experiment and simulation. Experiments demonstrate that the hollow kagome nanolattices in uniaxial tension always fail at the same load when the ratio of notch length (a) to sample width (w) is no greater than 1/3, with no correlation between failure occurring at or away from the notch. For notches with (a/w) > 1/3, the samples fail at lower peak loads and this is attributed to the increased compliance as fewer unit cells span the un-notched region. Finite element simulations of the kagome tension samples show that the failure is governed by tensile loading for (a/w) < 1/3 but as (a/w) increases, bending begins to play a significant role in the failure. This work explores the flaw sensitivity of hollow alumina kagome nanolattices in tension, using experiments and simulations, and demonstrates that the discrete-continuum duality of architected structural meta-materials gives rise to their flaw insensitivity even when made entirely of intrinsically brittle materials.\r\n", "doi": "10.7907/Z9D21VH2", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9162", "collection": "thesis", "collection_id": "9162", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09212015-105224808", "type": "thesis", "title": "Force Chains, Friction, and Flow: Behavior of Granular Media across Length Scales", "author": [ { "family_name": "Hurley", "given_name": "Ryan Colt", "clpid": "Hurley-Ryan-Colt" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lamb", "given_name": "Michael P.", "clpid": "Lamb-M-P" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "We study the behavior of granular materials at three length scales. At the smallest length scale, the grain-scale, we study inter-particle forces and \"force chains\". Inter-particle forces are the natural building blocks of constitutive laws for granular materials. Force chains are a key signature of the heterogeneity of granular systems. Despite their fundamental importance for calibrating grain-scale numerical models and elucidating constitutive laws, inter-particle forces have not been fully quantified in natural granular materials. We present a numerical force inference technique for determining inter-particle forces from experimental data and apply the technique to two-dimensional and three-dimensional systems under quasi-static and dynamic load. These experiments validate the technique and provide insight into the quasi-static and dynamic behavior of granular materials.
\r\n\r\nAt a larger length scale, the mesoscale, we study the emergent frictional behavior of a collection of grains. Properties of granular materials at this intermediate scale are crucial inputs for macro-scale continuum models. We derive friction laws for granular materials at the mesoscale by applying averaging techniques to grain-scale quantities. These laws portray the nature of steady-state frictional strength as a competition between steady-state dilation and grain-scale dissipation rates. The laws also directly link the rate of dilation to the non-steady-state frictional strength.
\r\n\r\nAt the macro-scale, we investigate continuum modeling techniques capable of simulating the distinct solid-like, liquid-like, and gas-like behaviors exhibited by granular materials in a single computational domain. We propose a Smoothed Particle Hydrodynamics (SPH) approach for granular materials with a viscoplastic constitutive law. The constitutive law uses a rate-dependent and dilation-dependent friction law. We provide a theoretical basis for a dilation-dependent friction law using similar analysis to that performed at the mesoscale. We provide several qualitative and quantitative validations of the technique and discuss ongoing work aiming to couple the granular flow with gas and fluid flows.
", "doi": "10.7907/Z91Z429J", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:8861", "collection": "thesis", "collection_id": "8861", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05172015-152006825", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 13609562, "license": "other", "mime_type": "application/pdf", "url": "/8861/1/thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Collective Behavior of Asperities as a Model for Friction and Adhesion", "author": [ { "family_name": "Hulikal Sampath Kumaran", "given_name": "Srivatsan", "clpid": "Hulikal-Sampath-Kumaran-Srivatsan" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Understanding friction and adhesion in static and sliding contact of surfaces is important in numerous physical phenomena and technological applications. Most surfaces are rough at the microscale, and thus the real area of contact is only a fraction of the nominal area. The macroscopic frictional and adhesive response is determined by the collective behavior of the population of evolving and interacting microscopic contacts. This collective behavior can be very different from the behavior of individual contacts. It is thus important to understand how the macroscopic response emerges from the microscopic one.
\r\n\r\nIn this thesis, we develop a theoretical and computational framework to study the collective behavior. Our philosophy is to assume a simple behavior of a single asperity and study the collective response of an ensemble. Our work bridges the existing well-developed studies of single asperities with phenomenological laws that describe macroscopic rate-and-state behavior of frictional interfaces. We find that many aspects of the macroscopic behavior are robust with respect to the microscopic response. This explains why qualitatively similar frictional features are seen for a diverse range of materials.
\r\n\r\nWe first show that the collective response of an ensemble of one-dimensional independent viscoelastic elements interacting through a mean field reproduces many qualitative features of static and sliding friction evolution. The resulting macroscopic behavior is different from the microscopic one: for example, even if each contact is velocity-strengthening, the macroscopic behavior can be velocity-weakening. The framework is then extended to incorporate three-dimensional rough surfaces, long- range elastic interactions between contacts, and time-dependent material behaviors such as viscoelasticity and viscoplasticity. Interestingly, the mean field behavior dominates and the elastic interactions, though important from a quantitative perspective, do not change the qualitative macroscopic response. Finally, we examine the effect of adhesion on the frictional response as well as develop a force threshold model for adhesion and mode I interfacial cracks.
", "doi": "10.7907/Z94M92HM", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8877", "collection": "thesis", "collection_id": "8877", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05212015-174045815", "primary_object_url": { "basename": "Xin_Ning_Thesis_2015.pdf", "content": "final", "filesize": 39630150, "license": "other", "mime_type": "application/pdf", "url": "/8877/1/Xin_Ning_Thesis_2015.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Imperfection Insensitive Thin Shells", "author": [ { "family_name": "Ning", "given_name": "Xin", "clpid": "Ning-Xin" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The buckling of axially compressed cylindrical shells and externally pressurized spherical shells is extremely sensitive to even very small geometric imperfections. In practice this issue is addressed by either using overly conservative knockdown factors, while keeping perfect axial or spherical symmetry, or adding closely and equally spaced stiffeners on shell surface. The influence of imperfection-sensitivity is mitigated, but the shells designed from these approaches are either too heavy or very expensive and are still sensitive to imperfections. Despite their drawbacks, these approaches have been used for more than half a century.
\r\n\r\nThis thesis proposes a novel method to design imperfection-insensitive cylindrical shells subject to axial compression. Instead of following the classical paths, focused on axially symmetric or high-order rotationally symmetric cross-sections, the method in this thesis adopts optimal symmetry-breaking wavy cross-sections (wavy shells). The avoidance of imperfection sensitivity is achieved by searching with an evolutionary algorithm for smooth cross-sectional shapes that maximize the minimum among the buckling loads of geometrically perfect and imperfect wavy shells. It is found that the shells designed through this approach can achieve higher critical stresses and knockdown factors than any previously known monocoque cylindrical shells. It is also found that these shells have superior mass efficiency to almost all previously reported stiffened shells.
\r\n\r\nExperimental studies on a design of composite wavy shell obtained through the proposed method are presented in this thesis. A method of making composite wavy shells and a photogrametry technique of measuring full-field geometric imperfections have been developed. Numerical predictions based on the measured geometric imperfections match remarkably well with the experiments. Experimental results confirm that the wavy shells are not sensitive to imperfections and can carry axial compression with superior mass efficiency.
\r\n\r\nAn efficient computational method for the buckling analysis of corrugated and stiffened cylindrical shells subject to axial compression has been developed in this thesis. This method modifies the traditional Bloch wave method based on the stiffness matrix method of rotationally periodic structures. A highly efficient algorithm has been developed to implement the modified Bloch wave method. This method is applied in buckling analyses of a series of corrugated composite cylindrical shells and a large-scale orthogonally stiffened aluminum cylindrical shell. Numerical examples show that the modified Bloch wave method can achieve very high accuracy and require much less computational time than linear and nonlinear analyses of detailed full finite element models.
\r\n\r\nThis thesis presents parametric studies on a series of externally pressurized pseudo-spherical shells, i.e., polyhedral shells, including icosahedron, geodesic shells, and triambic icosahedra. Several optimization methods have been developed to further improve the performance of pseudo-spherical shells under external pressure. It has been shown that the buckling pressures of the shell designs obtained from the optimizations are much higher than the spherical shells and not sensitive to imperfections.
\r\n", "doi": "10.7907/Z91J97P9", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8854", "collection": "thesis", "collection_id": "8854", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05112015-075223217", "primary_object_url": { "basename": "mbengue_cheikh_2015_thesis.pdf", "content": "final", "filesize": 3849313, "license": "other", "mime_type": "application/pdf", "url": "/8854/1/mbengue_cheikh_2015_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Storm Track Response to Perturbations in Climate", "author": [ { "family_name": "Mbengue", "given_name": "Cheikh Oumar", "clpid": "Mbengue-Cheikh-Oumar" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Teixeira", "given_name": "Joao", "clpid": "Teixeira-Joao" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis advances our understanding of midlatitude storm tracks and how they respond to perturbations in the climate system. The midlatitude storm tracks are regions of maximal turbulent kinetic energy in the atmosphere. Through them, the bulk of the atmospheric transport of energy, water vapor, and angular momentum occurs in midlatitudes. Therefore, they are important regulators of climate, controlling basic features such as the distribution of surface temperatures, precipitation, and winds in midlatitudes. Storm tracks are robustly projected to shift poleward in global-warming simulations with current climate models. Yet the reasons for this shift have remained unclear. Here we show that this shift occurs even in extremely idealized (but still three-dimensional) simulations of dry atmospheres. We use these simulations to develop an understanding of the processes responsible for the shift and develop a conceptual model that accounts for it.
\r\n\r\nWe demonstrate that changes in the convective static stability in the deep tropics alone can drive remote shifts in the midlatitude storm tracks. Through simulations with a dry idealized general circulation model (GCM), midlatitude storm tracks are shown to be located where the mean available potential energy (MAPE, a measure of the potential energy available to be converted into kinetic energy) is maximal. As the climate varies, even if only driven by tropical static stability changes, the MAPE maximum shifts primarily because of shifts of the maximum of near-surface meridional temperature gradients. The temperature gradients shift in response to changes in the width of the tropical Hadley circulation, whose width is affected by the tropical static stability. Storm tracks generally shift in tandem with shifts of the subtropical terminus of the Hadley circulation.
\r\n\r\nWe develop a one-dimensional diffusive energy-balance model that links changes in the Hadley circulation to midlatitude temperature gradients and so to the storm tracks. It is the first conceptual model to incorporate a dynamical coupling between the tropical Hadley circulation and midlatitude turbulent energy transport. Numerical and analytical solutions of the model elucidate the circumstances of when and how the storm tracks shift in tandem with the terminus of the Hadley circulation. They illustrate how an increase of only the convective static stability in the deep tropics can lead to an expansion of the Hadley circulation and a poleward shift of storm tracks.
\r\n\r\nThe simulations with the idealized GCM and the conceptual energy-balance model demonstrate a clear link between Hadley circulation dynamics and midlatitude storm track position. With the help of the hierarchy of models presented in this thesis, we obtain a closed theory of storm track shifts in dry climates. The relevance of this theory for more realistic moist climates is discussed.
", "doi": "10.7907/Z9FT8J05", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8841", "collection": "thesis", "collection_id": "8841", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04302015-143917971", "primary_object_url": { "basename": "zachary_aitken_2015_thesis.pdf", "content": "final", "filesize": 3688241, "license": "other", "mime_type": "application/pdf", "url": "/8841/1/zachary_aitken_2015_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Effect of Microstructural Interfaces on the Mechanical Response of Crystalline Metallic Materials", "author": [ { "family_name": "Aitken", "given_name": "Zachary Howard", "clpid": "Aitken-Zachary-Howard" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Goddard", "given_name": "William A., III", "clpid": "Goddard-W-A-III" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "div_eng" } ], "abstract": "Advances in nano-scale mechanical testing have brought about progress in the understanding of physical phenomena in materials and a measure of control in the fabrication of novel materials. In contrast to bulk materials that display size-invariant mechanical properties, sub-micron metallic samples show a critical dependence on sample size. The strength of nano-scale single crystalline metals is well-described by a power-law function, \u03c3\u03b1D-n, where D is a critical sample size and n is a experimentally-fit positive exponent. This relationship is attributed to source-driven plasticity and demonstrates a strengthening as the decreasing sample size begins to limit the size and number of dislocation sources. A full understanding of this size-dependence is complicated by the presence of microstructural features such as interfaces that can compete with the dominant dislocation-based deformation mechanisms. In this thesis, the effects of microstructural features such as grain boundaries and anisotropic crystallinity on nano-scale metals are investigated through uniaxial compression testing. We find that nano-sized Cu covered by a hard coating displays a Bauschinger effect and the emergence of this behavior can be explained through a simple dislocation-based analytic model. Al nano-pillars containing a single vertically-oriented coincident site lattice grain boundary are found to show similar deformation to single-crystalline nano-pillars with slip traces passing through the grain boundary. With increasing tilt angle of the grain boundary from the pillar axis, we observe a transition from dislocation-dominated deformation to grain boundary sliding. Crystallites are observed to shear along the grain boundary and molecular dynamics simulations reveal a mechanism of atomic migration that accommodates boundary sliding. We conclude with an analysis of the effects of inherent crystal anisotropy and alloying on the mechanical behavior of the Mg alloy, AZ31. Through comparison to pure Mg, we show that the size effect dominates the strength of samples below 10 \u03bcm, that differences in the size effect between hexagonal slip systems is due to the inherent crystal anisotropy, suggesting that the fundamental mechanism of the size effect in these slip systems is the same.", "doi": "10.7907/Z9C24TCP", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8819", "collection": "thesis", "collection_id": "8819", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04132015-160812309", "type": "thesis", "title": "A Variational Framework for Spectral Discretization of the Density Matrix in Kohn-Sham Density Functional Theory", "author": [ { "family_name": "Wang", "given_name": "Xin C.", "orcid": "0000-0003-3854-4831", "clpid": "Wang-Xin-C" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Minnich", "given_name": "Austin J.", "clpid": "Minnich-A-J" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Kohn-Sham density functional theory (KSDFT) is currently the main work-horse of quantum\r\nmechanical calculations in physics, chemistry, and materials science. From a mechanical\r\nengineering perspective, we are interested in studying the role of defects in the\r\nmechanical properties in materials. In real materials, defects are typically found at \r\nvery small concentrations e.g., vacancies occur at parts per million,\r\ndislocation density in metals ranges from $10^{10} m^{-2}$ to $10^{15} m^{-2}$,\r\nand grain sizes vary from nanometers to micrometers in polycrystalline materials, etc. In order to model materials at\r\nrealistic defect concentrations using DFT, we would need\r\nto work with system sizes beyond millions of atoms. Due to the cubic-scaling\r\ncomputational cost with respect to the number of atoms in conventional DFT implementations, such system sizes are\r\nunreachable. Since the early 1990s, there has been a huge interest in developing DFT\r\nimplementations that have linear-scaling computational cost. A promising\r\napproach to achieving linear-scaling cost is to approximate the density matrix in\r\nKSDFT. The focus of this\r\nthesis is to provide a firm mathematical framework to study the convergence of\r\nthese approximations. We reformulate the Kohn-Sham density\r\nfunctional theory as a nested variational problem in the density matrix,\r\nthe electrostatic potential, and a field dual to the electron density. The\r\ncorresponding functional is linear in the density matrix and thus amenable to\r\nspectral representation. Based on this reformulation, we introduce a new\r\napproximation scheme, called spectral binning, which does not require smoothing\r\nof the occupancy function and thus applies at arbitrarily low temperatures. We\r\nproof convergence of the approximate solutions with respect to spectral binning\r\nand with respect to an additional spatial discretization of the domain. For a\r\nstandard one-dimensional benchmark problem, we present numerical experiments for\r\nwhich spectral binning exhibits excellent convergence characteristics and\r\noutperforms other linear-scaling methods. ", "doi": "10.7907/Z99021QK", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8811", "collection": "thesis", "collection_id": "8811", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04022015-231633336", "primary_object_url": { "basename": "KozachkovHenry2015Thesis.pdf", "content": "final", "filesize": 7371634, "license": "other", "mime_type": "application/pdf", "url": "/8811/1/KozachkovHenry2015Thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Modifying Bulk Metallic Glasses: Composites and Configurational States", "author": [ { "family_name": "Kozachkov", "given_name": "Henry", "clpid": "Kozachkov-Henry" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Hofmann", "given_name": "Douglas C.", "clpid": "Hofmann-D-C" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Hofmann", "given_name": "Douglas C.", "clpid": "Hofmann-D-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Faber", "given_name": "Katherine T.", "clpid": "Faber-K-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Bulk metallic glasses (BMGs) maybe be considered to share some of the same inherent trade-offs as engineering ceramics. While BMGs typically exhibit high yield strengths, and while some have surprising fracture toughness, they exhibiting little to no tensile ductility, and fail in a brittle manner under uniaxial loading. Speaking broadly, there are two complimentary approaches to improving on these shortcomings: 1) create bulk metallic glass matrix composites (BMGMCs) and 2) improve the properties of a monolithic BMG. The structure of this thesis mirrors this division, with chapters 2-7 focusing on creating and processing amorphous metal matrix composites, and chapter 8 focusing on modifying the properties of a monolithic BGM by altering its configurational state through irradiation.", "doi": "10.7907/Z9JQ0XZ4", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8947", "collection": "thesis", "collection_id": "8947", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302015-221358386", "type": "thesis", "title": "Exploring the Kinetics of Domain Switching in Ferroelectrics for Structural Applications", "author": [ { "family_name": "Wojnar", "given_name": "Charles Stanley", "clpid": "Wojnar-Charles-Stanley" } ], "thesis_advisor": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The complex domain structure in ferroelectrics gives rise to electromechanical coupling, and its evolution (via domain switching) results in a time-dependent (i.e. viscoelastic) response. Although ferroelectrics are used in many technological applications, most do not attempt to exploit the viscoelastic response of ferroelectrics, mainly due to a lack of understanding and accurate models for their description and prediction. Thus, the aim of this thesis research is to gain better understanding of the influence of domain evolution in ferroelectrics on their dynamic mechanical response.\r\n\r\nThere have been few studies on the viscoelastic properties of ferroelectrics, mainly due to a lack of experimental methods. Therefore, an apparatus and method called Broadband Electromechanical Spectroscopy (BES) was designed and built. BES allows for the simultaneous application of dynamic mechanical and electrical loading in a vacuum environment. Using BES, the dynamic stiffness and loss tangent in bending and torsion of a particular ferroelectric, viz. lead zirconate titanate (PZT), was characterized for different combinations of electrical and mechanical loading frequencies throughout the entire electric displacement hysteresis. Experimental results showed significant increases in loss tangent (by nearly an order of magnitude) and compliance during domain switching, which shows promise as a new approach to structural damping.\r\n\r\nA continuum model of the viscoelasticity of ferroelectrics was developed, which incorporates microstructural evolution via internal variables and associated kinetic relations. For the first time, through a new linearization process, the incremental dynamic stiffness and loss tangent of materials were computed throughout the entire electric displacement hysteresis for different combinations of mechanical and electrical loading frequencies. The model accurately captured experimental results.\r\n\r\nUsing the understanding gained from the characterization and modeling of PZT, two applications of domain switching kinetics were explored by using Micro Fiber Composites (MFCs). Proofs of concept of set-and-hold actuation and structural damping using MFCs were demonstrated.", "doi": "10.7907/Z9HD7SM7", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8936", "collection": "thesis", "collection_id": "8936", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292015-170754625", "primary_object_url": { "basename": "150601_citThesis_Rauls.pdf", "content": "final", "filesize": 27724774, "license": "other", "mime_type": "application/pdf", "url": "/8936/1/150601_citThesis_Rauls.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Shock Wave Behavior of Particulate Composites", "author": [ { "family_name": "Rauls", "given_name": "Michael Brian", "clpid": "Rauls-Michael-Brian" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Neel", "given_name": "Christopher H.", "clpid": "Neel-C-H" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Material heterogeneity at some scale is common in present engineering and structural materials as a means of strength improvement, weight reduction, and performance enhancement in a great many applications such as impact and blast protection, construction, and aerospace. While the benefits of transitioning toward composites in practical applications is obvious, the methods of measurement and optimization required to handle spatial heterogeneity and bridge length scale differences across multiple orders of magnitude are not. This is especially true as loading rates transition into the shock regime. Composite materials, such as concrete, have advantages afforded to them by their microstructure that allow them to dissipate and scatter impact energy. The mechanical mismatch between constituent phases in composites (mortar and cement paste in concrete, crystals and binder in polymer bonded explosives, ceramic powder and epoxy in potting materials, etc.) provides the interfaces required for shock wave reflection. The degree to which a shock is disrupted from its accepted form as a propagating discontinuity in stress and particle velocity is highly dependent upon the size, shape, and density of the interfaces present.
\r\n\r\nThe experimental and computer aided simulations in this thesis seek to establish a scaling relationship between composite microstructure and shock front disruption in terms of particulate size and density through the use of multi-point heterodyne velocity interferometry. A model particulate composite has been developed to mimic the wave reflection properties of materials such as Ultra High Performace Composite (UHPC) concrete and polymer bonded explosives, while also being simple to source and manufacture repeatably. Polymethyl Methacrylate (PMMA), a thermoplastic polymer, and silica glass spheres satisfy the manufacturing constraints with a shock impedance mismatch of 4.1, when placed in-between the shock impedance of UHPC concretes (~ 10) and polymer bondedexplosives (~ 2). The flexibility afforded by the model composite allows for the use of mono-disperse bead particle diameter distributions centered at 5 discrete diameters centered in the range associated with high scattering effectiveness (5-50 times the shock thickness in the pure matrix material). Shock front disruption is measured at multiple points on the rear surface of a plate impact target to observe shock spreading and spatial heterogeneity in material response due to random particle placement.
\r\n\r\nShock rise times are reported for composites of 30% and 40% glass spheres by volume, with glass spheres of 100, 300, 500, 700, and 1000 micron diameter. Composites with single mode as well as bi-modal bead diameter distributions are subjected to plate impact loading at an average pressure of 5 GPa. In single mode composites, a linear dependence of shock wave rise time on particle diameter is observed, with a constant of proportionality equal to the bulk shock speed in the material. Bi-modal bead diameter composites were fabricated in order to achieve higher volume fractions without composite degradation. The addition of a second phase to a base 30% glass by volume composite mix results in significant increases in shock wave rise time for base mixes of 500 micron beads, while a point of maximum scattering effectiveness is observed for base mixes\r\nof 1000 micron diameter beads.
\r\n\r\nA comprehensive two dimensional series of CTH hydrocode simulations has been completed in tandem with experiments. An evaluation of the discrepancies in simulation and experimental results is presented. Shock disruption mechanisms and matrix/interface damage effects are discussed as possible sources of error and potential avenues for model improvement. The scaling arguments and model deficiency corrections made in this thesis have the potential to drive the development of new approaches of modeling shock waves in heterogeneous materials as well as optimization of microstructure for maximum shock front disruption.
", "doi": "10.7907/Z98P5XHC", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8917", "collection": "thesis", "collection_id": "8917", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292015-080950114", "primary_object_url": { "basename": "VikasTrivedi_2015_Thesis.pdf", "content": "final", "filesize": 112953202, "license": "other", "mime_type": "application/pdf", "url": "/8917/1/VikasTrivedi_2015_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "From Molecules to Organs : Microscopy and Multi-Scale Nature of Development", "author": [ { "family_name": "Trivedi", "given_name": "Vikas", "orcid": "0000-0003-0953-0553", "clpid": "Trivedi-Vikas" } ], "thesis_advisor": [ { "family_name": "Fraser", "given_name": "Scott E.", "orcid": "0000-0002-5377-0223", "clpid": "Fraser-S-E" }, { "family_name": "Pierce", "given_name": "Niles A.", "orcid": "0000-0003-2367-4406", "clpid": "Pierce-N-A" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" }, { "family_name": "Fraser", "given_name": "Scott E.", "orcid": "0000-0002-5377-0223", "clpid": "Fraser-S-E" }, { "family_name": "Pierce", "given_name": "Niles A.", "orcid": "0000-0003-2367-4406", "clpid": "Pierce-N-A" } ], "local_group": [ { "literal": "div_bbe" } ], "abstract": "Morphogenesis is a phenomenon of intricate balance and dynamic interplay between processes occurring at a wide range of scales (spatial, temporal and energetic). During development, a variety of physical mechanisms are employed by tissues to simultaneously pattern, move, and differentiate based on information exchange between constituent cells, perhaps more than at any other time during an organism's life. To fully understand such events, a combined theoretical and experimental framework is required to assist in deciphering the correlations at both structural and functional levels at scales that include the intracellular and tissue levels as well as organs and organ systems. Microscopy, especially diffraction-limited light microscopy, has emerged as a central tool to capture the spatio-temporal context of life processes. Imaging has the unique advantage of watching biological events as they unfold over time at single-cell resolution in the intact animal. In this work I present a range of problems in morphogenesis, each unique in its requirements for novel quantitative imaging both in terms of the technique and analysis. Understanding the molecular basis for a developmental process involves investigating how genes and their products- mRNA and proteins-function in the context of a cell. Structural information holds the key to insights into mechanisms and imaging fixed specimens paves the first step towards deciphering gene function. The work presented in this thesis starts with the demonstration that the fluorescent signal from the challenging environment of whole-mount imaging, obtained by in situ hybridization chain reaction (HCR), scales linearly with the number of copies of target mRNA to provide quantitative sub-cellular mapping of mRNA expression within intact vertebrate embryos. The work then progresses to address aspects of imaging live embryonic development in a number of species. While processes such as avian cartilage growth require high spatial resolution and lower time resolution, dynamic events during zebrafish somitogenesis require higher time resolution to capture the protein localization as the somites mature. The requirements on imaging are even more stringent in case of the embryonic zebrafish heart that beats with a frequency of ~ 2-2.5 Hz, thereby requiring very fast imaging techniques based on two-photon light sheet microscope to capture its dynamics. In each of the hitherto-mentioned cases, ranging from the level of molecules to organs, an imaging framework is developed, both in terms of technique and analysis to allow quantitative assessment of the process in vivo. Overall the work presented in this thesis combines new quantitative tools with novel microscopy for the precise understanding of processes in embryonic development. ", "doi": "10.7907/Z9SX6B59", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8902", "collection": "thesis", "collection_id": "8902", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282015-145339959", "primary_object_url": { "basename": "JohnSteeves_Thesis_2015_Final.pdf", "content": "final", "filesize": 35918418, "license": "other", "mime_type": "application/pdf", "url": "/8902/1/JohnSteeves_Thesis_2015_Final.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Multilayer Active Shell Mirrors", "author": [ { "family_name": "Steeves", "given_name": "John Bradley", "clpid": "Steeves-John-Bradley" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Burdick", "given_name": "Joel Wakeman", "orcid": "0000-0002-3091-540X", "clpid": "Burdick-J-W" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents a novel active mirror technology based on carbon fiber composites and replication manufacturing processes. Multiple additional layers are implemented into the structure in order to provide the reflective layer, actuation capabilities and electrode routing. The mirror is thin, lightweight, and has large actuation capabilities. These features, along with the associated manufacturing processes, represent a significant change in design compared to traditional optics. Structural redundancy in the form of added material or support structures is replaced by thin, unsupported lightweight substrates with large actuation capabilities.
\r\n\r\nSeveral studies motivated by the desire to improve as-manufactured figure quality are performed. Firstly, imperfections in thin CFRP laminates and their effect on post-cure shape errors are studied. Numerical models are developed and compared to experimental measurements on flat laminates. Techniques to mitigate figure errors for thicker laminates are also identified. A method of properly integrating the reflective facesheet onto the front surface of the CFRP substrate is also presented. Finally, the effect of bonding multiple initially flat active plates to the backside of a curved CFRP substrate is studied. Figure deformations along with local surface defects are predicted and characterized experimentally. By understanding the mechanics behind these processes, significant improvements to the overall figure quality have been made.
\r\n\r\nStudies related to the actuation response of the mirror are also performed. The active properties of two materials are characterized and compared. Optimal active layer thicknesses for thin surface-parallel schemes are determined. Finite element simulations are used to make predictions on shape correction capabilities, demonstrating high correctabiliity and stroke over low-order modes. The effect of actuator saturation is studied and shown to significantly degrade shape correction performance.
\r\n\r\nThe initial figure as well as actuation capabilities of a fully-integrated active mirror prototype are characterized experimentally using a Projected Hartmann test. A description of the test apparatus is presented along with two verification measurements. The apparatus is shown to accurately capture both high-amplitude low spatial-frequency figure errors as well as those at lower amplitudes but higher spatial frequencies. A closed-loop figure correction is performed, reducing figure errors by 94%.
", "doi": "10.7907/Z99W0CFB", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8749", "collection": "thesis", "collection_id": "8749", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12202014-233824767", "primary_object_url": { "basename": "Heyden_Stefanie_Thesis_2014.pdf", "content": "final", "filesize": 29532095, "license": "other", "mime_type": "application/pdf", "url": "/8749/1/Heyden_Stefanie_Thesis_2014.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Micromechanical Damage and Fracture in Elastomeric Polymers", "author": [ { "family_name": "Heyden", "given_name": "Stefanie", "clpid": "Heyden-Stefanie" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Weinberg", "given_name": "Kerstin", "clpid": "Weinberg-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis aims at a simple one-parameter macroscopic model of distributed damage and fracture of polymers that is amenable to a straightforward and efficient numerical implementation. The failure model is motivated by post-mortem fractographic observations of void nucleation, growth and coalescence in polyurea stretched to failure, and accounts for the specific fracture energy per unit area attendant to rupture of the material.
\r\n\r\nFurthermore, it is shown that the macroscopic model can be rigorously derived, in the sense of optimal scaling, from a micromechanical model of chain elasticity and failure regularized by means of fractional strain-gradient elasticity. Optimal scaling laws that supply a link between the single parameter of the macroscopic model, namely the critical energy-release rate of the material, and micromechanical parameters pertaining to the elasticity and strength of the polymer chains, and to the strain-gradient elasticity regularization, are derived. Based on optimal scaling laws, it is shown how the critical energy-release rate of specific materials can be determined from test data. In addition, the scope and fidelity of the model is demonstrated by means of an example of application, namely Taylor-impact experiments of polyurea rods. Hereby, optimal transportation meshfree approximation schemes using maximum-entropy interpolation functions are employed.
\r\n\r\nFinally, a different crazing model using full derivatives of the deformation gradient and a core cut-off is presented, along with a numerical non-local regularization model. The numerical model takes into account higher-order deformation gradients in a finite element framework. It is shown how the introduction of non-locality into the model stabilizes the effect of strain localization to small volumes in materials undergoing softening. From an investigation of craze formation in the limit of large deformations, convergence studies verifying scaling properties of both local- and non-local energy contributions are presented.
", "doi": "10.7907/Z9HX19NS", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8734", "collection": "thesis", "collection_id": "8734", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12042014-104112714", "primary_object_url": { "basename": "lim_kengwit_2015_thesis_ch1.pdf", "content": "final", "filesize": 2414247, "license": "other", "mime_type": "application/pdf", "url": "/8734/1/lim_kengwit_2015_thesis_ch1.pdf", "version": "v17.0.0" }, "type": "thesis", "title": "Discrete Modeling of Granular Media: A NURBS-based Approach", "author": [ { "family_name": "Lim", "given_name": "Keng-Wit", "clpid": "Lim-Keng-Wit" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This dissertation is concerned with the development of a new discrete element method (DEM) based on Non-Uniform Rational Basis Splines (NURBS). With NURBS, the new DEM is able to capture sphericity and angularity, the two particle morphological measures used in characterizing real grain geometries. By taking advantage of the parametric nature of NURBS, the Lipschitzian dividing rectangle (DIRECT) global optimization procedure is employed as a solution procedure to the closest-point projection problem, which enables the contact treatment of non-convex particles. A contact dynamics (CD) approach to the NURBS-based discrete method is also formulated. By combining particle shape flexibility, properties of implicit time-integration, and non-penetrating constraints, we target applications in which the classical DEM either performs poorly or simply fails, i.e., in granular systems composed of rigid or highly stiff angular particles and subjected to quasistatic or dynamic flow conditions. The CD implementation is made simple by adopting a variational framework, which enables the resulting discrete problem to be readily solved using off-the-shelf mathematical programming solvers. The capabilities of the NURBS-based DEM are demonstrated through 2D numerical examples that highlight the effects of particle morphology on the macroscopic response of granular assemblies under quasistatic and dynamic flow conditions, and a 3D characterization of material response in the shear band of a real triaxial specimen.", "doi": "10.7907/Z9W093V4", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8699", "collection": "thesis", "collection_id": "8699", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10162014-104834622", "primary_object_url": { "basename": "Thevamaran_Thesis.pdf", "content": "final", "filesize": 138368522, "license": "other", "mime_type": "application/pdf", "url": "/8699/76/Thevamaran_Thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Rate and Microstructure Effects on the Dynamics of Carbon Nanotube Foams", "author": [ { "family_name": "Thevamaran", "given_name": "Ramathasan", "orcid": "0000-0001-5058-6167", "clpid": "Thevamaran-Ramathasan" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Soft hierarchical materials often present unique functional properties that are sensitive to the geometry and organization of their micro- and nano-structural features across different lengthscales. Carbon Nanotube (CNT) foams are hierarchical materials with fibrous morphology that are known for their remarkable physical, chemical and electrical properties. Their complex microstructure has led them to exhibit intriguing mechanical responses at different length-scales and in different loading regimes. Even though these materials have been studied for mechanical behavior over the past few years, their response at high-rate finite deformations and the influence of their microstructure on bulk mechanical behavior and energy dissipative characteristics remain elusive.
\r\n\r\nIn this dissertation, we study the response of aligned CNT foams at the high strain-rate regime of 102 - 104 s-1. We investigate their bulk dynamic response and the fundamental deformation mechanisms at different lengthscales, and correlate them to the microstructural characteristics of the foams. We develop an experimental platform, with which to study the mechanics of CNT foams in high-rate deformations, that includes direct measurements of the strain and transmitted forces, and allows for a full field visualization of the sample\u2019s deformation through high-speed microscopy.
\r\n\r\nWe synthesize various CNT foams (e.g., vertically aligned CNT (VACNT) foams, helical CNT foams, micro-architectured VACNT foams and VACNT foams with microscale heterogeneities) and show that the bulk functional properties of these materials are highly tunable either by tailoring their microstructure during synthesis or by designing micro-architectures that exploit the principles of structural mechanics. We also develop numerical models to describe the bulk dynamic response using multiscale mass-spring models and identify the mechanical properties at length scales that are smaller than the sample height.
\r\n\r\nThe ability to control the geometry of microstructural features, and their local interactions, allows the creation of novel hierarchical materials with desired functional properties. The fundamental understanding provided by this work on the key structure-function relations that govern the bulk response of CNT foams can be extended to other fibrous, soft and hierarchical materials. The findings can be used to design materials with tailored properties for different engineering applications, like vibration damping, impact mitigation and packaging.
", "doi": "10.7907/Z9DB7ZRG ", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8199", "collection": "thesis", "collection_id": "8199", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04222014-195027916", "type": "thesis", "title": "Experimental Investigation of Thrust Fault Rupture Mechanics", "author": [ { "family_name": "Gabuchian", "given_name": "Vahe", "orcid": "0000-0001-7457-5921", "clpid": "Gabuchian-Vahe" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Bhat", "given_name": "Harsha S.", "clpid": "Bhat-H-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Thrust fault earthquakes are investigated in the laboratory by generating dynamic shear ruptures along pre-existing frictional faults in rectangular plates. A considerable body of evidence suggests that dip-slip earthquakes exhibit enhanced ground motions in the acute hanging wall wedge as an outcome of broken symmetry between hanging and foot wall plates with respect to the earth surface. To understand the physical behavior of thrust fault earthquakes, particularly ground motions near the earth surface, ruptures are nucleated in analog laboratory experiments and guided up-dip towards the simulated earth surface. The transient slip event and emitted radiation mimic a natural thrust earthquake. High-speed photography and laser velocimeters capture the rupture evolution, outputting a full-field view of photo-elastic fringe contours proportional to maximum shearing stresses as well as continuous ground motion velocity records at discrete points on the specimen. Earth surface-normal measurements validate selective enhancement of hanging wall ground motions for both sub-Rayleigh and super-shear rupture speeds. The earth surface breaks upon rupture tip arrival to the fault trace, generating prominent Rayleigh surface waves. A rupture wave is sensed in the hanging wall but is, however, absent from the foot wall plate: a direct consequence of proximity from fault to seismometer. Signatures in earth surface-normal records attenuate with distance from the fault trace. Super-shear earthquakes feature greater amplitudes of ground shaking profiles, as expected from the increased tectonic pressures required to induce super-shear transition. Paired stations measure fault parallel and fault normal ground motions at various depths, which yield slip and opening rates through direct subtraction of like components. Peak fault slip and opening rates associated with the rupture tip increase with proximity to the fault trace, a result of selective ground motion amplification in the hanging wall. Fault opening rates indicate that the hanging and foot walls detach near the earth surface, a phenomenon promoted by a decrease in magnitude of far-field tectonic loads. Subsequent shutting of the fault sends an opening pulse back down-dip. In case of a sub-Rayleigh earthquake, feedback from the reflected S wave re-ruptures the locked fault at super-shear speeds, providing another mechanism of super-shear transition.", "doi": "10.7907/Z9J96497", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8049", "collection": "thesis", "collection_id": "8049", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01072014-174255218", "primary_object_url": { "basename": "Scott_Roberts_thesis_2013_Complete_Thesis.pdf", "content": "final", "filesize": 28846404, "license": "other", "mime_type": "application/pdf", "url": "/8049/141/Scott_Roberts_thesis_2013_Complete_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Developing and Characterizing Bulk Metallic Glasses for Extreme Applications", "author": [ { "family_name": "Roberts", "given_name": "Scott Nolan", "clpid": "Roberts-Scott-Nolan" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Hofmann", "given_name": "Douglas C.", "clpid": "Hofmann-D-C" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Hofmann", "given_name": "Douglas C.", "clpid": "Hofmann-D-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Metallic glasses have typically been treated as a \u201cone size fits all\u201d type of material. Every alloy is considered to have high strength, high hardness, large elastic limits, corrosion resistance, etc. However, similar to traditional crystalline materials, properties are strongly dependent upon the constituent elements, how it was processed, and the conditions under which it will be used. An important distinction which can be made is between metallic glasses and their composites. Charpy impact toughness measurements are performed to determine the effect processing and microstructure have on bulk metallic glass matrix composites (BMGMCs). Samples are suction cast, machined from commercial plates, and semi-solidly forged (SSF). The SSF specimens have been found to have the highest impact toughness due to the coarsening of the dendrites, which occurs during the semi-solid processing stages. Ductile to brittle transition (DTBT) temperatures are measured for a BMGMC. While at room temperature the BMGMC is highly toughened compared to a fully glassy alloy, it undergoes a DTBT by 250 K. At this point, its impact toughness mirrors that of the constituent glassy matrix. In the following chapter, BMGMCs are shown to have the capability of being capacitively welded to form single, monolithic structures. Shear measurements are performed across welded samples, and, at sufficient weld energies, are found to retain the strength of the parent alloy. Cross-sections are inspected via SEM and no visible crystallization of the matrix occurs.
\r\n\r\nNext, metallic glasses and BMGMCs are formed into sheets and eggbox structures are tested in hypervelocity impacts. Metallic glasses are ideal candidates for protection against micrometeorite orbital debris due to their high hardness and relatively low density. A flat single layer, flat BMG is compared to a BMGMC eggbox and the latter creates a more diffuse projectile cloud after penetration. A three tiered eggbox structure is also tested by firing a 3.17 mm aluminum sphere at 2.7 km/s at it. The projectile penetrates the first two layers, but is successfully contained by the third.
\r\n\r\nA large series of metallic glass alloys are created and their wear loss is measured in a pin on disk test. Wear is found to vary dramatically among different metallic glasses, with some considerably outperforming the current state-of-the-art crystalline material (most notably Cu\u2084\u2083Zr\u2084\u2083Al\u2087Be\u2087). Others, on the other hand, suffered extensive wear loss. Commercially available Vitreloy 1 lost nearly three times as much mass in wear as alloy prepared in a laboratory setting. No conclusive correlations can be found between any set of mechanical properties (hardness, density, elastic, bulk, or shear modulus, Poisson\u2019s ratio, frictional force, and run in time) and wear loss. Heat treatments are performed on Vitreloy 1 and Cu\u2084\u2083Zr\u2084\u2083Al\u2087Be\u2087. Anneals near the glass transition temperature are found to increase hardness slightly, but decrease wear loss significantly. Crystallization of both alloys leads to dramatic increases in wear resistance. Finally, wear tests under vacuum are performed on the two alloys above. Vitreloy 1 experiences a dramatic decrease in wear loss, while Cu\u2084\u2083Zr\u2084\u2083Al\u2087Be\u2087 has a moderate increase. Meanwhile, gears are fabricated through three techniques: electrical discharge machining of 1 cm by 3 mm cylinders, semisolid forging, and copper mold suction casting. Initial testing finds the pin on disk test to be an accurate predictor of wear performance in gears.
\r\n\r\nThe final chapter explores an exciting technique in the field of additive manufacturing. Laser engineered net shaping (LENS) is a method whereby small amounts of metallic powders are melted by a laser such that shapes and designs can be built layer by layer into a final part. The technique is extended to mixing different powders during melting, so that compositional gradients can be created across a manufactured part. Two compositional gradients are fabricated and characterized. Ti 6Al\u00ac 4V to pure vanadium was chosen for its combination of high strength and light weight on one end, and high melting point on the other. It was inspected by cross-sectional x-ray diffraction, and only the anticipated phases were present. 304L stainless steel to Invar 36 was created in both pillar and as a radial gradient. It combines strength and weldability along with a zero coefficient of thermal expansion material. Only the austenite phase is found to be present via x-ray diffraction. Coefficient of thermal expansion is measured for four compositions, and it is found to be tunable depending on composition.
", "doi": "10.7907/FPNT-FT46", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8431", "collection": "thesis", "collection_id": "8431", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292014-191924394", "type": "thesis", "title": "High Strain Composites and Dual-Matrix Composite Structures", "author": [ { "family_name": "Maqueda Jim\u00e9nez", "given_name": "Ignacio", "clpid": "Maqueda-Jim\u00e9nez-Ignacio" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Murphey", "given_name": "Thomas", "clpid": "Murphey-T" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Most space applications require deployable structures due to the limiting size of current launch vehicles. Specifically, payloads in nanosatellites such as CubeSats require very high compaction ratios due to the very limited space available in this typo of platform. Strain-energy-storing deployable structures can be suitable for these applications, but the curvature to which these structures can be folded is limited to the elastic range. Thanks to fiber microbuckling, high-strain composite materials can be folded into much higher curvatures without showing significant damage, which makes them suitable for very high compaction deployable structure applications. However, in applications that require carrying loads in compression, fiber microbuckling also dominates the strength of the material. A good understanding of the strength in compression of high-strain composites is then needed to determine how suitable they are for this type of application.
\r\n\r\nThe goal of this thesis is to investigate, experimentally and numerically, the microbuckling in compression of high-strain composites. Particularly, the behavior in compression of unidirectional carbon fiber reinforced silicone rods (CFRS) is studied. Experimental testing of the compression failure of CFRS rods showed a higher strength in compression than the strength estimated by analytical models, which is unusual in standard polymer composites. This effect, first discovered in the present research, was attributed to the variation in random carbon fiber angles respect to the nominal direction. This is an important effect, as it implies that microbuckling strength might be increased by controlling the fiber angles. With a higher microbuckling strength, high-strain materials could carry loads in compression without reaching microbuckling and therefore be suitable for several space applications.
\r\n\r\nA finite element model was developed to predict the homogenized stiffness of the CFRS, and the homogenization results were used in another finite element model that simulated a homogenized rod under axial compression. A statistical representation of the fiber angles was implemented in the model. The presence of fiber angles increased the longitudinal shear stiffness of the material, resulting in a higher strength in compression. The simulations showed a large increase of the strength in compression for lower values of the standard deviation of the fiber angle, and a slight decrease of strength in compression for lower values of the mean fiber angle. The strength observed in the experiments was achieved with the minimum local angle standard deviation observed in the CFRS rods, whereas the shear stiffness measured in torsion tests was achieved with the overall fiber angle distribution observed in the CFRS rods.
\r\n\r\nHigh strain composites exhibit good bending capabilities, but they tend to be soft out-of-plane. To achieve a higher out-of-plane stiffness, the concept of dual-matrix composites is introduced. Dual-matrix composites are foldable composites which are soft in the crease regions and stiff elsewhere. Previous attempts to fabricate continuous dual-matrix fiber composite shells had limited performance due to excessive resin flow and matrix mixing. An alternative method, presented in this thesis uses UV-cure silicone and fiberglass to avoid these problems. Preliminary experiments on the effect of folding on the out-of-plane stiffness are presented. An application to a conical log-periodic antenna for CubeSats is proposed, using origami-inspired stowing schemes, that allow a conical dual-matrix composite shell to reach very high compaction ratios.
", "doi": "10.7907/Z34C-NY82", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8436", "collection": "thesis", "collection_id": "8436", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302014-012121120", "primary_object_url": { "basename": "Lyon- Thesis Final.pdf", "content": "final", "filesize": 5226346, "license": "other", "mime_type": "application/pdf", "url": "/8436/1/Lyon- Thesis Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A Multi-Scale Approach to Shaping Carbon Nanotube Structures for Hollow Microneedles", "author": [ { "family_name": "Lyon", "given_name": "Bradley Joseph", "clpid": "Lyon-Bradley-Joseph" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Tai", "given_name": "Yu-Chong", "orcid": "0000-0001-8529-106X", "clpid": "Tai-Yu-Chong" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The concept of a carbon nanotube microneedle array is explored in this thesis from multiple perspectives including microneedle fabrication, physical aspects of transdermal delivery, and in vivo transdermal drug delivery experiments. Starting with standard techniques in carbon nanotube (CNT) fabrication, including catalyst patterning and chemical vapor deposition, vertically-aligned carbon nanotubes are utilized as a scaffold to define the shape of the hollow microneedle. Passive, scalable techniques based on capillary action and unique photolithographic methods are utilized to produce a CNT-polymer composite microneedle. Specific examples of CNT-polyimide and CNT-epoxy microneedles are investigated. Further analysis of the transport properties of polymer resins reveals general requirements for applying arbitrary polymers to the fabrication process.
\r\n\r\nThe bottom-up fabrication approach embodied by vertically-aligned carbon nanotubes allows for more direct construction of complex high-aspect ratio features than standard top-down fabrication approaches, making microneedles an ideal application for CNTs. However, current vertically-aligned CNT fabrication techniques only allow for the production of extruded geometries with a constant cross-sectional area, such as cylinders. To rectify this limitation, isotropic oxygen etching is introduced as a novel fabrication technique to create true 3D CNT geometry. Oxygen etching is utilized to create a conical geometry from a cylindrical CNT structure as well as create complex shape transformations in other CNT geometries.
\r\n\r\nCNT-polymer composite microneedles are anchored onto a common polymer base less than 50 \u00b5m thick, which allows for the microneedles to be incorporated into multiple drug delivery platforms, including modified hypodermic syringes and silicone skin patches. Cylindrical microneedles are fabricated with 100 \u00b5m outer diameter and height of 200-250 \u00b5m with a central cavity, or lumen, diameter of 30 \u00b5m to facilitate liquid drug flow. In vitro delivery experiments in swine skin demonstrate the ability of the microneedles to successfully penetrate the skin and deliver aqueous solutions.
\r\n\r\nAn in vivo study was performed to assess the ability of the CNT-polymer microneedles to deliver drugs transdermally. CNT-polymer microneedles are attached to a hand actuated silicone skin patch that holds a liquid reservoir of drugs. Fentanyl, a potent analgesic, was administered to New Zealand White Rabbits through 3 routes of delivery: topical patch, CNT-polymer microneedles, and subcutaneous hypodermic injection. Results demonstrate that the CNT-polymer microneedles have a similar onset of action as the topical patch. CNT-polymer microneedles were also vetted as a painless delivery approach compared to hypodermic injection. Comparative analysis with contemporary microneedle designs demonstrates that the delivery achieved through CNT-polymer microneedles is akin to current hollow microneedle architectures. The inherent advantage of applying a bottom-up fabrication approach alongside similar delivery performance to contemporary microneedle designs demonstrates that the CNT-polymer composite microneedle is a viable architecture in the emerging field of painless transdermal delivery.
\r\n", "doi": "10.7907/BJGT-TB74", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:7993", "collection": "thesis", "collection_id": "7993", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10162013-221817628", "primary_object_url": { "basename": "Landry_Fokoua_Djodom_Thesis.pdf", "content": "final", "filesize": 1589012, "license": "other", "mime_type": "application/pdf", "url": "/7993/1/Landry_Fokoua_Djodom_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Optimal Scaling in Ductile Fracture", "author": [ { "family_name": "Fokoua Djodom", "given_name": "Landry", "clpid": "Fokoua-Djodom-Landry" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Weinberg", "given_name": "Kerstin", "clpid": "Weinberg-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This work is concerned with the derivation of optimal scaling laws, in the sense of matching lower and upper bounds on the energy, for a solid undergoing ductile fracture. The specific problem considered concerns a material sample in the form of an infinite slab of finite thickness subjected to prescribed opening displacements on its two surfaces. The solid is assumed to obey deformation-theory of plasticity and, in order to further simplify the analysis, we assume isotropic rigid-plastic deformations with zero plastic spin. When hardening exponents are given values consistent with observation, the energy is found to exhibit sublinear growth. We regularize the energy through the addition of nonlocal energy terms of the strain-gradient plasticity type. This nonlocal regularization has the effect of introducing an intrinsic length scale into the energy. We also put forth a physical argument that identifies the intrinsic length and suggests a linear growth of the nonlocal energy. Under these assumptions, ductile fracture emerges as the net result of two competing effects: whereas the sublinear growth of the local energy promotes localization of deformation to failure planes, the nonlocal regularization stabilizes this process, thus resulting in an orderly progression towards failure and a well-defined specific fracture energy. The optimal scaling laws derived here show that ductile fracture results from localization of deformations to void sheets, and that it requires a well-defined energy per unit fracture area. In particular, fractal modes of fracture are ruled out under the assumptions of the analysis. The optimal scaling laws additionally show that ductile fracture is cohesive in nature, i.e., it obeys a well-defined relation between tractions and opening displacements. Finally, the scaling laws supply a link between micromechanical properties and macroscopic fracture properties. In particular, they reveal the relative roles that surface energy and microplasticity play as contributors to the specific fracture energy of the material. Next, we present an experimental assessment of the optimal scaling laws. We show that when the specific fracture energy is renormalized in a manner suggested by the optimal scaling laws, the data falls within the bounds predicted by the analysis and, moreover, they ostensibly collapse---with allowances made for experimental scatter---on a master curve dependent on the hardening exponent, but otherwise material independent.", "doi": "10.7907/B1TW-2D81", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8630", "collection": "thesis", "collection_id": "8630", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08092014-195153430", "primary_object_url": { "basename": "Kristen John - Thesis - 2014.pdf", "content": "final", "filesize": 21801585, "license": "other", "mime_type": "application/pdf", "url": "/8630/1/Kristen John - Thesis - 2014.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Strength of Tantalum at High Pressures through Richtmyer-Meshkov Laser Compression Experiments and Simulations", "author": [ { "family_name": "John", "given_name": "Kristen Kathleen", "clpid": "John-Kristen-Kathleen" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Strength at extreme pressures (>1 Mbar or 100 GPa) and high strain rates (106-108 s-1) of materials is not well characterized. The goal of the research outlined in this thesis is to study the strength of tantalum (Ta) at these conditions. The Omega Laser in the Laboratory for Laser Energetics in Rochester, New York is used to create such extreme conditions. Targets are designed with ripples or waves on the surface, and these samples are subjected to high pressures using Omega\u2019s high energy laser beams. In these experiments, the observational parameter is the Richtmyer-Meshkov (RM) instability in the form of ripple growth on single-mode ripples. The experimental platform used for these experiments is the \u201cride-along\u201d laser compression recovery experiments, which provide a way to recover the specimens having been subjected to high pressures. Six different experiments are performed on the Omega laser using single-mode tantalum targets at different laser energies. The energy indicates the amount of laser energy that impinges the target. For each target, values for growth factor are obtained by comparing the profile of ripples before and after the experiment. With increasing energy, the growth factor increased.
\r\n \r\nEngineering simulations are used to interpret and correlate the measurements of growth factor to a measure of strength. In order to validate the engineering constitutive model for tantalum, a series of simulations are performed using the code Eureka, based on the Optimal Transportation Meshfree (OTM) method. Two different configurations are studied in the simulations: RM instabilities in single and multimode ripples. Six different simulations are performed for the single ripple configuration of the RM instability experiment, with drives corresponding to laser energies used in the experiments. Each successive simulation is performed at higher drive energy, and it is observed that with increasing energy, the growth factor increases. Overall, there is favorable agreement between the data from the simulations and the experiments. The peak growth factors from the simulations and the experiments are within 10% agreement. For the multimode simulations, the goal is to assist in the design of the laser driven experiments using the Omega laser. A series of three-mode and four-mode patterns are simulated at various energies and the resulting growth of the RM instability is computed. Based on the results of the simulations, a configuration is selected for the multimode experiments. These simulations also serve as validation for the constitutive model and the material parameters for tantalum that are used in the simulations.
\r\n\r\nBy designing samples with initial perturbations in the form of single-mode and multimode ripples and subjecting these samples to high pressures, the Richtmyer-Meshkov instability is investigated in both laser compression experiments and simulations. By correlating the growth of these ripples to measures of strength, a better understanding of the strength of tantalum at high pressures is achieved.
\r\n", "doi": "10.7907/NE7Y-CK04", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8043", "collection": "thesis", "collection_id": "8043", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12182013-094108778", "primary_object_url": { "basename": "patterson_keith_2013_thesis.pdf", "content": "final", "filesize": 21789439, "license": "other", "mime_type": "application/pdf", "url": "/8043/1/patterson_keith_2013_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Lightweight Deformable Mirrors for Future Space Telescopes", "author": [ { "family_name": "Patterson", "given_name": "Keith D.", "clpid": "Patterson-Keith-D" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents a concept for ultra-lightweight deformable mirrors based on a thin substrate of optical surface quality coated with continuous active piezopolymer layers that provide modes of actuation and shape correction. This concept eliminates any kind of stiff backing structure for the mirror surface and exploits micro-fabrication technologies to provide a tight integration of the active materials into the mirror structure, to avoid actuator print-through effects. Proof-of-concept, 10-cm-diameter mirrors with a low areal density of about 0.5 kg/m² have been designed, built and tested to measure their shape-correction performance and verify the models used for design. The low cost manufacturing scheme uses replication techniques, and strives for minimizing residual stresses that deviate the optical figure from the master mandrel. It does not require precision tolerancing, is lightweight, and is therefore potentially scalable to larger diameters for use in large, modular space telescopes. Other potential applications for such a laminate could include ground-based mirrors for solar energy collection, adaptive optics for atmospheric turbulence, laser communications, and other shape control applications.
\r\n\r\nThe immediate application for these mirrors is for the Autonomous Assembly and Reconfiguration of a Space Telescope (AAReST) mission, which is a university mission under development by Caltech, the University of Surrey, and JPL. The design concept, fabrication methodology, material behaviors and measurements, mirror modeling, mounting and control electronics design, shape control experiments, predictive performance analysis, and remaining challenges are presented herein. The experiments have validated numerical models of the mirror, and the mirror models have been used within a model of the telescope in order to predict the optical performance. A demonstration of this mirror concept, along with other new telescope technologies, is planned to take place during the AAReST mission.
\r\n\r\n", "doi": "10.7907/S7JS-A837", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8235", "collection": "thesis", "collection_id": "8235", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05142014-151151819", "primary_object_url": { "basename": "Nisha_Mohan_2014.pdf", "content": "final", "filesize": 4550555, "license": "other", "mime_type": "application/pdf", "url": "/8235/1/Nisha_Mohan_2014.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Extracting Material Response from Simple Mechanical Tests on Hardening-Softening-Hardening Viscoplastic Solids", "author": [ { "family_name": "Mohan", "given_name": "Nisha", "clpid": "Mohan-Nisha" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Compliant foams are usually characterized by a wide range of desirable mechanical properties. These properties include viscoelasticity at different temperatures, energy absorption, recoverability under cyclic loading, impact resistance, and thermal, electrical, acoustic and radiation-resistance. Some foams contain nano-sized features and are used in small-scale devices. This implies that the characteristic dimensions of foams span multiple length scales, rendering modeling their mechanical properties difficult. Continuum mechanics-based models capture some salient experimental features like the linear elastic regime, followed by non-linear plateau stress regime. However, they lack mesostructural physical details. This makes them incapable of accurately predicting local peaks in stress and strain distributions, which significantly affect the deformation paths. Atomistic methods are capable of capturing the physical origins of deformation at smaller scales, but suffer from impractical computational intensity. Capturing deformation at the so-called meso-scale, which is capable of describing the phenomenon at a continuum level, but with some physical insights, requires developing new theoretical approaches.
\r\n\r\nA fundamental question that motivates the modeling of foams is \u2018how to extract the intrinsic material response from simple mechanical test data, such as stress vs. strain response?\u2019 A 3D model was developed to simulate the mechanical response of foam-type materials. The novelty of this model includes unique features such as the hardening-softening-hardening material response, strain rate-dependence, and plastically compressible solids with plastic non-normality. Suggestive links from atomistic simulations of foams were borrowed to formulate a physically informed hardening material input function. Motivated by a model that qualitatively captured the response of foam-type vertically aligned carbon nanotube (VACNT) pillars under uniaxial compression [2011,\u201cAnalysis of Uniaxial Compression of Vertically Aligned Carbon Nanotubes,\u201d J. Mech.Phys. Solids, 59, pp. 2227\u20132237, Erratum 60, 1753\u20131756 (2012)], the property space exploration was advanced to three types of simple mechanical tests: 1) uniaxial compression, 2) uniaxial tension, and 3) nanoindentation with a conical and a flat-punch tip. The simulations attempt to explain some of the salient features in experimental data, like
\r\n1) The initial linear elastic response.
\r\n2) One or more nonlinear instabilities, yielding, and hardening.
The model-inherent relationships between the material properties and the overall stress-strain behavior were validated against the available experimental data. The material properties include the gradient in stiffness along the height, plastic and elastic compressibility, and hardening. Each of these tests was evaluated in terms of their efficiency in extracting material properties. The uniaxial simulation results proved to be a combination of structural and material influences. Out of all deformation paths, flat-punch indentation proved to be superior since it is the most sensitive in capturing the material properties.
", "doi": "10.7907/MMTW-FF91", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8051", "collection": "thesis", "collection_id": "8051", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01152014-115401299", "primary_object_url": { "basename": "Victoria_Richmond_2014.pdf", "content": "final", "filesize": 33064756, "license": "other", "mime_type": "application/pdf", "url": "/8051/1/Victoria_Richmond_2014.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Techniques for Strength Measurement at High Pressures and Strain-Rates using Transverse Waves", "author": [ { "family_name": "Richmond", "given_name": "Victoria Stolyar", "clpid": "Richmond-Victoria-Stolyar" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The study of the strength of a material is relevant to a variety of applications including automobile collisions, armor penetration and inertial confinement fusion. Although dynamic behavior of materials at high pressures and strain-rates has been studied extensively using plate impact experiments, the results provide measurements in one direction only. Material behavior that is dependent on strength is unaccounted for. The research in this study proposes two novel configurations to mitigate this problem.
\r\n\r\nThe first configuration introduced is the oblique wedge experiment, which is comprised of a driver material, an angled target of interest and a backing material used to measure in-situ velocities. Upon impact, a shock wave is generated in the driver material. As the shock encounters the angled target, it is reflected back into the driver and transmitted into the target. Due to the angle of obliquity of the incident wave, a transverse wave is generated that allows the target to be subjected to shear while being compressed by the initial longitudinal shock such that the material does not slip. Using numerical simulations, this study shows that a variety of oblique wedge configurations can be used to study the shear response of materials and this can be extended to strength measurement as well. Experiments were performed on an oblique wedge setup with a copper impactor, polymethylmethacrylate driver, aluminum 6061-t6 target, and a lithium fluoride window. Particle velocities were measured using laser interferometry and results agree well with the simulations.
\r\n\r\nThe second novel configuration is the y-cut quartz sandwich design, which uses the anisotropic properties of y-cut quartz to generate a shear wave that is transmitted into a thin sample. By using an anvil material to back the thin sample, particle velocities measured at the rear surface of the backing plate can be implemented to calculate the shear stress in the material and subsequently the strength. Numerical simulations were conducted to show that this configuration has the ability to measure the strength for a variety of materials.
\r\n", "doi": "10.7907/SH60-5659", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:7765", "collection": "thesis", "collection_id": "7765", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292013-162505920", "primary_object_url": { "basename": "Gdoutos-thesis-final-HQ.pdf", "content": "final", "filesize": 7658192, "license": "other", "mime_type": "application/pdf", "url": "/7765/1/Gdoutos-thesis-final-HQ.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Thin Metastructures with Engineered Thermal Expansion", "author": [ { "family_name": "Gdoutos", "given_name": "Eleftherios E.", "clpid": "Gdoutos-Eleftherios-E" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Shapiro", "given_name": "Andrew A.", "clpid": "Shapiro-A-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The geometry and constituent materials of metastructures can be used to engineer the thermal expansion coefficient. In this thesis, we design, fabricate, and test thin thermally stable metastructures consisting of bi-metallic unit cells and show how the coefficient of thermal expansion (CTE) of these metastructures can be finely and coarsely tuned by varying the CTE of the constituent materials and the unit cell geometry. Planar and three-dimensional finite element method modeling is used to drive the design and inform experiments, and predict the response of these metastructures. We demonstrate computationally the significance of out-of-plane effects in the metastructure response. We develop an experimental setup using digital image correlation and an infrared camera to experimentally measure full displacement and temperature fields during testing and accurately measure the metastructures\u2019 CTE. We experimentally demonstrate high aspect ratio metastructures of Ti/Al and Kovar/Al which exhibit near-zero and negative CTE, respectively. We demonstrate robust fabrication procedures for thermally stable samples with high aspect ratios in thin foil and thin film scales. We investigate the lattice structure and mechanical properties of thin films comprising a near-zero CTE metastructure. The mechanics developed in this work can be used to engineer metastructures of arbitrary CTE and can be extended to three dimensions.", "doi": "10.7907/M69R-3A76", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7488", "collection": "thesis", "collection_id": "7488", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02202013-185004693", "primary_object_url": { "basename": "thesis_alejandrolopezortega_021513.pdf", "content": "final", "filesize": 17502749, "license": "other", "mime_type": "application/pdf", "url": "/7488/1/thesis_alejandrolopezortega_021513.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Simulation of Richtmyer-Meshkov Flows for Elastic-Plastic Solids in Planar and Converging Geometries Using an Eulerian Framework", "author": [ { "family_name": "Lopez Ortega", "given_name": "Alejandro", "clpid": "Lopez-Ortega-Alejandro" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bruno", "given_name": "Oscar P.", "clpid": "Bruno-O-P" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents a numerical and analytical study of two problems of interest involving shock waves propagating through elastic-plastic media: the motion of converging (imploding) shocks and the Richtmyer-Meshkov (RM) instability. Since the stress conditions encountered in these cases normally produce large deformations in the materials, an Eulerian description, in which the spatial coordinates are fixed, is employed. This formulation enables a direct comparison of similarities and differences between the present study of phenomena driven by shock-loading in elastic-plastic solids, and in fluids, where they have been studied extensively. In the first application, Whitham's shock dynamics (WSD) theory is employed for obtaining an approximate description of the motion of an elastic-plastic material processed by a cylindrically/spherically converging shock. Comparison with numerical simulations of the full set of equations of motion reveal that WSD is an accurate tool for characterizing the evolution of converging shocks at all stages. The study of the Richtmyer-Meshkov flow (i.e., interaction between the interface separating two materials of different density and a shock wave incoming at an angle) in solids is performed by means of analytical models for purely elastic solids and numerical simulations when plasticity is included in the material model. To this effect, an updated version of a previously developed multi-material, level-set-based, Eulerian framework for solid mechanics is employed. The revised code includes the use of a multi-material HLLD Riemann problem for imposing material boundary conditions, and a new formulation of the equations of motion that makes use of the stretch tensor while avoiding the degeneracy of the stress tensor under rotation. Results reveal that the interface separating two elastic solids always behaves in a stable oscillatory or decaying oscillatory manner due to the existence of shear waves, which are able to transport the initial vorticity away from the interface. In the case of elastic-plastic materials, the interface behaves at first in an unstable manner similar to a fluid. Ejecta formation is appreciated under certain initial conditions while in other conditions, after an initial period of growth, the interface displays a quasi-stationary long-term behavior due to stress relaxation. The effect of secondary shock-interface interactions (re-shocks) in converging geometries is also studied. A turbulent mixing zone, similar to what is observed in gas--gas interfaces, is created, especially when materials with low strength driven by moderate to strong shocks are considered.", "doi": "10.7907/4WJ6-D795", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7747", "collection": "thesis", "collection_id": "7747", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232013-150116734", "primary_object_url": { "basename": "Hesham_Azizgolshani_Thesis.pdf", "content": "final", "filesize": 15952345, "license": "other", "mime_type": "application/pdf", "url": "/7747/1/Hesham_Azizgolshani_Thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Tissue Engineering Active Biological Machines: Bio-Inspired Design, Directed Self-Assembly, and Characterization of Muscular Pumps Simulating the Embryonic Heart", "author": [ { "family_name": "Azizgolshani", "given_name": "Hesham", "clpid": "Azizgolshani-Hesham" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Fraser", "given_name": "Scott E.", "orcid": "0000-0002-5377-0223", "clpid": "Fraser-S-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Biological machines are active devices that are comprised of cells and other biological components. These functional devices are best suited for physiological environments that support cellular function and survival. Biological machines have the potential to revolutionize the engineering of biomedical devices intended for implantation, where the human body can provide the required physiological environment. For engineering such cell-based machines, bio-inspired design can serve as a guiding platform as it provides functionally proven designs that are attainable by living cells. In the present work, a systematic approach was used to tissue engineer one such machine by exclusively using biological building blocks and by employing a bio-inspired design. Valveless impedance pumps were constructed based on the working principles of the embryonic vertebrate heart and by using cells and tissue derived from rats. The function of these tissue-engineered muscular pumps was characterized by exploring their spatiotemporal and flow behavior in order to better understand the capabilities and limitations of cells when used as the engines of biological machines.", "doi": "10.7907/2C8T-TB84", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7900", "collection": "thesis", "collection_id": "7900", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06172013-144232973", "primary_object_url": { "basename": "Rebecca_Rought_Thesis.pdf", "content": "final", "filesize": 11692721, "license": "other", "mime_type": "application/pdf", "url": "/7900/1/Rebecca_Rought_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Identification of Thermally-Tagged Coherent Structures in the Zero Pressure Gradient Turbulent Boundary Layer", "author": [ { "family_name": "Rought", "given_name": "Rebecca Lynn", "clpid": "Rought-Rebecca-Lynn" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A zero pressure gradient boundary layer over a flat plate is subjected to step changes in thermal condition at the wall, causing the formation of internal, heated layers. The resulting temperature fluctuations and their corresponding density variations are associated with turbulent coherent structures. Aero-optical distortion occurs when light passes through the boundary layer, encountering the changing index of refraction resulting from the density variations. Instantaneous measurements of streamwise velocity, temperature and the optical deflection angle experienced by a laser traversing the boundary layer are made using hot and cold wires and a Malley probe, respectively. Correlations of the deflection angle with the temperature and velocity records suggest that the dominant contribution to the deflection angle comes from thermally-tagged structures in the outer boundary layer with a convective velocity of approximately 0.8U\u221e. An examination of instantaneous temperature and velocity and their temporal gradients conditionally averaged around significant optical deflections shows behavior consistent with the passage of a heated vortex. Strong deflections are associated with strong negative temperature gradients, and strong positive velocity gradients where the sign of the streamwise velocity fluctuation changes. The power density spectrum of the optical deflections reveals associated structure size to be on the order of the boundary layer thickness. A comparison to the temperature and velocity spectra suggests that the responsible structures are smaller vortices in the outer boundary layer as opposed to larger scale motions. Notable differences between the power density spectra of the optical deflections and the temperature remain unresolved due to the low frequency response of the cold wire.", "doi": "10.7907/Z2V0-9V82", "publication_date": "2013", "thesis_type": "engd", "thesis_year": "2013" }, { "id": "thesis:7895", "collection": "thesis", "collection_id": "7895", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06122013-030022149", "primary_object_url": { "basename": "Leonard_Thesis_Library_June.pdf", "content": "final", "filesize": 12837288, "license": "other", "mime_type": "application/pdf", "url": "/7895/1/Leonard_Thesis_Library_June.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Controlling Wave Propagation through Nonlinear Engineered Granular Systems", "author": [ { "family_name": "Leonard", "given_name": "Andrea Beth", "clpid": "Leonard-Andrea-Beth" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "We study the fundamental dynamic behavior of a special class of ordered granular systems in order to design new, structured materials with unique physical properties. The dynamic properties of granular systems are dictated by the nonlinear, Hertzian, potential in compression and zero tensile strength resulting from the discrete material structure. Engineering the underlying particle arrangement of granular systems allows for unique dynamic properties, not observed in natural, disordered granular media. While extensive studies on 1D granular crystals have suggested their usefulness for a variety of engineering applications, considerably less attention has been given to higher-dimensional systems. The extension of these studies in higher dimensions could enable the discovery of richer physical phenomena not possible in 1D, such as spatial redirection and anisotropic energy trapping. We present experiments, numerical simulation (based on a discrete particle model), and in some cases theoretical predictions for several engineered granular systems, studying the effects of particle arrangement on the highly nonlinear transient wave propagation to develop means for controlling the wave propagation pathways. The first component of this thesis studies the stress wave propagation resulting from a localized impulsive loading for three different 2D particle lattice structures: square, centered square, and hexagonal granular crystals. By varying the lattice structure, we observe a wide range of properties for the propagating stress waves: quasi-1D solitary wave propagation, fully 2D wave propagation with tunable wave front shapes, and 2D pulsed wave propagation. Additionally the effects of weak disorder, inevitably present in real granular systems, are investigated. The second half of this thesis studies the solitary wave propagation through 2D and 3D ordered networks of granular chains, reducing the effective density compared to granular crystals by selectively placing wave guiding chains to control the acoustic wave transmission. The rapid wave front amplitude decay exhibited by these granular networks makes them highly attractive for impact mitigation applications. The agreement between experiments, numerical simulations, and applicable theoretical predictions validates the wave guiding capabilities of these engineered granular crystals and networks and opens a wide range of possibilities for the realization of increasingly complex granular material design.", "doi": "10.7907/NF5J-5W42", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7889", "collection": "thesis", "collection_id": "7889", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06112013-114509037", "primary_object_url": { "basename": "Prakhar_Thesis.pdf", "content": "final", "filesize": 7647370, "license": "other", "mime_type": "application/pdf", "url": "/7889/1/Prakhar_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Sustainable Energy Solutions for Irrigation and Harvesting in Developing Countries", "author": [ { "family_name": "Mehrotra", "given_name": "Prakhar", "clpid": "Mehrotra-Prakhar" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "clpid": "McKeon-B-J" }, { "family_name": "Hornung", "given_name": "Hans G.", "clpid": "Hornung-H-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "One of the critical problems currently being faced by agriculture industry in developing nations is the alarming rate of groundwater depletion. Irrigation accounts for over 70% of the total groundwater withdrawn everyday. Compounding this issue is the use of polluting diesel generators to pump groundwater for irrigation. This has made irrigation not only the biggest consumer of groundwater but also one of the major contributors to green house gases. The aim of this thesis is to present a solution to the energy-water nexus. To make agriculture less dependent on fossil fuels, the use of a solar-powered Stirling engine as the power generator for on-farm energy needs is discussed. The Stirling cycle is revisited and practical and ideal Stirling cycles are compared. Based on agricultural needs and financial constraints faced by farmers in developing countries, the use of a Fresnel lens as a solar-concentrator and a Beta-type Stirling engine unit is suggested for sustainable power generation on the farms. To reduce the groundwater consumption and to make irrigation more sustainable, the conceptual idea of using a Stirling engine in drip irrigation is presented. To tackle the shortage of over 37 million tonnes of cold-storage in India, the idea of cost-effective solar-powered on-farm cold storage unit is discussed.", "doi": "10.7907/KV5Y-4960", "publication_date": "2013", "thesis_type": "engd", "thesis_year": "2013" }, { "id": "thesis:7685", "collection": "thesis", "collection_id": "7685", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05082013-161911202", "primary_object_url": { "basename": "Szelengowicz_Ivan_Thesis_v6.pdf", "content": "final", "filesize": 16194524, "license": "other", "mime_type": "application/pdf", "url": "/7685/1/Szelengowicz_Ivan_Thesis_v6.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Analysis and Optimization of Stress Wave Propagation in Two-Dimensional Granular Crystals with Defects", "author": [ { "family_name": "Szelengowicz", "given_name": "Ivan Michel Nicolas", "clpid": "Szelengowicz-Ivan-Michel-Nicolas" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Bruno", "given_name": "Oscar P.", "clpid": "Bruno-O-P" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Granular crystals are compact periodic assemblies of elastic particles in Hertzian contact whose dynamic response can be tuned from strongly nonlinear to linear by the addition of a static precompression force. This unique feature allows for a wide range of studies that include the investigation of new fundamental nonlinear phenomena in discrete systems such as solitary waves, shock waves, discrete breathers and other defect modes. In the absence of precompression, a particularly interesting property of these systems is their ability to support the formation and propagation of spatially localized soliton-like waves with highly tunable properties. The wealth of parameters one can modify (particle size, geometry and material properties, periodicity of the crystal, presence of a static force, type of excitation, etc.) makes them ideal candidates for the design of new materials for practical applications. This thesis describes several ways to optimally control and tailor the propagation of stress waves in granular crystals through the use of heterogeneities (interstitial defect particles and material heterogeneities) in otherwise perfectly ordered systems. We focus on uncompressed two-dimensional granular crystals with interstitial spherical intruders and composite hexagonal packings and study their dynamic response using a combination of experimental, numerical and analytical techniques. We first investigate the interaction of defect particles with a solitary wave and utilize this fundamental knowledge in the optimal design of novel composite wave guides, shock or vibration absorbers obtained using gradient-based optimization methods.", "doi": "10.7907/TE86-1A15", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7682", "collection": "thesis", "collection_id": "7682", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05082013-152249157", "primary_object_url": { "basename": "Niema_Pahlevan_2013_Thesis.pdf", "content": "final", "filesize": 4219382, "license": "other", "mime_type": "application/pdf", "url": "/7682/1/Niema_Pahlevan_2013_Thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "A Systems Approach to Cardiovascular Health and Disease with a Focus on Aortic Wave Dynamics", "author": [ { "family_name": "Pahlevan", "given_name": "Niema Mohammed", "clpid": "Pahlevan-Niema-Mohammed" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Pierce", "given_name": "Niles A.", "orcid": "0000-0003-2367-4406", "clpid": "Pierce-N-A" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Hou", "given_name": "Thomas Y.", "orcid": "0000-0001-6287-1133", "clpid": "Hou-T-Y" }, { "family_name": "Hussain", "given_name": "Fazle", "orcid": "0000-0002-2209-9270", "clpid": "Hussain-F" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Cardiovascular diseases (CVDs) have reached an epidemic proportion in the US and worldwide with serious consequences in terms of human suffering and economic impact. More than one third of American adults are suffering from CVDs. The total direct and indirect costs of CVDs are more than $500 billion per year. Therefore, there is an urgent need to develop noninvasive diagnostics methods, to design minimally invasive assist devices, and to develop economical and easy-to-use monitoring systems for cardiovascular diseases. In order to achieve these goals, it is necessary to gain a better understanding of the subsystems that constitute the cardiovascular system. The aorta is one of these subsystems whose role in cardiovascular functioning has been underestimated. Traditionally, the aorta and its branches have been viewed as resistive conduits connected to an active pump (left ventricle of the heart). However, this perception fails to explain many observed physiological results. My goal in this thesis is to demonstrate the subtle but important role of the aorta as a system, with focus on the wave dynamics in the aorta.
\r\n\r\nThe operation of a healthy heart is based on an optimized balance between its pumping characteristics and the hemodynamics of the aorta and vascular branches. The delicate balance between the aorta and heart can be impaired due to aging, smoking, or disease. The heart generates pulsatile flow that produces pressure and flow waves as it enters into the compliant aorta. These aortic waves propagate and reflect from reflection sites (bifurcations and tapering). They can act constructively and assist the blood circulation. However, they may act destructively, promoting diseases or initiating sudden cardiac death. These waves also carry information about the diseases of the heart, vascular disease, and coupling of heart and aorta. In order to elucidate the role of the aorta as a dynamic system, the interplay between the dominant wave dynamic parameters is investigated in this study. These parameters are heart rate, aortic compliance (wave speed), and locations of reflection sites. Both computational and experimental approaches have been used in this research. In some cases, the results are further explained using theoretical models.
\r\n \r\nThe main findings of this study are as follows: (i) developing a physiologically realistic outflow boundary condition for blood flow modeling in a compliant vasculature; (ii) demonstrating that pulse pressure as a single index cannot predict the true level of pulsatile workload on the left ventricle; (iii) proving that there is an optimum heart rate in which the pulsatile workload of the heart is minimized and that the optimum heart rate shifts to a higher value as aortic rigidity increases; (iv) introducing a simple bio-inspired device for correction and optimization of aortic wave reflection that reduces the workload on the heart; (v) deriving a non-dimensional number that can predict the optimum wave dynamic state in a mammalian cardiovascular system; (vi) demonstrating that waves can create a pumping effect in the aorta; (vii) introducing a system parameter and a new medical index, Intrinsic Frequency, that can be used for noninvasive diagnosis of heart and vascular diseases; and (viii) proposing a new medical hypothesis for sudden cardiac death in young athletes.
\r\n", "doi": "10.7907/Z9DR2SFM", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7869", "collection": "thesis", "collection_id": "7869", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072013-143355354", "primary_object_url": { "basename": "Mihaly_Jonathan_2013_thesis.pdf", "content": "final", "filesize": 49496744, "license": "other", "mime_type": "application/pdf", "url": "/7869/1/Mihaly_Jonathan_2013_thesis.pdf", "version": "v9.0.0" }, "type": "thesis", "title": "Investigation of Hypervelocity Impact Phenomena Using Real-time Concurrent Diagnostics", "author": [ { "family_name": "Mihaly", "given_name": "Jonathan Michael", "clpid": "Mihaly-Jonathan-Michael" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Hypervelocity impact of meteoroids and orbital debris poses a serious and growing threat to spacecraft. To study hypervelocity impact phenomena, a comprehensive ensemble of real-time concurrently operated diagnostics has been developed and implemented in the Small Particle Hypervelocity Impact Range (SPHIR) facility. This suite of simultaneously operated instrumentation provides multiple complementary measurements that facilitate the characterization of many impact phenomena in a single experiment. The investigation of hypervelocity impact phenomena described in this work focuses on normal impacts of 1.8 mm nylon 6/6 cylinder projectiles and variable thickness aluminum targets. The SPHIR facility two-stage light-gas gun is capable of routinely launching 5.5 mg nylon impactors to speeds of 5 to 7 km/s. Refinement of legacy SPHIR operation procedures and the investigation of first-stage pressure have improved the velocity performance of the facility, resulting in an increase in average impact velocity of at least 0.57 km/s. Results for the perforation area indicate the considered range of target thicknesses represent multiple regimes describing the non-monotonic scaling of target perforation with decreasing target thickness. The laser side-lighting (LSL) system has been developed to provide ultra-high-speed shadowgraph images of the impact event. This novel optical technique is demonstrated to characterize the propagation velocity and two-dimensional optical density of impact-generated debris clouds. Additionally, a debris capture system is located behind the target during every experiment to provide complementary information regarding the trajectory distribution and penetration depth of individual debris particles. The utilization of a coherent, collimated illumination source in the LSL system facilitates the simultaneous measurement of impact phenomena with near-IR and UV-vis spectrograph systems. Comparison of LSL images to concurrent IR results indicates two distinctly different phenomena. A high-speed, pressure-dependent IR-emitting cloud is observed in experiments to expand at velocities much higher than the debris and ejecta phenomena observed using the LSL system. In double-plate target configurations, this phenomena is observed to interact with the rear-wall several micro-seconds before the subsequent arrival of the debris cloud. Additionally, dimensional analysis presented by Whitham for blast waves is shown to describe the pressure-dependent radial expansion of the observed IR-emitting phenomena. Although this work focuses on a single hypervelocity impact configuration, the diagnostic capabilities and techniques described can be used with a wide variety of impactors, materials, and geometries to investigate any number of engineering and scientific problems.", "doi": "10.7907/V3A7-7686", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7859", "collection": "thesis", "collection_id": "7859", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072013-023915252", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 14694385, "license": "other", "mime_type": "application/pdf", "url": "/7859/1/thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Interplay of Martensitic Phase Transformation and Plastic Slip in Polycrystals ", "author": [ { "family_name": "Richards", "given_name": "Andrew Walter", "clpid": "Richards-Andrew-Walter" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lebensohn", "given_name": "Ricardo A.", "clpid": "Lebensohn-R-A" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Inspired by key experimental and analytical results regarding Shape Memory Alloys (SMAs), we propose a modelling framework to explore the interplay between martensitic phase transformations and plastic slip in polycrystalline materials, with an eye towards computational efficiency. The resulting framework uses a convexified potential for the internal energy density to capture the stored energy associated with transformation at the meso-scale, and introduces kinetic potentials to govern the evolution of transformation and plastic slip. The framework is novel in the way it treats plasticity on par with transformation.
\r\n\r\nWe implement the framework in the setting of anti-plane shear, using a staggered implicit/explict update: we first use a Fast-Fourier Transform (FFT) solver based on an Augmented Lagrangian formulation to implicitly solve for the full-field displacements of a simulated polycrystal, then explicitly update the volume fraction of martensite and plastic slip using their respective stick-slip type kinetic laws. We observe that, even in this simple setting with an idealized material comprising four martensitic variants and four slip systems, the model recovers a rich variety of SMA type behaviors. We use this model to gain insight into the isothermal behavior of stress-stabilized martensite, looking at the effects of the relative plastic yield strength, the memory of deformation history under non-proportional loading, and several others.
\r\n\r\nWe extend the framework to the generalized 3-D setting, for which the convexified potential is a lower bound on the actual internal energy, and show that the fully implicit discrete time formulation of the framework is governed by a variational principle for mechanical equilibrium. We further propose an extension of the method to finite deformations via an exponential mapping. We implement the generalized framework using an existing Optimal Transport Mesh-free (OTM) solver. We then model the $\\alpha$--$\\gamma$ and $\\alpha$--$\\varepsilon$ transformations in pure iron, with an initial attempt in the latter to account for twinning in the parent phase. We demonstrate the scalability of the framework to large scale computing by simulating Taylor impact experiments, observing nearly linear (ideal) speed-up through 256 MPI tasks. Finally, we present preliminary results of a simulated Split-Hopkinson Pressure Bar (SHPB) experiment using the $\\alpha$--$\\varepsilon$ model.
\r\n", "doi": "10.7907/MM8X-BZ69", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7851", "collection": "thesis", "collection_id": "7851", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06062013-102758695", "primary_object_url": { "basename": "thesis_Ha.pdf", "content": "final", "filesize": 5156109, "license": "other", "mime_type": "application/pdf", "url": "/7851/1/thesis_Ha.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Coupled Effects of Mechanics, Geometry, and Chemistry on Bio-Membrane Behavior", "author": [ { "family_name": "Giang", "given_name": "Ha Thanh", "clpid": "Giang-Ha-Thanh" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Lipid bilayer membranes are models for cell membranes--the structure that helps regulate cell function. Cell membranes are heterogeneous, and the coupling between composition and shape gives rise to complex behaviors that are important to regulation. This thesis seeks to systematically build and analyze complete models to understand the behavior of multi-component membranes.
\r\n\r\nWe propose a model and use it to derive the equilibrium and stability conditions for a general class of closed multi-component biological membranes. Our analysis shows that the critical modes of these membranes have high frequencies, unlike single-component vesicles, and their stability depends on system size, unlike in systems undergoing spinodal decomposition in flat space. An important implication is that small perturbations may nucleate localized but very large deformations. We compare these results with experimental observations.
\r\n\r\nWe also study open membranes to gain insight into long tubular membranes that arise for example in nerve cells. We derive a complete system of equations for open membranes by using the principle of virtual work. Our linear stability analysis predicts that the tubular membranes tend to have coiling shapes if the tension is small, cylindrical shapes if the tension is moderate, and beading shapes if the tension is large. This is consistent with experimental observations reported in the literature in nerve fibers. Further, we provide numerical solutions to the fully nonlinear equilibrium equations in some problems, and show that the observed mode shapes are consistent with those suggested by linear stability. Our work also proves that beadings of nerve fibers can appear purely as a mechanical response of the membrane.
", "doi": "10.7907/BVSK-K782", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7817", "collection": "thesis", "collection_id": "7817", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012013-192515668", "primary_object_url": { "basename": "Aria_thesis_2013.pdf", "content": "final", "filesize": 25006301, "license": "other", "mime_type": "application/pdf", "url": "/7817/1/Aria_thesis_2013.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Control of Wettability of Carbon Nanotube Array by Reversible Dry Oxidation for Superhydrophobic Coating and Supercapacitor Applications", "author": [ { "family_name": "Aria", "given_name": "Adrianus Indrat", "clpid": "Aria-Adrianus-Indrat" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Heath", "given_name": "James R.", "orcid": "0000-0001-5356-4385", "clpid": "Heath-J-R" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "In this thesis, dry chemical modification methods involving UV/ozone, oxygen plasma, and vacuum annealing treatments are explored to precisely control the wettability of CNT arrays. By varying the exposure time of these treatments the surface concentration of oxygenated groups adsorbed on the CNT arrays can be controlled. CNT arrays with very low amount of oxygenated groups exhibit a superhydrophobic behavior. In addition to their extremely high static contact angle, they cannot be dispersed in DI water and their impedance in aqueous electrolytes is extremely high. These arrays have an extreme water repellency capability such that a water droplet will bounce off of their surface upon impact and a thin film of air is formed on their surface as they are immersed in a deep pool of water. In contrast, CNT arrays with very high surface concentration of oxygenated functional groups exhibit an extreme hydrophilic behavior. In addition to their extremely low static contact angle, they can be dispersed easily in DI water and their impedance in aqueous electrolytes is tremendously low. Since the bulk structure of the CNT arrays are preserved during the UV/ozone, oxygen plasma, and vacuum annealing treatments, all CNT arrays can be repeatedly switched between superhydrophilic and superhydrophobic, as long as their O/C ratio is kept below 18%.
\r\n\r\nThe effect of oxidation using UV/ozone and oxygen plasma treatments is highly reversible as long as the O/C ratio of the CNT arrays is kept below 18%. At O/C ratios higher than 18%, the effect of oxidation is no longer reversible. This irreversible oxidation is caused by irreversible changes to the CNT atomic structure during the oxidation process. During the oxidation process, CNT arrays undergo three different processes. For CNT arrays with O/C ratios lower than 40%, the oxidation process results in the functionalization of CNT outer walls by oxygenated groups. Although this functionalization process introduces defects, vacancies and micropores opening, the graphitic structure of the CNT is still largely intact. For CNT arrays with O/C ratios between 40% and 45%, the oxidation process results in the etching of CNT outer walls. This etching process introduces large scale defects and holes that can be obviously seen under TEM at high magnification. Most of these holes are found to be several layers deep and, in some cases, a large portion of the CNT side walls are cut open. For CNT arrays with O/C ratios higher than 45%, the oxidation process results in the exfoliation of the CNT walls and amorphization of the remaining CNT structure. This amorphization process can be implied from the disappearance of C-C sp2 peak in the XPS spectra associated with the pi-bond network.
\r\n\r\nThe impact behavior of water droplet impinging on superhydrophobic CNT arrays in a low viscosity regime is investigated for the first time. Here, the experimental data are presented in the form of several important impact behavior characteristics including critical Weber number, volume ratio, restitution coefficient, and maximum spreading diameter. As observed experimentally, three different impact regimes are identified while another impact regime is proposed. These regimes are partitioned by three critical Weber numbers, two of which are experimentally observed. The volume ratio between the primary and the secondary droplets is found to decrease with the increase of Weber number in all impact regimes other than the first one. In the first impact regime, this is found to be independent of Weber number since the droplet remains intact during and subsequent to the impingement. Experimental data show that the coefficient of restitution decreases with the increase of Weber number in all impact regimes. The rate of decrease of the coefficient of restitution in the high Weber number regime is found to be higher than that in the low and moderate Weber number. Experimental data also show that the maximum spreading factor increases with the increase of Weber number in all impact regimes. The rate of increase of the maximum spreading factor in the high Weber number regime is found to be higher than that in the low and moderate Weber number. Phenomenological approximations and interpretations of the experimental data, as well as brief comparisons to the previously proposed scaling laws, are shown here.
\r\n\r\nDry oxidation methods are used for the first time to characterize the influence of oxidation on the capacitive behavior of CNT array EDLCs. The capacitive behavior of CNT array EDLCs can be tailored by varying their oxygen content, represented by their O/C ratio. The specific capacitance of these CNT arrays increases with the increase of their oxygen content in both KOH and Et4NBF4/PC electrolytes. As a result, their gravimetric energy density increases with the increase of their oxygen content. However, their gravimetric power density decreases with the increase of their oxygen content. The optimally oxidized CNT arrays are able to withstand more than 35,000 charge/discharge cycles in Et4NBF4/PC at a current density of 5 A/g while only losing 10% of their original capacitance.
", "doi": "10.7907/B6MF-FX89", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7793", "collection": "thesis", "collection_id": "7793", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312013-133431536", "primary_object_url": { "basename": "Notbohm_Jacob_2013_thesis.pdf", "content": "final", "filesize": 9411328, "license": "other", "mime_type": "application/pdf", "url": "/7793/1/Notbohm_Jacob_2013_thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Dynamics of Cell\u2013Matrix Mechanical Interactions in Three Dimensions ", "author": [ { "family_name": "Notbohm", "given_name": "Jacob K.", "clpid": "Notbohm-Jacob-K" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The forces cells apply to their surroundings control biological processes such as growth, adhesion, development, and migration. In the past 20 years, a number of experimental techniques have been developed to measure such cell tractions. These approaches have primarily measured the tractions applied by cells to synthetic two-dimensional substrates, which do not mimic in vivo conditions for most cell types. Many cell types live in a fibrous three-dimensional (3D) matrix environment. While studying cell behavior in such 3D matrices will provide valuable insights for the mechanobiology and tissue engineering communities, no experimental approaches have yet measured cell tractions in a fibrous 3D matrix.
\r\n\r\nThis thesis describes the development and application of an experimental technique for quantifying cellular forces in a natural 3D matrix. Cells and their surrounding matrix are imaged in three dimensions with high speed confocal microscopy. The cell-induced matrix displacements are computed from the 3D image volumes using digital volume correlation. The strain tensor in the 3D matrix is computed by differentiating the displacements, and the stress tensor is computed by applying a constitutive law. Finally, tractions applied by the cells to the matrix are computed directly from the stress tensor.
\r\n\r\nThe 3D traction measurement approach is used to investigate how cells mechanically interact with the matrix in biologically relevant processes such as division and invasion. During division, a single mother cell undergoes a drastic morphological change to split into two daughter cells. In a 3D matrix, dividing cells apply tensile force to the matrix through thin, persistent extensions that in turn direct the orientation and location of the daughter cells. Cell invasion into a 3D matrix is the first step required for cell migration in three dimensions. During invasion, cells initially apply minimal tractions to the matrix as they extend thin protrusions into the matrix fiber network. The invading cells anchor themselves to the matrix using these protrusions, and subsequently pull on the matrix to propel themselves forward.
\r\n\r\nLastly, this thesis describes a constitutive model for the 3D fibrous matrix that uses a finite element (FE) approach. The FE model simulates the fibrous microstructure of the matrix and matches the cell-induced matrix displacements observed experimentally using digital volume correlation. The model is applied to predict how cells mechanically sense one another in a 3D matrix. It is found that cell-induced matrix displacements localize along linear paths. These linear paths propagate over a long range through the fibrous matrix, and provide a mechanism for cell-cell signaling and mechanosensing. The FE model developed here has the potential to reveal the effects of matrix density, inhomogeneity, and anisotropy in signaling cell behavior through mechanotransduction.
", "doi": "10.7907/AXD0-2D10", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7785", "collection": "thesis", "collection_id": "7785", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302013-233635296", "primary_object_url": { "basename": "bp_thesis_2013.pdf", "content": "final", "filesize": 15518061, "license": "other", "mime_type": "application/pdf", "url": "/7785/1/bp_thesis_2013.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Fracture of Materials Undergoing Solid-Solid Phase Transformation", "author": [ { "family_name": "Penmecha", "given_name": "Bharat Prasad", "clpid": "Penmecha-Bharat-Prasad" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A large number of technologically important materials undergo solid-solid phase transformations. Examples range from ferroelectrics (transducers and memory devices), zirconia (Thermal Barrier Coatings) to nickel superalloys and (lithium) iron phosphate (Li-ion batteries). These transformations involve a change in the crystal structure either through diffusion of species or local rearrangement of atoms. This change of crystal structure leads to a macroscopic change of shape or volume or both and results in internal stresses during the transformation. In certain situations this stress field gives rise to cracks (tin, iron phosphate etc.) which continue to propagate as the transformation front traverses the material. In other materials the transformation modifies the stress field around cracks and effects crack growth behavior (zirconia, ferroelectrics). These observations serve as our motivation to study cracks in solids undergoing phase transformations. Understanding these effects will help in improving the mechanical reliability of the devices employing these materials.
\r\n\r\nIn this thesis we present work on two problems concerning the interplay between cracks and phase transformations. First, we consider the directional growth of a set of parallel edge cracks due to a solid-solid transformation. We conclude from our analysis that phase transformations can lead to formation of parallel edge cracks when the transformation strain satisfies certain conditions and the resulting cracks grow all the way till their tips cross over the phase boundary. Moreover the cracks continue to grow as the phase boundary traverses into the interior of the body at a uniform spacing without any instabilities. There exists an optimal value for the spacing between the cracks. We ascertain these conclusion by performing numerical simulations using finite elements.
\r\n\r\nSecond, we model the effect of the semiconducting nature and dopants on cracks in ferroelectric perovskite materials, particularly barium titanate. Traditional approaches to model fracture in these materials have treated them as insulators. In reality, they are wide bandgap semiconductors with oxygen vacancies and trace impurities acting as dopants. We incorporate the space charge arising due the semiconducting effect and dopant ionization in a phase field model for the ferroelectric. We derive the governing equations by invoking the dissipation inequality over a ferroelectric domain containing a crack. This approach also yields the driving force acting on the crack. Our phase field simulations of polarization domain evolution around a crack show the accumulation of electronic charge on the crack surface making it more permeable than was previously believed so, as seen in recent experiments. We also discuss the effect the space charge has on domain formation and the crack driving force.
", "doi": "10.7907/FNTG-9T08", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7752", "collection": "thesis", "collection_id": "7752", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05252013-065528171", "primary_object_url": { "basename": "GRG_Full_Thesis.pdf", "content": "final", "filesize": 23139681, "license": "other", "mime_type": "application/pdf", "url": "/7752/49/GRG_Full_Thesis.pdf", "version": "v9.0.0" }, "type": "thesis", "title": "Study of the Origins of Toughness in Amorphous Metals", "author": [ { "family_name": "Garrett", "given_name": "Glenn Robert", "clpid": "Garrett-Glenn-Robert" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Goddard", "given_name": "William A., III", "clpid": "Goddard-W-A-III" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Demetriou", "given_name": "Marios D.", "clpid": "Demetriou-M-D" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Amorphous metals that form fully glassy parts over a few millimeters in thickness are still relatively new materials. Their glassy structure gives them particularly high strengths, high yield strains, high hardness values, high resilience, and low damping losses, but this can also result in an extremely low tolerance to the presence of flaws in the material. Since this glassy structure lacks the ordered crystal structure, it also lacks the crystalline defect (dislocations) that provides the micromechanism of toughening and flaw insensitivity in conventional metals. Without a sufficient and reliable toughness that results in a large tolerance of damage in the material, metallic glasses will struggle to be adopted commercially. Here, we identify the origin of toughness in metallic glass as the competition between the intrinsic toughening mechanism of shear banding ahead of a crack and crack propagation by the cavitation of the liquid inside the shear bands. We present a detailed study over the first three chapters mainly focusing on the process of shear banding; its crucial role in giving rise to one of the most damage-tolerant materials known, its extreme sensitivity to the configurational state of a glass with moderate toughness, and how the configurational state can be changed with the addition of minor elements. The last chapter is a novel investigation into the cavitation barrier in glass-forming liquids, the competing process to shear banding. The combination of our results represents an increased understanding of the major influences on the fracture toughness of metallic glasses and thus provides a path for the improvement and development of tougher metallic glasses. ", "doi": "10.7907/Z9JD4TRM", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7164", "collection": "thesis", "collection_id": "7164", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06122012-184825377", "type": "thesis", "title": "Mechanics of Viscoelastic Thin-Walled Structures", "author": [ { "family_name": "Kwok", "given_name": "Kawai", "orcid": "0000-0001-6876-4267", "clpid": "Kwok-Kawai" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Thin-walled structures made of polymers and reinforced polymer composites are prominent candidates for constructing large lightweight structures. A major challenge in designing polymer-based thin-walled structures is their time and temperature dependent behavior originating from material viscoelasticity and its interaction with the highly geometrically nonlinear response due to thinness of the walls. Although polymer viscoelasticity and geometric nonlinearity have been extensively studied, the mechanics of structures exhibiting both phenomena are not well understood.
\r\n\r\nThis thesis presents a combination of experimental, numerical, and analytical investigations of the behavior of viscoelastic thin-walled structures. The first goal of this research is to establish general methods of analysis for two types of structural components, namely composite shells and polymer membranes, that will serve as the basis for full-scale structural analysis. The second goal is to demonstrate the capability of the developed methods by analyzing time and temperature dependent behavior of deployable structures and balloon structures.
\r\n\r\nIn the study of deployable structures, the deployment and shape recovery processes after stowage are investigated. Fundamental features of viscoelastic deployable structures are studied first with homogeneous polymer beams and shells. A simple closed-form solution describing the shape evolution of a beam after stowage is proposed. The effects of rate and temperature on the bending instability of shells are revealed. Building on the understanding gained from the analysis of homogeneous structures, modeling techniques are developed for polymer composite structures. A micromechanical viscoelastic model for carbon fiber reinforced polymer thin shells is established through finite element homogenization and applied to evaluate the effects of long-term stowage in a representative composite deployable structure.
\r\n\r\nIn the study of balloon structures, a membrane model is developed to study polymer balloon films with stress concentrations due to thickness variation. A nonlinear viscoelastic constitutive model is first formulated for the film material. The wrinkling instability behavior is incorporated into the model through correction of stress and strain states in the presence of wrinkling. Stress concentration factors in balloon films are predicted and measured with the membrane model and full-field displacement measurement techniques, respectively.
", "doi": "10.7907/RSSF-1C35", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7203", "collection": "thesis", "collection_id": "7203", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09132012-125328533", "type": "thesis", "title": "Modeling, Simulation, and Design of Self-Assembling Space Systems: Accurate Collision Detection, Robust Time Integration, and Optimal Control", "author": [ { "family_name": "Johnson", "given_name": "Gwendolyn Brook", "clpid": "Johnson-G-B" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Leyendecker", "given_name": "Sigrid", "clpid": "Leyendecker-S" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "Keck Institute for Space Studies" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Motivated by issues inherent in modeling and designing self-assembling systems (e.g. multiple collisions, collisions between non-smooth bodies, clumping and jamming behaviors, etc.), the goal of this thesis is to develop robust numerical tools that enable ecient and accurate direct simulation of self assembling systems and the application of optimal control methods to this type of system. The systems will be alternately modeled using linear nite elements, rigid bodies, or chains of rigid bodies. To this end, this work begins with development of a linear programming based collision detection algorithm for general convex polyhedral bodies. The resulting linear program has several features which render it extremely useful in determining the force system at the time of contact in numerical collision integrators. With robust collision detection in hand, three related numerical integration methods for dynamics with collisions are treated; a direct potential-based approach, and exact collision integrator in a discrete variational setting, and a decomposition-based algorithm, again in the discrete variational setting. Finally, several control problems are treated in the Discrete Mechanics and Optimal Control{Constrained (DMOCC) framework in which collisions between non-smooth bodies either need to be avoided or explicitly included in the optimal control problem. A globally stable feedback controller and a family of trajectories for spacecraft docking are also developed and tested with an accurate representation of an optimized CubeSat docking system.", "doi": "10.7907/73S0-Y593", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7009", "collection": "thesis", "collection_id": "7009", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05072012-134317580", "primary_object_url": { "basename": "thesis_officially_submitted_w_emfont_120508.pdf", "content": "final", "filesize": 8536549, "license": "other", "mime_type": "application/pdf", "url": "/7009/7/thesis_officially_submitted_w_emfont_120508.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Hierarchical Structures of Aligned Carbon Nanotubes as Low-Density Energy-Dissipative Materials", "author": [ { "family_name": "Raney", "given_name": "Jordan Robert", "clpid": "Raney-Jordan-Robert" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Carbon nanotubes (CNTs) are known to have remarkable properties, such as a specific strength two orders of magnitude higher than that of steel. It has remained a challenge, however, to achieve useful bulk properties from CNTs. Toward that goal, here we develop low-density bulk materials (0.1-0.4 g cm-3) entirely or nearly entirely from CNTs. These consist of nominally-aligned arrays of CNTs that display a dissipative compressive response, with a notable stress-strain hysteresis. The compressive properties of CNT arrays are examined in detail. This analysis reveals interesting features in the mechanical response, such as strain localization (resulting from a gradient in physical properties along the height), recovery after compression, non-linear viscoelasticity, and behavior under repeated compression that depends on the strain of previous cycles (similar to the Mullins effect in rubbers). We observe that in compression the energy dissipation of these materials is more than 200 times that of polymeric foams of comparable density.
\r\n\r\nNext, materials based on CNT arrays are studied as exemplary of hierarchical materials (materials with distinct structure at multiple length scales). Hierarchical materials have pushed the limits of traditional material tradeoffs (e.g., the typical trend that increased strength requires increased weight). Techniques are developed to separately vary the structure of CNT arrays at nanometer, micrometer, and millimeter length scales, and the effects on the bulk material response are examined. Structure can be modified during CNT synthesis, such as by varying the composition of the flow gas or by manipulating the input rate of chemical precursors; it can also be modified post-synthesis, e.g., by the in situ synthesis of nanoparticles in the interstices of the CNT arrays or by the assembly of multilayer structures of multiple CNT arrays connected by polymeric or metallic interlayers.
\r\n\r\nFinally, a mathematical model is applied to capture the complexities of the mechanical response. This one-dimensional, multiscale, bistable spring model is able to match the global stress-strain response as well as local effects such as strain localization and Mullins-like behavior. A technique is developed to reliably discern the model's material parameters based on in situ optical data from the experiments.
", "doi": "10.7907/25R0-JT92", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:7022", "collection": "thesis", "collection_id": "7022", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05102012-091402754", "primary_object_url": { "basename": "ThesisMain_2011_10_9.pdf", "content": "final", "filesize": 50866200, "license": "other", "mime_type": "application/pdf", "url": "/7022/1/ThesisMain_2011_10_9.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Non-Destructive Evaluation of Material System Using Highly Nonlinear Acoustic Waves", "author": [ { "family_name": "Khatri", "given_name": "Devvrath", "clpid": "Khatri-Devvrath" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Yang", "given_name": "Jinkyu", "clpid": "Yang-Jinkyu" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A chain of granular particles is one of the most studied examples of highly nonlinear systems deriving its response from the nonlinear Hertzian contact interaction between particles. Interest in these systems derives from their tunable dynamic response, encompassing linear, weakly nonlinear, and strongly nonlinear regimes, controlled by varying the static and dynamic load applied. In chains with a very weak (or zero) static precompression, the system supports the formation and propagation of highly nonlinear solitary waves (HNSWs). The dual-nonlinear interaction between particles (i.e., a power-law type contact potential in compression, and zero strength in tension) combined with discreteness of the system, makes the granular system highly tunable. The propagation properties of these waves, such as traveling pulse width, wave speed, number of separated pulses (single or train of pulses), etc., can be controlled by modifying one or many of the parameters, like the particle's dimension, material properties, static and dynamic force amplitude, the type and duration of the initial excitation applied to the system, and/or the periodicity of the chain. The ability to control the wave properties in such chains has been proposed for several different practical engineering applications.
\r\n\r\nThe dynamic properties of these granular chains have been conventionally studied using discrete particle models (DPMs) which consider the particles in the chains as point masses connected by nonlinear Hertzian springs with the neighboring particles. Although, this is a good approximation under proper circumstances, it does not capture many features of the three dimensional elastic particles such as the elastic wave propagation within the particles, the local deformation of the particles in the vicinity of the contact point, the corresponding changes in the contact area, and the collective vibrations of the particles among others. This thesis focuses on the development of a nite element model (FEM)using the commercially available software Abaqus, which takes into account many of these characteristic features. The nite element model discretizes particles by considering them as three-dimensional deformable bodies of revolution and describes the nonlinear dynamic response of one-dimensional granular chains composed of particles with various geometries and orientations. We showed that particles' geometries and orientations provide additional design parameters for controlling the dynamic response of the system, compared to chains composed of spherical particles. We also showed that the tunable and compact nature of these waves can be used to tailor the properties of HNSWs for specfic application, such as information carriers for actuation and sensing of mechanical properties and boundary effects of adjoining media in Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM). Using experiments and numerics, we characterized interface dynamics between granular media and adjoining linear elastic media, and found that the coupling produced temporary localization of the incident waves at the boundaries between the two media and their decomposition into reflected waves. We monitored the formation of reflected solitary waves propagating back from the interface and found that their properties are sensitive to the geometric and material properties of the adjoining media. The work done in this research enhances our understanding of the basic physics and tunability of nonlinear granular media, and further establishes a theoretical and numerical foundation\r\nin the applications of HNSWs as information carriers.
", "doi": "10.7907/P3VR-Q582", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:7141", "collection": "thesis", "collection_id": "7141", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06062012-202646378", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 50336648, "license": "other", "mime_type": "application/pdf", "url": "/7141/1/Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Clefted Equilibrium Shapes of Superpressure Balloon Structures", "author": [ { "family_name": "Deng", "given_name": "Xiaowei", "clpid": "Deng-Xiaowei" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents a numerical and analytical study of the clefted equilibrium shape of superpressure balloon structures. Lobed superpressure balloons have shown a tendency to deploy into unexpected asymmetric shapes, hence their design has to strike a balance between the lower stresses achieved by increasing lobing and the risk of incomplete deployment. Extensive clefting is a regular feature of balloons that are incompletely inflated, and is regularly seen during launch and ascent. Our particular interest in the research is in clefts that remain once a balloon has reached its float altitude and is fully pressurized.
\r\n\r\nA simplified simulation technique for orthotropic viscoelastic membranes is presented in the thesis. Wrinkling is detected by a combined stress-strain criterion and an iterative scheme searches for the wrinkle angle using a pseudoelastic material stiffness matrix based on a nonlinear viscoelastic constitutive model. This simplified model has been implemented in ABAQUS/Explicit and is able to compute the behavior of a membrane structure by superposition of a small number of response increments. The model has been tested against a published solution for a time-independent isotropic membrane under simple shear and also against experimental results on StratoFilm 420 under simple shear.
\r\n\r\n\r\nA fully three-dimensional finite element model of balloon structures incorporating wrinkling and frictionless contact, able to simulate the shapes taken up by lobed superpressure balloons during the final stages of their ascent has been established. Two different methods have been considered to predict the clefts: (i) deflation and\r\ninflation method and (ii) constraint shift method. In method (i), the starting configuration is obtained by deflating an initially symmetric balloon subject to uniform pressure. The deflation simulation is continued until the differential pressure at the bottom of the balloon has become negative, at which point the balloon is extensively clefted. The balloon is then inflated by increasing the bottom pressure while maintaining a uniform vertical ressure gradient, and the evolution of the shape and stress distribution of the balloon is studied. Two different designs of uperpressure balloons are investigated: a flat facet balloon and a ighly lobed balloon. It is found that the flat facet balloon follows essentially the same path during deflation and inflation, and hence will deploy into a unique, symmetric shape. For the lobed balloon it is found that it follows different paths during deflation and inflation, and deploys into an alternate, clefted equilibrium shape.
\r\n\r\nCompared to method (i), method (ii) is computationally a more efficient clefting test. The test consists in setting up the balloon in its symmetrically inflated configuration, then breaking the symmetry of this shape by artificially introducing a clefting imperfection, and finally determining the equilibrium shape of the balloon. The clefting imperfection is computed by shifting the constraint at the bottom of the balloon and removing the pressure in the bottom region, below the shifted constraint. The clefting test is applied successfully to three 27~m diameter superpressure balloons that have been tested indoors by NASA, of which one had remained clefted when it was inflated and the other two had deployed completely.
\r\n\r\nIn addition to numerical simulations, formulation of a new cleft factor, employed as an indicator of tendency to S-cleft for superpressure balloons based on constant-stress design has been established through dimensional analysis. The cleft factor, defined as the ratio of clefted volume to cyclically symmetrical volume, is expressed in the form of power law relation of the dimensionless groups. An example illustrates how to calculate the coefficients of the analytical formula and analyze sensitivity of design parameters to clefting.
", "doi": "10.7907/YYTP-2005", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:7144", "collection": "thesis", "collection_id": "7144", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072012-032023169", "primary_object_url": { "basename": "Mello_Michael_2012_thesis.pdf", "content": "final", "filesize": 420337387, "license": "other", "mime_type": "application/pdf", "url": "/7144/1/Mello_Michael_2012_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Identifying the Unique Ground Motion Signatures of Supershear Earthquakes: Theory and Experiments", "author": [ { "family_name": "Mello", "given_name": "Michael", "orcid": "0000-0003-2129-9235", "clpid": "Mello-Michael" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The near-field ground motion signatures associated with sub-Rayleigh and supershear ruptures are investigated using the laboratory earthquake experiment originally developed by Rosakis and co-workers (Xia et al., 2004, 2005a; Lu et al., 2007; Rosakis et al., 2007). Heterodyne laser interferometers enable continuous, high-bandwidth measurements of fault-normal (FN), fault-parallel (FP), and vertical (V) particle velocity ``ground motion\" records at discrete locations on the surface of a Homalite-100 test specimen as a sub-Rayleigh or a supershear rupture sweeps along the frictional fault. Photoelastic interference fringes, acquired using high-speed digital photography, provide a synchronized, spatially resolved, whole field view of the advancing rupture tip and surrounding maximum shear stress field.
\r\n\r\nThe first phase of experimental investigations examine and verify the ground motion signatures of supershear ruptures. Experimental results demonstrate that a shear Mach front produced by a stable supershear rupture is characterized by a dominant FP velocity component. The situation is shown to reverse in the sub-Rayleigh rupture speed regime whereby the FN particle velocity component dominates the ground motion record. Additional distinguishing particle velocity signatures, consistent with theoretical and numerical predictions, and repeatedly observed in experimental records are, (1) a pronounced peak in the FP velocity record induced by the leading dilatational field, which sweeps the measurement station in advance of the shear Mach front, and (2) a pronounced velocity swing in the FN record associated with the arrival of a trailing Rayleigh sub-Rayleigh (secondary) rupture, which follows the arrival of the shear Mach front. Analysis of the particle velocity records also confirms 2D steady-state theoretical predictions pertaining to the separation, attenuation, and radiation partitioning of the shear and dilatational portions of the rupture velocity field components.
\r\n\r\nThe second phase of our experimental investigations re-examine the 2002, Mw7.9, Denali fault earthquake and the remarkable set of near-source ground motion records obtained at (PS10), located approximately 85 km east of the epicenter and just 3 km north of the fault along the Alaska pipeline. Motivated by the analysis and interpretation of these records by (Ellsworth et al., 2004; Dunham and Archuleta, 2004, 2005), we attempt to mimic the Denali strike-slip rupture scenario and replicate the PS10 ground motion signatures using a laboratory earthquake experiment. The experiments feature a left-to-right (west-to-east) propagating right lateral rupture within a Homalite-100 test specimen with particle velocity data collected at a near-field station situated just above (north of) the fault. Both sub-Rayleigh and supershear laboratory earthquake experiments are conducted using the Denali PS10 configuration in order to compare and contrast the resulting particle velocity signatures. Supershear laboratory records capture all of the prominent features displayed within the PS10 ground motion records. Noted velocity signatures are correlated to the location of the rupture fronts and their noted arrival times in the synchronized photoelastic image sequence. Scaling relationships are also presented which transform the laboratory records through six orders of magnitude in time, to match the scale of the PS10 ground motion records. The strong correlation between the scaled experimental records and the actual PS10 ground motion records support the hypothesis that the Denali strike-slip fault exhibited a supershear burst.
\r\n\r\nFinally, we present a 2D steady state, stress-velocity formulation that relates the FP and FN particle velocity records measured close to the fault, to the evolution of the stress tensor at the same location. A locally steady-state condition is assumed within a restricted time interval in order to invoke these relationships and estimate the dynamic stresses, \u03c3xx(t) and \u03c4(t), at the near-fault station. Dynamic stress measurements enable a new class of friction investigations using the laboratory earthquake configuration. Experimental findings are presented, which capture the temporal and spatial distributions of \u03c3xx and \u03c4, evolution of the dynamic friction coefficient, and velocity weakening behavior of a supershear slip-pulse.
", "doi": "10.7907/DJDD-2487", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:6396", "collection": "thesis", "collection_id": "6396", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05162011-131929134", "primary_object_url": { "basename": "Boechler_Nicholas_Thesis_16May2011.pdf", "content": "final", "filesize": 3272506, "license": "other", "mime_type": "application/pdf", "url": "/6396/1/Boechler_Nicholas_Thesis_16May2011.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Granular Crystals: Controlling Mechanical Energy with Nonlinearity and Discreteness", "author": [ { "family_name": "Boechler", "given_name": "Nicholas Sebastian", "orcid": "0000-0001-9639-1533", "clpid": "Boechler-Nicholas-Sebastian" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Cross", "given_name": "Michael Clifford", "clpid": "Cross-M-C" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Painter", "given_name": "Oskar J.", "clpid": "Painter-O" }, { "family_name": "Davis", "given_name": "Gregory L.", "clpid": "Davis-G-L" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The presence of structural discreteness and periodicity can affect the propagation of phonons, sound, and other mechanical waves. A fundamental property of many of the periodic structures and materials designed for this purpose is the presence of complete band gaps in their dispersion relation. Waves with frequencies in the band gap cannot propagate and are reflected by the material. Like the concept of a band gap, the functionality of these periodic structures has historically been based on concepts from linear dynamics. Nonlinear systems can offer increased flexibility over linear systems including new ways to localize energy, convert energy between frequencies, and tune the response of the system. Granular crystals are arrays of elastic particles that interact nonlinearly via Hertzian contact, and are a type of nonlinear periodic structure whose response to dynamic excitations can be tuned to encompass linear, weakly nonlinear, and strongly nonlinear regimes. Drawing on ideas from condensed matter physics and nonlinear science, this thesis focuses on how the nonlinearity and structural discreteness of granular crystals can be used to control mechanical energy. The dynamic response of one-dimensional granular crystals composed of compressed elastic spheres (or cylinders) is studied using a combination of experimental, numerical, and analytical techniques. The discovery of fundamental physical phenomena occurring in the linear and weakly nonlinear regimes is described, along with how such phenomena can be used to create new ways to control the propagation of mechanical wave energy. The specific mechanisms investigated include tunable frequency band gaps, discrete breathers, nonlinear localized defect modes, and bifurcations. These mechanisms are utilized to create novel devices for tunable vibration filtering, energy harvesting and conversion, and tunable acoustic rectification.", "doi": "10.7907/BAHK-WD44", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6271", "collection": "thesis", "collection_id": "6271", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03152011-154253229", "primary_object_url": { "basename": "ThesisMain.pdf", "content": "final", "filesize": 6546066, "license": "other", "mime_type": "application/pdf", "url": "/6271/1/ThesisMain.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Mechanics of Thin Carbon Fiber Composites with a Silicone Matrix", "author": [ { "family_name": "Lopez Jimenez", "given_name": "Francisco", "clpid": "Lopez-Jimenez-Francisco" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents an experimental, numerical and analytical study of the behavior of thin fiber composites with a silicone matrix. The main difference with respect to traditional composites with epoxy matrix is the fact that the soft matrix allows the fibers to microbuckle without breaking. This process acts as a stress relief mechanism during folding, and allows the material to reach very high curvatures, which makes them particularly interesting as components of space deployable structures. The goal of this study is to characterize the behavior and understand the mechanics of this type of composite.
\r\n\r\nExperimental testing of the bending behavior of unidirectional composites with a silicone matrix shows a highly non-linear moment vs. curvature relationship, as well as strain softening under cyclic loading. These effects are not usually observed in composites with an epoxy matrix. In the case of tension in the direction transverse to the fibers, the behavior shows again non-linearity and strain softening, as well as an initial stiffness much higher than what would be expected based on the traditional estimates for fiber composites.
\r\n\r\nThe micro mechanics of the material have been studied with a finite element model. It uses solid elements and a random fiber arrangement produced with a reconstruction process based on micrographs of the material cross section. The simulations capture the macroscopic non-linear response, as well as the fiber microbuckling, and show how microbuckling reduces the strain in the fibers. The model shows good agreement for the bending stiffness of specimens with low fiber volume fraction, but it overestimates the effect of the matrix for more densely packed fibers. This is due to the high matrix strain that derives from the assumption of perfect bonding between fiber and matrix. In the case of tension transverse to the fibers, the model shows a much better agreement with experiments than traditional composite theory, and shows that the reason for the observed high stiffness is the incompressibility of the matrix. In order to capture the strain softening due to fiber debonding, cohesive elements have been introduced between the fibers and the matrix. This allows the model to capture quantitatively the non-linear behavior in the case of loading transverse to the fibers, and the damage due to cyclic loading. A single set of parameters for the cohesive elements produce good agreement with the experimental results for very different values of the fiber volume fraction, and could also be used in the analysis of more complicated loading cases, such as bending or biaxial tension.
\r\n\r\nIn addition to the simulations, a homogenized analytical model has also been created. It extends previous analysis of composites with a soft matrix to the case of very thin composites. It provides a good qualitative description of the material behavior, and it helps understand the mechanics that take place within the material, such as the equilibrium of energy terms leading to a finite wave length, as opposed to microbuckling under compression.
", "doi": "10.7907/A773-KF92", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6389", "collection": "thesis", "collection_id": "6389", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05122011-154526450", "type": "thesis", "title": "Damage Evolution in Composite Materials and Sandwich Structures Under Impulse Loading", "author": [ { "family_name": "Silva", "given_name": "Michael Lee", "clpid": "Silva-Michael-Lee" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Shukla", "given_name": "Arun", "clpid": "Shukla-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Damage evolution in composite materials is a rather complex phenomenon. There are numerous failure modes in composite materials stemming from the interaction of the various constituent materials and the particular loading conditions. This thesis is concerned with investigating damage evolution in sandwich structures under repeated transient loading conditions associated with impulse loading due to hull slamming of high-speed marine craft. To fully understand the complex stress interactions, a full field technique to reveal stress or strain is required. Several full field techniques exist but are limited to materials with particular optical properties. A full field technique applicable to most materials is known as thermoelastic stress analysis (TSA) and reveals the variation in sum of principal stresses of a cyclically loaded sample by correlating the stresses to a small temperature change occurring at the loading frequency. Digital image correlation (DIC) is another noncontact full field technique that reveals the deformation field by tracking the motion of subsets of a random speckle pattern during the loading cycles.
\r\n\r\nA novel experimental technique to aid in the study of damage progression that combines TSA and DIC simultaneously utilizing a single infrared camera is presented in this thesis. A technique to reliably perform DIC with an infrared (IR) camera is developed utilizing variable emissivity paint. The thermal data can then be corrected for rigid-body motion and deformation such that each pixel represents the same material point in all frames. TSA is then performed on this corrected data, reducing motion blur and increasing accuracy. This combined method with a single infrared camera has several advantages, including a straightforward experimental setup without the need to correct for geometric effects of two spatially separate cameras. Additionally, there is no need for external lighting in TSA as the measured electromagnetic radiation is emitted by the sample\u2019s thermal fields.
\r\n\r\nThe particular stress resolution of TSA will depend on properties of the material of interest but the noise floor for the temperature variation is universal to the camera utilized. For the camera system in this thesis, the noise floor was found to be fairly frequency independent with a magnitude of 0.01 oC, giving the minimum measurable stress for 2024 aluminum alloy of 3.6 MPa and for Nylon of 0.84 MPa. The average displacement range found during a static DIC test with IR images was 0.1 pixels. The maximum displacement variation at 1 Hz was 0.018 pixels. The average variation in strain at 1 Hz was 25 microstrain comparable to traditional DIC measurements in the visible optical regime.
\r\n\r\nThe combined TSA-DIC method in IR was validated with several benchmark example problems including plate structures with holes, cracks, and bimaterials. The validated technique was applied to foam-core sandwich composite beams under repeated simulated wave slamming loading. There are numerous failure modes in sandwich composite materials and the full field stress and strain from TSA and DIC, respectively, allow for improved failure analysis and prediction. Understanding damage in sandwich structures under impulse loading is a complex open area of research and the combined TSA-DIC method provides further insight into the failure process.
\r\n", "doi": "10.7907/CRX1-7D43", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6422", "collection": "thesis", "collection_id": "6422", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05242011-022845109", "primary_object_url": { "basename": "draft2.pdf", "content": "final", "filesize": 9777877, "license": "other", "mime_type": "application/pdf", "url": "/6422/1/draft2.pdf", "version": "v11.0.0" }, "type": "thesis", "title": "Repeatability of Joint-Dominated Deployable Masts", "author": [ { "family_name": "Stohlman", "given_name": "Olive Remington", "clpid": "Stohlman-Olive-Remington" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Kim", "given_name": "Yunjin", "clpid": "Kim-Yunjin" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Deployable masts are a class of structure that can be stowed in a small volume and expanded into long, slender, and stable booms. Their greatest benefit as space structures is their packing ratio: masts can typically be packed to a fraction of their deployed length at a diameter only modestly wider than their deployed width. This thesis is concerned with precision deployable masts, which can be stowed and deployed with repeatability of the tip position of better than 1 mm over 60 m. The methods of investigation are experimental measurements of a sample mast and numerical modeling of the mast with specially attention to hysteretic joints.
\r\n\r\nA test article of an ADAM mast was used for the experimental work. Two categories of experi- ment were pursued: measurements of mast components as inputs to the model, and measurements of full bays as validation cases for the model. Measurements of the longeron ball end joint friction, cable preload, and latch behavior are of particular note, and were evaluated for their variability. Further measurements were made of a bay in torsion and a short two-bay mast in shear, showing that there is residual displacement in this mast after shear loading is applied and released.
\r\n\r\nThe modeling approach is described in detail, with attention to the treatment of the mast latches, which lock the structure in its deployed configuration. A user element subroutine was used within the framework of the Abaqus finite element analysis solver to model the behavior of the latches with high fidelity.
\r\n\r\nValidation cases for the model are presented in comparison with experimental observations of a two-bay mast. These cases show that the model captures a number of important and complex nonlinear effects of the hysteretic mast components. Parametric studies of the impacts of component behaviors and modeling practices are explored, emphasizing the impacts of part variability and the idealization of the mast latching mechanisms.
", "doi": "10.7907/D3AR-G573", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6450", "collection": "thesis", "collection_id": "6450", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262011-111659863", "primary_object_url": { "basename": "Meier_Thesis2011_Final.pdf", "content": "final", "filesize": 3809795, "license": "other", "mime_type": "application/pdf", "url": "/6450/1/Meier_Thesis2011_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A Novel Experimental Study of a Valveless Impedance Pump for Applications at Lab-On-Chip, Microfluidic, and Biomedical Device Size Scales", "author": [ { "family_name": "Meier", "given_name": "John Allen", "clpid": "Meier-John-Allen" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In 1954, Gerhart Liebau demonstrated a simple valveless pumping phenomenon utilizing the periodic compression of a compliant tube and some systematic asymmetry to pump water out of a bucket. Liebau's goal was to explain peculiarities seen in the human circulatory system. In the years that have followed, the Liebau phenomenon has been studied in a variety of open and closed loop configurations, through experimental, computational, and analytical studies.
\r\n\r\nRecent advances in microfluidic and microelectromechanical systems (MEMS) technology have enabled a wide range of small scale engineering systems. The further development of many important systems is limited by the absence of an appropriate means of fluid transport. Valveless pumps based on the Liebau phenomenon show great promise, particularly in lab-on-chip (LOC), biological, and medical applications in which biocompatibility and the ability to move sensitive molecules without damage are key design requirements.
\r\n\r\nThe purpose of this thesis is to synthesize previous studies of the Liebau phenomenon and produce the first extensive experimental study of a novel valveless pump at size scales and geometries that are relevant to lab-on-chip, microfluidic, and biomedical device applications. For the first time, detailed, dynamic pressure and flow data have been recorded during the operation of these valveless pumps for a large range of operating parameters. This dynamic data allowed us to identify new flow regimes and observe previously undocumented pump behaviors and performance. Parameters investigated include pump material properties and geometry, working fluid density and viscosity, pump excitation properties (amplitude, offset, location, and frequency), and flow loop/system properties. A critical relationship between the relative volumetric compliance of the valveless pump to the system it acts upon is identified, and the implications for practical implementation of valveless pumps at small size scales are discussed.
", "doi": "10.7907/DBKG-EJ21", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6165", "collection": "thesis", "collection_id": "6165", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11022010-080434454", "type": "thesis", "title": "Multiscale Modeling and Simulation of Damage by Void Nucleation and Growth", "author": [ { "family_name": "Reina Romo", "given_name": "Celia", "clpid": "Reina-Romo-Celua" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Marian", "given_name": "Jaime", "clpid": "Marian-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Voids are observed to be generated under sufficient loading in many materials, ranging from polymers and metals to biological tissues. The presence of these voids can have drastic implications at the macroscopic level including strong material softening and more incipient fracture. Developing tools to appropriately account for these effects is therefore very desirable.
\r\n\r\nThis thesis is concerned with both, the appearance of voids (nucleation process) and the modeling and simulation of materials in the presence of voids. A particular nucleation mechanism based on vacancy aggregation in high purity metallic single crystals is analyzed. A multiscale model is developed in order to obtain an approximate value of the time required for vacancies to form sufficiently large clusters for further growth by plastic deformation. It is based on quantum mechanical results, kinetic Monte Carlo methods and continuum mechanics estimates calibrated with quasi-continuum results. The ultimate goal of these simulations is to determine the feasibility of this nucleation mechanism under shock loading conditions, where the temperature and tensions are high and vacancy diffusion is promoted.
\r\n\r\nOn the other hand, the effective behavior of materials with pre-existent voids is analyzed within the general framework of continuum mechanics and is therefore applicable to any material. The overall properties of the heterogeneous material are obtained through a two-level characterization: a representative volume element consisting of a hollow sphere is used to describe the \"microscopic\" fields, and an equivalent homogeneous material is used for the \"macroscopic\" behavior. A variational formulation of this two-scale model is presented. It provides a consistent definition of the macro-variables under general loading conditions, extending the well-known static averaging results so as to include microdynamic effects under finite deformations. This variational framework also provides a suitable starting point for time discretization and consistent definitions within discrete time. The spatial boundary value problem resulting from this multiscale model is solved with a particular spherical shell element specially developed for this problem. The approximation space is based on spherical harmonics, which respects the symmetries of the porous material and allows the representation of the fields on the sphere with very few degrees of freedom. Numerical tools, such as the exact representation of the boundary conditions and an exact quadrature rule, are also provided. The resulting numerical model is verified extensively, demonstrating good convergence results, and its applicability is shown through several material point calculations and a full two-scale finite element implementation.
", "doi": "10.7907/WFYW-AS22", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6458", "collection": "thesis", "collection_id": "6458", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262011-172059575", "primary_object_url": { "basename": "Thesis_CK2.pdf", "content": "final", "filesize": 42353671, "license": "other", "mime_type": "application/pdf", "url": "/6458/1/Thesis_CK2.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Mechanics of Peeling: Cohesive Zone Law and Stability", "author": [ { "family_name": "Kovalchick", "given_name": "Christopher", "clpid": "Kovalchick-Christopher" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The measurement of interface mechanical properties between an adhesive layer and a substrate is significant for optimization of a high-quality interface. A common method for measuring these properties is the peel test. While there are many interesting applications of peel in such areas as cell and gecko adhesion, the focus here is to obtain a better understanding of the fundamental mechanics underlying the problem.
\r\n\r\nThe mechanics of the peel test is examined through experiments, finite element simulations, and theoretical analysis with the aim of developing governing relations to describe the role of fracture in the peel test for elastic adhesive tapes. An inverse formulation is developed to extract a cohesive zone law from a set of experimental peel tests using a theoretical framework based upon non-linear beam theory. Through extracting a cohesive zone law, the adhesion energy during a peel test is determined along with the force distribution in the process zone. This local method of determining the adhesion energy is compared to a global method used by Rivlin in the context of finite deformations, showing good agreement.
\r\n\r\nThe effect of rate-dependency in the peel test is also examined experimentally, with the results used to derive a rate-dependent power-law for the adhesion energy in a peel test as a function of the peel rate. The effects of varying different geometrical parameters during the peel test and how they affect the force distribution and adhesion energy are also presented. Finally, a study of the stability in the peel test, including the role of compliance through several newly developed force-controlled experimental configurations is discussed. The stiffness of the system is varied by altering the magnitude and direction of the applied load during a test. This change in stiffness can be tuned in order to trigger or delay the onset of instability. Theoretical stability criteria are also presented to in order to develop insights of the role of parameters investigated experimentally.
", "doi": "10.7907/W2KT-CY70", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6473", "collection": "thesis", "collection_id": "6473", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292011-200916324", "primary_object_url": { "basename": "phanish_PhD_thesis.pdf", "content": "final", "filesize": 2123034, "license": "other", "mime_type": "application/pdf", "url": "/6473/1/phanish_PhD_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Coarse-Graining Kohn-Sham Density Functional Theory", "author": [ { "family_name": "Suryanarayana", "given_name": "Phanish", "clpid": "Suryanarayana-Phanish" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Defects, though present in relatively minute concentrations, play a significant role in determining macroscopic properties. Even vacancies, the simplest and most common type of defect, are fundamental to phenomena like creep, spall and radiation ageing. This necessitates an accurate characterization of defects at physically relevant concentrations, which is typically in parts per million. This represents a unique challenge since both the electronic structure of the defect core as well as the long range elastic field need to be resolved simultaneously. Unfortunately, accurate ab-initio electronic structure calculations are limited to a few hundred atoms, which is orders of magnitude smaller than that necessary for a complete description. Thus, defects represent a truly challenging multiscale problem.
\r\n\r\nDensity functional theory developed by Hohenberg, Kohn and Sham (DFT) is a widely accepted, reliable ab-initio method for computing a wide range of material properties. We present a real-space, non-periodic, finite-element and max-ent formulation for DFT. We transform the original variational problem into a local saddle-point problem, and show its well-posedness by proving the existence of minimizers. Further, we prove the convergence of finite-element approximations including numerical quadratures. Based on domain decomposition, we develop parallel finite-element and max-ent implementations of this formulation capable of performing both all-electron and pseudopotential calculations. We assess the accuracy of the formulation through selected test cases and demonstrate good agreement with the literature.
\r\n\r\nTraditional implementations of DFT solve for the wavefunctions, a procedure which has cubic-scaling with respect to the number of atoms. This places serious limitations on the size of the system which can be studied. Further, they are not amenable to coarse-graining since the wavefunctions need to be orthonormal, a global constraint. To overcome this, we develop a linear-scaling method for DFT where the key idea is to directly evaluate the electron density without solving for the individual wavefunctions. Based on this linear-scaling method, we develop a numerical scheme to coarse-grain DFT derived solely based on approximation theory, without the introduction of any new equations and resultant spurious physics. This allows us to study defects at a fraction of the original computational cost, without any significant loss of accuracy. We demonstrate the efficiency and efficacy of the proposed methods through examples. This work enables the study of defects like vacancies, dislocations, interfaces and crack tips using DFT to be computationally viable.
", "doi": "10.7907/GCKH-EX20", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6427", "collection": "thesis", "collection_id": "6427", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05242011-143955754", "primary_object_url": { "basename": "Brown_Thesis.pdf", "content": "final", "filesize": 9215095, "license": "other", "mime_type": "application/pdf", "url": "/6427/1/Brown_Thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "High Pressure Hugoniot Measurements in Solids Using Mach Reflections", "author": [ { "family_name": "Brown", "given_name": "Justin Lee", "clpid": "Brown-Justin-Lee" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Hornung", "given_name": "Hans G.", "clpid": "Hornung-H-G" }, { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Shock compression experiments provide access to high pressures in a laboratory setting. Matter at extreme pressures is often studied by utilizing a well controlled planar impact between two flat plates to generate a one dimensional shock wave. While these experiments are a powerful tool in equation of state (EOS) development, they are inherently limited by the velocity of the impacting plate. In an effort to dramatically increase the range of pressures which can be studied with available impact velocities, a new experimental technique is examined. The target plate is replaced by a composite assembly consisting of two concentric cylinders and is designed such that the initial shock velocity in a well characterized outer cylinder is higher than in the inner cylinder material of interest. After impact, conically converging shocks are generated at the interface due to the impedance mismatch between the two materials and the axisymmetric geometry. Upon convergence, an irregular reflection occurs and the conical analog of a Mach reflection develops. This Mach reflection grows until it reaches a steady state, for which an extremely high pressure state is concentrated behind the Mach stem. The reflection is studied using a combination of analytical, numerical, and experimental techniques. Ideas from gas dynamics, such as shock polars, are connected to the classic treatment of one-dimensional shocks in solids to form a simple method for treating the oblique reflections in the Mach lens configuration. Numerical simulations provide detailed full-field solutions and illustrate a methodology for extracting EOS information. The technique is validated experimentally by studying the shock response of copper and iron. Two different confining materials, 6061-T6 aluminum and molybdenum, are used to drive the converging shock waves for which the high pressure state is measured through a combination of velocity interferometry and impedance matching techniques.", "doi": "10.7907/SC1V-PK42", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6017", "collection": "thesis", "collection_id": "6017", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08302010-215155302", "primary_object_url": { "basename": "MLWinterrose_PhD_Thesis.pdf", "content": "final", "filesize": 8577517, "license": "other", "mime_type": "application/pdf", "url": "/6017/1/MLWinterrose_PhD_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Quantum Mechanical Simulation and X-Ray Scattering Applied to Pressure-Induced Invar Anomaly in Magnetic Iron Alloy", "author": [ { "family_name": "Winterrose", "given_name": "Michael Lon", "clpid": "Winterrose-Michael-Lon" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Jackson", "given_name": "Jennifer M.", "clpid": "Jackson-J-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The Invar effect has remained at the forefront of materials research since Charles-\u00c9douard Guillaume discovered the vanishing thermal expansion of Fe-Ni alloys in 1897. More recently, a pressure-induced Invar effect was discovered in Fe-Ni alloys, and the relationship between classical and pressure induced Invar phenomena has added complexity to the century-old struggle to comprehend the microscopic origins of Invar behavior.
\r\n\r\nIn this thesis I present our recent discovery of pressure-induced Invar behavior in Pd\u2083Fe with the ordered L1\u2082 structure. Nuclear forward scattering measurements show that the ferromagnetic ground state in Pd\u2083Fe is destabilized with pressure, collapsing around 10GPa (V/V0=0.96) to a lowspin magnetic state. From high-pressure synchrotron x-ray diffraction measurements we find a large volume collapse at ambient temperature to accompany the collapse of ferromagnetism. After the volume collapse there is a significant increase in the bulk modulus. Using nuclear resonant inelastic x-ray scattering to study the 57Fe phonon partial density of states (PDOS) at high pressures, we find the pressure-induced magnetic transition to cause an anomalous relative softening of the average phonon frequency. Heating our sample to 650K in a furnace at a pressure of 7GPa, synchrotron xray diffraction measurements reveal negligible thermal expansion from 300 to 523 K, demonstrating pressure-induced Invar behavior in Pd\u2083Fe.
\r\n\r\nDensity functional theory calculations identify a ferromagnetic ground state in Pd\u2083Fe with large moments at the Fe sites. These calculations show that the application of pressure counteracts the band-filling effect of Pd. By tuning the position of the top of the 3d band with respect to the Fermi level, pressure-induced Invar behavior resembles classical Invar behavior that is controlled by chemical composition. This insight marks the first step towards a unification of our understanding vii of classical and pressure-induced Invar behavior. Pressure drives the majority-spin t2g antibonding electronic states closer to the Fermi level. The transition to the low-spin state occurs as these t2g states move across the Fermi level, transferring charge to the minority-spin eg nonbonding electronic states. This charge transfer reduces the internal electronic pressure in the material, giving a volume reduction in the low-spin state. The movement of the t2g states with increasing pressure results in a greater number of states at the Fermi level, increasing screening efficiency and softening the first nearest-neighbor Fe-Pd longitudinal force constants in the low-spin state. The measured and calculated magnetic transition pressures differ significantly, despite sharing similar elastic properties in both the ferromagnetic and low-spin states. The magnitude of the disagreement between theoretical and experimental magnetic transition pressures suggests a spin-disordered state exists at high pressures in Pd\u2083Fe. A shape discrepancy between the calculated and measured high-pressure Fe PDOS suggests significant short-range spin correlations exist in this spin-disordered state.
", "doi": "10.7907/2CBN-7H49", "publication_date": "2011-06-15", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6103", "collection": "thesis", "collection_id": "6103", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10052010-230939247", "primary_object_url": { "basename": "MarcialGonzalez-Thesis.pdf", "content": "final", "filesize": 9980691, "license": "other", "mime_type": "application/pdf", "url": "/6103/1/MarcialGonzalez-Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Energy and Force Stepping Integrators in Lagrangian Mechanics", "author": [ { "family_name": "Gonzalez", "given_name": "Marcial", "clpid": "Gonzalez-Marcial" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The overarching goal of this thesis is to develop new numerical time integration schemes for Lagrangian mechanics that better cope with the challenges of understanding the dynamic behavior of materials. We specifically address the formulation of convergent time integration schemes that exhibit good long-term behavior---such as conferred by symplecticity and exact conservation properties---and that have the ability to automatically and asynchronously modulate the time step in different regions of the domain. We achieve these properties in a progression of three developments: (i) energy-stepping, (ii) force-stepping, and (iii) asynchronous energy-stepping integrators. These developments are based on a new method of approximation for Lagrangian mechanics, proposed in this thesis, that consists of replacing the Lagrangian of the system by a sequence of approximate Lagrangians that can be solved exactly. Then, energy-stepping integrators result from replacing the potential energy by a piecewise constant approximation, force-stepping integrators result from replacing the potential energy by a piecewise affine approximation, and asynchronous energy-stepping integrators result from replacing localized potential energies by piecewise constant approximations. Throughout the dissertation, the properties of these time integrators are theoretically predicted and born out by a number of selected examples of application. Furthermore, we address the challenges of understanding the propagation of solitary waves in granular crystals at low impact velocity conditions by investigating the role of energy-trapping effects with the numerical time integration schemes developed in this work.\r\n", "doi": "10.7907/SP10-A207", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6187", "collection": "thesis", "collection_id": "6187", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11222010-061455728", "primary_object_url": { "basename": "Hurtado-Thesis.pdf", "content": "final", "filesize": 1915111, "license": "other", "mime_type": "application/pdf", "url": "/6187/1/Hurtado-Thesis.pdf", "version": "v9.0.0" }, "type": "thesis", "title": "Multiscale Modeling of Microcrystalline Materials", "author": [ { "family_name": "Hurtado Sepulveda", "given_name": "Daniel Esteban", "clpid": "Hurtado-Sepulveda-Daniel-Esteban" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Materials with micrometer dimensions and their distinct mechanical properties have generated a great interest in the material science community over the last couple of decades. There is strong experimental evidence showing that microcrystalline materials are capable of achieving much higher yield and fracture strength values than bulk mesoscopic samples as they decrease in size. Several theories have been proposed to explain the size effect found in micromaterials, but a predictive physics-based model suitable for numerical simulations remains an open avenue of research. Since the successful design of micro-electro-mechanical systems (MEMS) and novel engineered materials hinges upon the mechanical properties at the micrometer scale, there is a compelling need for a quantitative and accurate characterization of the size effects exhibited by metallic micromaterials.
\r\n\r\nThis work is concerned with the multiscale material modeling and simulation of strength in crystalline materials with micrometer dimensions. The elasto-viscoplastic response is modeled using a continuum crystal plasticity formulation suitable for large-deformation problems. Crystallographic dislocation motion is accounted for by stating the crystal kinematics within the framework of continuously distributed dislocation theory. The consideration of the dislocation self-energy and the step formation energy in the thermodynamic formulation of the constitutive relations renders the model non-local and introduces a length scale. Exploiting the concept of total variation we are able to recover an equivalent model that is local under a staggered approach, and therefore amenable to time integration using variational constitutive updates. Numerical simulations of compression tests in nickel micropillars using the proposed multiscale framework quantitatively capture the size dependence found in experimental results, showcasing the predictive capabilities of the model.
\r\n", "doi": "10.7907/FHZT-3A33", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6172", "collection": "thesis", "collection_id": "6172", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11052010-113423550", "primary_object_url": { "basename": "ThesisLuigiPerotti.pdf", "content": "final", "filesize": 8769735, "license": "other", "mime_type": "application/pdf", "url": "/6172/1/ThesisLuigiPerotti.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Modeling the Behavior of Fiber Reinforced Sandwich Structures Subjected to Underwater Explosions", "author": [ { "family_name": "Perotti", "given_name": "Luigi Emanuele", "clpid": "Perotti-Luigi-Emanuele" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Krishnan", "given_name": "Swaminathan", "clpid": "Krishnan-S" }, { "family_name": "Pandolfi", "given_name": "Anna", "clpid": "Pandolfi-A" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Fiber composite material panels and sandwich panels possess both a high resistance to weight ratio and a high stiffness to weight ratio. Due to these features, fiber composite panels are used widely in aeronautic and marine structures, where the improvement of the structural performance while keeping a low weight is crucial. Sandwich structures, consisting of a foam core enclosed by two external layers of fiber reinforced material, seem to be promising in minimizing the total weight, maintaining structural rigidity and improving the resistance under exceptional loads, such as those due to explosions. Full scale experiments to test the performance of real fiber composite sandwich structures subjected to underwater explosions would be very complex and expensive. Therefore, the capability to numerically simulate the response of sandwich structures undergoing explosive loading will provide a powerful and unique tool to analyze and optimize their design by investigating the influence of different parameters. Obviously, small scale laboratory tests will still be essential to validate and calibrate the computational model before its use.
\r\n\r\nThe present research focuses on the development of a computational scheme to model the behavior of large sandwich panels subjected to underwater explosions. The description of the sandwich requires the definition of the material behavior of the components, i.e., the foam core and the external sheets, of the structural behavior of the thin shell structure, and of the interaction with the surrounding fluid. Several finite kinematics material models taken from the recent literature have been used, and a new simple model for fiber reinforced composites has been developed and validated. The thin shell structure is modeled with an existing in-house built non-local shell finite element code (SFC), equipped with fracturing capabilities. The coupling between the behavior of the shells and the action of the fluid as a consequence of an underwater explosion is modeled here with the aid of an existing fluid-solid interaction (FSI) code. In this study, the FSI code has been expanded in order to include the possibility of simulating fiber composite materials. New algorithms and new control indicators, such as global measures of energy dissipation, have also been developed. The new capabilities of the fluid-solid coupled solver have been verified and validated before applying the solver to realistic problems. In the applications part of the present research, two different methods for applying the pressure load due to an underwater explosion are compared. The first method is simpler, and consists in applying a prescribed pressure profile without considering FSI. In the second method, the explosive charge is modeled as a spherical energy deposition and the full FSI is considered. The simpler method is used to assess the role of different design parameters of the face sheets on the overall response of sandwich panels when subjected to impulsive loads. Subsequently, the best sandwich design obtained from these initial simulations is used for the evaluation of the mechanical performance of the hull section of an existing Argentinean navy vessel. The final application of the proposed computational scheme is a parametric analysis of the hull section, considering different weights of the explosive charge and different distances of the explosion location from the hull wall.
\r\n\r\nFinally, with awareness of the limits of the adopted approach, several alternative schemes to improve the dynamical analysis of sandwich panels impulsively loaded are presented and discussed. In particular, two different kinds of shell finite elements are introduced. The proposed shell elements are based on alternative approximation schemes, which may model in a more realistic way the behavior of sandwich structures under extreme loads.
\r\n", "doi": "10.7907/83JD-HN76", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:5809", "collection": "thesis", "collection_id": "5809", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05142010-174001426", "primary_object_url": { "basename": "main.pdf", "content": "final", "filesize": 26326921, "license": "other", "mime_type": "application/pdf", "url": "/5809/1/main.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Detonation Induced Strain in Tubes", "author": [ { "family_name": "Karnesky", "given_name": "James Alan", "clpid": "Karnesky-James-Alan" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" }, { "family_name": "Blanquart", "given_name": "Guillaume", "clpid": "Blanquart-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "abstract": "When a detonation wave propagates through a piping system, it acts as a traveling pressure load to the pipe wall. The detonation wave must be followed by an expansion wave in order to bring the combustion products to zero velocity at the ignition end. When it reaches a closed end-wall, a reflected shock is formed which propagates back into the tube with a decaying pressure. The present study aims to develop predictive models for the stresses and strains produced in such a situation. To this end, two series of experiments are discussed. The first series used strain gauges and a laser vibrometer to measure the elastic response of the tube to the incident detonation in thin aluminum tubes. The second series used strain gauges and high speed video to measure the plastic response of steel tubes to incident detonations and reflected shocks. In these experiments a novel mode of plastic deformation was discovered in which the residual plastic deformation in the tube wall had a periodic sinusoidal pattern.
\r\n\r\nA semi-empirical model of the pressure history was developed for use as a boundary condition in models of the mechanical response of the tube. This model was tested against experiment, and it was found that the pressure and arrival time could not be simultaneously predicted from the simple model. This and the general form of the pressure traces in the experiment seem to suggest an interaction between the reflected shock and the boundary layer behind the detonation resulting in a possible bifurcation in the reflected shock wave.
\r\n\r\nWith these considerations in mind, the model was applied to single degree of freedom and finite element models of the tube wall. The ripples observed in the experiment were present in the 1-D single degree of freedom models, indicating that they are a result of the interaction of the reflected shock wave with the elastic oscillations set in motion by the detonation wave. Strain-rate hardening was found to be an important consideration under detonation loading conditions. With proper consideration of rate hardening, a single material model may be used to arrive at reasonable predictions the plastic strains resulting from detonations and reflections at initial pressures of 2 and 3 bar initial pressures.
", "doi": "10.7907/GTKC-FY91", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5868", "collection": "thesis", "collection_id": "5868", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272010-173243262", "primary_object_url": { "basename": "thesis_SBane.pdf", "content": "final", "filesize": 7713493, "license": "other", "mime_type": "application/pdf", "url": "/5868/1/thesis_SBane.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Spark Ignition: Experimental and Numerical Investigation With Application to Aviation Safety", "author": [ { "family_name": "Bane", "given_name": "Sally Page Moffett", "orcid": "0000-0002-4764-3228", "clpid": "Bane-Sally-Page-Moffett" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "McKeon", "given_name": "Beverley J.", "clpid": "McKeon-B-J" }, { "family_name": "Blanquart", "given_name": "Guillaume", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "abstract": "Determining the risk of accidental ignition of flammable mixtures is a topic of tremendous importance in industry and aviation safety. The concept of minimum ignition energy (MIE) has traditionally formed the basis for studying ignition hazards of fuels. However, in recent years, particularly in the aviation safety industry, the viewpoint has changed to one where ignition is statistical in nature. Approaching ignition as statistical rather than a threshold phenomenon appears to be more consistent with the inherent variability in the engineering test data.
\r\n\r\nIgnition tests were performed in lean hydrogen-based aviation test mixtures and in two hexane-air mixtures using low-energy capacitive spark ignition systems. Tests were carried out using both short, fixed sparks (1 to 2 mm) and variable length sparks up to 10 mm. The results were analyzed using statistical tools to obtain probability distributions for ignition versus spark energy and spark energy density (energy per unit spark length). Results show that a single threshold MIE value does not exist, and that the energy per unit length may be a more appropriate parameter for quantifying the risk of ignition than only the energy. The probability of ignition versus spark charge was also investigated, and the statistical results for the spark charge and spark energy density were compared. It was found that the test results were less variable with respect to the spark charge than the energy density. However, variability was still present due to phenomena such as plasma instabilities and cathode effects that are caused by the electrodynamics.
\r\n\r\nWork was also done to develop a two-dimensional numerical model of spark ignition that accurately simulates all physical scales of the fluid mechanics and chemistry. In this work a two-dimensional model of spark discharge in air and spark ignition was developed using the non-reactive and reactive Navier-Stokes equations. One-step chemistry models were used to allow for highly resolved simulations, and methods for calculating effective one-step parameters were developed using constant pressure explosion theory. The one-step model was tuned to accurately simulate the flame speed, temperature, and straining behavior using one-dimensional flame computations. The simulations were performed with three different electrode geometries to investigate the effect of the geometry on the fluid mechanics of the evolving spark kernel and on flame formation. The computational results were compared with high-speed schlieren visualization of spark and ignition kernels. It was found that the electrode geometry had a significant effect on the fluid motion following spark discharge and hence influences the ignition process.
\r\n", "doi": "10.7907/W1NB-5W06", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5883", "collection": "thesis", "collection_id": "5883", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282010-141343271", "type": "thesis", "title": "Shape Changing Transformations: Interactions with Plasticity and Electrochemical Processes", "author": [ { "family_name": "Roumi", "given_name": "Farshid", "clpid": "Roumi-Farshid" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Solids undergo phase transformations where the crystal structure changes with temperature, chemical potential, stress, applied electric fields, or other external parameters. These occur by either long-range diffusion of atoms (diffusional phase transformation) or by some form of cooperative, homogeneous movement of many atoms that results in changes in crystal structure (displacive phase transformation). In the latter case, these movements are usually less than the interatomic distances, and the atoms maintain their coordination. The most common example of displacive phase transformations is martensitic transformation. The martensitic transformation in steel is economically very important and can result in very different behavior in the product. Other examples of martensitic transformations are shape memory alloys which are lightweight, solid-state alternatives to conventional actuators such as hydraulic, pneumatic, and motor-based systems.
\r\n\r\nThe martensitic transformation usually only depends on temperature and stress and, in contrast to diffusion-based transformations, is not time dependent. In shape memory alloys the transformation is reversible. On the other hand in steel, the martensite formation from austenite by rapidly cooling carbon-steel is not reversible; so steel does not have shape memory properties.
\r\n\r\nIn Chapters 2 and 3, we study the interesting yet very complicated behavior of martensitic transformation interactions with plastic deformations. A good example here is steel, which has been known for thousands of years but still is believed to be a very complicated material. Steel can show different behavior depending on its complex microstructure. Thus understanding the formation mechanisms is crucial for the interpretation and optimization of its properties. As an example, low alloyed steels with transformation induced plasticity (TRIP), metastable austenite steels, are known for strong hardening and excellent elongation and strength. It is suggested that the strain-induced transformation of small amounts of untransformed (retained) austenite into martensite during plastic deformation is a key to this excellent behavior.
\r\n \r\nIn Chapters 4 and 5, we study the interactions of solid-solid phase transformations with electrochemical processes. It is suggested that electronic and ionic structures depends on lattice parameters, thus it is expected that structural transformations can lead to dramatic changes in material properties. These transformations can also change the energy barrier and hysteresis. It is known that compatible interfaces can reduce elastic energy and hysteresis, thus may extend the life of the system. Solid-solid transformations change the crystalline structure. These geometry changes can have long range effects and cause stresses in the whole material. The generated stress field itself changes the total free energy, due to the change in elastic energy, and thus, the electrochemical potential and processes are affected. An example is olivine phosphates which are candidates for cathode material in Li-ion batteries. These materials undergo an orthorhombic to orthorhombic phase transition. Experiments in the literature have suggested that elastic compatibility can affect rates of charge/discharge in the battery. Our theory provides some insight into this observation.
", "doi": "10.7907/P94H-4B23", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5859", "collection": "thesis", "collection_id": "5859", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272010-133830557", "primary_object_url": { "basename": "AndoCIT2010.pdf", "content": "final", "filesize": 2480169, "license": "other", "mime_type": "application/pdf", "url": "/5859/1/AndoCIT2010.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Effects of Polydispersity in Bubbly Flows", "author": [ { "family_name": "Ando", "given_name": "Keita", "orcid": "0000-0002-9572-8242", "clpid": "Ando-Keita" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis concerns the dynamics of bubbly flows with a distribution of equilibrium bubble sizes. The main goal is to formulate the physical and numerical models of continuum bubbly flows that enable us to efficiently compute the average mixture dynamics. Numerical simulations are conducted to quantify the effects of bubble size distributions on the averaged dynamics for several model flows.
\r\n\r\nFirst, the ensemble-averaged conservation laws for polydisperse bubbly flows are derived. One-way-coupled flow computations are conducted to illustrate that the different-sized bubbles can oscillate with different frequencies. The resulting phase cancellations can be regarded as an apparent damping of the averaged dynamics of polydisperse flows. A high-order-accurate finite-volume method is then developed to compute the flow, paying special attention to issues of wave dispersion and stiffness.
\r\n\r\nNext, computations of one-dimensional shock propagation through bubbly liquids are performed. The numerical experiments reveal that the bubble size distribution has a profound impact on the averaged shock structure. If the distribution is sufficiently broad, the apparent damping due to the phase cancellations can dominate over the single-bubble-dynamic dissipation (due to thermal, viscous, and compressibility effects) and the averaged shock dynamics become insensitive to the individual bubble dynamics. One-dimensional cloud cavitation caused by fluid-structure interaction is also solved to investigate the collapse of cavitation clouds with both monodisperse and polydisperse nuclei. The phase cancellations among the cavitation bubbles with broad nuclei size distributions are found to eliminate violent cloud collapse in the averaged dynamics.
\r\n\r\nFinally, shock propagation through a bubbly liquid-filled, deformable tube is considered. The quasi-one-dimensional conservation law that takes into account structural deformation is formulated and steady shock relations are derived. The results are compared to water-hammer experiments; the present shock theory gives better agreement with the measured wave speeds than linear theory. This indicates that the gas-phase nonlinearity needs to be included to accurately predict the propagation speeds of finite-amplitude waves in a deformable tube filled with a bubbly liquid.
", "doi": "10.7907/SW8K-Y135", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5472", "collection": "thesis", "collection_id": "5472", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12222009-004407068", "primary_object_url": { "basename": "Wiest_Thesis_12-22-09.pdf", "content": "final", "filesize": 9158622, "license": "other", "mime_type": "application/pdf", "url": "/5472/15/Wiest_Thesis_12-22-09.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Thermoplastic Forming and Related Studies of the Supercooled Liquid Region of Metallic Glasses ", "author": [ { "family_name": "Wiest", "given_name": "Aaron", "clpid": "Wiest-Aaron" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Atwater", "given_name": "Harry Albert", "clpid": "Atwater-H-A" }, { "family_name": "Conner", "given_name": "Robert Dale", "clpid": "Conner-R-D" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The thermoplastic formability (TPF) of metallic glasses was found to be related to the calorimetrically measured crystallization temperature minus the glass transition temperature, Tg - Tx = \u0394T. Alloy development in the ZrTiBe system identified a composition with \u0394T = 120 \u00b0C. Many alloys with \u0394T > 150 \u00b0C and one alloy, Zr35Ti30Be27.5Cu7.5, with \u0394T = 165 \u00b0C were discovered by substituting Be with small amounts of fourth alloying elements. The viscosity as a function of temperature, \u03b7(T), and time temperature transformation (TTT) measurements for the new alloy are presented and combined to create \u03b7TT plots (viscosity time transformation) that are useful in determining what viscosities are available for a required processing time. \u03b7TT plots are created for many alloys used in TPF in the literature and it is found that for processes requiring 60 - 300 s, Zr35Ti30Be27.5Cu7.5 provides an order of magnitude lower viscosity for processing than the other metallic glasses. Injection molding is demonstrated with Zr35Ti30Be27.5Cu7.5 and the part shows improved mechanical properties over die cast specimens of the same geometry. Changes of slope in \u03b7(T) measurements were observed and investigated in some quaternary compositions and found to be present in ternary compositions as well. Traditionally metallic glasses show a single discontinuity in heat capacity at the glass transition temperature. Alloys with the changes in slope of \u03b7(T) were found to show two discontinuities in heat capacity with the changes in slope of \u03b7(T) roughly correlating with the observed Tg values. These two Tg values were assumed to arise from two glassy phases present in the alloy. Further heat capacity analysis found systematic trends in the magnitude of the heat capacity discontinuities with composition and the single phase compositions of a metastable miscibility gap were discovered. Microscopic evidence of the two phases is lacking so we must limit our claims to evidence of two relaxation phenomena existing and can\u2019t definitively claim two phases.
\r\n\r\nThe alloy development led to the discovery of alloys with densities near Ti that are among the highest strength to weight ratio materials known. Alloys with corrosion resistances in simulated sea water 10x greater than other Zr based glasses and commonly used marine metals were discovered. Glasses spanning 6 orders of magnitude in corrosion resistance to 37% w/w HCl were discovered. Corrosion fatigue in saline environments remains a problem for these compositions and prevents their utility as biomaterials despite good evidence of biocompatibility in in vitro and in vivo studies.
\r\n", "doi": "10.7907/W2G5-1S16", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5888", "collection": "thesis", "collection_id": "5888", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282010-183132978", "primary_object_url": { "basename": "Lamberson_Thesis_FINAL.pdf", "content": "final", "filesize": 19737417, "license": "other", "mime_type": "application/pdf", "url": "/5888/1/Lamberson_Thesis_FINAL.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Dynamic Optical Investigations of Hypervelocity Impact Damage", "author": [ { "family_name": "Lamberson", "given_name": "Leslie Elise", "orcid": "0000-0002-1340-4667", "clpid": "Lamberson-Leslie-Elise" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "One of the prominent threats in the endeavor to develop next-generation space assets is the risk of space debris impact in earth\u2019s orbit and micrometeoroid impact damage in near-earth orbit and deep space. To date, there is no study available which concentrates on the analysis of dynamic crack growth from hypervelocity impacts on such structures, resulting in their eventual catastrophic degradation. Experiments conducted using a unique two-stage light-gas gun facility have examined the in situ dynamic fracture of brittle polymers subjected to this high-energy-density event. Optical techniques of caustics and photoelasticity, combined with high-speed photography up to 100 million frames per second, analyze crack growth behavior of Mylar and Homalite 100 thin plates after impact by a 1.8 mm diameter nylon 6-6 right cylindrical slug at velocities ranging from 3 to 7 km/s (7000\u201315500 mph). Crack speeds in both polymers averaged between 0.2 and 0.47 cR, the Rayleigh wave speed (450\u20131000 mph). Shadow spots and surrounding caustics reveal time histories of the dynamic stress intensity factor, as well as the energy release rate ahead of the mode-I, or opening, crack tips. Results indicate that even under extreme impact conditions of out of-plane loading, highly localized heating, and energetic impact phenomena involving plasma formation and ejecta, the dynamic fracture process occurs during a deformation regime dominated by in-plane loading. These findings imply that the reliability of impacted, thin-walled, plate and shell space structures, idealized by the experimental configuration investigated, can be predicted by the well defined principles of classical dynamic fracture mechanics.", "doi": "10.7907/AQJH-3D60", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:950", "collection": "thesis", "collection_id": "950", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03132009-154225", "primary_object_url": { "basename": "BHansen_dis.pdf", "content": "final", "filesize": 4723699, "license": "other", "mime_type": "application/pdf", "url": "/950/1/BHansen_dis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Modeling Metallic Single Crystal Plastic Hardening Through the Evolution of Dislocation Subgrain Structures", "author": [ { "family_name": "Hansen", "given_name": "Benjamin Lee", "clpid": "Hansen-Benjamin-Lee" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Bronkhorst", "given_name": "Curt", "clpid": "Bronkhorst-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A single crystal plasticity theory for insertion into finite element simulation is formulated using sequential laminates to model subgrain dislocation structures. It is known that local models do not adequately account for latent hardening, as latent hardening is not only a material property, but a nonlocal property (e.g., grain size and shape). The addition of the nonlocal energy from the formation of subgrain structure dislocation walls and the boundary layer misfits provide both latent and self hardening of crystal slip. Latent hardening occurs as the formation of new dislocation walls limit motion of new mobile dislocations, thus hardening future slip systems. Self hardening is accomplished by evolution of the subgrain structure length scale. No multiple slip hardening terms are included.
\r\n\r\nThe substructure length scale is computed by minimizing the nonlocal energy. The minimization of the nonlocal energy is a competition between the dislocation wall and boundary layer energy. The nonlocal terms are also directly minimized within the subgrain model as they impact deformation response. The geometrical relationship between the dislocation walls and slip planes affecting dislocation mean free path is accounted for giving a first-order approximation to shape effects. A coplanar slip model is developed due to requirements when modeling the subgrain structure. This subgrain structure plasticity model is noteworthy as all material parameters are experimentally determined rather than fit. The model also has an inherit path dependency due to the formation of the subgrain structures. Validation is accomplished by comparison to single crystal tension test results.
\r\n", "doi": "10.7907/C052-3119", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:1336", "collection": "thesis", "collection_id": "1336", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04102009-170632", "primary_object_url": { "basename": "00_Maskarinec_thesis.pdf", "content": "final", "filesize": 27675925, "license": "other", "mime_type": "", "url": "/1336/1/00_Maskarinec_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Engineering Protein-Based Biomaterials with Biological and Mechanical Cues to Direct Cellular Behavior", "author": [ { "family_name": "Maskarinec", "given_name": "Stacey Ann", "clpid": "Maskarinec-Stacey-Ann" } ], "thesis_advisor": [ { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" } ], "thesis_committee": [ { "family_name": "Grubbs", "given_name": "Robert H.", "clpid": "Grubbs-R-H" }, { "family_name": "Asthagiri", "given_name": "Anand R.", "clpid": "Asthagiri-A-R" }, { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "Biomaterials play critical roles in tissue engineering, reconstructive surgery, and regenerative medicine. Constructing biomaterials through protein engineering offers a unique and powerful solution to the challenges posed by the creation of well-defined, multi-functional materials that must provide structural support, as well as guide cell and tissue behavior. This thesis describes the construction, characterization, and application of protein-based biomaterials composed of photoreactive artificial extracellular matrix (aECM) proteins. These proteins consist of mechanical and biological motifs derived from natural proteins: an elastin-like backbone containing a photocrosslinking site dictates the mechanical properties of the material, and an RGD binding domain from fibronectin mediates cell attachment. aECM proteins were assembled at the genetic level and expressed in a bacterial host. Manipulation of an aminoacyl-tRNA synthetase of the expression strain permitted the incorporation of the photoreactive noncanonical amino acid, para-azidophenylalanine (pN3Phe), within the elastin subunit of the protein.
\r\n\r\nAn evaluation of the mechanical and biological properties of photoreactive aECM proteins has been performed. Nanoindentation experiments using atomic force microscopy (AFM) demonstrated that the elastic moduli of photocrosslinked protein films lie within the range of native elastins and that the mechanical properties of films can be tuned by altering the incorporation of pN3Phe in the protein or adjusting the irradiation dosage. Cell spreading experiments showed that cell attachment to protein films is sequence-specific to the presence of the RGD cell-binding domain. Patterning of protein films using standard photolithography methods allowed for the generation of cellular arrays.
\r\n\r\nA molecular characterization of cellular response to engineered aECM proteins has been initiated using mRNA microarrays and the proteomic technique, BONCAT (bio-orthogonal noncanonical amino acid tagging). These methods provide information regarding changes in the cellular expression of mRNA transcripts and proteins in response to the biochemical composition of protein-based materials.
\r\n\r\nCell-mediated deformation of the extracellular matrix in all three spatial dimensions was tracked and quantified using a newly developed method that combines time-lapse laser scanning confocal imaging and digital volume correlation. Analysis of the displacement profiles of migrating cells illustrates that, in addition to in-plane (x,y) forces, cells also exert significant normal (z) forces. A new \"push-pull\" aspect of cell migration was detected in the normal displacement profiles of monitored cells suggesting that cells simultaneously \"push\" and \u201cpull\u201d the underlying matrix while exploring their microenvironment.
\r\n", "doi": "10.7907/751E-J376", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:1808", "collection": "thesis", "collection_id": "1808", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05142009-135909", "primary_object_url": { "basename": "Julian_Rimoli_Thesis.pdf", "content": "final", "filesize": 3261270, "license": "other", "mime_type": "application/pdf", "url": "/1808/3/Julian_Rimoli_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "A Computational Model for Intergranular Stress Corrosion Cracking", "author": [ { "family_name": "Rimoli", "given_name": "Julian Jose", "orcid": "0000-0002-8707-2968", "clpid": "Rimoli-Julian-Jose" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Krishnan", "given_name": "Swaminathan", "clpid": "Krishnan-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Stress corrosion cracking (SCC) is a very common failure mechanism characterized by a slow, environmentally induced crack propagation in structural components. Time-to-failure tests and crack-growth-rate tests are widespread practices for studying the response of various materials undergoing SCC. However, due to the large amount of factors affecting the phenomenon and the scattered data, they do not provide enough information for quantifying the effects of main SCC mechanisms. This thesis is concerned with the development of a novel 3-dimensional, multiphysics model for understanding the intergranular SCC of polycrystalline materials under the effect of impurity-enhanced decohesion. This new model is based upon: (i) a robust algorithm capable of generating the geometry of polycrystals for objects of arbitrary shape; (ii) a continuum finite element model of the crystals including crystal plasticity; (iii) a grain boundary diffusion model informed with first-principles computations of diffusion coefficients; and (iv) an intergranular cohesive model described by concentration-dependent constitutive relations also derived from first-principles. Results are validated and compared against crack-growth-rate and initiation time tests.\r\n", "doi": "10.7907/K1HJ-DZ56", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:2176", "collection": "thesis", "collection_id": "2176", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272009-094456", "primary_object_url": { "basename": "00A_Entire_Thesis_Hyperlinks.pdf", "content": "final", "filesize": 137255648, "license": "other", "mime_type": "application/pdf", "url": "/2176/1/00A_Entire_Thesis_Hyperlinks.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Phase-Shifting Full-Field Interferometric Methods for In-Plane Tensorial Stress Determination for Fracture Studies", "author": [ { "family_name": "Kramer", "given_name": "Sharlotte Lorraine Bolyard", "orcid": "0000-0001-6015-8385", "clpid": "Kramer-Sharlotte-Lorraine-Bolyard" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Anisotropic fracture criteria can be established with understanding of full-field stresses near a crack. The anisotropy of the stresses implies that the full in-plane tensorial stress is required, but current experimental optical techniques only give the sum or difference of principal stresses, motivating development of experimental methods that combines two experimental techniques to determine all of the stress components, such as the proposed hybrid experimental method of phase-shifting photoelasticity and transmission Coherent Gradient Sensing (CGS). This thesis establishes this method for stress determination around cracks in photoelastic materials.
\r\n\r\nThis experimental method first requires a new theory for the use of CGS, a wavefront shearing interferometry technique, for photoelastic materials. The first analysis of transmission wavefront shearing interferometry for photoelastic materials is experimentally demonstrated using CGS in full field for a compressed polycarbonate plate with a side V-shaped notch with good agreement with theoretical data. For the hybrid experimental method, a six-step phase-shifting photoelasticity method determines principal stress directions and the difference of principal stresses, and the transmission CGS method utilizes a standard four-step phase-shifting method to measure the x and y first derivatives of the sum of principal stresses, which are numerically integrated for the sum of principal stresses. The full-field principal stresses may then be separated, followed by the Cartesian and polar coordinate stresses using the principal stress directions and the polar angle. The method is first demonstrated for in-plane tensorial stress determination for a compressed polycarbonate plate with a side V-shaped notch with good comparison to theoretical stress fields. The CGS-photoelasticity experimental method is then applied to determine stresses around Mode I-dominant cracks in Homalite-100. The experimental stress fields have excellent agreement with the full-field 2D asymptotic crack solution using the Mode I and Mode II stress intensity factor values calculated from the experimental data. With this foundation of stress determination around cracks in photoelastic materials and with some future analysis, this experimental method can be extended to determine stresses in anisotropic crystals for fracture studies.
\r\n", "doi": "10.7907/M9NV-T722", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:208", "collection": "thesis", "collection_id": "208", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01162009-072031", "primary_object_url": { "basename": "yogev2008.pdf", "content": "final", "filesize": 14831508, "license": "other", "mime_type": "application/pdf", "url": "/208/1/yogev2008.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Computational Evolutionary Embryogeny", "author": [ { "family_name": "Yogev", "given_name": "Or", "clpid": "Yogev-Or" } ], "thesis_advisor": [ { "family_name": "Antonsson", "given_name": "Erik K.", "clpid": "Antonsson-E-K" }, { "family_name": "Shapiro", "given_name": "Andrew A.", "clpid": "Shapiro-A-A" } ], "thesis_committee": [ { "family_name": "Antonsson", "given_name": "Erik K.", "clpid": "Antonsson-E-K" }, { "family_name": "Shapiro", "given_name": "Andrew A.", "clpid": "Shapiro-A-A" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Evolution and development (Evo-Devo), are the two main processes which produce all of the different kinds of phenotypes we see in nature. Evolutionary process is responsible for eliminating the genetic information of weak phenotypes through natural selection, and also for exploring novel genotypes through genetic operations; crossover, mutation. The development process is the process of using the set of rules (codons) written in a genome, to turn a single set (zygote) into a mature phenotype. In this thesis, evolutionary and developmental processes are used to evolve the configurations of three-dimensional structures in silico to achieve desired performances. Although natural systems utilize the combination of both evolution and development processes to produce remarkable performance and diversity, this approach has not yet been applied extensively to the design of continuous three-dimensional load-supporting structures. Beginning with a single artificial cell containing information analogous to a DNA sequence, a structure is grown according to the rules encoded in the sequence. Each artificial cell in the structure contains the same sequence of growth and development rules, and each artificial cell is an element in a finite element mesh representing the structure of the mature individual. Rule sequences are evolved over many generations through selection and survival of individuals in a population.
\r\n\r\nModularity and symmetry are visible in nearly every natural and engineered structure. Understanding of the evolution and expression of symmetry and modularity is emerging from recent biological research. Initial evidence of these attributes is present in the phenotypes that are developed from the artificial evolution, although neither characteristic is imposed nor selected for directly.
\r\n\r\nThe computational evolutionary development approach presented here shows promise for synthesizing novel configurations of high-performance systems. The approach may advance system design to a new paradigm, where current design strategies have difficulty producing useful solutions. In addition to a new design approach perse, this model gives us the ability to explore the development process, from the standpoint of complex systems analysis. The phenotypes in our system have been grown under a highly stochastic environment, which serves as a triggered mechanism for gene expression. Still, evolution was able to find solutions which are robust to these stochastic elements, both at the phenotype level (the phenotype ability to function under the environment) and the growth process itself. In addition we have also explored the effects of symmetric and nonsymmetric environment over the topology of the phenotypes; we have found strong evidence that indicates a high correlation between the two. Finally we have also established a tool which enables us to understand the relationship between the environment and the degree of modularity of the phenotype.
\r\n", "doi": "10.7907/N4XG-F402", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:3438", "collection": "thesis", "collection_id": "3438", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09102008-101837", "primary_object_url": { "basename": "Hofmann_PhD.pdf", "content": "final", "filesize": 7507739, "license": "other", "mime_type": "application/pdf", "url": "/3438/1/Hofmann_PhD.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Designing Bulk Metallic Glass Matrix Composites with High Toughness and Tensile Ductility", "author": [ { "family_name": "Hofmann", "given_name": "Douglas Clayton", "clpid": "Hofmann-Douglas-Clayton" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Atwater", "given_name": "Harry Albert", "clpid": "Atwater-H-A" }, { "family_name": "Demetriou", "given_name": "Marios D.", "clpid": "Demetriou-M-D" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Metallic glasses have been the subject of intense scientific study since the 1960s, owing to their unique properties such as high strength, large elastic limit, high hardness, and amorphous microstructure. However, bulk metallic glasses have not been used in the high strength structural applications for which they have so much potential, owing to a highly localized failure mechanism that results in catastrophic failure during unconfined loading. In this thesis, bulk metallic glass matrix composites are designed with the combined benefits of high yield strengths and tensile ductility. This milestone is achieved by first investigating the length scale of the highly localized deformation, known as shear bands, that governs fracture in all metallic glasses. Under unconfined loading, a shear band grows to a certain length that is dependent on the fracture toughness of the glass before a crack nucleates and fracture occurs. Increasing the fracture toughness and ductility involves adding microstructural stabilization techniques that prevent shear bands from lengthening and promotes formation of multiple shear bands. To accomplish this, we develop in-situ formed bulk metallic glass matrix-composites with soft crystalline dendrites whose size and distribution are controlled through a novel semi-solid processing technique. The new alloys have a dramatically increased room-temperature ductility and a fracture toughness that appears to be similar to the toughest steels. Owing to their low modulus, the composites are therefore among the toughest known materials, a claim that has recently been confirmed independently by a fracture mechanics group. We extend our toughening strategy to a titanium-vanadium-based glass-dendrite composite system with density as low as 4.97 g/cm3. The new low-density composites rival the mechanical properties of the best structural crystalline Ti alloys. We demonstrate new processing techniques available in the highly toughened composites: room temperature cold rolling, work hardening, and thermoplastic forming. This thesis is a proven road map for developing metallic glass composites into real structural engineering materials.\r\n", "doi": "10.7907/CJZ7-VV41", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:3823", "collection": "thesis", "collection_id": "3823", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09292008-204618", "primary_object_url": { "basename": "Akellythesis090408.pdf", "content": "final", "filesize": 1100957, "license": "other", "mime_type": "application/pdf", "url": "/3823/1/Akellythesis090408.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "A Constitutive Relation for Shape-Memory Alloys", "author": [ { "family_name": "Kelly", "given_name": "Alex", "clpid": "Kelly-Alex" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Murray", "given_name": "Richard M.", "clpid": "Murray-R-M" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" }, { "family_name": "Murray", "given_name": "Richard M.", "clpid": "Murray-R-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The novel nonlinear thermoelastic behavior of shape-memory alloys (SMAs) makes them increasingly desirable as components in many advanced technological applications. In order to incorporate these materials into engineering designs, it is important to develop an understanding of their constitutive response. The purpose of this thesis is to develop a constitutive model of shape-memory polycrystals that is faithful to the underlying micromechanics while remaining simple enough for utility in engineering analysis and design.
\r\n\r\nWe present a model in which the material microstructure is represented macroscopically as a recoverable transformation strain that is constrained by the texture of the polycrystal. The point of departure in this model is the recognition that the mechanics of the onset of martensitic transformation are fundamentally different from those of its saturation. Consequently, the constraint on the set of recoverable strains varies throughout the transformation process. The effects of constraint geometry on the constitutive response of SMAs are studied. Several well known properties of SMAs are demonstrated. Finally the model is simply implemented in a commercial finite-element package as a proof of the concept.
\r\n", "doi": "10.7907/YMT5-AX47", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:3567", "collection": "thesis", "collection_id": "3567", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09162008-023546", "type": "thesis", "title": "A Critical Appraisal of Nanoindentation with Application to Elastic-Plastic Solids and Soft Materials", "author": [ { "family_name": "Poon", "given_name": "Poh Chieh Benny", "clpid": "Poon-Poh-Chieh-Benny" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Rittel", "given_name": "Daniel", "clpid": "Rittel-D" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This study examines the accuracy of the extracted elastic properties using nanoindentation. Since the conventional method to extract these properties utilizes Sneddon\u2019s elastic solution, this study first considers indentations of linearly elastic solids for direct comparison. The study proposes a criterion for a converged specimen\u2019s geometry and modifies Sneddon\u2019s equation to account for the finite tip radius and specimen compressibility effects. A composite correction factor is derived to account for the violations of the underlying assumptions behind Sneddon\u2019s derivation. This factor is a function of indentation depth, and a critical depth is derived beyond which the finite tip radius effect will be insignificant. Techniques to identify the radius of curvature of the indenter and to decouple the elastic constants for linear elastic materials are proposed. Experimental results on nanoindentation of natural latex are reported and discussed in light of the proposed modified relation and techniques.
\r\n\r\nThe second part of the study examines the accuracy of the extracted material properties in elastic-plastic nanoindentations. The study establishes that the accurate determination of the projected area of contact, A, is crucial. However, the conventional method to determine A is largely limited to elastic materials, hence a new electrical resistance method is proposed to measure A for elastic-plastic materials. With an accurate A, the error associated with the extracted elastic material properties is reduced by more than 50% in some cases. This error remains to be a function of the material\u2019s Poisson\u2019s ratio, which is identified to influence the amount of residual stresses at the plastic imprint.
\r\n\r\nFinally, this study examines the accuracy of the extracted material properties in the nanoindentation of soft materials using an Atomic Force Microscope (AFM). The effects of cantilever stiffness, preload, and surface interaction forces are observed to influence the measurements. Three set of experiments were performed to decouple these effects. The effect of a preload resembles a shift of nanoindentation load-displacement curve, while the cantilever stiffness is observed to have significant influence on the measurement of the surface forces. Lastly, a novel technique to account for these effects is proposed, in order to accurately extract the material properties of interest.
", "doi": "10.7907/J1WM-BW36", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:3701", "collection": "thesis", "collection_id": "3701", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09222008-161217", "primary_object_url": { "basename": "PHD_thesis.pdf", "content": "final", "filesize": 53260660, "license": "other", "mime_type": "application/pdf", "url": "/3701/1/PHD_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Wrinkling of Dielectric Elastomer Membranes", "author": [ { "family_name": "Zheng", "given_name": "Ling", "clpid": "Zheng-Ling" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Wrinkling of thin membranes due to different in-plane loading and boundary conditions has drawn attention of researchers in structural engineering since the development of thin webs for early aircraft structures. More recently, prestressed lightweight membrane structures have been proposed for future space missions, for example solar sails, the next generation space telescope sunshield and space-based radar systems. These structures are often partially wrinkled during operation. The formation of wrinkles alters the load paths and the structural stiffness of the membranes. More importantly its occurrence degrades the surface accuracy of these structures, which is a key design parameter.
\r\n\r\nThis dissertation focuses on wrinkling of thin rectangular membranes subjected to uniaxial tension and investigates the onset and profiles of wrinkles using both experimental and numerical approaches.
\r\n\r\nAn optical method, which integrates fringe projection method with four-frame phase-shifting technique, pre-conditioned conjugate gradient phase unwrapping algorithm and series-expansion carrier removal technique was developed in order to measure the full-field out-of-plane displacement of membranes, and an optical system was constructed including a uniaxial tension testbed, a LCD projector and a CCD camera. A series of uniaxial tensile tests were carried out on silicone rubber membranes of varying dimensions and aspect ratios in order to investigate the effect of geometric factors such as membrane dimension and aspect ratio on wrinkling onset; and a series of measurements were performed on each membrane at several desired strain levels to understand the evolution of the wrinkles, in particular wrinkle amplitude and wavelength.
\r\n\r\nA numerical study was carried out using the commercial finite element software ABAQUS to further understand the important characteristics of wrinkling of thin membranes observed in the physical model. Geometrically nonlinear finite element models of membrane structures were constructed with thin-shell elements. A series of simulations were carried out for different membrane dimensions. The critical buckling load and buckling modes was predicted for each dimension using a pre-buckling eigenvalue analysis. The desirable buckling mode was selected and introduced into the structure as a geometric imperfection. The formation and growth of wrinkles were simulated in the post-buckling analysis.
\r\n\r\nFinally, an idea of suppressing wrinkle instabilities of dielectric elastomer membranes using through-thickness electric field was proposed and verified in both experiment and numerical simulations.
", "doi": "10.7907/RTAB-GX13", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:4240", "collection": "thesis", "collection_id": "4240", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10242008-070701", "primary_object_url": { "basename": "LuXiao_Thesis_Oct08.pdf", "content": "final", "filesize": 7721293, "license": "other", "mime_type": "application/pdf", "url": "/4240/1/LuXiao_Thesis_Oct08.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Combined Experimental and Numerical Study of Spontaneous Dynamic Rupture on Frictional Interfaces", "author": [ { "family_name": "Lu", "given_name": "Xiao", "clpid": "Lu-Xiao" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The process of spontaneous dynamic frictional sliding along the interface of two elastic solids is of great interest to a number of disciplines in engineering and sciences. Applications include frictional rupture processes in earthquakes, delamination of layered composite materials, and sliding between soft membranes in biological systems. The transient nature of rupture dynamics presents an array of fascinating yet challenging questions, including the nucleation process, the mechanism of interface failure, and the speed and mode of rupture propagation.
\r\n\r\nThis thesis presents such a combined experimental and theoretical study aimed at understanding the conditions for selecting pulse-like vs. crack-like rupture modes and subshear vs. supershear rupture speeds. There are two major contributions in this work. The first one is high-resolution experimental study of the rupture modes on a frictional interface. The study presents first experimental observations of spontaneous pulse-like ruptures in a homogeneous linear-elastic setting that mimics crustal earthquakes, reveals how different rupture modes are selected based on the level of fault prestress, demonstrates that both rupture modes can transition to supershear speeds, and advocates, based on comparison with theoretical studies, importance of velocity-weakening friction for earthquake dynamics. The second major contribution is the numerical modeling of the rupture experiments that reveal the importance of the rupture nucleation mechanism and friction formulations. The modeling of sub-Rayleigh to supershear transition has demonstrated the influence of rupture nucleation mechanism on supershear transition distance, as well as on the mechanism of supershear transition. The modeling of pulse-like to crack-like rupture mode transition has confirmed the necessity of velocity weakening friction for producing pulse-like rupture to match the experimental observations.
\r\n", "doi": "10.7907/BGGT-MC04", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:41", "collection": "thesis", "collection_id": "41", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01062009-001830", "primary_object_url": { "basename": "Thesis_Jin-Yoo_Suh.pdf", "content": "final", "filesize": 8712088, "license": "other", "mime_type": "application/pdf", "url": "/41/4/Thesis_Jin-Yoo_Suh.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Fracture Toughness Study on Bulk Metallic Glasses and Novel Joining Method Using Bulk Metallic Glass Solder", "author": [ { "family_name": "Suh", "given_name": "Jin-Yoo", "orcid": "0000-0003-3786-6652", "clpid": "Suh-Jin-Yoo" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" }, { "family_name": "Conner", "given_name": "Robert Dale", "clpid": "Conner-R-D" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The fracture toughness of three new compositional variants of the Zr-Ti-Be-LTM (Late Transition Metal) family of bulk metallic glasses (BMG's) are studied in the as-cast and annealed condition. Quaternary Zr-Ti-Cu-Be alloys consistently had linear elastic fracture toughness values greater than 80 MPa\u2219m1/2, while Vitreloy 1, a Zr-Ti-Cu-Ni-Be alloy, had an average fracture toughness of 48.5 MPa\u2219m1/2 with a large amount of scatter. The addition of iron to Vitreloy 1 reduced the fracture toughness to 25 MPa\u2219m1/2. The Zr-Ti-Cu-Be alloy, having fracture toughness KQ = 85 MPa\u2219m1/2 as cast, was annealed at various time/temperature combinations. When the alloy was annealed 50C below Tg, the fracture toughness dropped to 6 MPa\u2219m1/2, while DSC and X-ray showed the alloy to still be amorphous. Fracture surfaces were analyzed using scanning electron microscopy. The tougher samples have shown evidence of highly jagged patterns at the beginning stage of crack propagation, and the length scale and roughness of this jagged pattern correlate well with the measured fracture toughness values. These jagged patterns, the main source of energy dissipation in the sample, are attributed to the formation of shear bands inside the sample. This observation provides a strong evidence of significant \u201cplastic zone\u201d screening at the crack tip.
\r\n\r\nUnlike the unstable fracture behavior of monolithic BMG's, ductile phase containing in-situ BMG composite shows stable crack growth behavior. Application of ductile BMG as a matrix for an in-situ composite with controlled microstructural characteristic length scales maximizes the toughening effect. In order to characterize this highly toughened BMG composite, the elastic-plastic fracture mechanics concept is introduced and the J-parameter is evaluated.
\r\n\r\nA novel thermoplastic bonding concept is demonstrated based on the unique rheological behavior and pattern-replication ability of bulk metallic glass forming liquids. In this approach, the bulk metallic glass is heated above Tg to the \u201csupercooled liquid\u201d region while a small normal force is applied to the joint. This results in liquid reflow, wetting and a strong bond. Complete wetting between copper substrates and a layer of platinum based bulk metallic glass leads to an atomistically intimate void-free interface.
\r\n", "doi": "10.7907/EQ77-D286", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:5193", "collection": "thesis", "collection_id": "5193", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05212009-173044", "primary_object_url": { "basename": "Thesis_Bo_Li.pdf", "content": "final", "filesize": 17993982, "license": "other", "mime_type": "application/pdf", "url": "/5193/1/Thesis_Bo_Li.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The Optimal Transportation Method in Solid Mechanics", "author": [ { "family_name": "Li", "given_name": "Bo", "orcid": "0000-0002-0127-8210", "clpid": "Li-Bo" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This dissertation is concerned with the development of a robust and efficient meshless method, the Optimal Transportation Method (OTM), for general solid flows involving extremely large deformation, fast, transient loading and hydrodynamic phenomena. This method is a Lagrangian particle method through an integration of optimal transportation theory with meshless interpolation and material point integrations. The theoretical framework developed in this thesis generalized the Benamou-Brenier differential formulation of optimal transportation problems and leads to a multi-field variational characterization of solid flows, including elasticity, inelasticity, equation of state, and general geometries and boundary conditions. To this end, the accuracy, robustness and versatility of OTM is assessed and demonstrated with convergence and stability test, Taylor anvil test and a series of full three-dimensional simulations of high/hyper-velocity impact examples with the aid of a novel meshless dynamic contact algorithm presented in this thesis.\r\n", "doi": "10.7907/FAT3-0247", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:5207", "collection": "thesis", "collection_id": "5207", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272009-165712", "primary_object_url": { "basename": "Ajay_Thesis.pdf", "content": "final", "filesize": 1453432, "license": "other", "mime_type": "application/pdf", "url": "/5207/1/Ajay_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Simulation of Dynamic Interface Fracture Using Spectral Boundary Integral Method", "author": [ { "family_name": "Harish", "given_name": "Ajay Bangalore", "clpid": "Harish-Ajay-Bangalore" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Simulation of three-dimensional dynamic fracture events constitutes one of the most challenging topics in the field of computational mechanics. Spontaneous dynamic fracture along the interface of two elastic solids is of great importance and interest to a number of disciplines in engineering and science. Applications include dynamic fractures in aircraft structures, earthquakes, thermal shocks in nuclear containment vessels and delamination in layered composite materials.
\r\n\r\nThis thesis presents numerical modeling of laboratory experiments on dynamic shear rupture, giving an insight into the experimental nucleation conditions. We describe a methodology of dynamic rupture simulation using spectral boundary integral method, including the theoretical background, numerical implementation and cohesive zone models relevant to the dynamic fracture problem. The developed numerical implementation is validated using the simulation of Lamb's problem of step loading on an elastic half space and mode I crack propagation along a bonded interface. Then the numerical model and its comparison with experimental measurements is used to investigate the initiation procedure of the dynamic rupture experiments. The inferred parameters of the initiation procedure can be used in future studies to model the experimental results on supershear transition and rupture models.
\r\n", "doi": "10.7907/B6P7-F707", "publication_date": "2009", "thesis_type": "engd", "thesis_year": "2009" }, { "id": "thesis:5220", "collection": "thesis", "collection_id": "5220", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06012009-104937", "type": "thesis", "title": "The Optimal Transportation Meshfree Method for General Fluid Flows and Strongly Coupled Fluid-Structure Interaction Problems", "author": [ { "family_name": "Habbal", "given_name": "Feras", "clpid": "Habbal-Feras" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Owhadi", "given_name": "Houman", "clpid": "Owhadi-H" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis develops a novel meshfree numerical method for simulating general fluid flows. Drawing from concepts in optimal mass transport theory and in combination with the notion of material point sampling and meshfree interpolation, the optimal transport meshfree (OTM) method provides a rigorous mathematical framework for numerically simulating three-dimensional general fluid flows with general, and possibly moving boundaries (as in fluid-structure interaction simulations). Specifically, the proposed OTM method generalizes the Benamou-Brenier differential formulation of optimal mass transportation problems which leads to a multi-field variational characterization of general fluid flows including viscosity, equations of state and general geometries and boundary conditions. With the use of material point sampling in conjunction with local max-entropy shape functions, the OTM method leads to a meshfree formulation bearing a number of salient features. Compared with other meshfree methods that face significant challenges to enforce essential boundary conditions as well as couple to other methods, such as the finite element method, the OTM method provides a new paradigm in meshfree methods. The OTM method is numerically validated by simulating the classical Riemann benchmark example for Euler flow. Furthermore, in order to highlight the ability of the OTM to simulate Navier-Stokes flows within general, moving three-dimensional domains, and naturally couple with finite elements, an illustrative strongly coupled FSI example is simulated. This illustrative FSI example, consisting of a gas-inflated sphere impacting the ground, is simulated as a toy model of the final phase of NASA's landing scheme devised for Mars missions, where a network of airbags are deployed to dissipate the energy of impact.\r\n\r\n", "doi": "10.7907/MHQX-3Z52", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:638", "collection": "thesis", "collection_id": "638", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02142009-181805", "primary_object_url": { "basename": "Thesis_YiLiu_Sep08.pdf", "content": "final", "filesize": 7225175, "license": "other", "mime_type": "application/pdf", "url": "/638/1/Thesis_YiLiu_Sep08.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Three-Dimensional Elastodynamic Modeling of Frictional Sliding with Application to Intersonic Transition", "author": [ { "family_name": "Liu", "given_name": "Yi", "clpid": "Liu-Yi" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Spontaneous slip on frictional interfaces involves both short-lived inertially-driven events and long-term quasi-static sliding. An example of considerable practical importance is the response of faults in the Earth's crust to tectonic loading. The response combines earthquakes that cause destructive ground motions and aseismic slip. Numerical models are needed to study the physics and mechanics of such complex behavior. In part, the models can help understand the observed slip patterns and interpret them in terms of constitutive properties of rocks determined in the lab.
\r\n\r\nThis thesis contains two main contributions. The first one is the development and implementation of a 3D methodology for simulations of spontaneous long-term interface slip punctuated by rapid inertially driven ruptures. Our approach is the first one to combine long-term deformation histories and the resulting stress redistribution on faults with full inclusion of inertial effects during simulated earthquakes in the context of 3D models. It reproduces all stages of earthquake cycles, from accelerating slip before dynamic instability, to rapid inertially driven propagation of earthquake rupture, to post-seismic slip, and to interseismic creep, including aseismic transients. The second main contribution is the discovery of the potentially dominating effect of favorable heterogeneity on intersonic transition in earthquakes, in both 2D models of single dynamic ruptures and 3D models of long-term fault slip. Studies of intersonic ruptures are practically important as they have the potential to cause strong ground motion farther from the fault than subsonic ruptures. Our conclusion that rheological boundaries promote transition to intersonic speeds in 3D rupture models is completely unexpected, as the neighboring stably slipping regions inhibit fast, inertially driven slip. The result could not be established in earlier studies, as it requires the computational methodology developed here that combines inertial effects, long-term slip histories, and 3D fault models. The thesis also develops test problems for dynamic rupture propagation and evaluates simplified quasi-dynamic approaches.
\r\n\r\nThe obtained results emphasize that dynamic ruptures should be considered in the context of the entire slip history of the fault, as such approach allows dynamic ruptures to occur under stress conditions established by prior slip, which leads to characteristic stress distributions that are not considered in single-event simulations. The developed 3D methodology can be applied to a number of problems in earthquake physics and mechanics that involve interaction of seismic and aseismic slip.
\r\n", "doi": "10.7907/JWCV-8V74", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:5215", "collection": "thesis", "collection_id": "5215", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05292008-115810", "primary_object_url": { "basename": "Rinderknecht_thesis.pdf", "content": "final", "filesize": 26701577, "license": "other", "mime_type": "application/pdf", "url": "/5215/16/Rinderknecht_thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Development of a Microimpedance Pump for Pulsatile Flow Transport - Part 1: Flow Characteristics of the Microimpedance Pump. Part 2: A Systematic Study of Steady and Pulsatile Transport in Microscale Cavities", "author": [ { "family_name": "Rinderknecht", "given_name": "Derek Gresham", "orcid": "0000-0001-6387-8433", "clpid": "Rinderknecht-Derek- Gresham" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Microfluidics offers an effective means to carry out a wide range of transport processes within a controlled microenvironment by drawing on the benefits imparted by increasing surface area to volume ratio at the microscale. Critical to the impact of microfluidics on integrated devices in the fields of bioengineering and biomedicine is the ability to transport fluids and biomolecules effectively particularly at the size scales involved. In this context a bio-inspired pumping mechanism, the valveless impedance pump, was explored for applications in microfluidics ranging from micro total analysis systems to microchannel cooling. Adhering to the basic principles of the impedance pump mechanism, pumps have been constructed at a variety of size scales from a few centimeters to a few hundred microns. The micro impedance pump is valveless, bidirectional, and can be constructed simply from a wide range of materials. Depending on the size of the pump flow rates range from nL/min to mL/min and pressures can be generated that exceed 20 kPa. Another benefit of the impedance pump is the pulsatile flow output which can be used in the context of microfluidic applications to enhance transport at low Reynolds numbers as well as metering in drug delivery.
\r\n\r\nPulsatile flow was therefore investigated as a method of augmenting transport in microfluidic systems. Micro PIV was used to study the affect of both steady and pulsatile flows on transport at low Reynolds number was examined in microscale rectangular cavities. Ventilation of the cavity contents was examined in terms of the residence time or average time a particle remains in the cavity region. Lagrangian coherent structures (LCS) were applied to empirical velocity fields to determine the impact of unsteadiness on time dependent boundaries to fluid transport present in the flow. Experimental results show that there are both frequencies which are beneficial and detrimental to cavity ventilation as well as certain frequencies which more evenly distribute particles originating in the cavity throughout the freestream.
\r\n", "doi": "10.7907/0K4J-0548", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2322", "collection": "thesis", "collection_id": "2322", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05302008-161653", "primary_object_url": { "basename": "WinstonJacksonThesis.pdf", "content": "final", "filesize": 40556815, "license": "other", "mime_type": "application/pdf", "url": "/2322/1/WinstonJacksonThesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Characterization of Soft Polymers and Gels Using the Pressure-Bulge Technique", "author": [ { "family_name": "Jackson", "given_name": "Winston Paul", "clpid": "Jackson-Winston-Paul" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A method to characterize the bulk hydrated properties of soft polymers and hydrogels, whose moduli are in the low MPa regime, using the pressure-bulge technique is presented. The pressure-bulge technique has been used extensively in the characterization of thin films, particularly for the case of metals. The extension of the plane-strain and circular bulge techniques to determine the Young's modulus and Poisson's ratio of bulk latex and silicone rubber sheets are shown here, in addition to the viscoelastic behavior of 5% agarose gels in the time domain using relaxation tests.
\r\n\r\nThe membranes are clamped between two stainless steel plates that are connected to a liquid pressure chamber. A syringe connected to a linear actuator causes changes in the pressure and displacement, and a pressure sensor and confocal displacement sensor are used to monitor these changes in real time. The theory presented converts the measured pressure and displacement data into stress and stretch data, using a geometrically nonlinear analysis, and the elastic/viscoelastic properties are then determined from this data.
\r\n\r\nThe results from the bulge tests are compared with data from uniaxial tension tests on hydrated specimens, and the data comparison with respect to each of the materials tested show good agreement between the two measurements. These results show promise regarding the use of pressure-displacement techniques to characterize other soft material systems, including biological polymers and tissues, as well as cell-to-matrix and cell-to-cell interactions under varying mechanical loading conditions of cell substrates.
\r\n", "doi": "10.7907/DMZ9-RE14", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:4238", "collection": "thesis", "collection_id": "4238", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10242007-131150", "primary_object_url": { "basename": "Tamer_thesis.pdf", "content": "final", "filesize": 5243266, "license": "other", "mime_type": "application/pdf", "url": "/4238/1/Tamer_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Constitutive models for polymers and soft biological tissues", "author": [ { "family_name": "El Sayed", "given_name": "Tamer", "clpid": "ElSayed-Tamer" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Soft materials such as polymers and biological tissues have several engineering and biomechanical applications. These materials exhibit complex mechanical behavior, characterized by large strains, hysteresis, rate sensitivity, stress softening (Mullins effect), and deviatoric and volumetric plasticity. The need to accurately predict the behavior of such materials has been a tremendous challenge for scientists and engineers.
\r\n\r\nThis thesis presents a seamless, fully variational constitutive model capable of capturing all of the above complex characteristics. Also, this work describes a fitting procedure based on the use of Genetic Algorithms, which proves to be necessary for the multi-modal, non-convex optimization required to identify fitting material parameters.
\r\n\r\nThe capabilities of the presented model are demonstrated via several fits of experimental tests on a wide range of materials. These tests involve monotonic and cyclic loading of polyurea, high-density polyethylene, and brain tissue, and also involve cyclic hysteresis, softening, rate effects, shear, and cavitation plasticity.
\r\n\r\nApplication to ballistic impact on a polyurea retrofitted DH36 steel plate is simulated and validated, utilizing the soft material model presented in this thesis for the polymer and a porous plasticity model for the metal. Localization elements are also included in this application to capture adiabatic shear bands. Moreover, computational capability for assessing the blast performance of metal/elastomer composite shells utilizing the soft material model for the elastomer is also presented.
\r\n\r\nAnother implemented application is in the area of traumatic brain injuries under impact/acceleration loading. Clinically observed brain damage is reproduced utilizing the model presented in this work and a predictive capability of the distribution, intensity, and reversibility/irreversibility of brain tissue damage is demonstrated.
\r\n", "doi": "10.7907/KH16-4S81", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:4802", "collection": "thesis", "collection_id": "4802", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12052007-141434", "primary_object_url": { "basename": "Thesis_CK.pdf", "content": "final", "filesize": 1220651, "license": "other", "mime_type": "application/pdf", "url": "/4802/1/Thesis_CK.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Bayesian Learning for Earthquake Engineering Applications and Structural Health Monitoring", "author": [ { "family_name": "Oh", "given_name": "Chang Kook", "clpid": "Oh-Chang-Kook" } ], "thesis_advisor": [ { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" } ], "thesis_committee": [ { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Porter", "given_name": "Keith A.", "clpid": "Porter-K-A" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Parallel to significant advances in sensor hardware, there have been recent developments of sophisticated methods for quantitative assessment of measured data that explicitly deal with all of the involved uncertainties, including inevitable measurement errors. The existence of these uncertainties often causes numerical instabilities in inverse problems that make them ill-conditioned.
\r\n\r\nThe Bayesian methodology is known to provide an efficient way to alleviate this ill-conditioning by incorporating the prior term for regularization of the inverse problem, and to provide probabilistic results which are meaningful for decision making.
\r\n\r\nIn this work, the Bayesian methodology is applied to inverse problems in earthquake engineering and especially to structural health monitoring. The proposed methodology of Bayesian earning using automatic relevance determination (ARD) prior, including its kernel version called the Relevance Vector Machine, is presented and applied to earthquake early warning, earthquake ground motion attenuation estimation, and structural health monitoring, using either a Bayesian classification or regression approach.
\r\n\r\nThe classification and regression are both performed in three phases: (1) Phase I (feature extraction phase): Determine which features from the data to use in a training dataset; (2) Phase II (training phase): Identify the unknown parameters defining a model by using a training dataset; and (3) Phase III (prediction phase): Predict the results based on the features from new data.
\r\n\r\nThis work focuses on the advantages of making probabilistic predictions obtained by Bayesian methods to deal with all uncertainties and the good characteristics of the proposed method in terms of computationally efficient training, and, especially, prediction that make it suitable for real-time operation. It is shown that sparseness (using only smaller number of basis function terms) is produced in the regression equations and classification separating boundary by using the ARD prior along with Bayesian model class selection to select the most probable (plausible) model class based on the data. This model class selection procedure automatically produces optimal regularization of the problem at hand, making it well-conditioned.
\r\n\r\nSeveral applications of the proposed Bayesian learning methodology are presented. First, automatic near-source and far-source classification of incoming ground motion signals is treated and the Bayesian learning method is used to determine which ground motion features are optimal for this classification. Second, a probabilistic earthquake attenuation model for peak ground acceleration is identified using selected optimal features, especially taking a non-linearly involved parameter into consideration. It is shown that the Bayesian learning method an be utilized to estimate not only linear coefficients but also a non-linearly involved parameter to provide an estimate for an unknown parameter in the kernel basis functions for elevance Vector Machine. Third, the proposed method is extended to a general case of regression problems with vector outputs and applied to structural health monitoring applications. It is concluded that the proposed vector output RVM shows promise for estimating damage locations and their severities from change of modal properties such as natural frequencies and mode shapes.
", "doi": "10.7907/RFD5-7Y72", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:1929", "collection": "thesis", "collection_id": "1929", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05212008-221749", "primary_object_url": { "basename": "00_jwolf_full_thesis.pdf", "content": "final", "filesize": 27155612, "license": "other", "mime_type": "application/pdf", "url": "/1929/1/00_jwolf_full_thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "A Plasticity Model to Predict the Effects of Confinement on Concrete", "author": [ { "family_name": "Wolf", "given_name": "Julie Anne", "clpid": "Wolf-Julie-Anne" } ], "thesis_advisor": [ { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" } ], "thesis_committee": [ { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A plasticity model to predict the behavior of confined concrete is developed. The model is designed to implicitly account for the increase in strength and ductility due to confining a concrete member. The concrete model is implemented into a finite element (FE) model. By implicitly including the change in the strength and ductility in the material model, the confining material can be explicitly included in the FE model. Any confining material can be considered, and the effects on the concrete of failure in the confinement material can be modeled. Test data from a wide variety of different concretes utilizing different confinement methods are used to estimate the model parameters. This allows the FE model to capture the generalized behavior of concrete under multiaxial loading. The FE model is used to predict the results of tests on reinforced concrete members confined by steel hoops and fiber reinforced polymer (FRP) jackets. Loading includes pure axial load and axial load-moment combinations. Variability in the test data makes the model predictions difficult to compare but, overall, the FE model is able to capture the effects of confinement on concrete. Finally, the FE model is used to compare the performance of steel hoop to FRP confined sections, and of square to circular cross sections. As expected, circular sections are better able to engage the confining material, leading to higher strengths. However, higher strains are seen in the confining material for the circular sections. This leads to failure at lower axial strain levels in the case of the FRP confined sections. Significant differences are seen in the behavior of FRP confined members and steel hoop confined members. Failure in the FRP members is always determined by rupture in the composite jacket. As a result, the FRP members continue to take load up to failure. In contrast, the steel hoop confined sections exhibit extensive strain softening before failure. This comparison illustrates the usefulness of the concrete model as a tool for designers. Overall, the concrete model provides a flexible and powerful method to predict the performance of confined concrete.", "doi": "10.7907/XMA5-MK90", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:4214", "collection": "thesis", "collection_id": "4214", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10222007-134413", "primary_object_url": { "basename": "GangChowThesis.pdf", "content": "final", "filesize": 3022357, "license": "other", "mime_type": "application/pdf", "url": "/4214/1/GangChowThesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Laser Tweezers for Moving Live Dissociated Neurons", "author": [ { "family_name": "Chow", "given_name": "Gang", "clpid": "Chow-Gang" } ], "thesis_advisor": [ { "family_name": "Pine", "given_name": "Jerome", "clpid": "Pine-J" } ], "thesis_committee": [ { "family_name": "Pine", "given_name": "Jerome", "clpid": "Pine-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Kasdan", "given_name": "Harvey L.", "clpid": "Kasdan-H-L" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A laser tweezers system for transporting dissociated neurons into small \"cages\" on a culture dish was constructed, and it was studied extensively.
\r\n\r\nThe system consists of an inverted microscope, a 1064 nm or 980 nm laser module, a beam expander, a motorized mechanical stage, a CCD camera, and steering mirrors. A laser beam is generated by the IR laser module, and the beam is expanded by the beam expander to match the size of the back aperture of the objective. The beam is then steered into the objective where it is focused to a point. The system uses this single, tightly focused laser beam to trap a neuron. Once a neuron is trapped and lifted, the mechanical stage is moved to locate the neuron above its destination. The system will know the location of the neurocages and will automatically move neurons to their destination.
\r\n\r\nNewly dissociated neurons will attach to most substrate surfaces eagerly, and the lifting of a neuron is impossible when it is attached to the surface. Many possible surfaces were investigated, and it was discovered that the surface can best be made \"non-sticky\" for more than an hour, by coating the surface with Poly-2-hydroxyethyl methacrylate (PolyHEMA). The neural survival at different laser intensities, exposure times, and wavelengths were studied. The results show that neural survival depends strongly on laser wavelengths, and a 980 nm laser is less damaging than a 1064 nm laser. For 980 nm, perfect survival after irradiation is independent of laser power up to our maximum of 130 mW for exposure time up to 4 minutes. At 17 mW, almost all neurons can be lifted off a PolyHEMA substrate. The maximum speed for moving a neuron through the medium at different laser intensities was studied, and was 250 um/s at 100 mW for 980 nm. The studies have shown that a laser tweezers system is suitable for transporting live dissociated neurons over millimeter distance in less than a minute. The neural survival in neurocages on glass substrate was then studied. The survival and growth over time for neurons loaded into cages was found to be no different than for that of a control culture.
\r\n", "doi": "10.7907/9A72-G624", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:3765", "collection": "thesis", "collection_id": "3765", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09252007-100851", "primary_object_url": { "basename": "01TitlePage_Ack.pdf", "content": "final", "filesize": 195001, "license": "other", "mime_type": "application/pdf", "url": "/3765/1/01TitlePage_Ack.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Simulations, Modeling, and Designs of Bulk Metallic Glasses", "author": [ { "family_name": "Duan", "given_name": "Gang", "clpid": "Duan-Gang" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Goddard", "given_name": "William A., III", "clpid": "Goddard-W-A-III" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Samwer", "given_name": "Konrad", "clpid": "Samwer-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Bulk metallic glasses usually have very high yield strength, at least double that of ordinary commercially used crystalline materials, and high elastic strain limit, roughly 2% in tension or compression, due to their disordered atomic structure. Unfortunately, the Achilles heel of metallic glasses is their rather limited ductility and low resistance to the propagation of a crack especially in tension. Many research efforts have been devoted to understanding the deformation and fracture behavior of bulk metallic glasses. One interesting observation is that the properties of metallic glasses are well correlated with each other. The challenge is to understand these correlations, and to utilize such understanding to design novel glasses with good glass forming ability and mechanical toughness.
\r\n\r\nFollowing the Cooperative Shear Model, we investigated the temperature, volume, and configurational dependence of elastic properties by constructing an effective tight-binding force field for a Cu-Zr binary alloy system, and carrying out molecular dynamics simulations. We determined the isothermal Equation of State in a wide range of temperatures and pressures. Pressure-induced cavitation was observed and negative pressure is critical for triggering cavitation. The cavitation barrier height was estimated from the classical nucleation theory. The intrinsic origin of cavitation and its connection to Poisson\u2019s ratio or the ratio of G/B are investigated. The relationship to the deformation and fracture behavior of glasses is discussed.
\r\n\r\nWe designed several novel bulk metallic-glass-forming systems using the link between fragility, elastic properties, and glass forming ability as a guiding tool. The compositional dependence of thermal and elastic properties of Cu-Zr-Be ternary bulk-metallic-glass forming alloys was systematically studied. Lightweight Ti-based bulk amorphous structural metals with more than double the specific strength of conventional titanium alloys have been discovered. We report a novel class of bulk amorphous alloys with benchmark thermoplastic processability, having good glass forming ability, exceptional thermal stability, unexpectedly large Angell fragility number, and good mechanical toughness. Starting from the two binary bulk glass formers in the Cu-Zr system, we systematically investigated the compositional dependence of glass formation, thermal, elastic, and mechanical properties in the Cu-Zr-Ag ternary alloys.
\r\n", "doi": "10.7907/ZH3G-JF43", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:5216", "collection": "thesis", "collection_id": "5216", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05292008-163638", "primary_object_url": { "basename": "Christian_Franck_Thesis.pdf", "content": "final", "filesize": 19389049, "license": "other", "mime_type": "application/pdf", "url": "/5216/1/Christian_Franck_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Quantitative Characterization of 3D Deformations of Cell Interactions with Soft Biomaterials", "author": [ { "family_name": "Franck", "given_name": "Christian", "clpid": "Franck-Christian" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "In recent years, the importance of mechanical forces in directing cellular function has been recognized as a significant factor in biological and physiological processes. In fact, these physical forces are now viewed equally as important as biochemical stimuli in controlling cellular response. Not only do these cellular forces, or cell tractions, play an important role in cell migration, they are also significant to many other physiological and pathological processes, both at the tissue and organ level, including wound healing, inflammation, angiogenesis, and embryogenesis. A complete quantification of cell tractions during cell-material interactions can lead to a deeper understanding of the fundamental role these forces play in cell biology. Thus, understanding the function and role of a cell from a mechanical framework can have important implications towards the development of new implant materials and drug treatments.
\r\n\r\nPrevious research has contributed significant descriptions of cell-tissue interactions by quantifying cell tractions in two-dimensional environments; however, most physiological processes are three-dimensional in nature. Recent studies have shown morphological differences in cells cultured on two-dimensional substrates versus three-dimensional matrices, and that the intrinsic extracellular matrix interactions and migration behavior are different in three dimensions versus two dimensions. Hence, measurement techniques are needed to investigate cellular behavior in all three dimensions.
\r\n\r\nThis thesis presents a full-field imaging technique capable of quantitatively measuring cell traction forces in all three spatial dimensions, and hence addresses the need of a three-dimensional quantitative imaging technique to gain insight into the fundamental role of physical forces in biological processes. The technique combines laser scanning confocal microscopy (LSCM) with digital volume correlation (DVC) to track the motion of fluorescent particles during cell-induced or externally applied deformations. This method is validated by comparing experimentally measured non-uniform deformation fields near hard and soft spherical inclusions under uniaxial compression with the corresponding analytical solution. Utilization of a newly developed computationally efficient stretch-correlation and deconvolution algorithm is shown to improve the overall measurement accuracy, in particular under large deformations.
\r\n\r\nUsing this technique, the full three-dimensional substrate displacement fields are experimentally determined during the migration of individual fibroblast cells on polyacrylamide gels. This is the first study to show the highly three-dimensional structure of cell-induced displacement and traction fields. These new findings suggest a three-dimensional push-pull cell motility, which differs from the traditional theories based on two-dimensional data. These results provide new insight into the dynamic cell-matrix force exchange or mechanotransduction of migrating cells, and will aid in the development of new three-dimensional cell motility and adhesion models.
\r\n\r\nAs this study reveals, the mechanical interactions of cells and their extracellular matrix appear to be highly three-dimensional. It also shows that the LSCM-DVC technique is well suited for investigating the mechanics of cell-matrix interactions while providing a platform to access detailed information of the intricate biomechanical coupling for many cellular responses. Thus, this method has the capability to provide direct quantitative experimental data showing how cells interact with their surroundings in three dimensions and might stimulate new avenues of scientific thought in understanding the fundamental role physical forces play in regulating cell behavior.
", "doi": "10.7907/VMN5-SP86", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:1961", "collection": "thesis", "collection_id": "1961", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05222008-090253", "primary_object_url": { "basename": "ARAthesis.pdf", "content": "final", "filesize": 4584211, "license": "other", "mime_type": "application/pdf", "url": "/1961/1/ARAthesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Surface Deformation in a Liquid Environment Resulting from Single Particle Collisions", "author": [ { "family_name": "Ruiz-Angulo", "given_name": "Angel", "orcid": "0000-0002-7292-3002", "clpid": "Ruiz-Angulo-Angel" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Multiphase flows are fairly complex and they are usually studied as a bulk. In this thesis, these flows are approached by looking at single particle interactions (particle-particle and particle-wall). This work presents experimental measurements of the approach and rebound of a particle colliding with a ``deformable' surface in a viscous liquid. The complex interaction between the fluid and the solid phases is coupled through the dynamics of the flow as well as the deformation process. A simple pendulum experiment was used to produced single controlled collisions; steel particles were used to impact different aluminum alloy samples (Al-6061, Al-2024, and Al-7075) using different aqueous mixtures of glycerol and water as a viscous fluid. The velocity of the particle before and after the collision was estimated by post-processing the particle position recorded with a high speed camera. For the combination of materials proposed, the elastic limit is reached at relatively low velocities. The deformations produced by the collision were analyzed using an optical profilometer. The measurements showed that the size of the indentations is independent of the fluid media. It was found that the size of the indentations was the same for collisions in air than for the rest of the collisions using various viscous fluids. The results show that the plastic deformation is only a function of the impact velocity and the material properties. The normal coefficient of restitution and deformation parameters account for losses due to lubrication effect and inelasticity, identifying then, the dominant energy loss mechanism during the collision process.
\r\n\r\nAccording to the strain imposed in the samples due to the collision, the deformations were either elastic or elastic-plastic. The equivalent load due to the impact velocities used in this work did not reach the fully-plastic regime. For the collisions in air, different models were used to compare the experimental results showing that the elastic-plastic regime is not well characterized by only the material properties and the impact velocity. The time-resolved contact force was measured during the process of the indentation for the dry collision experiments using a quartz load transducer.
\r\n\r\nThe experiments clearly show four different regimes depending on the impact Stokes number: lubrication effect and elastic deformation, lubrication effect and elastic-plastic deformation, elastic deformation with no hydrodynamic effects, and elastic-plastic deformation with negligible lubrication effect. An analysis of the erosion of ductile materials during immersed collisions is presented. The size of the crater formed by the impact of a single particle against a ductile target can be estimated from theory, and these estimates agree well with experimental measurements.
", "doi": "10.7907/WVJT-TG10", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2067", "collection": "thesis", "collection_id": "2067", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05252007-000127", "primary_object_url": { "basename": "Dissertation_SamanthaDaly.pdf", "content": "final", "filesize": 2774907, "license": "other", "mime_type": "application/pdf", "url": "/2067/1/Dissertation_SamanthaDaly.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Deformation and Fracture of Thin Sheets of Nitinol", "author": [ { "family_name": "Daly", "given_name": "Samantha Hayes", "orcid": "0000-0002-7297-1696", "clpid": "Daly-Samantha-Hayes" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Molinari", "given_name": "Alain", "clpid": "Molinari-A" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Rittel", "given_name": "Daniel", "clpid": "Rittel-D" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Nickel-Titanium (Nitinol) is a Shape Memory Alloy (SMA) that exhibits superelasticity (pseudoelasticity) and shape memory by a solid-solid state diffusion-less phase transformation. Phase transformation and the resulting strain localization in Nitinol has long been a topic of study, both for its inherent scientific interest and also because of the large number of practical applications of this bimetallic alloy. Although Nitinol devices are extensively used in the medical industry, there is a fundamental gap in the amount of high-quality quantitative experimental data detailing strain localization. The numerous applications of shape memory alloys provide the motivation to understand the deformation and failure mechanisms of these materials, particularly their fatigue and fracture behavior. By using an in-situ optical technique called Digital Image Correlation (DIC), quantitative measures of strain localization in Nitinol are presented for the first time in both deformation and failure modes. In addition, a finite element small-scale transformation analysis near a crack tip in Nitinol subjected to mode-I loading under plane stress conditions is performed for the first time. The experimental results and finite element analysis provide new and detailed insights concerning the structure of phase transformation and crack tip fields in Nitinol.", "doi": "10.7907/RATX-WG46", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:2074", "collection": "thesis", "collection_id": "2074", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05252007-154233", "primary_object_url": { "basename": "Motahari_Thesis.pdf", "content": "final", "filesize": 2238732, "license": "other", "mime_type": "application/pdf", "url": "/2074/1/Motahari_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Study of Constitutive Behavior of Ferroelectrics via Self-Consistent Modeling and Neutron Diffraction", "author": [ { "family_name": "Motahari", "given_name": "Seyed-Maziar", "clpid": "Motahari-Seyed-Maziar" } ], "thesis_advisor": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The central goal of this study is to develop a reliable self-consistent model to describe the constitutive behavior of polycrystalline ferroelectrics and to predict their lattice strain and texture evolution. Starting with the model developed by Huber et al. formulations and refinements were added to increase both the functionality and the accuracy of the model\u2019s results. These refinements include methods for calculating lattice strain, tracking the number of domains contributing to diffraction patterns, locking the domain switching at a specified level, inputting initial grain orientation distribution, and a correction for a major flaw in the previous model: the phenomenon of reverse domain switching.
\r\n\r\nTo validate the model\u2019s predictions, in-situ neutron diffraction experiments were conducted on polycrystalline BaTiO\u2083 under uniaxial compression. It was found that the data analysis required a close inspection due to lattice strain anisotropy and leading to a systematic study of different analysis methods: the single peak method, the regular whole-pattern Rietveld method (with no strain anisotropy), and the improved Rietveld method which offers limited strain anisotropy analysis. The latter was judged to be the most appropriate for ferroelectrics and it was further improved by new formulations to permit lattice strain anisotropy analysis for tetragonal and hexagonal crystal structures.
\r\n\r\nThe comparison of model predictions and diffraction data from BaTiO\u2083 yielded the following observations: (i) domain switching starts at very low stresses (<\t 10 MPa) and proceeds gradually; (ii) domains with c-axes closer to the loading axis start switching earlier and experience more switching; (iii) lattice-plane-specific (hkl) strains, with the exception of (111), exhibit apparent hardening after switching starts. The level of agreement between the model and the experimental data was satisfactory, particularly considering the relative simplicity of the model. Keeping in mind the basic assumptions present in the model, it can be a useful analytical tool in the study of ferroelectric constitutive behavior when combined with diffraction experiments.
", "doi": "10.7907/55PX-GT55", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:1822", "collection": "thesis", "collection_id": "1822", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05152007-121823", "primary_object_url": { "basename": "Vikram-Thesis-defended.pdf", "content": "final", "filesize": 3682057, "license": "other", "mime_type": "application/pdf", "url": "/1822/1/Vikram-Thesis-defended.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Electronic Structure Calculations at Macroscopic Scales", "author": [ { "family_name": "Gavini", "given_name": "Vikram", "orcid": "0000-0002-9451-2300", "clpid": "Gavini-Vikram" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knap", "given_name": "Jaroslaw", "clpid": "Knap-J" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Electronic structure calculations, especially those using density-functional theory have provided many insights into various materials properties in the recent decade. However, the computational complexity associated with electronic structure calculations has restricted these investigations to periodic geometries with small cell-sizes (computational domains) consisting of few atoms (about 200 atoms). But material properties are influenced by defects---vacancies, dopants, dislocations, cracks, free surfaces---in small concentrations (parts per million). A complete description of such defects must include both the electronic structure of the core at the fine (sub-nanometer) scale and also elastic and electrostatic interactions at the coarse (micrometer and beyond) scale. This in turn requires electronic structure calculations at macroscopic scales, involving millions of atoms, well beyond the current capability. This thesis presents the development of a seamless multi-scale scheme, Quasi-Continuum Orbital-Free Density-Functional Theory (QC-OFDFT) to address this significant issue. This multi-scale scheme has enabled for the first time a calculation of the electronic structure of multi-million atom systems using orbital-free density-functional theory, thus, paving the way to an accurate electronic structure study of defects in materials.
\r\n\r\nThe key ideas in the development of QC-OFDFT are (i) a real-space variational formulation of orbital-free density-functional theory, (ii) a nested finite-element discretization of the formulation, and (iii) a systematic means of adaptive coarse-graining retaining full resolution where necessary, and coarsening elsewhere with no patches, assumptions, or structure. The real-space formulation and the finite-element discretization gives freedom from periodicity, which is important in the study of defects in materials. More importantly, the real-space formulation and its finite-element discretization support unstructured coarse-graining of the basis functions, which is exploited to advantage in developing the QC-OFDFT method. This method has enabled for the first time a calculation of the electronic structure of samples with millions of atoms subjected to arbitrary boundary conditions. Importantly, the method is completely seamless, does not require any ad hoc assumptions, uses orbital-free density-functional theory as its only input, and enables convergence studies of its accuracy. From the viewpoint of mathematical analysis, the convergence of the finite-element approximation is established rigorously using Gamma-convergence, thus adding strength and validity to the formulation.
\r\n\r\nThe accuracy of the proposed multi-scale method under modest computational cost, and the physical insights it offers into properties of materials with defects, have been demonstrated by the study of vacancies in aluminum. One of the important results of this study is the strong cell-size effect observed on the formation energies of vacancies, where cells as large as tens of thousands of atoms were required to obtain convergence. This indicates the prevalence of long-range physics in materials with defects, and the need to calculate the electronic structure of materials at macroscopic scales, thus underscoring the importance of QC-OFDFT.
\r\n\r\nFinally, QC-OFDFT was used to study a problem of great practical importance: the embrittlement of metals subjected to radiation. The brittle nature of metals exposed to radiation is associated with the formation of prismatic dislocation loops---dislocation loops whose Burgers vector has a component normal to their plane. QC-OFDFT provides an insight into the mechanism of prismatic dislocation loop nucleation, which has remained unclear to date. This study, for the first time using electronic structure calculations, establishes vacancy clustering as an energetically favorable process. Also, from direct numerical simulations, it is demonstrated that vacancy clusters collapse to form stable prismatic dislocation loops. This establishes vacancy clustering and collapse of these clusters as a possible mechanism for prismatic dislocation loop nucleation. The study also suggests that prismatic loops as small as those formed from a 7-vacancy cluster are stable, thus shedding new light on the nucleation size of these defects which was hitherto unknown.
\r\n", "doi": "10.7907/1R69-YY30", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:2558", "collection": "thesis", "collection_id": "2558", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06122006-161234", "primary_object_url": { "basename": "Kaushik-Dayal_PhD-Thesis.pdf", "content": "final", "filesize": 1822282, "license": "other", "mime_type": "application/pdf", "url": "/2558/2/Kaushik-Dayal_PhD-Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Nonlocal Microstructural Mechanics of Active Materials", "author": [ { "family_name": "Dayal", "given_name": "Kaushik", "orcid": "0000-0002-0516-3066", "clpid": "Dayal-Kaushik" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis deals with two aspects of the mechanics of symmetry-breaking defects such as phase boundaries, inclusions and free surfaces, and their role in the macroscopic response of active materials. We first examine the problem of kinetics using a nonlocal theory, and then study the role of geometry in active materials with fields that are not confined to the material.
\r\n\r\nClassical PDE continuum models of active materials are not closed, and require nucleation and kinetic information or regularization as additional constitutive input. We examine this problem in the peridynamic formulation, a nonlocal continuum model that uses integral equations to account for long-range forces that are important at small scales, and allows resolution of the structure of interfaces. Our analysis shows that kinetics is inherent to the theory. Viewing nucleation as a dynamic instability at small times, we obtain interesting scaling results and insight into nucleation in regularized theories. We also exploit the computational ease of this theory to study an unusual mechanism that allows a phase boundary to bypass an inclusion.
\r\n\r\nShifting focus to problems of an applied nature, we consider issues in the design of ferroelectric optical/electronic circuit elements. Free surfaces and electrodes on these devices generate electrical fields that must be resolved over all space, and not just within the body. These fields greatly enhance the importance of geometry in understanding the electromechanical response of these materials, and give rise to strong size and shape dependence. We describe a computational method that transforms this problem into a local setting in an accurate and efficient manner. We apply it to three examples: closure domains, a ferroelectric slab with segmented electrodes and a notch subjected to electro-mechanical loading.
", "doi": "10.7907/YGR6-H428", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:807", "collection": "thesis", "collection_id": "807", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02282007-154751", "primary_object_url": { "basename": "thesis_main.pdf", "content": "final", "filesize": 5878517, "license": "other", "mime_type": "application/pdf", "url": "/807/1/thesis_main.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Charge-Exchange Collision Dynamics and Ion Engine Grid Geometry Optimization", "author": [ { "family_name": "Morris", "given_name": "Bradford S.", "clpid": "Morris-Bradford-S" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Culick", "given_name": "Fred E. C.", "clpid": "Culick-F-E-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Johnson", "given_name": "Lee K.", "clpid": "Johnson-L-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Astronomy Department" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "abstract": "The development of a new three-dimensional model for determining the absolute energy distribution of ions at points corresponding to spacecraft surfaces to the side of an ion engine is presented. The ions resulting from elastic collisions, both charge-exchange (CEX) and direct, between energetic primary ions and thermal neutral xenon atoms are accounted for. Highly resolved energy distributions of CEX ions are found by integration over contributions from all points in space within the main beam formed by the primary ions.
\r\n\r\nThe sputtering rate due to impingement of these ions on a surface is calculated. The CEX ions that obtain significant energy (10 eV or more) in the collision are responsible for the majority of the sputtering, though this can depend on the specific material being sputtered. In the case of a molybdenum surface located 60 cm to the side of a 30 cm diameter grid, nearly 90% of the sputtering is due to the 5% of ions with the highest collision exit energies. Previous models that do not model collision energetics cannot predict this. The present results agree with other models and predict that the majority of the ion density is due to collisions where little to no energy is transferred.
\r\n\r\nThe sputtering model is combined with a grid-structure model in an optimization procedure where the sputtering rate at specified locations is minimized by adjustment of parameters defining the physical shape of the engine grids. Constraints are imposed that require that the deflection of the grid under a specified load does not exceed a maximum value, in order to ensure survivability of the grids during launch. To faciliate faster execution of the calculations, simplifications based on the predicted behavior of the CEX ions are implemented. For diametrically opposed sputtering locations, a rounded barrel-vault shape reduces the expected sputtering rate by up to 30% in comparison to an NSTAR-shaped grid.
", "doi": "10.7907/W996-M220", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:262", "collection": "thesis", "collection_id": "262", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01212007-235934", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 10737452, "license": "other", "mime_type": "application/pdf", "url": "/262/1/Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Amorphous Metallic Foam: Synthesis and Mechanical Properties", "author": [ { "family_name": "Veazey", "given_name": "Christopher Thomas", "clpid": "Veazey-Christopher-Thomas" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Atwater", "given_name": "Harry Albert", "clpid": "Atwater-H-A" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Bulk metallic glass alloys were processed into foam by several synthesis routes. These methods utilize the thermodynamic stability and thermoplastic formability of the supercooled liquid state to produce low-density homogeneous foams. The cellular structure is shown to evolve by growth of randomly distributed spherical bubbles towards polyhedral-like cells separated by microscopic intracellular membranes exhibiting random orientations and aspect ratios. The ability of amorphous metals to develop such random cellular morphologies is attributed primarily to the high ductility exhibited by their softened state, which enables large superplastic membrane elongations during foaming.
\r\n\r\nUpon loading, moderate porosity foams are known to deform plastically by recurring non-linear yielding transitions followed by non-catastrophic collapse events. The ability of these foams to yield non-catastrophically is a result of the plastic deformability of amorphous metals in sub-millimeter dimensions. Nonlinear yielding is found to be accommodated by clusters involving 4\u20136 cells, which yield by intracellular membrane buckling and ultimately collapse plastically to produce a localized plastic collapse band. By comparison, high-porosity foams deform plastically by multiple recurring non-catastrophic collapse events without undergoing macroscopic failure. The numerous minor collapse events are associated with localized ligament collapse, and the few major collapse events are associated with the cooperative collapse of several adjacent ligaments and the formation of a collapse band. On average, the serrated flow responses between major events appear to be self-similar and resemble the recurring nonlinear yielding responses exhibited by moderate porosity foams.
", "doi": "10.7907/JXDW-9224", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:4500", "collection": "thesis", "collection_id": "4500", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11102006-182329", "primary_object_url": { "basename": "tracysthesis7_21.pdf", "content": "final", "filesize": 5410487, "license": "other", "mime_type": "application/pdf", "url": "/4500/1/tracysthesis7_21.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Mechanical Characterization of Damage and Failure in Polymeric Foams and Glass/Epoxy Composites", "author": [ { "family_name": "Kidd", "given_name": "Theresa Hiromi", "clpid": "Kidd-Theresa-Hiromi" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The mechanical characterization including evolution of damage and failure of foams and composites are becoming increasingly important, as they form the basic components of sandwich structures. Sandwich structures consist of two faceplates that surround a core material. In many modern applications, faceplates and cores are typically comprised of composite materials and polymeric foam, respectively. Knowledge of the failure behavior of these individual components is necessary for understanding the failure behavior and design of sandwich structures. A systematic investigation of the damage evolution and failure behavior of foams and composites was conducted using a variety of experimental techniques.
\r\n\r\nIn-situ ultrasonic measurements were used to track the damage behavior in PVC polymeric foams with densities ranging from 130 to 250 kg/m\u00b3. The wave speeds were measured by two quartz piezoelectric shear transducers with a resonant frequency of 5 MHz in the transmission mode. A fixture was developed and constructed to protect the transducers during compression, while allowing them to take sound speed measurements of the sample along the axis of the load train. This fixture was placed in a servo-hydraulic MTS (Materials Testing System) machine, where the load-displacement response of the foam was recorded. A digital image correlation (DIC) method was used to capture the progression of failure under compression. Two dominant failure modes, elastic buckling and plastic collapse, were identified \u2013 and their onsets corresponded to the change in elastic wave speeds in the material, measured by the in-situ ultrasonic technique.
\r\n\r\nThe transverse response of S-Glass/Epoxy unidirectional composites was investigated under varying degrees of confinement and strain rates. The experimental setup utilizes a fixture that allowed for independent measurement of the three principal stresses in a confined specimen. A servo-hydraulic materials testing system and a Kolsky (split Hopkinson) pressure bar generated strain rates between 10\u207b\u00b3 to 10\u2074 s\u207b\u00b9. Post-test scanning electron microscopy (SEM) observations suggest that under transverse loading at low-strain rates, confinement contributes to localized band formation. In addition, micrographs indicated that macroscopic transverse failure is dominated by shear stress, and occurs within these localized bands. These shear dominated failure bands were found inclined in a direction approximately 35\u00b0 to the direction of loading. Implications of this orientation deviation of failure bands from maximum shear trajectories at 45\u00b0 are discussed in reference to the state of confinement.
", "doi": "10.7907/G25Y-KE07", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:4498", "collection": "thesis", "collection_id": "4498", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11102006-152125", "primary_object_url": { "basename": "Yashashree_Thesis.pdf", "content": "final", "filesize": 1539400, "license": "other", "mime_type": "application/pdf", "url": "/4498/1/Yashashree_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Coarse-Graining of Atomistic Description at Finite Temperature", "author": [ { "family_name": "Kulkarni", "given_name": "Yashashree", "clpid": "Kulkarni-Yashashree" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knap", "given_name": "Jaroslaw", "clpid": "Knap-J" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis presents a computational method for seamlessly bridging the atomistic and the continuum realms at finite temperature. The theoretical formulation is based on the static theory of the quasicontinuum and extends it to model non-equilibrium finite temperature material response.
\r\n\r\nAt non-zero temperature, the problem of coarse-graining is compounded by the presence of multiple time scales in addition to multiple spatial scales. We address this problem by first averaging over the thermal motion of atoms to obtain an effective temperature-dependent energy on the macroscopic scale. Two methods are proposed to this end. The first method is developed as a variational mean field approximation which yields local thermodynamic potentials such as the internal energy, the free energy, and the entropy as phase averages of appropriate phase functions. The chief advantage of this theory is that it accounts for the anharmonicity of the interaction potentials, albeit numerically, unlike many methods based on statistical mechanics which require the quasi-harmonic approximation for computational feasibility. Furthermore, the theory reduces to the classical canonical ensemble approach of Gibbs under the quasi-harmonic approximation for perfect, isotropic, infinite crystals subjected to uniform temperature. In the second method, based on perturbation analysis, the internal energy is derived as an effective Hamiltonian of the atomistic system by treating the thermal fluctuations as perturbations about an equilibrium configuration.
\r\n\r\nThese energy functionals are then introduced into the quasicontinuum theory, which facilitates spatial coarse-graining of the atomistic description. Finally, a variational formulation for simulating rate problems, such as heat conduction, using the quasicontinuum method is developed. This is achieved by constructing a joint incremental energy functional whose Euler-Lagrange equations yield the equilibrium equations as well as the time-discretized heat equation.
\r\n\r\nWe conclude by presenting the results for numerical validation tests for the thermal expansion coefficient and the specific heat for some materials and compare them with classical theory, molecular dynamics results, and experimental data. Some illustrative examples of thermo-mechanical coupled problems such as heat conduction in a deformable solid, adiabatic tension test, and finite temperature nanoindentation are also presented which show qualitative agreement with expected behavior and demonstrate the applicability of the method.
", "doi": "10.7907/W9M0-HX47", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:3697", "collection": "thesis", "collection_id": "3697", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09222006-131741", "primary_object_url": { "basename": "mye_thesis_etd.pdf", "content": "final", "filesize": 25286751, "license": "other", "mime_type": "application/pdf", "url": "/3697/1/mye_thesis_etd.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Textured Ferroelectric Thin Films: Synthesis, Characterization, and Influence of Compositional Grading on the Dielectric Behavior", "author": [ { "family_name": "El-Naggar", "given_name": "Mohamed Y.", "clpid": "El-Naggar-Mohamed-Y" } ], "thesis_advisor": [ { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" } ], "thesis_committee": [ { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Haile", "given_name": "Sossina M.", "clpid": "Haile-S-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This dissertation focuses on two topics concerning the connections between structure and property in ferroelectric thin films. First, the synthesis of highly oriented ferroelectric thin films is addressed, where the texture is needed to generate high strains that rely on electromechanical domain switching. The ferroelectric films are integrated with oxide electrodes onto single crystal MgO and Si substrates using biaxially-textured MgO as buffer layers. The second topic focuses on modeling the dielectric behavior of compositionally graded ferroelectrics.
\r\n\r\nThe functional ferroelectric (Pb,Ba)TiO3 films are deposited by metalorganic chemical vapor deposition (MOCVD). SrRuO3, grown by pulsed laser deposition (PLD), is a lattice-matching electrode. Both the ferroelectric and oxide electrode layers are found to inherit the biaxial texture of the underlying MgO template, which can be deposited by ion beam assisted deposition (IBAD) directly on Si-based substrates. In addition, we demonstrated control of the ferroelectric film stoichiometry using a spectroscopic control loop that monitors the ultraviolet spectra of the gas-phase MOCVD precursors during growth. Detailed studies of the microstructural details of these films will be presented.
\r\n\r\nThe second topic of this thesis explores the dielectric behavior of functionally graded ferroelectric thin films. Homogenous ferroelectrics offer the possibility of engineering a tunable dielectric response for components in microwave circuits. However, this approach often leads to an undesired temperature sensitivity. Compositionally-graded (Ba,Sr)TiO3 ferroelectric films have been explored as a means of redressing this sensitivity, but experimental observations vary depending on geometry and other details. A continuum model is presented to calculate the capacitive response of graded ferroelectric films with realistic electrode geometries by accurately accounting for the polarization distribution and long-range electrostatic interactions. We show preferred designs that are extremely effective in obtaining high and temperature-stable dielectric properties.
", "doi": "10.7907/WMJW-H724", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:5200", "collection": "thesis", "collection_id": "5200", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05252007-140239", "primary_object_url": { "basename": "Thesis_submitted_2007-05-26.pdf", "content": "final", "filesize": 72916510, "license": "other", "mime_type": "application/pdf", "url": "/5200/1/Thesis_submitted_2007-05-26.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "On the Development of Defocusing Digital Particle Image Velocimetry with Full Characterization", "author": [ { "family_name": "Graff", "given_name": "Emilio Casta\u00f1o", "clpid": "Graff-Emilio-Casta\u00f1o" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Yang", "given_name": "Changhuei", "orcid": "0000-0001-8791-0354", "clpid": "Yang-Changhuei" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Defocusing Digital Particle Image Velocimetry is the first volumetric, three-dimensional PIV method ever put into practice. This manuscript contains the details of its development, a detailed analysis of its performance (both through simulation and real measurements), and a series of experimental demonstrations of the capability of the technique. The system is capable of resolving upwards of 7,000 vectors per pair with an absolute error on the order of 0.03% of the volume size.", "doi": "10.7907/Z9HM56F8", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:1916", "collection": "thesis", "collection_id": "1916", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05212007-043942", "primary_object_url": { "basename": "BoonratChapter0.pdf", "content": "final", "filesize": 109746, "license": "other", "mime_type": "application/pdf", "url": "/1916/3/BoonratChapter0.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Development, Characterization, and Applications of Gold and Platinum Bulk Metallic Glasses", "author": [ { "family_name": "Lohwongwatana", "given_name": "Boonrat", "orcid": "0000-0002-3131-8076", "clpid": "Lohwongwatana-Boonrat" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Haile", "given_name": "Sossina M.", "clpid": "Haile-S-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The development of bulk metallic glass alloys is presented with various elemental selection criteria, design strategies, and experimental techniques. The focus was later drawn towards the development of noble bulk metallic glasses based on gold and platinum. To formulate a good bulk glass forming composition, we found that the gold alloys had to be optimized using uncommon approaches. One strategy was to primarily increase the glass transition temperature of the alloy, instead of lowering the melting temperature. The resulting gold bulk metallic glass alloy could be cast fully amorphous up to 5 mm thick. However, the best gold glass former also exhibited many anomalous behaviors; for example, a very high strain rate could induce phase separation in the bulk glass forming liquid. A detail study on the strain rate induced crystallization was carried out systematically to pinpoint the exact conditions that would cause an anomaly.
\r\n\r\nAdditionally, a variety of comparative studies were conducted on the gold and platinum bulk metallic glass alloys, including elastic constants measurement, heat capacity measurement, viscosity measurement using three-point beam bending, and time to crystallization study in order to construct a Time-Temperature Transformation diagram.
\r\n\r\nThe last chapter switches gears to the engineering and technology aspect of gold and platinum bulk metallic glasses. The thermoplastic soldering technique is introduced as a novel method for joining any two materials at temperatures lower than that of brazing or welding processes. The proposed technique is a new alternative to the lead-free soldering process available to the electronic industry.
", "doi": "10.7907/F215-BD47", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:2451", "collection": "thesis", "collection_id": "2451", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06042007-171342", "primary_object_url": { "basename": "Full_thesis_v3_FINAL.pdf", "content": "final", "filesize": 7011552, "license": "other", "mime_type": "application/pdf", "url": "/2451/1/Full_thesis_v3_FINAL.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Measuring Stress in Thin-Film - Substrate Systems Featuring Spatial Nonuniformities of Film Thickness and/or Misfit Strain", "author": [ { "family_name": "Brown", "given_name": "Michal Amaris", "clpid": "Brown-Michal-Amaris" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Streit", "given_name": "Dwight", "clpid": "Streit-D" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "It is very important to be able to accurately determine the film stress distribution in a thin film structure, since stress can lead directly to failure and as such is intimately related to reliability and process yield. The most common way of inferring film stress caused by a given process is by measuring system curvature before and after the process; the change in curvature is directly related to the stress caused by that process, usually through the Stoney formula. This formula was derived based on a number of restrictive assumptions. Two of these are the assumptions of a spatially uniform film thickness and a spatially uniform misfit strain; taken together, these assumptions imply constant curvature and film stress over the entire wafer. In practice, these conditions are rarely met, and yet the Stoney formula is still the film stress measurement standard.
\r\nRecently, extensions to this formula were derived which allow for spatial non-uniformities in film thickness and misfit strain. The resulting Stoney-like relations which relate film stress and wafer curvature, known as the HR relations, require knowledge of not only the curvature at a single point, but also full-field curvature information. In this work, the HR relations are verified by comparison with X-ray microdiffraction. Two independent XRD measurements are used; one measures substrate curvature and the other determines film stress. Since these measurements are independent, the substrate curvature data are used as an input to the Stoney and HR stress/curvature relations. The resulting film stresses are then compared with XRD film stress data. From this, it is established that the HR relations result in substantially more accurate film stress predictions than does the Stoney analysis.
\r\n\r\nNext, a full-field curvature measurement technique, Coherent Gradient Sensing, is introduced as an ideally suited measurement tool for inferring film stress through the HR analysis. CGS measurements are taken of several progressively more interesting test wafers and the curvature is used through the HR relations to determine film stress.
", "doi": "10.7907/9GD9-A088", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:1661", "collection": "thesis", "collection_id": "1661", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05072007-150354", "primary_object_url": { "basename": "01_Title.pdf", "content": "final", "filesize": 21885, "license": "other", "mime_type": "application/pdf", "url": "/1661/1/01_Title.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Experimental Studies of Elasticity, Plastic Flow, and Anelasticity in Metallic-Glass-Forming-Liquids", "author": [ { "family_name": "Harmon", "given_name": "John Shelby, III", "clpid": "Harmon-John-Shelby-III" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Goddard", "given_name": "William A., III", "clpid": "Goddard-W-A-III" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A rheological law based on the concept of cooperatively-sheared flow zones is presented, in which the thermodynamic state variable controlling flow is identified to be the isoconfigurational shear modulus of the liquid. The law captures Newtonian as well as non-Newtonian viscosity data for glass-forming metallic liquids over a broad range of fragility. Acoustic measurements on specimens deformed at constant strain rates correlate with the measured steady-state viscosities, and define the relative effects of the \u201celastic\u201d and \u201ccooperative volume\u201d indices on the liquid fragility. The steady state deformation material properties are evaluated to obtain a relationship between the shear modulus and configurational enthalpy. Furthermore, the material properties are evaluated through steady state in an effort to probe the relaxation mechanisms governing flow.", "doi": "10.7907/53AD-3G15", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:420", "collection": "thesis", "collection_id": "420", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01312006-170959", "primary_object_url": { "basename": "ch0_final_R.pdf", "content": "final", "filesize": 223793, "license": "other", "mime_type": "application/pdf", "url": "/420/1/ch0_final_R.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Mechanical Characterization of Thin Films with Application to Ferroelectrics", "author": [ { "family_name": "Zhang", "given_name": "Rongjing", "clpid": "Zhang-Rongjing" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Molinari", "given_name": "Alain", "clpid": "Molinari-A" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Haile", "given_name": "Sossina M.", "clpid": "Haile-S-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "One important part of the motivation for this research work comes from the microelectromechanical systems (MEMS) technology. Its basic concept of high volume production and low unit cost can only be achieved when the devices made by microelectronics technique are reliable. The success in this area largely depends on the understanding of materials. However, the mechanical characterization is lagged behind the theoretical work and designing software development. The standard characterization method is still not established. For MEMS actuators, especially for active materials, the desired characterization system for obtaining mechanical properties requires load control feature and the capability of doing dynamic tests. However, there is no such method among the currently available tools for mechanical characterization.
\r\n\r\nThe other part of the motivation comes from the comprehensive research work of Caltech ferroelectric group. This group, which consists of nine faculty members, is aiming to develop new devices, especially new actuators, by the aid of multi-scale theory tools and selected experimental methods. The work presented in this dissertation is an important and key step of this ambitious project: the electromechanical characterization of devices. This will provide validation for the multi-scale materials modeling framework and help to increase the reliability of the actuators and devices.
\r\n\r\nIn this work, two techniques were developed for mechanical characterization, which satisfy the challenging requirements for thin film structures and devices: being able to do dynamic study on fragile ceramic thin film samples with load control feature. The first technique is a new method to characterize mechanical properties of released thin films under concentrated load. This technique can be used to apply load in the ?N?mN range with displacement measured with high accuracy of 0.1 ?m. The successful characterization of Si3N4 free-standing membranes demonstrated the capability and reliability of this new technique. The elastic modulus and residual stress of Si3N4 free-standing thin film were measured to be around 250 GPa and 450 MPa, respectively. These values were in close agreement with values obtained using a different technique as well as those found in the literature. This technique has the potential application on elastic-plastic characterization and characterization of other functional thin film materials such as shape memory alloys.
\r\n\r\nPressure bulge test technique, which is another type of load control method suitable for dynamic test, was also developed. The apparatus was designed to be compact to fit into the x-ray diffractometer for in-situ XRD study and had additional compatibility for polarized light microscopy study. Characterization of free standing thin film of single layer amorphous silicon nitride (Si3N4) and multi-layered PBT/Si3N4, and thick film of single crystal barium titanate (BaTiO3) showed the capability and reliability of this technique. Excellent agreement of the Si3N4 Young\u2019s modulus between these two developed methods gave the confidence for using these techniques to understand new materials.
\r\n\r\nIn situ x-ray diffraction study was carried out on the single crystal thick films which were loaded with distributed mechanical loading by pressure bulge setup. Direct evidence of 90o domain switching was obtained from the in situ XRD results with the intensity changing in both (002) and (200) orientations. Obvious changes in domain patterns were observed by using the polarized light microscope. The Young\u2019s modulus of this barium titanate single crystal thick film with thickness of 100 ?m was characterized before the XRD exam. Using this information, in-plane stress can be analyzed, and the relation between the driving force (the stress) and the microstructural change (volume fraction change in a-domain or c-domain) can be determined.
", "doi": "10.7907/CJR5-DK94", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1725", "collection": "thesis", "collection_id": "1725", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05112006-162948", "primary_object_url": { "basename": "Thesis_AmirSadjadpour.pdf", "content": "final", "filesize": 3787627, "license": "other", "mime_type": "application/pdf", "url": "/1725/1/Thesis_AmirSadjadpour.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Micromechanics-Inspired Three-Dimensional Constitutive Model for the Thermomechanical Response of Shape-Memory Alloys", "author": [ { "family_name": "Sadjadpour", "given_name": "Amir", "clpid": "Sadjadpour-Amir" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The goal of this thesis is to develop a full dimensional micromechanics-inspired constitutive model for polycrystalline shape-memory alloys. The model is presented in two forms: (1) The one-dimensional framework where we picture the ability of the model in capturing main properties of shape memory alloys such as superelasticity and shape-memory effect; (2) The full dimensional model where micromechanics origins of the model, the concepts emerged from those analysis and their relation to macroscopic properties in both single and polycrystals are presented.
\r\n\r\nWe use this framework to study the effects of the texture and anisotropy in the material behavior. Since phase transformation often competes with plasticity in shape-memory alloys, we incorporate that phenomenon into our model. We also demonstrate the ability of the model to predict the response of the material and track the phase transformation process for multi-axial, proportional and non-proportional loading and unloading experiments. We consider both stress-controlled and strain-controlled experiments and develop the model for isothermal, adiabatic and non-adiabatic thermal conditions. Adiabatic heating and loading rate both lead to the apparent hardening at high rates. We also visit this problem and examine the relative role of these two factors.
\r\n\r\nFinally we extend our model to study the reversible \"bcc\" to \"hcp\" martensitic phase transformation in pure iron. We consider a wide range of loading rates ranging from quasistatic to high rate dynamic loading and use our model to describe the evolution of the microstructure along with the effects of the rate hardening and thermal softening.
", "doi": "10.7907/MB1W-1V17", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:196", "collection": "thesis", "collection_id": "196", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01162006-005552", "primary_object_url": { "basename": "GLYKO_Thesis.pdf", "content": "final", "filesize": 21693932, "license": "other", "mime_type": "application/pdf", "url": "/196/1/GLYKO_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Experimental Study of Dynamic Frictional Sliding Modes along Incoherent Interfaces", "author": [ { "family_name": "Lykotrafitis", "given_name": "Georgios C.", "orcid": "0000-0003-2088-5478", "clpid": "Lykotrafitis-Georgios-C" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Tromp", "given_name": "Jeroen", "clpid": "Tromp-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Huang", "given_name": "Yonggang", "clpid": "Huang-Yonggang" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Dynamic sliding along incoherent (frictional) interfaces is investigated experimentally in a microsecond time scale. A bimaterial system comprised of Homalite and steel plates and a homogeneous system consisting of two Homalite plates are considered. The plates are held together by a uniform compressive stress while dynamic sliding is initiated by an impact-induced shear loading. The evolution of maximum shear stress contours is recorded by high-speed photography in conjunction with dynamic photoelasticity. Simultaneously with photoelasticity, a newly-developed technique based on laser interferometry is employed to locally measure the sliding speed at the interface.
\r\n\r\nThe response of the Homalite-steel bimaterial system differs according to whether the impact loading is applied to the Homalite plate or to the steel plate. In the first case, a disturbance traveling along the interface at a constant speed close to the Rayleigh wave speed of steel generates a shear Mach line crossing the P-wave front. Sliding initiates behind the P-wave front in the Homalite plate and it propagates at a supershear speed with respect to the shear wave speed of Homalite. A disturbance, traveling at constant speeds between the shear wave speed and the longitudinal wave speed of Homalite, appears behind the sliding tip. Wrinkle-like opening pulses, propagating along the bimaterial interface at a constant speed between the Rayleigh wave and the shear wave speed of Homalite, are also observed. When the impact loading is applied to the steel plate, sliding at a given point initiates with the arrival of the P-wave front there, so that the rupture is sonic with respect to steel and supersonic with respect to Homalite.
\r\n\r\nIn all the experiments performed on the bimaterial structure (Homalite-steel), sliding always occurred in a crack-like mode. In the case of a homogeneous system of Homalite plates however, direct physical evidence of different modes of sliding is recorded. Crack-like sliding, pulse-like sliding and mixed mode sliding in the form of pulses followed by a crack are discovered. Supersonic trailing pulses are also recorded. Behind the sliding tip, wrinkle-like opening pulses are developed for a wide range of impact speeds and confining stresses.
\r\n", "doi": "10.7907/0CCJ-5S66", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1268", "collection": "thesis", "collection_id": "1268", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04042006-205243", "primary_object_url": { "basename": "Nowatzki-Full_Text.pdf", "content": "final", "filesize": 2117273, "license": "other", "mime_type": "application/pdf", "url": "/1268/9/Nowatzki-Full_Text.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Characterization of Crosslinked Artificial Protein Films", "author": [ { "family_name": "Nowatzki", "given_name": "Paul John", "clpid": "Nowatzki-Paul-John" } ], "thesis_advisor": [ { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" } ], "thesis_committee": [ { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kornfield", "given_name": "Julia A.", "clpid": "Kornfield-J-A" }, { "family_name": "Davis", "given_name": "Mark E.", "clpid": "Davis-M-E" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "Genetically engineered artificial proteins are promising candidates for new biomaterials because their amino acid sequences can be precisely controlled. This work describes the characterization of crosslinked films of biomimetic artificial extracellular matrix (aECM) proteins with hybrid functions designed to meet materials needs in applications such as small diameter vascular grafts and corneal tissue implants. Elastin-derived polypeptides give the proteins flexibility, while RGD and CS5 peptide domains from fibronectin serve to adhere cells.
\r\n\r\nTechniques were sought to crosslink aECM proteins in ways that resulted in tunable mechanical properties. Hexamethylene diisocyanate was used to crosslink aECM proteins into uniform, transparent, highly-extensible hydrogel films with low water contents characteristic of native elastin. Their elastic moduli, 0.1 \u2013 1.1 MPa, depended on crosslinker concentration and aECM protein length, and spanned the observed range of elastin fibers.
\r\n\r\nThe suitability of biomaterials implants depends strongly on their susceptibility to proteolytic degradation in vivo. It was shown that small sequence changes in the elastin-like portion of aECM proteins were sufficient to decrease their rate of degradation by elastase sevenfold, illustrating a simple method to tune the protease sensitivity of designed proteins. The effects were seen in both soluble proteins and crosslinked films analyzed by measuring their decrease in elastic modulus during degradation.
\r\n\r\nAn aECM protein was examined for its effectiveness as a corneal onlay, or permanent contact lens. The protein was crosslinked into transparent, elastic, water-rich lenses and was implanted into rabbit corneas. The onlays were stable and well-tolerated, and full re-epithelialization occurred within 4-7 days. Histological examination revealed normal regenerating epithelial cell morphology on the anterior surface, good interfaces between the onlay and surrounding tissue, and only minimal inflammation.
\r\n\r\nTo create substrates for studying the coordinating effects of mechanical and biological signals on cell behavior, thin films were made from a photoreactive aECM protein containing the non-canonical amino acid para-azidophenylalanine. Atomic force microscopy (AFM) nanoindentation was used to calculate elastic modulus, and the technique was confirmed by bulk tensile measurements and finite element simulations. Film modulus could be tuned either by differential irradiation or variable incorporation of para-azidophenylalanine.
", "doi": "10.7907/AAWE-NV35", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:2113", "collection": "thesis", "collection_id": "2113", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262006-160244", "primary_object_url": { "basename": "ODelaire_Thesis.pdf", "content": "final", "filesize": 6881032, "license": "other", "mime_type": "application/pdf", "url": "/2113/1/ODelaire_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "The Phonon Entropy of Transition Metals and Alloys: Effects of Impurities and of a Martensitic Phase Transition", "author": [ { "family_name": "Delaire", "given_name": "Olivier", "orcid": "0000-0003-1230-2834", "clpid": "Delaire-Olivier" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Halevy", "given_name": "Itzhak", "clpid": "Halevy-I" }, { "family_name": "Goddard", "given_name": "William A., III", "clpid": "Goddard-W-A-III" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "For a fixed configuration of ions on a given crystalline lattice, low energy excitations around the static average configuration can be thermally activated and will contribute to the entropy of the system. As such, phonons, spin-waves or electronic excitations have their own entropic contribution. This thesis investigates the entropic effects of lattice vibrations in transition metal alloys, both from experimental and computational points of view.
\r\n\r\nUsing inelastic neutron scattering, it is shown that a few percent of substitutional impurities from the transition metal series strongly affect the phonon density of states (DOS) of pure vanadium. Alloying with 6% Pt solutes produces a strong stiffening of the phonon DOS, inducing a large and negative vibrational entropy of mixing, which overcomes the increase in configurational entropy. A systematic study of chemical trends for different transition metal impurities was conducted. A previously unknown correlation is established between the vibrational entropy of alloying and the difference in electronegativity of the solute and the host. Density-functional theory calculations were conducted and confirmed the occurrence of systematic charge-transfers correlating with the electronegativity, which affect the interatomic force-constants and the phonons.
\r\n\r\nThe effect of impurities on the anomalous temperature-dependence of phonons in vanadium is investigated. It is found that the solutes which affect the phonon density of states most strongly at room temperature also suppress the anomalous temperature behavior. Electron-phonon and phonon-phonon couplings are examined as potential sources of this effect, through a careful accounting of their contributions to the heat capacity, based on inelastic neutron scattering experiments, calorimetry measurements and electronic structure calculations.
\r\n\r\nFinally, the changes in the phonon DOS and the vibrational entropy across the low-temperature martensitic phase transformation in Fe71Ni29 are investigated. The respective contributions of the phonons and magnetism to the entropy of the direct and reverse transformation are evaluated from neutron scattering and differential scanning calorimetry measurements. A significant magnetic entropy is found in the reverse transformation, which is not present in the direct transformation. This result stresses the necessity to account for the respective contributions of all microscopic degrees of freedom in evaluating entropy changes in solid-solid phase transitions.
", "doi": "10.7907/SDJF-E025", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:2116", "collection": "thesis", "collection_id": "2116", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262006-173355", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 4890009, "license": "other", "mime_type": "application/pdf", "url": "/2116/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Shock-Induced Damage in Rocks: Application to Impact Cratering", "author": [ { "family_name": "Ai", "given_name": "Huirong (Anita)", "clpid": "Ai-Huirong-Anita" } ], "thesis_advisor": [ { "family_name": "Ahrens", "given_name": "Thomas J.", "clpid": "Ahrens-T-J" } ], "thesis_committee": [ { "family_name": "Saleeby", "given_name": "Jason B.", "clpid": "Saleeby-J-B" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Clayton", "given_name": "Robert W.", "clpid": "Clayton-R-W" }, { "family_name": "Ahrens", "given_name": "Thomas J.", "clpid": "Ahrens-T-J" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Shock-induced damage beneath impact craters is studied in this work. Two representative terrestrial rocks, San Marcos granite and Bedford limestone, are chosen as test target. Impacts into the rock targets with different combinations of projectile material, size, impact angle, and impact velocity are carried out at cm scale in the laboratory.
\r\n\r\nShock-induced damage and fracturing would cause large-scale compressional wave velocity reduction in the recovered target beneath the impact crater. The shock-induced damage is measured by mapping the compressional wave velocity reduction in the recovered target. A cm scale nondestructive tomography technique is developed for this purpose. This technique is proved to be effective in mapping the damage in San Marcos granite, and the inverted velocity profile is in very good agreement with the result from dicing method and cut open directly.
\r\n\r\nBoth compressional velocity and attenuation are measured in three orthogonal directions on cubes prepared from one granite target impacted by a lead bullet at 1200 m/s. Anisotropy is observed from both results, but the attenuation seems to be a more useful parameter than acoustic velocity in studying orientation of cracks.
\r\n\r\nOur experiments indicate that the shock-induced damage is a function of impact conditions including projectile type and size, impact velocity, and target properties. Combined with other crater phenomena such as crater diameter, depth, ejecta, etc., shock-induced damage would be used as an important yet not well recognized constraint for impact history.
\r\n\r\nThe shock-induced damage is also calculated numerically to be compared with the experiments for a few representative shots. The Johnson-Holmquist strength and failure model, initially developed for ceramics, is applied to geological materials. Strength is a complicated function of pressure, strain, strain rate, and damage. The JH model, coupled with a crack softening model, is used to describe both the inelastic response of rocks in the compressive field near the impact source and the tensile failure in the far field. The model parameters are determined either from direct static measurements, or from indirect numerical adjustment. The agreement between the simulation and experiment is very encouraging.
", "doi": "10.7907/39ZE-SY71", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:834", "collection": "thesis", "collection_id": "834", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03022006-005723", "primary_object_url": { "basename": "02thesis.pdf", "content": "final", "filesize": 3203305, "license": "other", "mime_type": "application/pdf", "url": "/834/2/02thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "High Temperature Deformation of Vitreloy Bulk Metallic Glasses and Their Composite", "author": [ { "family_name": "Tao", "given_name": "Min", "clpid": "Tao-Min" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Huang", "given_name": "Yonggang", "clpid": "Huang-Yonggang" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A complete understanding of the deformation mechanisms of BMGs and their composites requires investigation of the microstructural changes and their interplay with the mechanical behavior. In this dissertation, the deformation mechanisms of a series of Vitreloy glasses and their composites are experimentally investigated over a wide range of strain rates and temperatures, with focus on the supercooled liquid regime, by combining uniaxial mechanical testing with calorimetric and microscopic examinations. Various theories of deformation of metallic glasses and the composites are examined in light of the experimental data.
\r\n\r\nA comparative structural relaxation study was performed on two closely related Vitreloy alloys, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit 1) and Zr46.7Ti8.3Cu7.5Ni10Be27.5 (Vit 4). Differential scanning calorimetric studies on the specimens deformed in compression at constant-strain-rate in supercooled liquid regime showed that mechanical loading accelerated the spinodal phase separation and nanocrystallization process in Vit 1, while the relaxation in Vit 4 featured local chemical composition fluctuation accompanied by annealing out of free volume. The effect of the structural relaxation on their mechanical behavior was further studied via single and multiple jump-in-strain-rate tests.
\r\n\r\nThe deformation and viscosity of a new Vitreloy alloy were characterized using uniaxial compression tests in its supercooled liquid regime. A new theoretical model named Cooperative Shear Model, which correlates the evolution of the macroscopic mechanical/thermal variables such as shear modulus and viscosity with the configurational energies of atom clusters in an amorphous alloy, was critically examined in this investigation. The model was successful in predicting the Newtonian and non-Newtonian viscosities of the material, as well as the shear moduli of the deformed specimens, in a self-consistent manner.
\r\n\r\nThe plastic flow of an in-situ metallic glass composite, beta-Vitreloy, was investigated under uniaxial compression in its supercooled liquid regime and at various strain rates. The composite, with ~0.25 volume fraction of crystalline beta-phase dendrites exhibited superplastic behavior similar to that of amorphous Vit 1. Significant strain hardening was observed when the material was deformed at high temperatures and low strain rates. A dual-phase composite model was employed in finite element simulations to understand the effect of the composite microstructure on its mechanical behavior.
", "doi": "10.7907/27SN-R187", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1724", "collection": "thesis", "collection_id": "1724", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05112006-162905", "primary_object_url": { "basename": "Thesis_Matias_Zielonka_Final.pdf", "content": "final", "filesize": 8087385, "license": "other", "mime_type": "application/pdf", "url": "/1724/2/Thesis_Matias_Zielonka_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Configurational Forces and Variational Mesh Adaption in Solid Dynamics", "author": [ { "family_name": "Zielonka", "given_name": "Matias Gabriel", "clpid": "Zielonka-Matias-Gabriel" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis is concerned with the exploration and development of a variational finite element mesh adaption framework for non-linear solid dynamics and its conceptual links with the theory of dynamic configurational forces. The distinctive attribute of this methodology is that the underlying variational principle of the problem under study is used to supply both the discretized fields and the mesh on which the discretization is supported. To this end a mixed-multifield version of Hamilton's principle of stationary action and Lagrange-d'Alembert principle is proposed, a fresh perspective on the theory of dynamic configurational forces is presented, and a unifying variational formulation that generalizes the framework to systems with general dissipative behavior is developed. A mixed finite element formulation with independent spatial interpolations for deformations and velocities and a mixed variational integrator with independent time interpolations for the resulting nodal parameters is constructed. This discretization is supported on a continuously deforming mesh that is not prescribed at the outset but computed as part of the solution. The resulting space-time discretization satisfies exact discrete configurational force balance and exhibits excellent long term global energy stability behavior. The robustness of the mesh adaption framework is assessed and demonstrated with a set of examples and convergence tests.", "doi": "10.7907/V6RB-FR94", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:769", "collection": "thesis", "collection_id": "769", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02262005-161824", "primary_object_url": { "basename": "MyThesis_Xia_Text.pdf", "content": "final", "filesize": 4530246, "license": "other", "mime_type": "application/pdf", "url": "/769/1/MyThesis_Xia_Text.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Laboratory Investigations of Earthquake Dynamics", "author": [ { "family_name": "Xia", "given_name": "Kaiwen", "clpid": "Xia-Kaiwen" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" }, { "family_name": "Tromp", "given_name": "Jeroen", "clpid": "Tromp-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Earthquake represents one of most destructive geological hazards. In this thesis I will attempt to understand it through controlled laboratory experiments. The earthquake dynamic rupturing process itself is a complicated phenomenon, involving dynamic friction, wave propagation, and heat production. Because controlled experiments can produce results without assumptions needed in theoretical and numerical analysis, the experimental method is thus advantageous over theoretical and numerical methods.
\r\n\r\nOur laboratory fault is composed of carefully cut photoelastic polymer plates (Homalite-100, Polycarbonate) held together by uniaxial compression. As a unique unit of the experimental design, a controlled exploding wire technique provides the triggering mechanism of laboratory earthquakes. Three important components of real earthquakes (i.e., pre-existing fault, tectonic loading, and triggering mechanism) correspond to and are simulated by frictional contact, uniaxial compression, and the exploding wire technique. Dynamic rupturing processes are visualized using the photoelastic method and are recorded via a high-speed camera. Our experimental methodology, which is full-field, in situ, and non-intrusive, has better control and diagnostic capacity compared to other existing experimental methods.
\r\n\r\nUsing this experimental approach, we have investigated several problems: dynamics of earthquake faulting occurring along homogeneous faults separating identical materials, earthquake faulting along inhomogeneous faults separating materials with different wave speeds, and earthquake faulting along faults with a finite low wave speed fault core. We have observed supershear ruptures, rupture speed transition, directionality of rupture in faults with a material contrast, self-healing slip pulses in faults with a finite core, crack-like to pulse-like rupture transition in faults with a finite core.
", "doi": "10.7907/WQQX-6Q19", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:5084", "collection": "thesis", "collection_id": "5084", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12202004-182638", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 2056953, "license": "other", "mime_type": "application/pdf", "url": "/5084/1/Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Theory of Complex Lattice Quasicontinuum and Its Application to Ferroelectrics", "author": [ { "family_name": "Kowalewsky", "given_name": "Olga", "clpid": "Kowalewsky-Olga" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Complex lattice Quasicontinuum theory is developed and applied to the description of ferroelectric phenomena. Quasicontinuum theory is a multiscale theory that provides a unified description of materials by combining atomistic and continuum approaches. It provides a seamless transition between atomistics and continuum, but the description of the material is derived directly from the underlying atomic structure, using the computationally expensive atomistics only where needed, at the location of phenomena of atomistic origin.
\r\n\r\nComplex Lattice Quasicontinuum theory can be applied to complex lattice crystals consisting of many kinds of atoms. One highlight of it is treatment of each component lattice as separately and independently as possible. The component Quasicontinua are coupled through the microscopic forces within nodal clusters, making the complex atomistics of the heterogeneous lattice the basis of the description.
\r\n\r\nFerroelectrics are especially suited to the application of Quasicontinuum theory. The nature of defects in ferroelectric materials is atomistic, but their influence over the material is long ranged due to induced elastic fields. Many different ferroelectric phenomena involving the perovskite ferroelectrics Barium Titanate and Lead Titanate are investigated and simulated. For Barium Titanate: the 180 degree domain wall structure and quasistatic crack under load. For Lead Titanate: the 180 degree domain wall structure and a domain wall step.
\r\n\r\nThe results for the domain walls show that the domain wall thickness is atomistically small, of the order of few lattice constants, which is in agreement with recent ab initio molecular dynamics simulations, but we also observe long range effects resulting from the presence of the wall. During crack loading in the sample of Barium Titanate we observe polarization changes around the crack tip which are consistent with experimental observations of an increase of fracture toughness. The quasicontinuum study of a domain wall step gives an atomistical view into the equilibrium structure of the step.
\r\n\r\nQuasicontinuum is able to model these phenomena with atomistic precision around the defects and non-homogeneities, and also capture the influence of long-ranging effects in the samples. These studies could also give valuable modeling input for larger scale continuum approaches.
", "doi": "10.7907/rb0c-9534", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:4991", "collection": "thesis", "collection_id": "4991", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12142004-121255", "primary_object_url": { "basename": "Thesis_Arash.pdf", "content": "final", "filesize": 4619776, "license": "other", "mime_type": "application/pdf", "url": "/4991/1/Thesis_Arash.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Atomic Structure of Ferroelectric Domain Walls, Free Surfaces and Steps", "author": [ { "family_name": "Yavari", "given_name": "Arash", "orcid": "0000-0002-7088-7984", "clpid": "Yavari-Arash" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The goal of this thesis is to develop a general framework for lattice statics analysis of defects in ferroelectric Perovskites. The techniques presented here are general and can be easily applied to other systems as well. We present all the calculations and numerical examples for two technologically important ferroelectric materials, namely, PbTiO3 and BaTiO3. We use shell potentials, that are derived using quantum mechanics calculations, and analyze three types of defects: (i) 180\u00b0 and 90\u00b0 domain walls, (ii) free surfaces and (iii) steps in 180\u00b0 domain walls. Our formulation assumes that an interatomic potential is given. In other words, there is no need to have the force constants or restrict the number of nearest neighbor interactions a priori. Depending on the defect and symmetry, the discrete governing equations are reduced to those for representatives of some equivalence classes. The idea of symmetry reduction in lattice statics calculations is one of the contributions of this thesis. We call our formulation of lattice statics 'inhomogeneous lattice statics' as we consider the fact that close to defects force constants (stiffness matrices) change. For defects with one-dimensional symmetry reduction we solve the discrete governing equations directly using a novel method in the setting of the theory of difference equations. This will be compared with the solutions obtained using discrete Fourier transform. For defects with two-dimensional symmetry reduction we solve the discrete governing equations using discrete Fourier transform. We calculate the fully nonlinear solutions using modified Newton-Raphson iterations and call the method 'inhomogeneous anharmonic lattice statics'. This work is aimed to fill the gap between quantum mechanics ab initio calculations and continuum models (based on Landau-Ginzberg-Devonshire theory) of ferroelectric domain walls.", "doi": "10.7907/jdy3-1m77", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:4303", "collection": "thesis", "collection_id": "4303", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10292004-152709", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 2303147, "license": "other", "mime_type": "application/pdf", "url": "/4303/1/thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Atomistic Simulation of Barium Titanate", "author": [ { "family_name": "Zhang", "given_name": "Qingsong", "clpid": "Zhang-Qingsong" } ], "thesis_advisor": [ { "family_name": "Goddard", "given_name": "William A., III", "clpid": "Goddard-W-A-III" } ], "thesis_committee": [ { "family_name": "Haile", "given_name": "Sossina M.", "clpid": "Haile-S-M" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Atwater", "given_name": "Harry Albert", "clpid": "Atwater-H-A" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Cagin", "given_name": "Tahir", "clpid": "Cagin-Tahir" }, { "family_name": "Goddard", "given_name": "William A., III", "clpid": "Goddard-W-A-III" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "We present the Polarizable Charge Equilibration (P-QEq) force field to include self-consistent atomic polarization and charge transfer in molecular dynamics of materials. The short-range Pauli repulsion effects are described by two body potentials without exclusions. A linear self-consistent field solution to the charge transfer is proposed for charge transfer in large systems. The P-QEq is parameterized for BaTiO\u2083 based on quantum mechanics calculations (DFT with GGA) and applied to the study of the phase transitions, domain walls and oxygen vacancies.
\r\n\r\nFrozen phonon analysis reveals that the three high-temperature BaTiO\u2083 phases in the displacive model are unstable. Within their corresponding macroscopic phase symmetries, the smallest stable phase structures are achieved by antiferroelectric distortions from unstable phonons at the Brillouin zone boundaries. The antiferroelectric distortions soften phonons, reduce zero point energies and increase vibrational entropies. A correct BaTiO\u2083 phase transition sequence and comparable transition temperatures are obtained by free energy calculations. The inelastic coherent scattering functions of these phases agree with X-ray diffraction experiments.
\r\n\r\nBaTiO\u2083 180\u00b0 domain wall is Ba-centered with abrupt polarization switching across the wall. The center of BaTiO\u2083 90\u00b0 domain wall is close to its orthogonal phase. There are transition layers from the wall centers to the internal domains in the types of domain walls. Polarization variation in these transition layers induces polarization charge and free charge transfer. This effect causes a strong bipolar electric field in BaTiO\u2083 90\u00b0 domain wall.
\r\n\r\nOxygen vacancies are frozen at room temperature, and mobile near the Curie temperature. In the tetragonal phase, the broken Ti-O chains are frozen, reducing switchable polarization. Due to charge redistribution and local relaxation, oxygen vacancy interaction is short-range and anisotropic. Two oxygen vacancies can form a stable pair state, where two broken Ti-O chains are aligned parallel. Oxygen vacancy clusters can form dendritic structures as a result of local relaxation and charge interaction.
", "doi": "10.7907/SQ9J-4H73", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:388", "collection": "thesis", "collection_id": "388", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01282005-111916", "primary_object_url": { "basename": "Thesis_Chapters.pdf", "content": "final", "filesize": 1487780, "license": "other", "mime_type": "application/pdf", "url": "/388/1/Thesis_Chapters.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Study of Crystallization Behavior, Kinetics and Thermodynamics of Bulk Metallic Glasses Using Noncontact Electrostatic Levitation Technique", "author": [ { "family_name": "Mukherjee", "given_name": "Sundeep", "orcid": "0000-0002-1954-0045", "clpid": "Mukherjee-Sundeep" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Goddard", "given_name": "William A., III", "clpid": "Goddard-W-A-III" }, { "family_name": "Rhim", "given_name": "Won-Kyu", "clpid": "Rhim-Won-Kyu" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The exceptional stability against crystallization of recently developed bulk metallic glasses, together with the non-contact measurement techniques to investigate their thermophysical properties, have opened the doors for detailed studies of these systems in the deep undercooled liquid state. The primary objective of this thesis is identification of reliable predictive indicators of glass-forming ability based on the measured thermophysical properties and intrinsic crystallization behavior of bulk metallic glasses. A number of bulk metallic glass-forming alloys having widely different glass-forming abilities are chosen for this investigation. All of the measurements are carried out using the noncontact high vacuum electrostatic levitation technique to avoid any heterogeneous nucleation effects from container walls or environment.
\r\n\r\nThe measured kinetic properties, viscosity, and free volume are found to have the most pronounced influence on glass-forming ability, while the thermodynamics show weak correlation with the trends in glass-formation. The glass-forming melts show orders of magnitude higher viscosity compared to pure metals. Among the glass-forming alloys, better glass formers have higher melting temperature viscosity, higher fragility, and show a smaller change in volume upon crystallization compared to poor glass formers. The melting temperature viscosity is correlated with volume change upon crystallization in accordance with Cohen-Grest free-volume theory for a wide variety of alloys.
\r\n\r\nTo quantify the glass-forming ability, Time-Temperature-Transformation (TTT) curves of a number of glass-forming melts are measured over a wide temperature range between their glass-transition and melting temperatures. A pronounced effect of overheating is observed on the undercooling levels and crystallization time scales. This is attributed to oxide particles acting as heterogeneous nucleation sites unless dissolved by overheating above their melting points. The TTT curves, obtained after eliminating the heterogeneous influences, are found to scale with their respective glass transition temperatures. The TTT curves are analyzed within the framework of nucleation theory to obtain a quantitative correlation between crystal-melt interfacial tension, melt viscosity, and glass forming ability.
\r\n\r\nFinally, the crystallization behavior, microstructure, and melt viscosity of an in situ ductile phase reinforced amorphous matrix composite is investigated as a function of the processing temperature. The results for the in situ composite are compared with those of the monolithic bulk metallic glasses.
", "doi": "10.7907/33FY-8482", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:8", "collection": "thesis", "collection_id": "8", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01032005-140446", "primary_object_url": { "basename": "Thesis_YuXiao.pdf", "content": "final", "filesize": 3483791, "license": "other", "mime_type": "application/pdf", "url": "/8/1/Thesis_YuXiao.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "The Influence of Oxygen Vacancies on Domain Patterns in Ferroelectric Perovskites", "author": [ { "family_name": "Xiao", "given_name": "Yu", "clpid": "Xiao-Yu" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis investigates the role of oxygen vacancies in determining ferroelectric properties and domain patterns of ferroelectric perovskites. Being non-polar (paraelectric) above their Curie temperature but spontaneously polarized (ferroelectric) below it, ferroelectric perovskites offer a tantalizing potential for applications: large actuation through domain switching and memory storage via switchable electric polarization. Oxygen vacancies, commonly present and mobile at high temperature, are the primary defects and thus play a central role in these applications.
\r\n\r\nWe develop a model that combines the ferroelectric and semiconducting nature of ferroelectric perovskites. Oxygen vacancies act as n-type dopants and thus affect the semiconducting properties. We show that the ferroelectric and semiconducting features interact and lead to the formation of depletion layers near the electrodes and double layers at the 90\u00b0 domain walls. We find a potential drop across 90\u00b0 domain walls even in a perfect crystal. This potential drop marks the essential difference between a 90\u00b0 and an 180\u00b0 domain wall, drives the formation of a space charge double layer in a doped crystal, promotes electronic charge injection and trapping, and leads to the redistribution of oxygen vacancies at 90\u00b0 domain walls. The rearrangement of oxygen vacancies near 90\u00b0 domain walls may form a basis for domain memory and provides a potentially new mechanism for large electrostriction.
\r\n\r\nWe also rigorously justify the continuum theory by calculating the Coulomb energy of a spontaneously polarized solid starting from a periodic distribution of charges based on the classical interpretation of ferroelectrics and with a definite choice of polarization per unit cell. We prove that in the limit where the size of the body is large compared to the unit cell, the energy of Coulombic interactions may be approximated by a sum of a local part and a nonlocal part. The local part depends on the lattice structure, but is different from the Lorentz formula for a lattice of dipoles. The nonlocal part is identical to the Lorentz formula.
", "doi": "10.7907/5QSX-9Y68", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2394", "collection": "thesis", "collection_id": "2394", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06022005-180557", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 3765742, "license": "other", "mime_type": "application/pdf", "url": "/2394/1/Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Stereo Digital Particle Image Velocimetry Investigation of a Free Surface Mixing Layer", "author": [ { "family_name": "Dooley", "given_name": "Bradley Scott", "clpid": "Dooley-Bradley-Scott" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Ingersoll", "given_name": "Andrew P.", "orcid": "0000-0002-2035-9198", "clpid": "Ingersoll-A-P" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Shear flows in the vicinity of a free surface are a problem with numerous applications, perhaps the most obvious being the wakes of seagoing surface vessels. The flow behind a full-scale ship is extremely complex \u2013 so much so that it is frequently more instructive to consider simpler cases highlighting particular elements of the larger problem. To that end, an experimental investigation has been conducted to study the behavior of a turbulent plane mixing layer intersecting a free surface at low Froude number. The local Reynolds number, based on the velocity differential across the layer and the momentum thickness, was approximately 10,000.
\r\n\r\nThe technique of Stereoscopic Digital Particle Image Velocimetry (SDPIV) was implemented to obtain instantaneous three-component velocity measurements within planar slices of the steady-state, spatially developing mixing layer flow. Guided by previous studies of the same flow conditions, specific depths were chosen at a single downstream station for investigation \u2013 specifically those in and around counter-rotating streamwise vortices known to exist in the mean flow very near the free surface. 3,000 consecutive SDPIV image pairs were recorded at a rate of 15 per second at each location, giving ample data for Reynolds decomposition and spectral analysis of the velocity fields.
\r\n\r\nThe present study has found that the anisotropy known to exist in some other free surface flows, such as surface-parallel submerged jets, is also present in the case of the mixing layer. Power spectra of all three velocity components are shown to capture part of the inertial subrange; the isotropic energy cascade seen to be present away from the free surface is also seen to disappear near the surface, as surface-normal velocity fluctuations are severely attenuated.
\r\n\r\nAdditionally, a low-frequency spanwise oscillation is deduced from the velocity power spectra and cospectra in the immediate vicinity of the mean streamwise vortices. Not present at all at significant depth, the motions at this frequency are also observed to markedly decrease \u2013 in all components \u2013 at locations closer to the surface. These observations appear to have both parallels and key differences compared to previously observed meandering of model boat wakes, and the possibility that the oscillation stems from the vortex-pair instability is discussed.
", "doi": "10.7907/EH41-N436", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2158", "collection": "thesis", "collection_id": "2158", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-160315", "primary_object_url": { "basename": "01_preliminary_pages.pdf", "content": "final", "filesize": 129155, "license": "other", "mime_type": "application/pdf", "url": "/2158/1/01_preliminary_pages.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Development of Novel Binary and Multi-Component Bulk Metallic Glasses", "author": [ { "family_name": "Xu", "given_name": "Donghua", "orcid": "0000-0001-5018-5603", "clpid": "Xu-Donghua" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Conner", "given_name": "Robert Dale", "clpid": "Conner-R-D" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Goddard", "given_name": "William A., III", "orcid": "0000-0003-0097-5716", "clpid": "Goddard-W-A-III" }, { "family_name": "Nicolet", "given_name": "Marc-Aurele", "clpid": "Nicolet-M-A" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Bulk Metallic Glasses (BMGs) have been drawing increasing attention in recent years due to their scientific and engineering significance. A great deal of effort in this area has been devoted to developing BMGs in different alloy systems. BMGs based on certain late transition metals (e.g. Fe, Co, Ni, Cu) have many potential advantages over those based on early transition metals. These include even higher strength and elastic modulii, and lower materials cost, to name a few, which are highly preferable for a broad application of BMGs as engineering materials. Nevertheless, these ordinary-late-transition-metal-based BMGs generally have quite limited glass-forming ability (GFA). In particular, for the Ni-based and Cu-based alloys reported prior to this research, the maximum casting thickness allowed to retain their amorphous structures is only ~2 mm (or lower) and ~5 mm (or lower), respectively.
\r\n\r\nDuring this research it was first found that certain quinary Ni-based alloys in the Ni-Cu-Ti-Zr-Al system can be cast into 5 mm diameter amorphous rods. This critical casting thickness is the highest for any reported Ni-based BMG\u2019s indicating that these alloys are the easiest metallic glass formers based on Ni discovered to date. Secondly, certain binary alloys in the Cu-Zr and Cu-Hf systems were found to form bulk glasses with casting thickness as high as 2 mm. The discovery of these binary BMGs was very surprising since it had been widely considered that only multi-component (containing at least three elements) alloys could form bulk metallic glasses. These new binary BMGs provide interesting subjects for future theoretical studies such as molecular dynamics simulations since they possess both the simplicity of binary alloys and the good GFA of multi-component BMGs. In fact, these binary BMGs have led to the third but perhaps the most significant progress during this research, i.e., the discovery of a family of Cu-based BMGs in the Cu-Zr-Al-Y system which possess a critical casting thickness up to 1 cm. These quaternary Cu-based alloys, together with some complicated Fe-based alloys reported by two other groups during the course of this research, are the first centimeter level BMGs based on the ordinary late transition metals.
\r\n\r\nThis thesis first reviews the fundamentals related to BMG development, then reports in detail the formation and properties of the above-mentioned binary and multi-component BMGs based on Ni and Cu. A generalized geometric model for the critical-value problem of nucleation developed in this research is also presented.
", "doi": "10.7907/XD2M-WW51", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:1425", "collection": "thesis", "collection_id": "1425", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04202005-172426", "primary_object_url": { "basename": "Jijie_ZHOU_dissertation.pdf", "content": "final", "filesize": 7968243, "license": "other", "mime_type": "application/pdf", "url": "/1425/1/Jijie_ZHOU_dissertation.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Nanowicking: Multi-scale Flow Interaction with Nanofabric Structures", "author": [ { "family_name": "Zhou", "given_name": "Jijie", "clpid": "Zhou-Jijie" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Noca", "given_name": "Flavio", "clpid": "Noca-F" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Burdick", "given_name": "Joel Wakeman", "orcid": "0000-0002-3091-540X", "clpid": "Burdick-J-W" }, { "family_name": "Fraser", "given_name": "Scott E.", "orcid": "0000-0002-5377-0223", "clpid": "Fraser-S-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Dense arrays of aligned carbon nanotubes are designed into strips \u2014 nanowicks \u2014 as a miniature wicking element for liquid delivery and potential microfluidic chemical analysis devices. The delivery function of nanowicks enables novel fluid transport devices to run without any power input, moving parts or external pump. The intrinsically nanofibrous structure of nanowicks provides a sieving matrix for molecular separations, and a high surface-to-volume ratio porous bed to carry catalysts or reactive agents.
\r\n\r\nThis work also experimentally studies the spontaneous fluid transport along nanowicks. Liquid is conveyed through corner flow, surface flow, and interstitial flow through capillary force and the Marangoni effect. The main course for corner flow and surface flow follows Washburn behavior, and can deliver liquid centimeters away from the input blob with a speed on the order of millimeters per second depending on the nanowick configuration and the amount of input liquid. Corner flow can be minimized and even eliminated through proper nanowick and input design. Otherwise, corner flow interacts with surface flow in the first 2mm of the pathway closest to the input point. Interstitial flow dominates the late stage. It is driven by both capillary force and concentration-gradient-induced Marangoni force. The concentration gradient is determined by two competing rates: surfactant diffusion in solution and adsorption onto nanotube surfaces. The flow inside nanowicks may wick hundreds of microns in seconds or tens of seconds. A non-conventional advancing front may develop in the flow around nanowicks. They are seen as (i) Rayleigh instability-induced fingering in surface flow on millimeter-wide nanowicks, (ii) viscous instability-induced branching near almost-stagnant surface film at low surfactant concentration, and (iii) disjointed wetting domains at very low concentration.
", "doi": "10.7907/XMW0-DJ25", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2194", "collection": "thesis", "collection_id": "2194", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05282004-152537", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 2421464, "license": "other", "mime_type": "application/pdf", "url": "/2194/1/Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Thermomechanical Variational Principles for Dissipative Materials with Application to Strain Localization in Bulk Metallic Glasses", "author": [ { "family_name": "Yang", "given_name": "Qiang", "clpid": "Yang-Qiang" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis is concerned with variational principles for general coupled thermomechanical problems in dissipative materials including finite elastic and plastic deformation, non-Newtonian viscosity, rate sensitivity, arbitrary flow and hardening rule, as well as heat conduction. It is shown that there exists a potential function such that both the conservation of energy and balance of linear momentum are the Euler-Lagrange equations of its first variation. Inspired from the time-discretized version of the variational formulation, we present a procedure for variational thermomechanical update, which generalizes the isothermal approach under a variational thermodynamic framework. This variational formulation then serves as a basis for temperature change as well as constitutive updates.
\r\n\r\nAn important application of the variational formulation is to optimize the shear band thickness in strain localization processes. We show that this optimization takes the form of a configurational-force equilibrium and results in a well-defined band thickness. We further implement displacement discontinuities into a class of strain-localization finite elements. These elements consist of two surfaces, attached to the abutting volume elements, which can separate and slip relative to each other, and thus enable the accurate and efficient simulation of the dynamical formation of stain localization.
\r\n\r\nThe variational formulation also leads to a finite-deformation continuum modeling of bulk metallic glasses. It is shown that the strain softening of bulk metallic glasses is due to the increase of free volume (and thus the decrease of viscosity), while temperature rise accelerates the localization of the deformation. The model reproduces the constitutive behavior of Vitreloy 1 bulk metallic glass at various strain rates and temperatures.
", "doi": "10.7907/6FV2-KV63", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:755", "collection": "thesis", "collection_id": "755", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02252004-181515", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 92276037, "license": "other", "mime_type": "application/pdf", "url": "/755/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Three-Dimensional Nonlinear Analysis of Tall Irregular Steel Buildings Subject to Strong Ground Motion", "author": [ { "family_name": "Krishnan", "given_name": "Swaminathan", "orcid": "0000-0002-2594-1523", "clpid": "Krishnan-Swaminathan" } ], "thesis_advisor": [ { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" } ], "thesis_committee": [ { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Iwan", "given_name": "Wilfred D.", "clpid": "Iwan-W-D" } ], "local_group": [ { "literal": "Earthquake Engineering Research Laboratory" }, { "literal": "div_eng" } ], "abstract": "Strong ground motion from a nearby fault has frequency content in the same range as the natural frequencies of tall buildings. This may have serious repercussions and is the topic of this dissertation.Buildings are designed per building code standards. But, are the code provisions adequate? Strong motion from large earthquakes has been recorded only in recent times in the near-source region. Have the current codes used this information to update tall structure design guidelines? Considerable damage has been observed in tall buildings from the Northridge, Kobe, Turkey, and Taiwan earthquakes. How will tall buildings designed per the latest code regulations perform if they were to be shaken by any of these earthquakes? This thesis attempts to answer these questions.
\r\n\r\nTall buildings by their nature are computationally intensive to analyze. They consist of thousands of degrees of freedom and when subjected to strong ground motion from a nearby source, exhibit inelastic response. Modeling this inelastic response requires an iterative approach that is computationally expensive. Furthermore, a large class of buildings, classified as irregular, exhibits complex behavior that can be studied only when the structures are modeled in their entirety. To this end, a three-dimensional analysis program, FRAME3D, has been developed incorporating two special beam-column elements -- the plastic hinge element and the elastofiber element that can model beams and columns in buildings accurately and efficiently, a beam-column joint element that can model inelastic joint deformation, and 4-noded elastic plane-stress elements to model floor slabs acting as diaphragms forcing the lateral force resisting frames in a building to act as one unit. The program is capable of performing time-history analyses of buildings in their entirety.
\r\n\r\nSix 19-story irregular steel moment frame buildings (with buildings 2A and 3A being variants of buildings 2 and 3, respectively) have been designed per the latest code (Uniform Building Code, 1997). Two of these buildings have reentrant corners and the other two have torsional irregularity. Their strength and ductility are assessed by performing pushover analyses on them. To assess their performance under strong shaking, FRAME3D models of these buildings are subjected to near-source strong motion records from the Iran earthquake (Mw = 7.3, Tabas Station) of 1978, the Northridge earthquake (Mw = 6.7, Sylmar Station) of 1994 and the Kobe earthquake (Mw = 6.9, Takatori Station) of 1995. None of the buildings collapsed under these strong events in the computer analyses. However, when compared against the acceptable limits for various performance levels in FEMA 356 document, the damage in terms of plastic deformation at the ends of beams and columns and at joints would render the buildings inadequate in terms f life safety in quite a few cases and would even indicate possible collapse in a couple of cases. Thus, in these terms, the code falls short of achieving its life safety objective, and the near-source factors introduced in the code in 1997 in recognition of the special features of near-source ground motion seem to be inadequate.
\r\n\r\nThe ductility demand, in terms of plastic rotation at the ends of beams and columns and in joints, on these buildings during this class of earthquakes is up to 6% of a radian, which is far greater than a typical limiting plastic rotation of 3% associated with fracture and consequent failure of large wide-flanged steel sections during experiments. Thus, if strength degradation due to fractures, local buckling, etc., were to be included in the analysis, then the results would likely to be worse, as far as the ability of these buildings to withstand these earthquakes without collapse is concerned.
\r\n\r\nDue to damage localization, the peak drifts observed in the structure far exceeded the inelastic drift limit in the code of 0.02 (in some cases up to 3 times). This points to serious non-structural damage to facades, interior dry wall, etc. Furthermore, large roof permanent offsets after the events indicate significant post-earthquake repair requiring considerable disruption and building closure.
\r\n\r\nColumn yielding was minimal thus validating the strong-column weak-beam criterion in the code. Redundancy factors used to assess the redundancy in the system need to take into account the case of torsionally sensitive structures where frames in both principal directions are simultaneously activated. Stress concentration was not observed at the reentrant corners in L-shaped buildings.
\r\n\r\nFinally, the data catalogued in this work could be useful for future code development and tall structure design guidelines.
", "doi": "10.7907/A00K-RQ42", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:4059", "collection": "thesis", "collection_id": "4059", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10132003-150122", "primary_object_url": { "basename": "KlugDissertation.pdf", "content": "final", "filesize": 4089540, "license": "other", "mime_type": "application/pdf", "url": "/4059/1/KlugDissertation.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Director-Field Theory of DNA Packaging in Bacteriophage Viruses", "author": [ { "family_name": "Klug", "given_name": "William Scott", "clpid": "Klug-William-Scott" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis is concerned with the formulation of a continuum theory of packaging of DNA in bacterial viruses based on a director-field representation of the encapsidated DNA. The point values of the director field give the local direction and density of the DNA. The continuity of the DNA strand requires that the director field be divergence-free and tangent to the capsid wall. The energy of the DNA is defined as a functional of the director field which accounts for bending, torsion, and for electrostatic interactions through a density-dependent interaction energy. The operative principle which determines the encapsidated DNA conformation is assumed to be energy minimization.
\r\n\r\nThe director-field theory is used for the direct formulation and study of two low-energy DNA conformations: the inverse spool and torsionless toroidal solenoids. Analysis of the inverse spool configuration yields predictions of the interaxial spacing and the dependence of the packing force on the packed genome fraction which are found to be in agreement with experiments. Further analysis shows that torsionless toroidal solenoids can achieve lower energy than the inverse spool configuration.
\r\n\r\nAlso, the theory is adapted to a framework of numerical optimization, wherein all fields are discretized on a computational lattice, and energy minimizing configurations are sought via simulated annealing and the nonlinear conjugate gradient method. It is shown that the inverse spool conformation is stable in all regions of the virus capsid except in a central core, where the DNA tends to buckle out of the spooling plane.
", "doi": "10.7907/E0V6-4Y97", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:935", "collection": "thesis", "collection_id": "935", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03122004-105618", "primary_object_url": { "basename": "Swift_Thesis.pdf", "content": "final", "filesize": 2773011, "license": "other", "mime_type": "application/pdf", "url": "/935/1/Swift_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Neutron Diffraction Study of In Situ-Reinforced Silicon Nitride during Creep", "author": [ { "family_name": "Swift", "given_name": "Geoffrey Allen", "clpid": "Swift-Geoffrey-Allen" } ], "thesis_advisor": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" } ], "thesis_committee": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ahn", "given_name": "Channing C.", "clpid": "Ahn-C-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Neutron diffraction was used to study the high temperature creep of in situ-reinforced silicon nitride (ISR Si3N4). Full pattern and single peak fitting methods were used to calculate the average and hkl-specific mechanical properties, including thermal expansion coefficient, creep exponent, Young?s modulus, and Poisson?s ratio. This is both the first in-depth study using time-of-flight neutron diffraction to examine materials at such temperatures (1473 and 1648 K) and the first in situ microstructural study of creeping silicon nitride. Two commercial grades of ISR Si3N4 were tested, AS800 and GS44.
\r\n\r\nThe refractory grain boundary phase of AS800 prevented the onset of creep in the vacuum environment of the SMARTS furnace. However, the high temperature stress-strain data allowed determination of the 1648 K single crystal elastic stiffness tensor, the first such calculation from neutron diffraction strain data. Also determined was the coefficient of thermal expansion (CTE) tensor. The 1648 K stiffness tensor indicated a less stiff C33 component compared to a room temperature stiffness tensor. This lesser value is due either to microstructural or thermal effects.
\r\n\r\nCreep was observed for GS44. A stress step-up test and four constant stress creep tests were performed. Large strains were measured by an extensometer, though of much less magnitude than literature creep studies, with the difference attributed to the vacuum environment protecting the grain boundary phase from suffering reduced viscosity. This is supportive evidence of the long-held notion that the grain boundary phase is the primary determinate of creep behavior. The diffraction strains, though of significantly lower magnitude than the extensometer strains, were measured as non-constant, though the silicon nitride lattice parameters behaved in an unexpected manner. The two lattice parameters were seen to fork from a nearly common initial strain, with the c lattice parameter indicating tensile strain and the a lattice parameter compressive. The relative changes, however, corresponded to an essentially constant unit cell volume, as computed from simple geometry. None of the potential inelastic strain effects on diffraction peaks, such as peak broadening, were observed, further supporting the notion that the grain boundary phase is the source of strain. Neither was there any measured preferred orientation evolution due to creep. The creep exponent of GS44 was calculated as 3.18, a greater value than in literature, likely due to the same creep inhibition of the vacuum furnace.
\r\n\r\nThe classic Norton Equation for creep matched well with the steady state creep rates as a function of applied stress, while a newer model by Luecke and Wiederhorn, incorporating multiple facets specific to Si3N4, matched the data comparably, though with an additional empirical stress dependence incorporated. The effect of performing these experiments in a vacuum rather than in air likely prevented as accurate prediction by their model as with Norton?s. This result is based on the much-reduced creep strain measured compared to literature measurements of the same material at like temperature and stress. However, given the large disparity between the extensometer creep strain and the diffraction creep strain, it is clear that the grain boundary phase experiences the bulk of the deformation. Subsequent SEM observations of tested samples indicated no microstructural change due to the short duration of creep experiments. As with AS800, the GS44 CTE tensor was found, while the stiffness tensor was incalculable due to extreme non-linearity of single peak data.
", "doi": "10.7907/A3PQ-FC67", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:1799", "collection": "thesis", "collection_id": "1799", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05142004-144712", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 9091331, "license": "other", "mime_type": "application/pdf", "url": "/1799/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Constrained Sequential Lamination: Nonconvex Optimization and Material Microstructure", "author": [ { "family_name": "Fago", "given_name": "Matthew Justin", "clpid": "Fago-Matthew-Justin" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A practical algorithm has been developed to construct, through sequential lamination, the partial relaxation of multiwell energy densities such as those characteristic of shape memory alloys. The resulting microstructures are in static and configurational equilibrium, and admit arbitrary deformations. The laminate topology evolves during deformation through branching and pruning operations, while a continuity constraint provides a simple model of metastability and hysteresis. In cases with strict separation of length scales, the method may be integrated into a finite element calculation at the subgrid level. This capability is demonstrated with a calculation of the indentation of a Cu-Al-Ni shape memory alloy by a spherical indenter.
\r\n\r\nIn verification tests the algorithm attained the analytic solution in the computation of three benchmark problems. In the fourth case, the four-well problem (of, e.g., Tartar), results indicate that the method for microstructural evolution imposes an energy barrier for branching, hindering microstructural development in some cases. Although this effect is undesirable for purely mathematical problems, it is reflective of the activation energies and metastabilities present in applications involving natural processes.
\r\n\r\nThe method was further used to model Shield's tension test experiment, with initial calculations generating reasonable transformation strains and microstructures that compared well with the sequential laminates obtained experimentally.
", "doi": "10.7907/P1PK-E179", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:1409", "collection": "thesis", "collection_id": "1409", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04192004-120604", "primary_object_url": { "basename": "Can_Aydiner_Thesis.pdf", "content": "final", "filesize": 2896990, "license": "other", "mime_type": "application/pdf", "url": "/1409/1/Can_Aydiner_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Investigation of Thermal Tempering in Bulk Metallic Glasses", "author": [ { "family_name": "Aydiner", "given_name": "Cahit Can", "orcid": "0000-0001-8256-6742", "clpid": "Aydiner-Cahit-Can" } ], "thesis_advisor": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" } ], "thesis_committee": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Bulk metallic glasses are recent advanced materials which generate residual stresses due to rapid cooling from their surfaces during processing. These stresses arise from the thermal gradients that form within the sample at and above the glass transition region. A typical processing of BMGs involves feeding the alloy melt into a mold followed by severe quenching. The formation and nature of these stresses are analogous to the residual stresses due to the thermal tempering of silicate glasses. This analytical-experimental study investigates the thermal tempering phenomenon in BMGs for the first time.
\r\n\r\nOne of the best glass forming metallic alloys, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy 1TM), is employed in this study. First, the best technique for the high-resolution measurement of residual stresses in BMGs is determined to be the crack compliance method. Second, the formation of the stresses is modeled with three different levels of viscoelastic phenomenology, namely, an instant freezing model, a viscoelastic model and a structural model. The first is a simplistic analytical model to estimate residual stresses whereas the structural model accounts for the temperature history dependence of the glassy structure. The constitutive laws for the viscoelastic and structural models are incorporated into the finite element method (ABAQUSTM software package) allowing the application of these models to complex geometries. To increase the accuracy of the analysis, the 'correct' temperature evolution in the sample during processing has to be input to these 'mechanical' models. Therefore, the heat transfer problem during the casting process of the BMG is analyzed in detail. Accuracy also requires a detailed knowledge of the thermal parameters of the material as a function of temperature; thus, some attention is also devoted to their measurement.
\r\n \r\nAt the end, calculated and measured stresses are compared and good agreement is achieved. BMGs are demonstrated to be capable of generating very high (around 400 MPa) compression on their surfaces. The study also yielded valuable physical insight into the thermal tempering process itself. It is seen that this process exhibits significant discrepancies in BMGs compared to its analogy in silicate glasses. For instance, the transient tensile stresses that develop in the latter are shown to be lacking in the BMGs. Another discrepancy between the two materials is that the density of BMGs is uniform across the sample cross section in contrast to that found in silicate glasses. Overall, this investigation developed sufficient understanding of the thermal tempering phenomenon in BMGs to establish it as a viable process to manipulate properties.
\r\n", "doi": "10.7907/ZC9Z-5Y06", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:1276", "collection": "thesis", "collection_id": "1276", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04062004-165940", "primary_object_url": { "basename": "twc_thesis_electronic.pdf", "content": "final", "filesize": 9410907, "license": "other", "mime_type": "application/pdf", "url": "/1276/1/twc_thesis_electronic.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Gaseous Detonation-Driven Fracture of Tubes", "author": [ { "family_name": "Chao", "given_name": "Tong Wa", "clpid": "Chao-Tong-Wa" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "abstract": "An experimental investigation of fracture response of aluminum 6061-T6 tubes under internal gaseous detonation loading has been carried out. The pressure load, with speeds exceeding 2 km/s, can be characterized as a pressure peak (ranging from 2 to 6 MPa) followed by an expansion wave. The unique combination of this particular traveling load and tube geometry produced fracture data not available before in the open literature. Experimental data of this type are useful for studying the fluid-structure-fracture interaction and various crack curving and branching phenomena, and also for validation for multi-physics and multi-scale modeling.
\r\n\r\nAxial surface flaws were introduced to control the crack initiation site. Fracture threshold models were developed by combining a static fracture model and an extensively studied dynamic amplification factor for tubes under internal traveling loads. Experiments were also performed on hydrostatically loaded preflawed aluminum 6061-T6 tubes for comparison. Significantly different fracture behavior was observed and the difference was explained by fluid dynamics and energy considerations. The experiments yielded comparison on crack speeds, strain, and pressure histories.
\r\n\r\nIn other experiments, the specimens were also pre-torqued to control the propagation direction of the cracks. Measurements were made on the detonation velocity, strain history, blast pressure from the crack opening, and crack speeds. The curved crack paths were digitized. The Chapman-Jouguet pressure, initial axial flaw length, and torsion level were varied to obtain different crack patterns. The incipient crack kinking angle was found to be consistent with fracture under mixed-mode loading. High-speed movies of the fracture events and blast wave were taken and these were used in interpreting the quantitative data.
\r\n\r\nNumerical simulations were performed using the commercial explicit finite-element software LS-Dyna. The detonation wave was modeled as a traveling boundary load. Both non-fracturing linear elastic simulations and elastoplastic simulations with fracture were conducted on three-dimensional models. The simulated fracture was compared directly with an experiment with the same conditions. The overall qualitative fracture behavior was captured by the simulation. The forward and backward cracks were observed to branch in both the experiment and simulation.
", "doi": "10.7907/TEZP-YC46", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:2044", "collection": "thesis", "collection_id": "2044", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05252004-131315", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 7686920, "license": "other", "mime_type": "application/pdf", "url": "/2044/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Energy-Minimizing Microstructures in Multiphase Elastic Solids", "author": [ { "family_name": "Chenchiah", "given_name": "Isaac Vikram", "orcid": "0000-0002-8618-620X", "clpid": "Chenchiah-Isaac-Vikram" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Hou", "given_name": "Thomas Y.", "clpid": "Hou-T-Y" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis concerns problems of microstructure and its macroscopic consequences in multiphase elastic solids, both single crystals and polycrystals.
\r\n\r\nThe elastic energy of a two-phase solid is a function of its microstructure. Determining the infimum of the energy of such a solid and characterizing the associated extremal microstructures is an important problem that arises in the modeling of the shape memory effect, microstructure evolution (precipitation, coarsening, etc.), homogenization of composites and optimal design. Mathematically, the problem is to determine the relaxation under fixed volume fraction of a two-well energy.
\r\n\r\nWe compute the relaxation under fixed volume fraction for a two-well linearized elastic energy in two dimensions with no restrictions on the elastic moduli and transformation strains; and show that there always exist rank-I or rank-II laminates that are extremal. By minimizing over the volume fraction we obtain the quasiconvex envelope of the energy. We relate these results to experimental observations on the equilibrium morphology and behavior under external loads of precipitates in Nickel superalloys. We also compute the relaxation under fixed volume fraction for a two-well linearized elastic energy in three dimensions when the elastic moduli are isotropic (with no restrictions on the transformation strains) and show that there always exist rank-I, rank-II or rank-III laminates that are extremal.
\r\n\r\nShape memory effect is the ability of a solid to recover on heating apparently plastic deformation sustained below a critical temperature. Since utility of shape memory alloys critically depends on their polycrystalline behavior, understanding and predicting the recoverable strains of shape memory polycrystals is a central open problem in the study of shape memory alloys. Our contributions to the solution of this problem are twofold:
\r\n\r\nWe prove a dual variational characterization of the recoverable strains of shape memory polycrystals and show that dual (stress) fields could be signed Radon measures with finite mass supported on sets with Lebesgue measure zero. We also show that for polycrystals made of materials undergoing cubic-tetragonal transformations the strains fields associated with macroscopic recoverable strains are related to the solutions of hyperbolic partial differential equations.
", "doi": "10.7907/RXE5-9A33", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:2187", "collection": "thesis", "collection_id": "2187", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05282004-105848", "primary_object_url": { "basename": "Rogan_Thesis.pdf", "content": "final", "filesize": 5224115, "license": "other", "mime_type": "application/pdf", "url": "/2187/1/Rogan_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Investigation of the Multiscale Constitutive Behavior of Ferroelectric Materials Using Advanced Diffraction Techniques", "author": [ { "family_name": "Rogan", "given_name": "Robert Cashman", "clpid": "Rogan-Robert-Cashman" } ], "thesis_advisor": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" } ], "thesis_committee": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" }, { "family_name": "Fultz", "given_name": "Brent T.", "clpid": "Fultz-B-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Ferroelectric ceramics are widely used in a diverse set of devices including sensors, actuators, and transducers. The technological importance of ferroelectrics originates from their large electromechanical coupling. Ferroelectric materials exhibit a complicated behavior in response to both electrical and mechanical loads which produce large internal stresses that eventually lead to failure. Efforts to model and predict the behavior of ferroelectrics have been hindered by the lack of suitable constitutive relations that accurately describe the electromechanical response of these materials. While many measurements have been conducted on the macroscopic response of single-crystals or polycrystals, multiaxial (and multiscale) data about the in situ internal strain and texture response of these materials is lacking; this information is critical to the development of accurate models, and diffraction techniques which directly measure internal crystal strains and material texture are aptly suited to supply it.
\r\n\r\nA neutron diffraction technique was employed which allowed for the simultaneous measurement of material texture and lattice strains in directions parallel and transverse to an applied mechanical load. By comparing the behaviors of single-phase tetragonal, single-phase rhombohedral, and dual-phase morphotropic compositions, information concerning mechanics of average macroscopic behavior was inferred. In an effort to probe more of the multiaxial constitutive behavior, a high-energy X-ray diffraction technique was employed. Using transmission geometry and a 2-D image plate detector, 36 different directions of sample behavior were measured simultaneously. Polychromatic scanning X-ray microdiffraction was used to investigate the microscale three-dimensional strain tensor in single-crystals. One investigation yielded the first ever direct measurement of the tri-axial strain fields associated with single domain walls in ferroelectrics. The second investigation recorded the domain switching mechanisms activated to accommodate indentation-induced fracture stresses. Finally, 3-D XRD was used to probe the mesoscale constitutive behavior of single, embedded grains of BaTiO3 within a polycrystalline matrix.
\r\n\r\nThe experimental methods described in this thesis provide access to two-dimensional and three-dimensional multiaxial constitutive strain behavior in ferroelectrics for each of the microscopic, mesoscopic, and macroscopic length scales. Results from each of these length scales will provide critical data for models attempting to accurately describe the behavior of ferroelectric materials.
\r\n", "doi": "10.7907/BT3T-F608", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:2298", "collection": "thesis", "collection_id": "2298", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05302003-171943", "primary_object_url": { "basename": "GJ-thesis-print.pdf", "content": "final", "filesize": 2308374, "license": "other", "mime_type": "application/pdf", "url": "/2298/2/GJ-thesis-print.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Collisional Dynamics of Macroscopic Particles in a Viscous Fluid", "author": [ { "family_name": "Joseph Gonzalez", "given_name": "Gustavo", "clpid": "Joseph-Gonzalez-Gustavo" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "clpid": "Hunt-M-L" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Brady", "given_name": "John F.", "clpid": "Brady-J-F" }, { "family_name": "Zenit Camacho", "given_name": "Jose Roberto", "clpid": "Zenit-Camacho-J-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis presents experimental measurements of the approach and rebound of a particle colliding with a wall in a viscous fluid. Steel, glass, nylon, and Delrin particles were used, with diameters ranging from 3 to 12 mm. The experiments were performed using a thick Zerodur or Lucite wall with various mixtures of glycerol and water. Normal and tangential coefficients of restitution were defined from the ratios of the respective velocity components at the point of contact just prior to and after impact. These coefficients account for losses due to lubrication effects and inelasticity.
\r\n\r\nThe experiments clearly show that the rebound velocity depends strongly on the impact Stokes number and weakly on the elastic properties of the materials. Below a Stokes number of approximately 10, no rebound of the particle occurs. Above a Stokes number of approximately 500, the normal coefficient of restitution asymptotically approaches the value for a dry collision. The data collapse onto a single curve of restitution coefficient as a function of Stokes number when normalized by the dry coefficient of restitution.
\r\n\r\nOblique collisions in a fluid are qualitatively similar to oblique collisions in a dry system, with a lowered friction coefficient dependent on surface roughness. For smooth surfaces the friction coefficient is drastically reduced due to lubrication effects. Values for the friction coefficient are predicted based on elastohydrodynamic lubrication theory. The particle surface roughness was found to affect the repeatability of some measurements, especially for low impact velocities.
\r\n\r\nA significant retardation of a particle approaching a target at a low Stokes number was observed and quantified. The distance at which the particle's trajectory varies due to the presence of the wall is dependent on the impact Stokes number. The observed slowdown can be predicted from hydrodynamic theory to a good approximation.
\r\n\r\nAn analysis of the erosion of ductile materials during immersed collisions is presented. The size of the crater formed by the impact of a single particle against a ductile target can be estimated from theory, and these estimates agree well with experimental measurements.
", "doi": "10.7907/FFFC-X753", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:2227", "collection": "thesis", "collection_id": "2227", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05292003-113845", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 960602, "license": "other", "mime_type": "application/pdf", "url": "/2227/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Variational Arbitrary Lagrangian-Eulerian Method", "author": [ { "family_name": "Thoutireddy", "given_name": "Pururav", "clpid": "Thoutireddy-Pururav" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis is concerned with the development of Variational Arbitrary Lagrangian-Eulerian method (VALE) method. VALE is essentially finite element method generalized to account for horizontal variations, in particular, variations in nodal coordinates. The distinguishing characteristic of the method is that the variational principle simultaneously supplies the solution, the optimal mesh and, in case problems of shape optimization, optimal shape. This is accomplished by rendering the functional associated with the variational principle stationary with respect to nodal field values as well as with respect to the nodal positions of triangulation of the domain of analysis. Stationarity with respect to the nodal positions has the effect of the equilibriating the energetic or configurational forces acting in the nodes. Further, configurational force equilibrium provides precise criterion for mesh optimality. The solution so obtained corresponds to minimum of energy functional (minimum principle) in static case and to the stationarity of action sum (discrete Hamilton's stationarity principle) in dynamic case, with respect to both nodal variables and nodal positions. Further, the resulting mesh adaption scheme is devoid of error estimates and mesh-to-mesh transfer interpolation errors. We illustrate the versatility and convergence characteristics of the method by way of selected numerical tests and applications, including the problem of semi-infinite crack, the shape optimization of elastic inclusions and free vibration of 1-d rod.", "doi": "10.7907/DQT0-5104", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:2077", "collection": "thesis", "collection_id": "2077", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262003-200254", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 15430957, "license": "other", "mime_type": "application/pdf", "url": "/2077/1/Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Variational Time Integrators in Computational Solid Mechanics", "author": [ { "family_name": "Lew", "given_name": "Adri\u00e1n Jos\u00e9", "clpid": "Lew-Adri\u00e1n-Jos\u00e9" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Sulsky", "given_name": "Deborah", "clpid": "Sulsky-D" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis develops the theory and implementation of variational integrators for computational solid mechanics problems, and to some extent, for fluid mechanics problems as well. Variational integrators for finite dimensional mechanical systems are succinctly reviewed, and used as the foundations for the extension to continuum systems. The latter is accomplished by way of a space-time formulation for Lagrangian continuum mechanics that unifies the derivation of the balance of linear momentum, energy and configurational forces, all of them as Euler-Lagrange equations of an extended Hamilton's principle. In this formulation, energy conservation and the path independence of the J- and L-integrals are conserved quantities emanating from Noether's theorem. Variational integrators for continuum mechanics are constructed by mimicking this variational structure, and a discrete Noether's theorem for rather general space-time discretizations is presented. Additionally, the algorithms are automatically (multi)symplectic, and the (multi)symplectic form is uniquely defined by the theory. For instance, in nonlinear elastodynamics the algorithms exactly preserve linear and angular momenta, whenever the continuous system does.
\r\n\r\nA class of variational algorithms is constructed, termed asynchronous variational integrators (AVI), which permit the selection of independent time steps in each element of a finite element mesh, and the local time steps need not bear an integral relation to each other. The conservation properties of both synchronous and asynchronous variational integrators are discussed in detail. In particular, AVI are found to nearly conserve energy both locally and globally, a distinguishing feature of variational integrators. The possibility of adapting the elemental time step to exactly satisfy the local energy balance equation, obtained from the extended variational principle, is analyzed. The AVI are also extended to include dissipative systems. The excellent accuracy, conservation and convergence characteristics of AVI are demonstrated via selected numerical examples, both for conservative and dissipative systems. In these tests AVI are found to result in substantial speedups, at equal accuracy, relative to explicit Newmark.
\r\n\r\nIn elastostatics, the variational structure leads to the formulation of discrete path-independent integrals and a characterization of the configurational forces acting in discrete systems. A notable example is a discrete, path-independent J-integral at the tip of a crack in a finite element mesh.
", "doi": "10.7907/6C74-GC16", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:2287", "collection": "thesis", "collection_id": "2287", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05302003-094155", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 17895455, "license": "other", "mime_type": "application/pdf", "url": "/2287/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "A Phase-Field Model of Dislocations in Ductile Single Crystals", "author": [ { "family_name": "Koslowski", "given_name": "Marisol", "orcid": "0000-0001-9650-2168", "clpid": "Koslowski-Marisol" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Molinari", "given_name": "Alain", "clpid": "Molinari-A" }, { "family_name": "Meiron", "given_name": "Daniel I.", "clpid": "Meiron-D-I" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A phase-field theory of dislocations, strain hardening and hysteresis in ductile single crystals is developed. The theory accounts for an arbitrary number and arrangement of dislocation lines over a slip plane; the long-range elastic interactions between dislocation lines; the core structure of the dislocations; the interaction between the dislocations and an applied resolved shear stress field; and the irreversible interactions with short-range obstacles, resulting in hardening, path dependency and hysteresis.
\r\n\r\nWe introduce a variational formulation for the statistical mechanics of dissipative systems. The influence of finite temperature as well as the mechanics in the phase-field theory are modeled with a Metropolis Monte Carlo algorithm and a mean field approximation.
\r\n\r\nA chief advantage of the present theory is that at zero temperature it is analytically tractable, in the sense that the complexity of the calculations may be reduced, with the aid of closed form analytical solutions, to the determination of the value of the phase field at point-obstacle sites. The theory predicts a range of behaviors which are in qualitative agreement with observation, including hardening and dislocation multiplication in single slip under monotonic loading; the Bauschinger effect under reverse loading; the fading memory effect; the evolution of the dislocation density under cycling loading; temperature softening; strain rate dependence; and others.
\r\n\r\nThe model also reproduces the formation of dislocation networks observed in grain boundaries for different crystal structures and orientations. Simultaneously with the stable configurations the theory naturally predicts the equilibrium dislocation density independently of initial values or sources.
", "doi": "10.7907/SFMJ-1B50", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:2300", "collection": "thesis", "collection_id": "2300", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05302003-174154", "primary_object_url": { "basename": "SNL_Thesis.pdf", "content": "final", "filesize": 3839672, "license": "other", "mime_type": "application/pdf", "url": "/2300/1/SNL_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "I. The Heterogeneities at the Core-Mantle and Inner-Core Boundaries from PKP Phases. II. The Static and Dynamic Behavior of Silica at High Pressures", "author": [ { "family_name": "Luo", "given_name": "Sheng-Nian", "clpid": "Luo-Sheng-Nian" } ], "thesis_advisor": [ { "family_name": "Clayton", "given_name": "Robert W.", "clpid": "Clayton-R-W" } ], "thesis_committee": [ { "family_name": "Clayton", "given_name": "Robert W.", "clpid": "Clayton-R-W" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Asimow", "given_name": "Paul David", "clpid": "Asimow-P-D" }, { "family_name": "Ahrens", "given_name": "Thomas J.", "clpid": "Ahrens-T-J" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Waveform and differential travel-time (DTT) of various PKP phases have been utilized to study the velocity variations at the core-mantle boundary (CMB) and the inner-core boundary (ICB). The spatial concentration of events and stations, and the significant variations in PKPab-PKPdf DTT and waveform of PKPab, indicate localized sharp lateral variation of velocity at the CMB as supported by simulations. Modeling of DTT's among PKiKP, PKIKP and PKP-B-diffracted (Bdiff) phases, and waveform of Bdiff supports that the ratio of relative velocity variations of S- and P-wave at the CMB is larger than 2, and that hemispheric P-wave velocity variations exist at the top of the inner core, and that D' structure is related to the ICB via core dynamics.
\r\n\r\nThe equation of state of stishovite is obtained by direct shock wave loading up to 235 GPa as K0T=306 \u00b1 5 GPa and K0T' = 5.0 \u00b1 0.2 where K0T is ambient bulk modulus and K0T' its pressure derivative. Phase diagram of silica (including melting curve) up to megabar pressure regime is established based on molecular dynamics (MD) simulations and dynamic and static experiments. Calculations show that perovskite is thermodynamically stable relative to the stishovite and periclase assemblage at lower mantle conditions. A detailed and quantitative examination is conducted on the thermodynamics and phase change mechanisms (including amorphization) that occur upon shock wave loading and unloading of silica. The systematics of maximum undercooling and superheating, are established by incorporating normalized energy barrier for nucleation and heating (cooling) rate, and validated at the atomic level with systematic MD simulations. By considering superheating in shock wave experiments, high-pressure melting curves for silica, alkali halides and transition metals are constructed based on the Lindemann law and the $ln2$ rule for the entropy of melting.
", "doi": "10.7907/D8HB-A439", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:6345", "collection": "thesis", "collection_id": "6345", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04212011-132230527", "primary_object_url": { "basename": "Zhu_w_2002.pdf", "content": "final", "filesize": 47725548, "license": "other", "mime_type": "application/pdf", "url": "/6345/1/Zhu_w_2002.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Nonlinearly Viscoelastic Response of Glassy Polymers", "author": [ { "family_name": "Zhu", "given_name": "Weidong", "clpid": "Zhu-Weidong" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Emri", "given_name": "Igor", "orcid": "0000-0003-3329-9312", "clpid": "Emri-Igor" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis consists of three chapters. After a brief introduction on the general aspects of polymer characterization and viscoelasticity in the first chapter, all major features of this research project are described in the following two chapters.\r\n\r\nThe second chapter deals exclusively with the nonlinearly thermo-mechanical creep behavior of (bisphenol A) polycarbonate under pure shear loading at different\r\ntemperatures (0 \u00b0C to 140 \u00b0C). The shear creep in the linearly viscoelastic range was measured with a torsiometer for reference purposes and a master curve, along with a shift factor curve, were deduced. While the master curve is well defined with no detectable deviation, the shift factor can be represented by two straight line segments interrupted at the \u03b2 transition temperature of polycarbonate. The shear creep tests in the nonlinearly\r\nviscoelastic range were conducted on an Arcan specimen geometry at different temperatures and under different stress levels, utilizing digital image correlation for the\r\nrecording of the creep strains. The difference between the nominal stress and the actual stress distribution in the Arcan specimen was explored via numerical simulations\r\n(ABAQUS) by assuming linear quasi-elastic and quasi-plastic analysis in place of the as yet uncertain material characterization. Isochronal plots were created from the creep data. Nonlinearly viscoelastic behavior starts to take effect near 1% strain at the temperatures considered. The applicability of the stress-clock representation for material characterization has been explored and is found to be dubious, at best, for this material. The \"yield-like\" behavior of polycarbonate has been examined in terms of the isochronal stress-strain response and a corresponding \"yield-like shear stress\" has been determined\r\nto be a monotonically decreasing function of the temperature, but again with an interruption at the \u03b2 transition temperature. Time-temperature trade-off as practiced for \"time-temperature shifting\" at small strains does not apply in the nonlinear domain. The results are generally in agreement with those found for Poly(Methyl Methacrylate), thus fostering the idea that the present results can be generalized -with additional work- to other amorphous polymers.\r\n\r\nThe third chapter focuses on the role of volumetric strain in nonlinearly viscoelastic behavior of polycarbonate. The creep responses of (bisphenol A) polycarbonate at 80 \u00b0C under combined two-dimensional shear and tensile/compressive\r\nstress states were measured on Arcan specimens in the nonlinearly viscoelastic regime. Of particular interest is the influence of the dilatational deformation component on the nonlinearly viscoelastic creep behavior. Because the nonlinear material response determines the stress distribution under fixed deformation or load, but is not known a priori, a re-estimation of the latter is essential to verify or adjust the stress state(s). This is\r\naccomplished by approximating isochronal stress-strain relations derived from shear creep behavior, encompassing the nonlinear domain, by a classical incremental elastoplastic\r\nmaterial description at appropriate times. Inasmuch as the two-dimensional character of the test configuration places limits on accessing three-dimensional information, a coherent representation of the results in terms of maximum shear and/or octahedral representation is examined. It is found that the creep behavior under shear and normal stress or deformation imposition differ significantly: when viewed as a response in terms of a maximum shear description, there are material responses under combined loading when either one or the other dominates. Once the response is formulated in terms of an octahedral description the representation becomes less sensitive to normal vs. shear\r\nbehavior. Within the precision underlying the measurements it is found that the shear and normal strain components accumulate under creep in nearly constant ratios. However,\r\nunder this scenario it is demonstrated quite clearly that the addition of negative dilatational stress (or deformation) to pure shear leads to distinctly lower creep rates. The converse is true, if positive dilatational stresses are added, though not monotonically so.\r\n", "doi": "10.7907/V7GW-0342", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:6380", "collection": "thesis", "collection_id": "6380", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05102011-141326530", "primary_object_url": { "basename": "Zhuang_s_2002.pdf", "content": "final", "filesize": 20980970, "license": "other", "mime_type": "application/pdf", "url": "/6380/1/Zhuang_s_2002.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Shock Wave Propagation in Periodically Layered Composites", "author": [ { "family_name": "Zhuang", "given_name": "Shiming", "clpid": "Zhuang-Shiming" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Grady", "given_name": "Dennis E.", "clpid": "Grady-D-E" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Mathematically, a shock wave is treated as a discontinuity in a medium. In reality, however, a shock wave is always structured, i.e., its front takes a finite time to rise from an initial material state to the final shocked state. The structuring of a shock front is due to the competition between the nonlinearity of material behavior and the dissipation processes occurring during the wave propagation. There are many mechanisms which may be responsible for the dissipation and/or dispersion of shock wave energy. In homogeneous media, such as metals, one common interpretation for the structuring of a shock wave is that the viscoplasticity processes (dislocation, twinning, etc.) are responsible for the dissipation of energy. While in heterogeneous composites, besides the viscous dissipative processes existing in each of its constituents, due to the existence of internal interfaces, the scattering induced by the interface during shock compression could be another important mechanism.\r\n\r\nIn this study, the interface scattering effects on shock wave propagation in heterogeneous media were investigated by subjecting periodically layered composites to planar impact loading with a flyer plate. The flyer plate was accelerated to a desired velocity using a powder gun loading system. In order to measure shock particle velocity time history at an internal or the free surface of the specimen, the so-called VISAR (Velocity Interferometry System for Any Reflector) diagnostic system was constructed and used during shock compression experiments. Manganin stress gages were embedded inside the specimen at selected internal interfaces to measure shock stress time history. To study the scattering mechanisms of the interface to waves, two-component composite specimens with different interface mechanical properties and heterogeneity were prepared and tested. Different types of composites were prepared with differing mechanical impedance. Specimens with different heterogeneity were obtained by changing the geometrical configuration (length scale) of the layered stack. Two-dimensional numerical simulations were also carried out to understand the process of shock wave evolution in the layered composites.\r\n\r\nExperimental and numerical studies show that periodically layered composites support steady structured shock waves. The influence of internal interfaces on the shock wave propagation is through the scattering mechanism, i.e., multiple reflection of waves in the layers and their interaction with the shock wave. The interface scattering affects both the bulk and the deviatoric response of the composite to shock compression. The influence of scattering on the bulk behavior is to slow down the velocity of the shock wave in the composites, while its influence on the deviatoric response is to structure the shock wave profile. If all the dissipative and dispersive effects are collectively termed as viscosity, which causes the shock front structuring, i.e., the shock front rise-time increasing, then the effective shock viscosity increases with the increase of interface impedance mismatch and decreases with the increase of interface density (interface area per unit volume) and shock loading strength. The existing mixture model for constructing the constitutive relation for composites based on the known properties of its component materials can only, at best, reasonably predict the response of the composites under strong shock loading conditions. In order to fully describe the response of a heterogeneous composite to shock compression loading, accurate physics-based constitutive relations need to be formulated to take into account the scattering effects induced by the heterogeneous microstructure.\r\n", "doi": "10.7907/988X-1V27", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:4745", "collection": "thesis", "collection_id": "4745", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12032004-161201", "primary_object_url": { "basename": "Nguyen_ot_2002.pdf", "content": "final", "filesize": 5217126, "license": "other", "mime_type": "application/pdf", "url": "/4745/1/Nguyen_ot_2002.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Cohesive Models of Fatigue Crack Growth and Stress-Corrosion Cracking", "author": [ { "family_name": "Nguyen", "given_name": "Olivier Thanh", "clpid": "Nguyen-Olivier-Thanh" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Pandolfi", "given_name": "Anna", "clpid": "Pandolfi-A" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The aim of this dissertation was to develop models of fatigue crack growth and stress-corrosion cracking by investigating cohesive theories of fracture. These models were integrated in a finite-element framework embedding a contact algorithm and techniques of remeshing and adaptive meshing.\r\n\r\nFor the fatigue model, we developed a phenomenological cohesive law which exhibits unloading-reloading hysteresis. This model qualitatively predicts fatigue crack growth rates in metals under constant amplitude regime for short and long cracks, as well as growth retardation due to overload. Quantitative predictions were obtained in the case of long cracks.\r\n\r\nWe developed a chemistry-dependent cohesive law which serves as a basis for the stress-corrosion cracking model. In order to determine this cohesive law, two approaches, based on energy relaxation and the renormalization group, were used for coarse-graining interplanar potentials. We analyzed the cohesive behavior of a large--but finite--number of interatomic planes and found that the macroscopic cohesive law adopts a universal asymptotic form. The resulting stress-corrosion crack growth rates agreed well with those observed experimentally in 'static' fatigue tests given in the literature.\r\n", "doi": "10.7907/C3KP-4M44", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:3529", "collection": "thesis", "collection_id": "3529", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09142005-105805", "primary_object_url": { "basename": "Chasiotis_i_2002.pdf", "content": "final", "filesize": 18575077, "license": "other", "mime_type": "application/pdf", "url": "/3529/1/Chasiotis_i_2002.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "The Strength of Polycrystalline Silicon at the Micro- and Nano-Scales with Applications to MEMS", "author": [ { "family_name": "Chasiotis", "given_name": "Ioannis", "clpid": "Chasiotis-Ioannis" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ustundag", "given_name": "Ersan", "orcid": "0000-0002-0812-7028", "clpid": "Ustundag-E" }, { "family_name": "Tai", "given_name": "Yu-Chong", "orcid": "0000-0001-8529-106X", "clpid": "Tai-Yu-Chong" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Three aspects concerning the reliability of MicroElectroMechanical Systems (MEMS) are discussed in this work. These aspects are: (1) the development of a new tensile testing technique for measuring the elastic modulus and rupture strengths of thin films, (2) an assessment of stress concentration and specimen size effects in failure of micron-sized specimens and (3) the consequences of Hydrofluoric Acid (HF) chemical treatment on the microstructural integrity and the tensile strength of polycrystalline silicon.
\r\n\r\nA new method for tensile testing of thin films by means of an improved apparatus has been developed to measure the elastic properties (Young's modulus, tensile strength) of surface micromachined polycrystalline silicon specimens. The newly designed tensile tester makes use of an Ultraviolet (UV) light curable adhesive to clamp micron-sized specimens. It permits for the first time the testing of thin film materials possessing high failure strength. The properties determination utilizes surface topologies of deforming specimens, acquired with an Atomic Force Microscope (AFM), for determining strain fields by means of Digital Image Correlation (DIC). This full-field, direct and local measurements technique provides the capability of testing any type of thin film materials with nanometer resolution. The gage section of the specimens tested in this study varied between 200 and 1000 \u00b5m in length, 6 and 50 \u00b5m in width, all for a nominal thickness of 2 \u00b5m.
\r\n\r\nThe dependence of fracture strength on micron and sub-micron sized geometries was studied by means of specimens containing various degrees of stress concentrations. A systematic study of small-scale size effects was thus performed by tensioning elliptically perforated specimens (minimum radius of curvature of 1 gm) so as to: (a) vary the stress concentration with constant radius of curvature, (b) increasing radius of curvature of micronotches relative to the grain size. The results demonstrate a strong influence of the size of the highly strained domain (decreasing notch radii) on the failure strength of MEMS scale specimens, while the effect of varying the stress concentration factor is rather insignificant. In addition, tests performed on unnotched tensile specimens of varying dimensions revealed a specimen size effect by which the values of strength scaled with the specimen length. The Young's modulus, however, is found to be rather insensitive to the specimen dimensions at the scale of microns.
\r\n\r\nIn an effort to assess the unexplained and puzzling large variation in properties reported for very small polysilicon specimens a study was conducted to search for a cause. Contrary to the common belief that 49% HF wet release represents a safe post-process for manufacturing polycrystalline silicon, this study has clearly identified the release process as a key item in determining thin film failure properties. It is found that surface roughness as characterized by groove formation at the grain boundaries depends distinctly on the HF release time. In addition, while the actual failure mechanism in polysilicon follows a transgranular fracture, moderate exposure in HF results in partial intergranular fracture at the film surface that is responsible for complete failure. Long exposures yield films of low mechanical strength that demonstrate clear intergranular failure.
", "doi": "10.7907/JPRZ-P277", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:601", "collection": "thesis", "collection_id": "601", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02112002-153745", "primary_object_url": { "basename": "DA_thesis.pdf", "content": "final", "filesize": 6062605, "license": "other", "mime_type": "application/pdf", "url": "/601/1/DA_thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Experimental Investigation of Quasistatic and Dynamic Fracture Properties of Titanium Alloys", "author": [ { "family_name": "Anderson", "given_name": "David Deloyd", "clpid": "Anderson-David-Deloyd" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ustundag", "given_name": "Ersan", "orcid": "0000-0002-0812-7028", "clpid": "Ustundag-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The goal of this work is to investigate the quasistatic and dynamic fracture properties of three titanium alloys: 6Al-4V titanium, 6Al-4V titanium ELI, and Timetal 5111. While standard tests exist for measuring quasistatic fracture toughness, the dynamic investigation requires that several measurement techniques are employed including Coherent Gradient Sensing (CGS), Crack Opening Displacement (COD), and the use of strain gages. The use of these methods with difficult engineering materials in the dynamic loading regime requires methodologies to be advanced beyond that previously required with model materials having properties ideal for experimental measurements techniques.
\r\n\r\nAfter a description of each measurement technique is given, stress intensity factor measurements made on 12.7 mm thick pre-cracked 6Al-4V titanium specimens are compared. These specimens were dynamically impacted in three point bend in a drop weight tower. Specimens with and without side-grooves were tested as each measurement technique allows. Side-grooves are useful to increase the degree of plane strain experienced in proximity of the crack tip, allowing plane strain (geometry independent) fracture toughnesses to be obtained from specimens that may be otherwise too thin in cross section. Resulting stress intensity factor-time histories from the different techniques are compared to verify that their results mutually agree.
\r\n\r\nAdvancements in employing CGS, a shearing interferometric technique, are described in more detail. First, the analysis of CGS interferograms is extended to allow experimental fringe data to be fit to very general analytical asymptotic crack tip solution to determine mixed mode stress intensity factors. As formulated in this work, the CGS technique can be used to measure stress intensity factors for non-uniformly propagating dynamic mixed mode cracks moving along arbitrary paths in homogeneous linear elastic isotropic materials. Other advancements are also detailed which improve analysis accuracy, objectivity, and efficiency.
\r\n\r\nFinally, with the equivalence of the three measurement technique results established, tests were performed on 8--17 mm thick pre-cracked three point bend specimens of the three materials to measure critical stress intensity values for crack initiation. Side-grooves are necessary for the more ductile 6Al-4V titanium ELI and Timetal 5111 materials to obtain plane strain fracture toughness values. It is found that both the 6Al-4V titanium ELI and Timetal 5111 alloys are 50-70% tougher than the 6Al-4V titanium, and for all three materials their initiation toughness does not vary significantly with loading rate over the domain tested.
", "doi": "10.7907/NHZS-D271", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:2515", "collection": "thesis", "collection_id": "2515", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06082005-151713", "primary_object_url": { "basename": "Lu_J_2002.pdf", "content": "final", "filesize": 14799070, "license": "other", "mime_type": "application/pdf", "url": "/2515/1/Lu_J_2002.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Mechanical Behavior of a Bulk Metallic Glass and Its Composite Over a Wide Range of Strain Rates and Temperatures", "author": [ { "family_name": "Lu", "given_name": "Jun", "clpid": "Lu-Jun" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ustundag", "given_name": "Ersan", "orcid": "0000-0002-0812-7028", "clpid": "Ustundag-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe development of bulk metallic glasses (BMG), which have exceptional mechanical properties such as high strength, high hardness and corrosion resistance, as well as good glass forming and shaping abilities, using relatively expensive materials and processing techniques, offers great opportunities to use this class of solids as structural amorphous materials (SAM). In this thesis, the mechanical behavior of a bulk metallic glass [...] (Vitreloy 1) and its composite [beta]-phase Vitreloy 1 composite, i.e., [...]) is investigated.\r\n\r\nThe stress-strain relations for Vitreloy 1 over a broad range of temperatures (from room temperature up to the crystallization temperature) and strain rates [...] were established in uniaxial compression using both quasi-static and dynamic Kolsky pressure bar loading systems. The effect of strain rate and temperature on steady state flow stress, viscosity and peak stress, as well as the effect of jump-in-strain-rate on the stress-strain behavior, were investigated. Based on the experimental results, boundaries between three main deformation modes are proposed, namely, Newtonian flow and nonlinear flow resulting in homogeneous deformation and shear-localized failure constituting inhomogeneous deformation. To characterize the constitutive behavior of the bulk metallic glass, a free volume based model as well as a fictive stress model are utilized to analyze the stress-strain behavior and a mechanism for shear band formation.\r\n\r\nA unique deformation characteristic of a bulk metallic glass is the shear localization of the material in response to external mechanical loading, which may lead to catastrophic shear failure immediately after yielding under uniaxial loading and at low temperatures. A dynamic indentation experimental setup was developed to evaluate the high-strain-rate inelastic post yield deformation behavior of Vitreloy 1 and its [beta]-phase composite. Time-resolved depth and load responses during the process of indentation on the materials were obtained. Both materials are found to be strain rate insensitive up to 2,000 [...]. Numerical simulations of the indentation experiments, using both pressure insensitive (J2 von Mises) and pressure dependent (Drucker-Prager) flow models, reveal that both materials are pressure (or normal stress) dependent. Intense multiple shear bands are observed in the indentation craters and are responsible for the observed overall inelastic deformation.\r\n\r\nTo further examine the inelastic deformation and as well as whether a pressure sensitive or normal stress is more appropriate for Vitreloy 1, multiaxial compression experiments using a confining sleeve technique were performed. In contrast to the catastrophic shear failure behavior in uniaxial compression, Vitreloy 1 exhibits large inelastic deformation of more than 10 percent under confinement, indicating the nature of ductile deformation under constrained conditions. It is found that the metallic glass follows a pressure dependent Tresca criterion, [...], and the coefficient of the pressure dependence, [beta], is 0.17. Multiple parallel shear bands are observed on the outer surfaces of the deformed specimens.\r\n\r\nMotivated by potential use of Vitreloy 1 in impact related applications, the shock compression characteristics of both Vitreloy 1 and [beta]-Vitreloy composite were studied using planar impact loading. A surprisingly low amplitude elastic precursor bulk wave, corresponding to the elastic response of the 'frozen structure' of the intact metallic glasses, was observed to precede the rate-dependent large deformation shock wave. A concave downward curvature after the initial increase of the [...] shock Hugoniots suggests that a phase-change-like transition occurred during shock compression. In addition, compression damage occurred due to the shear localization. The spalling inside Vitreloy 1 was induced by shear localization, while in [beta]-Vitreloy 1, it was due to debonding of the [beta]-phase boundary from the matrix. The spall strengths at strain rate of [...] were 2.35 GPa and 2.11 GPa for Vitreloy I and [beta]-Vitreloy 1, respectively.", "doi": "10.7907/3SZ8-Y947", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:6350", "collection": "thesis", "collection_id": "6350", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04252011-111825843", "primary_object_url": { "basename": "Xu_lr_2002.pdf", "content": "final", "filesize": 6045763, "license": "other", "mime_type": "application/pdf", "url": "/6350/1/Xu_lr_2002.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Dynamic Failure Characteristics in Layered Materials and Structures", "author": [ { "family_name": "Xu", "given_name": "Luoyu Roy", "clpid": "Xu-Luoyu-Roy" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Heaton", "given_name": "Thomas H.", "orcid": "0000-0003-3363-2197", "clpid": "Heaton-T-H" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Systematic investigations were carried out to understand the general nature of dynamic failure mechanisms in layered materials and structures such as composite and\r\nsandwich structures, thin films, layered armors and layered rock. A series of impact experiments on model-layered specimens were conducted using high-speed photography\r\nand dynamic photoelasticity.\r\n\r\nFor the first time, the sequence and interaction of two major dynamic failure modes in layered materials-inter-layer cracking and intra-layer cracking were revealed\r\nin real time. For heterogeneous three-layer systems, shear-dominated inter-layer cracking was always the first failure event for specimens subjected to low-speed impact. Interlayer cracking generally nucleated from interfacial locations where the inter-layer shear stress acquired a local maximum. Depending on impact speed and bond strength\r\ncharacteristics, inter-layer cracks were very transient and often became intersonic even under moderate impact speeds. Intra-layer cracking always initiated after the development of inter-layer cracks as a result of inter-layer crack kinking into the adjacent layer. The resulting intra-layer mode I cracks often accelerated and branched as they\r\nattained high speeds, causing core layer fragmentation. For homogenous-layered systems composed of bonded layers of Homalite, intra-layer cracks appeared in the form of cracks\r\nradiating from the impact site. As soon as these cracks approached an interface, interlayer cracks were often induced depending on the angle between the crack path and the interface. Direct experimental evidence of the dynamic equivalent of \"Cook-Gordon mechanism\" was recorded, i.e., two intersonic interfacial cracks nucleated and propagated along the interface before a fan of mode I incident cracks was ever able to reach the interface. Also, significant dependence of the failure characteristics on impact speeds and interfacial strengths was found. For the heterogeneous three-layer system subjected to a high impact speed, two clear shear shock waves associated with the intersonic inter-layer cracks were observed at the specimen center. Shock waves were also observed along the interface in heterogeneous three-layer systems featuring weak and ductile bonds. The impact momentum and loading duration were identified as two important parameters in damage spreading for a given impact energy.\r\n\r\nMotivated by the experimental observations of crack deflection/penetration at an interface, a novel wedge-loaded impact specimen was designed to explore the basic\r\nmechanics nature of this phenomenon. The deflection/penetration behavior of an incoming dynamic crack at an interface was found to depend on the interfacial angle and the interfacial fracture toughness. A dynamic fracture model, together with an energy criterion, were proposed and were found to agree reasonably well with the experimental observations.", "doi": "10.7907/wver-8342", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:6127", "collection": "thesis", "collection_id": "6127", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10112010-152127073", "primary_object_url": { "basename": "Coker_d_2001.pdf", "content": "final", "filesize": 23182405, "license": "other", "mime_type": "application/pdf", "url": "/6127/1/Coker_d_2001.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Dynamic Initiation and Propagation of Cracks in Unidirectional Composite Plates", "author": [ { "family_name": "Coker", "given_name": "Demirkan", "orcid": "0000-0001-7385-7089", "clpid": "Coker-Demirkan" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Dynamic crack growth along weak planes is a significant mode of failure in composites and other layered/sandwiched structures and is also the principal mechanism of shallow crustal earthquakes. In order to shed light on this phenomenon dynamic crack initiation and propagation characteristics of a model fiber-reinforced unidirectional graphite/epoxy composite plate was investigated experimentally. Dynamic fracture experiments were conducted by subjecting the composite plates to in-plane, symmetric and asymmetric, impact loading. The lateral shearing interferometric technique of coherent gradient sensing (CGS) in conjunction with high-speed photography was used to visualize the failure process in real time. It was found that mode-I cracks propagated subsonically with crack speeds increasing to the neighborhood of the Rayleigh wave speed of the composite. Also in mode-I, the dependence of the dynamic initiation fracture toughness on the loading rate was determined and was found to be constant for low loading rates and to increase rapidly above K̇dI > 10\u2075. The dynamic crack propagation toughness, KID, was observed to decrease with crack tip speed up to the Rayleigh wave speed of the composite.
\r\n\r\nFor asymmetric, mode-II, types of loading the results revealed highly unstable and intersonic shear-dominated crack growth along the fibers. These cracks propagated with unprecedented speeds reaching 7400 m/s which is the dilatational wave speed of the composite along the fibers. For intersonic crack growth, the interferograms featured a shock wave structure typical of disturbances traveling with speeds higher than one of the characteristic wave speeds in the solid. In addition high speed thermographic measurements are conducted that show concentrated hot spots behind the crack tip indicating non-uniform crack face frictional contact. In addition, shear dominated dynamic crack growth is investigated along composite/Homalite interfaces subjected to impact loading. The crack growth phenomenon was observed usivvvvng dynamic photoelasticity in conjunction with high-speed photography. Three quantized intersonic and supersonic crack tip speed regimes were identified. First conclusive evidence of crack growth at supersonic speeds with respect to lower speed material and sonic speeds with respect to the unidirectional composite was obtained. Furthermore, this investigation documents the first experimental observation of a mother/daughter crack mechanism allowing a subsonic crack to evolve into an intersonic crack.
\r\n\r\n", "doi": "10.7907/yrm2-4b88", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:6220", "collection": "thesis", "collection_id": "6220", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12212010-112400412", "primary_object_url": { "basename": "Huang_y_2001.pdf", "content": "final", "filesize": 10325405, "license": "other", "mime_type": "application/pdf", "url": "/6220/1/Huang_y_2001.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Scanning Tunneling Microscopy and Digital Image Correlation in Nanomechanics Investigations", "author": [ { "family_name": "Huang", "given_name": "Ying", "clpid": "Huang-Ying" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Probe microscopy (scanning tunneling microscopy and atomic force microscopy) and digital image correlation together serve as a potentially powerful tool for experimentally\r\ninvestigating the mechanical behaviors of materials at the sub-micron and nanometer scales. Based on the tunneling effect in quantum physics, the scanning tunneling\r\nmicroscope (STM) records surface topography quantitatively and can achieve angstrom resolution. The digital image correlation (DIC) extracts the displacements and gradients\r\nfrom the undeformed and deformed topographical images.\r\nIn this work, a calibration has been performed on the existing STM built \"in-house\" and the coefficients used in the STM system were confirmed. Major improvements on several\r\ncomponents of the system have been made, including constructing a new actuator probe to decouple its in-plane and out-of-plane movements, designing and implementing a new\r\nfirst-stage amplifier to reduce the noise output by a factor of 10 and modeling of the controller in the STM feedback loop.\r\nFurther, systematic study of the digital image correlation has been conducted. In the simple case of one-dimensional correlation, key parameters involved are the subset size,\r\nvariables in the displacement representation, frequency content of the signal and noise. The one-dimensional study was then extended to two dimensions. In addition to those\r\nkey parameters identified in the one-dimensional study, the sampling rate poses substantial influence on the correlation accuracy. Low amplitude, high frequency noise\r\nstill increases the correlation error significantly.", "doi": "10.7907/exy7-5z58", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:6354", "collection": "thesis", "collection_id": "6354", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04262011-100757709", "primary_object_url": { "basename": "Sane_s_2001.pdf", "content": "final", "filesize": 50485649, "license": "other", "mime_type": "application/pdf", "url": "/6354/1/Sane_s_2001.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Time-dependent compressibility of poly (methyl methacrylate) (PMMA) : an experimental and molecular dynamics investigation", "author": [ { "family_name": "Sane", "given_name": "Sandeep Bhalchandra", "clpid": "Sane-S-B" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis contains three chapters, which describe different aspects of an investigation of the bulk response of Poly(Methyl Methacrylate) (PMMA). The first chapter describes the physical measurements by means of a Belcher/McKinney-type apparatus. Used earlier for the measurement of the bulk response of Poly(Vinyl Acetate), it was now adapted for making measurements at higher temperatures commensurate with the glass transition\r\ntemperature of PMMA. The dynamic bulk compliance of PMMA was measured at atmospheric pressure over a wide range of temperatures and frequencies, from which the master curves for the bulk compliance were generated by means of the time-temperature superposition principle. It was found that the extent of the transition ranges for the bulk and shear response were comparable. Comparison of the shift factors for bulk and shear responses supports the idea that different molecular mechanisms contribute to shear and\r\nbulk deformations.\r\n\r\nThe second chapter delineates molecular dynamics computations for the bulk response for a range of pressures and temperatures. The model(s) consisted of 2256 atoms\r\nformed into three polymer chains with fifty monomer units per chain per unit cell. The time scales accessed were limited to tens of pico seconds. It was found that, in addition to the typical energy minimization and temperature annealing cycles for establishing equilibrium models, it is advantageous to subject the model samples to a cycle of\r\nrelatively large pressures (GPa-range) for improving the equilibrium state. On comparing the computations with the experimentally determined \"glassy\" behavior, one finds that,\r\nalthough the computations were limited to small samples in a physical sense, the primary limitation rests in the very short times (pico seconds). The molecular dynamics computations do not model the physically observed temperature sensitivity of PMMA, even if one employs a hypothetical time-temperature shift to account for the large\r\ndifference in time scales between experiment and computation. The values computed by the molecular dynamics method do agree with the values measured at the coldest\r\ntemperature and at the highest frequency of one kiloHertz.\r\n\r\nThe third chapter draws on measurements of uniaxial, shear and Poisson response conducted previously in our laboratory. With the availability of four time or frequency-dependent material functions for the same material, the process of interconversion between different material functions was investigated. Computed material functions were\r\nevaluated against the direct experimental measurements and the limitations imposed on successful interconversion due to the experimental errors in the underlying physical data\r\nwere explored. Differences were observed that are larger than the experimental errors would suggest.\r\n", "doi": "10.7907/saw5-7p32", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:6145", "collection": "thesis", "collection_id": "6145", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10152010-111044693", "primary_object_url": { "basename": "Good_NR_2001.pdf", "content": "final", "filesize": 3000007, "license": "other", "mime_type": "application/pdf", "url": "/6145/1/Good_NR_2001.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Influence of Texture on the Magnetoelastic Properties of Polycrystalline TbDy Alloys", "author": [ { "family_name": "Good", "given_name": "Nathan Ray", "clpid": "Good-Nathan-Ray" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Crocker", "given_name": "John C.", "clpid": "Crocker-J-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "An investigation into the influence of texture on the magneto elastic properties of cold-rolled polycrystalline terbium-dysprosium alloys has been performed. Significant influence of grain orientations on the thermal expansion, magnetostrictive and magnetomechanical damping properties of TbDy were observed.
\r\n\r\nDrop cast ingots of TbDy alloys were deformed by cold-rolling and annealing with the aim of reorienting grains for large magnetostrictive strains and damping capacities. Thermal expansion coefficients of single crystal and polycrystalline samples of TbDy were measured from 77 - 570 K. These measurements confirmed the expected strong reorientation of the c-axes of grains toward the direction of applied force during deformation. The anisotropy of thermal expansion between the rolling and applied stress directions provided a measure of the effectiveness of various sample preparations for maximizing magnetocrystalline anisotropy. Furthermore, the effects of magnetic phase transitions on thermal expansion through the Curie and N eel points of TbDy revealed thermal expansion anisotropy between grain orientations in the rolling and transverse directions of deformation.\r\nMagnetostrictive strains along the rolling direction of polycrystalline TbDy alloys were measured at 77 K. Saturation magnetostriction of up to 55% of previous single crystal results were observed. Minimal applied stress was required to obtain near maximum magnetostrictions for all of the samples tested. This suggests a preloading mechanism within the grain structure of polycrystalline TbDy not present in single crystals. Also in contrast with single crystal measurements, the performance of more economical commercial purity (99.7%) samples was seen to be somewhat higher than similarly prepared high purity (99.94%) samples. Resistance to deterioration of performance over multiple cycles was observed, as changes in magnetostriction over 150 cycles at 0.1 Hz were within measurement errors. By comparing thermal expansion anisotropy of TbDy samples with peak magnetostrictive strain, a clear proportionality between texture and magnetostrictive performance was established. Deviations from this pattern by samples with more deformation and annealing suggest microstructural mechanisms beyond average grain orientation impacting magnetostriction.
\r\n\r\nMagnetomechanical damping effects were observed for polycrystalline TbDy samples through compression stress-strain curves. Elastic moduli at 77 K was measured to be up to 80% less than at 300 K, with a large hysteresis present in the stress-strain curves of all samples tested below the Curie point. Damping capacity was measured as the stress-strain hysteresis loop divided by the total area under the stress-strain curve. Damping capacities up to 23% were measured for polycrystalline TbDy alloys. Larger magnetomechanical damping was observed at lower strains, with higher strains corresponded to saturation of magnetic domain realignment, smaller damping capacities and larger elastic moduli. Samples with larger magnetostrictions displayed larger damping capacities over a wide range of applied stresses, and also had consistently lower elastic moduli at 77K.
\r\n\r\nMechanisms of damping were investigated by fitting magnetostriction, elastic modulus and estimated strain of damping saturation of TbDy alloys to a model of magnetomechanical energy dissipation. This model relates magnetomechanical damping to magnetic hysteresis, neglecting microstructural influences not present in magnetostriction data. The damping capacities predicted by this model were approximately an order of magnitude higher than experimental results. This result suggests a prominent role of microstructural interactions in the domain realignments responsible for magnetoelastic damping.
", "doi": "10.7907/nh91-2f05", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:966", "collection": "thesis", "collection_id": "966", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03162005-130412", "primary_object_url": { "basename": "Lokhandwalla_m_2001.pdf", "content": "final", "filesize": 5622602, "license": "other", "mime_type": "application/pdf", "url": "/966/1/Lokhandwalla_m_2001.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Damage Mechanisms in Shock Wave Lithotripsy (SWL)", "author": [ { "family_name": "Lokhandwalla", "given_name": "Murtuza", "clpid": "Lokhandwalla-Murtuza" } ], "thesis_advisor": [ { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" } ], "thesis_committee": [ { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nShock wave lithotripsy is a 'non-invasive' therapy for treating kidney stones. Focused shock waves fragment stones to a size that can be passed naturally. There is, however, considerable tissue injury associated with this treatment, and the mechanisms of stone fragmentation and tissue injury are not well understood. This work investigates potential tissue damage mechanisms, with an aim towards modifying the wave-field parameters, so as to enhance stone fragmentation and minimize tissue damage.\r\n\r\nLysis of red blood cells (RBC's) due to in vitro exposure to shock waves was considered as a model of cellular level damage. Fluid flow-fields induced by a non-uniform shock wave, as well as radial expansion/implosion of a bubble was hypothesized to cause lysis of cells. Both the above flow-fields constitute an unsteady, extensional flow exerting inertial as well as viscous forces on the RBC membrane. The resultant membrane tension and the membrane areal strain ([Delta]A/A) due to the above flow-fields were estimated. Both were found to exert a significantly higher inertial force (50 - 100 mN/m) than the critical membrane tension (10 mN/m). Bubble-induced flow-field was estimated to last for a longer duration ([...]) compared to the shock-induced flow ([...]) and hence, was predicted to be lytically more effective, in typical in vitro experimental conditions. However, in vivo conditions severely constrain bubble growth, and cell lysis due to shock-induced shear could be dominant.\r\n\r\nHemolysis due to shock-induced shear, in absence of cavitation, was experimentally investigated. The lithotripter-generated shock wave was refocused by a parabolic reflector. This refocused wave-field had a tighter focus (smaller beam-width and a higher amplitude) than the lithotripter wave-field. Cavitation was eliminated by applying overpressure to the fluid. A novel passive cavitation detector (HP-PCD) operating at high overpressure (upto 7 MPa) was used to measure acoustic emission due to bubble activity. Aluminum foils were also used to differentiate cavitational from non-cavitational mode of damage. RBC's suspended in phosphate-buffered saline PBS) were exposed to the reflected wave-field from the parabolic reflector and also from a flat reflector, the latter serving as a control experiment. Exposure to the wave-field from the parabolic reflector increased hemolysis four-fold compared to untreated controls and was twice that of cell lysis with the flat reflector. This result corroborated the hypothesis of shock-induced shear as a cell damage mechanism in the absence of cavitation.\r\n", "doi": "10.7907/VZWS-7Z85", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:4218", "collection": "thesis", "collection_id": "4218", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10232001-192042", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 6568436, "license": "other", "mime_type": "application/pdf", "url": "/4218/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Investigation of Large Strain Actuation in Barium Titanate", "author": [ { "family_name": "Burcsu", "given_name": "Eric Noboru", "clpid": "Burcsu-Eric-Noboru" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Haile", "given_name": "Sossina M.", "orcid": "0000-0002-5293-6252", "clpid": "Haile-S-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Sensors and actuators based on ferroelectric materials have become indispensable in the fields of aerospace, high technology, and medical instruments. Most devices rely on the linear piezoelectric behavior of formulations of PZT which offer high bandwidth, linear actuation but very low strains of around 0.1%. The nonlinear electromechanical behavior of these materials is largely governed by the motion of domains and is highly affected by stress as well as electric field. The recent theories of Shu and Bhattacharya have sought to address some of the issues related to the structure and behavior of these materials at the mesoscale. One result of the theories is the prediction of another mode of actuation in ferroelectric crystals based on a combined electrical and mechanical loading that could result in strains of up to 6%.
\r\n\r\nDescriptions of the phenomenological theories of ferroelectrics are presented including the classical Landau-Ginsburg-Devonshire theory and the more recent theory of Shu and Bhattacharya. Predictions are made, based on the theory, of the electromechanical behavior of ferroelectric crystals that are addressed by the experiments. An experimental setup has been designed to investigate large strain actuation in single crystal ferroelectrics based on combined electrical and mechanical loading. An investigation of the stress dependence of the electrostrictive response has been carried out with in situ observations of the domain patterns under constant compressive stress and variable electric field. Experiments have been performed on initially single domain crystals of barium titanate with (100) and (001) orientation at compressive stresses between 0 and 5 MPa. Global strain and polarization histories have been recorded. The electrostrictive response is shown to be highly dependent on the level of applied stress with a maximum strain of 0.9% measured at a compressive stress of about 2 MPa. An unusual secondary hysteresis has been observed in the polarization signal at high levels of stress that indicates an intermediate structural configuration, possibly the orthorhombic state. Polarized light microscopy has been used to observe the evolution of the domain pattern simultaneously with the strain and polarization measurement. These results are discussed and suggestions for future work are proposed.
", "doi": "10.7907/XT3Y-Z860", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:6119", "collection": "thesis", "collection_id": "6119", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10082010-091323238", "type": "thesis", "title": "Micromechanical Aspects of Failure in Unidirectional Fiber Reinforced Composites", "author": [ { "family_name": "Oguni", "given_name": "Kenji", "orcid": "0000-0003-0425-9784", "clpid": "Oguni-Kenji" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Micromechanical aspects of failure in unidirectional fiber reinforced composites are investigated using combined experimental and analytical methods. Results from an experimental investigation on mechanical behavior of a unidirectional fiber reinforced polymer composite (E-glass/vinylester) with 50% fiber volume fraction under quasi-static uniaxial and proportional multiaxial compression are presented. Detailed examination of the specimen during and after the test reveals the failure mode transition from axial splitting to kink band formation as the loading condition changes from uniaxial to multiaxial compression.
\r\n\r\nMotivated by the experimental observations, an energy-based model is developed to provide an analytical estimate of the critical stress for axial splitting observed in unidirectional fiber reinforced composites under uniaxial compression in the fiber direction (also with weak lateral confinement). The analytic estimate for the compressive strength is used to illustrate its dependence on material properties, surface energy, fiber volume fraction, fiber diameter and lateral confining pressure.
\r\n\r\nTo understand the effect of flaws on the strength of unidirectional fiber reinforced composites, a fracture mechanics based model for failure is developed. Based on this model, failure envelope, dominant initial flaw orientation and failure mode for the composites under a wide range of stress states are predicted. Parametric study provides quantitative evaluation of the effect of various mechanical and physical properties on failure behavior and identifies their influence on strength.
\r\n\r\nFinally, results from an experimental investigation on the dynamic mechanical behavior of unidirectional E-glass/vinylester composites with 30%, 50% fiber volume fraction under uniaxial compression are presented. Limited experimental results are also presented for the 50% fiber volume fraction composite under dynamic proportional lateral confinement. Specimens are loaded in the fiber direction using a modified Kolsky (split Hopkinson) pressure bar. The results indicate that the compressive strength of the composite increases with increasing strain rate and confinement. Post-test scanning electron microscopy reveals that axial splitting is the dominant failure mechanism in the composites under uniaxial compression in the entire range of strain rates. Based on the experimental results and observations, the energy-based analytic model is extended to predict the compressive strength of these composites under dynamic uniaxial loading conditions.
", "doi": "10.7907/3VSA-QN96", "publication_date": "2000", "thesis_type": "phd", "thesis_year": "2000" }, { "id": "thesis:524", "collection": "thesis", "collection_id": "524", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02062008-080229", "primary_object_url": { "basename": "Deshpande_n_1999.pdf", "content": "final", "filesize": 10703760, "license": "other", "mime_type": "application/pdf", "url": "/524/1/Deshpande_n_1999.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "An Experimental Investigation of High-Shear-Strain-Rate Behavior of Metals", "author": [ { "family_name": "Deshpande", "given_name": "Nitin Ashok", "clpid": "Deshpande-Nitin-Ashok" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe present study investigated the mechanical behavior of metals under high-shear-strain-rates and large shear strains. Numerical and experimental investigation of different specimen geometries for shear testing of materials was carried out. Results of numerical simulations showed that shear stress-strain curves calculated from boundary measurement of load and displacement did not match with the constitutive law for the material. The yield stress for the shear stress-strain curve from the boundary measurement was considerably lower than that of the constitutive law whereas hardening exponent was almost the same for two curves. Considerable bending was observed in the shear zone. The results from the boundary measurements were close to the constitutive law for H specimen, an analogue of axisymmetric top hat specimen with top replaced by tool steel punch. A planar version of axisymmetric top hat specimen geometry was studied using finite element analysis. The plane specimen was chosen since it is suitable for temperature measurements in the shear zone. In order to reduce the bending, three types of constraints were considered in the experiments.\r\n\r\nQuasi-static and high strain rate experiments were carried out on different geometries in the strain rate range, 10[...] to 10[...] s[...]. Relatively rate insensitive material, 2024-T3 aluminum was used to establish the relationship between numerical and experimental results. Experimental results for the axisymmetric top hat specimen were found to be in good agreement with the finite element results, but there was discrepancy between the stress-strain curve from the boundary measurements and the constitutive law. Shear stress-strain curve from the quasi-static test for the plane specimen with external constraint reproduced the results of numerical simulation. A planar specimen with built-in constraint was fabricated using wire EDM. Both quasi-static and high strain rate results matched with the numerical simulation of the same specimen geometry. High shear strains of the order of 1.5 were reached in the experiments on the Kolsky pressure bar. Some amount of thermal softening was observed in high strain rate experiments.\r\n\r\nIt was concluded that both numerical simulations and experiments are required in order to obtain accurate constitutive behavior of the material using top hat specimen geometries. A relationship can be established between the numerical tests and the experiments by conducting the experiments at strain rates where the constitutive behavior of the metal is well known. This relationship then can be used to predict the constitutive law at higher strain rates from the experimental data obtained at high strain rates.", "doi": "10.7907/naah-mx91", "publication_date": "1999", "thesis_type": "engd", "thesis_year": "1999" }, { "id": "thesis:705", "collection": "thesis", "collection_id": "705", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02212008-114547", "primary_object_url": { "basename": "Shu_yc_1999.pdf", "content": "final", "filesize": 4376328, "license": "other", "mime_type": "application/pdf", "url": "/705/1/Shu_yc_1999.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Shape-Memory Effect in Bulk and Thin-Film Polycrystals", "author": [ { "family_name": "Shu", "given_name": "Yi-Chung", "clpid": "Shu-Yi-Chung" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Bruno", "given_name": "Oscar P.", "orcid": "0000-0001-8369-3014", "clpid": "Bruno-O-P" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Shape-memory effect (SME) is a phenomenon where deformation suffered below a critical temperature can be recovered on heating. About 20-30 alloys are known to exhibit SME in single crystals. However, the degree to which they retain their shape-memory behavior in polycrystals is widely varied. In particular, Ti-Ni and Cu-Zn-Al undergo cubic to monoclinic transformation and recover similar strains as single crystals; yet, the observed shape-memory behavior in the former is much better than that in the latter. We develop a model based on energy minimization to understand this difference. Using this model, we establish that texture is the very important reason why the strains recoverable in Ti-Ni are so much larger than those in Cu-based shape-memory alloys in rolled, extruded and drawn specimens. We find that even the qualitative behavior of combined tension-torsion can critically depend on the texture. The results are in good agreement with experimental observations.\r\n\r\nWe extend our analysis to the behavior of very thin films with three competing length scales: the film thickness, the length scales of heterogeneity and material microstructure. We start with three-dimensional nonhomogeneous nonlinear elasticity enhanced with an interfacial energy of the van der Waals type, and derive the effective energy density as all length scales tend to zero with given limiting ratios. We do not require any priori selection of asymptotic expansion or ansatz in deriving our results. Depending on the dominating length scale, the effective energy density can be identified by three procedures: averaging, homogenization and thin-film limit. We apply our theory to martensitic thin films and use a model example to show that the shape-memory behavior can crucially depend on the relative magnitudes of these length scales. Using this theory, we show that sputtering textures in both Ti-Ni and Cu-based shape-memory thin films are not favorable for large recoverable strain. We comment on multilayers made of shape-memory and elastic materials.\r\n\r\nFinally, we suggest textures for improved SME in bulk and thin-film polycrystals.", "doi": "10.7907/4NW5-3Q89", "publication_date": "1999", "thesis_type": "phd", "thesis_year": "1999" }, { "id": "thesis:7626", "collection": "thesis", "collection_id": "7626", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04232013-084302487", "type": "thesis", "title": "I. Rigid Body Penetration into Brittle Material. II. Phase Change Effect on Shock Wave Propagation", "author": [ { "family_name": "Liu", "given_name": "Cangli", "clpid": "Liu-Cangli" } ], "thesis_advisor": [ { "family_name": "Ahrens", "given_name": "Thomas J.", "clpid": "Ahrens-T-J" }, { "family_name": "Stock", "given_name": "Joann M.", "orcid": "0000-0003-4816-7865", "clpid": "Stock-J-M" } ], "thesis_committee": [ { "family_name": "Stock", "given_name": "Joann M.", "orcid": "0000-0003-4816-7865", "clpid": "Stock-J-M" }, { "family_name": "Ahrens", "given_name": "Thomas J.", "clpid": "Ahrens-T-J" }, { "family_name": "Clayton", "given_name": "Robert W.", "orcid": "0000-0003-3323-3508", "clpid": "Clayton-R-W" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Part I.
\r\n\r\nWe have developed a technique for measuring the depth time history of rigid body penetration into brittle materials (hard rocks and concretes) under a deceleration of ~ 105 g. The technique includes bar-coded projectile, sabot-projectile separation,\r\ndetection and recording systems. Because the technique can give very dense data on penetration depth time history, penetration velocity can be deduced. Error analysis shows that the technique has a small intrinsic error of ~ 3-4 % in time during penetration, and 0.3 to 0.7 mm in penetration depth. A series of 4140 steel projectile penetration into G-mixture mortar targets have been conducted using the Caltech 40 mm gas/ powder gun in the velocity range of 100 to 500 m/s.
\r\n\r\nWe report, for the first time, the whole depth-time history of rigid body penetration into brittle materials (the G-mixture mortar) under 105 g deceleration. Based on the experimental results, including penetration depth time history, damage of recovered target and projectile materials and theoretical analysis, we find:
\r\n\r\n1. Target materials are damaged via compacting in the region in front of a projectile and via brittle radial and lateral crack propagation in the region surrounding the penetration path. The results suggest that expected cracks in front of penetrators may be stopped by a comminuted region that is induced by wave propagation. Aggregate erosion on the projectile lateral surface is < 20% of the final penetration depth. This result suggests that the effect of lateral friction on the penetration process can be ignored.
\r\n\r\n2. Final penetration depth, Pmax, is linearly scaled with initial projectile energy per unit cross-section area, es , when targets are intact after impact. Based on the experimental data on the mortar targets, the relation is Pmax(mm) 1.15es (J/mm2 ) + 16.39.
\r\n\r\n3. Estimation of the energy needed to create an unit penetration volume suggests that the average pressure acting on the target material during penetration is ~ 10 to 20 times higher than the unconfined strength of target materials under quasi-static loading, and 3 to 4 times higher than the possible highest pressure due to friction and material strength and its rate dependence. In addition, the\r\nexperimental data show that the interaction between cracks and the target free surface significantly affects the penetration process.
\r\n\r\n4. Based on the fact that the penetration duration, tmax, increases slowly with es and does not depend on projectile radius approximately, the dependence of tmax on projectile length is suggested to be described by tmax(\u03bcs) = 2.08es (J/mm2 + 349.0 x m/(\u03c0R2), in which m is the projectile mass in grams and R is the projectile radius in mm. The prediction from this relation is in reasonable agreement with the experimental data for different projectile lengths.
\r\n\r\n5. Deduced penetration velocity time histories suggest that whole penetration history is divided into three stages: (1) An initial stage in which the projectile velocity change is small due to very small contact area between the projectile and target materials; (2) A steady penetration stage in which projectile velocity continues to decrease smoothly; (3) A penetration stop stage in which projectile deceleration jumps up when velocities are close to a critical value of ~ 35 m/s.
\r\n\r\n6. Deduced averaged deceleration, a, in the steady penetration stage for projectiles with same dimensions is found to be a(g) = 192.4v + 1.89 x 104, where v is initial projectile velocity in m/s. The average pressure acting on target materials during penetration is estimated to be very comparable to shock wave pressure.
\r\n\r\n7. A similarity of penetration process is found to be described by a relation between normalized penetration depth, P/Pmax, and normalized penetration time, t/tmax, as P/Pmax = f(t/tmax, where f is a function of t/tmax. After f(t/tmax is determined using experimental data for projectiles with 150 mm length, the penetration depth time history for projectiles with 100 mm length predicted by this relation is in good agreement with experimental data. This similarity also predicts that average deceleration increases with decreasing projectile length, that is verified by the experimental data.
\r\n\r\n8. Based on the penetration process analysis and the present data, a first principle model for rigid body penetration is suggested. The model incorporates the models for contact area between projectile and target materials, friction coefficient, penetration stop criterion, and normal stress on the projectile surface. The most important assumptions used in the model are: (1) The penetration process can be treated as a series of impact events, therefore, pressure normal to projectile surface is estimated using the Hugoniot relation of target material; (2) The necessary condition for penetration is that the pressure acting on target materials is not lower than the Hugoniot elastic limit; (3) The friction force on projectile lateral surface can be ignored due to cavitation during penetration. All the parameters involved in the model are determined based on independent experimental data. The penetration depth time histories predicted from the model are in good agreement with the experimental data.
\r\n\r\n9. Based on planar impact and previous quasi-static experimental data, the strain rate dependence of the mortar compressive strength is described by \u03c3f/\u03c30f = exp(0.0905(log(\u03ad/\u03ad_0) 1.14, in the strain rate range of 10-7/s to 103/s (\u03c30f and \u03ad are reference compressive strength and strain rate, respectively). The non-dispersive\r\nHugoniot elastic wave in the G-mixture has an amplitude of ~ 0.14 GPa and a velocity of ~ 4.3 km/s.
\r\n\r\nPart II.
\r\n\r\nStress wave profiles in vitreous GeO2 were measured using piezoresistance gauges in the pressure range of 5 to 18 GPa under planar plate and spherical projectile impact. Experimental data show that the response of vitreous GeO2 to planar shock loading\r\ncan be divided into three stages: (1) A ramp elastic precursor has peak amplitude of 4 GPa and peak particle velocity of 333 m/s. Wave velocity decreases from initial longitudinal elastic wave velocity of 3.5 km/s to 2.9 km/s at 4 GPa; (2) A ramp wave with amplitude of 2.11 GPa follows the precursor when peak loading pressure is 8.4 GPa. Wave velocity drops to the value below bulk wave velocity in this stage; (3) A shock wave achieving final shock state forms when peak pressure is > 6 GPa. The Hugoniot relation is D = 0.917 + 1.711u (km/s) using present data and the data of Jackson and Ahrens [1979] when shock wave pressure is between 6 and 40 GPa for \u03c10 = 3.655 gj cm3 . Based on the present data, the phase change from 4-fold to 6-fold coordination of Ge+4 with O-2 in vitreous GeO2 occurs in the pressure range of 4 to 15 \u00b1 1 GPa under planar shock loading. Comparison of the shock loading data for\r\nfused SiO2 to that on vitreous GeO2 demonstrates that transformation to the rutile structure in both media are similar. The Hugoniots of vitreous GeO2 and fused SiO2 are found to coincide approximately if pressure in fused SiO2 is scaled by the ratio of fused SiO2to vitreous GeO2 density. This result, as well as the same structure, provides the basis for considering vitreous Ge02 as an analogous material to fused SiO2 under shock loading. Experimental results from the spherical projectile impact demonstrate: (1) The supported elastic shock in fused SiO2 decays less rapidly than a linear elastic wave when elastic wave stress amplitude is higher than 4 GPa. The supported elastic shock in vitreous GeO2 decays faster than a linear elastic wave; (2) In vitreous GeO2 , unsupported shock waves decays with peak pressure in the phase transition range (4-15 GPa) with propagation distance, x, as \u03b1 1/x-3.35 , close to the prediction of Chen et al. [1998]. Based on a simple analysis on spherical wave propagation, we find that the different decay rates of a spherical elastic wave in fused SiO2 and vitreous GeO2 is predictable on the base of the compressibility variation with stress under one-dimensional strain condition in the two materials.
\r\n\r\n", "doi": "10.7907/w8gc-n615", "publication_date": "1999", "thesis_type": "phd", "thesis_year": "1999" }, { "id": "thesis:3655", "collection": "thesis", "collection_id": "3655", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09202002-154801", "primary_object_url": { "basename": "Karina_LM_1997.pdf", "content": "final", "filesize": 10074287, "license": "other", "mime_type": "application/pdf", "url": "/3655/1/Karina_LM_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Shock Wave Processing of Transitional Metal Silicides", "author": [ { "family_name": "Montilla Edmonds", "given_name": "Karina Luciel", "clpid": "Montilla-Edmonds-Karina-Luciel" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Ustundag", "given_name": "Ersan", "orcid": "0000-0002-0812-7028", "clpid": "Ustundag-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nShock wave consolidation is an innovative processing technique for the densification of initially porous media. A compressive shock wave is introduced in the material by the impact of a high velocity flyer plate. Densification is achieved via intense inhomogeneous plastic deformation, pore collapse, and localized melting around particle surface. The passage of the shock wave may also induce chemical reactions within the material. The chemical reactivity of the powders are enhanced through dislocation nucleation, plastic flow, grain fracture and mass mixing as a result of the shock wave.\r\n\r\nA systematic investigation is performed to examine the effects of particle size and porosity on the initiation of the Ti[subscript 5]Si[subscript 3] reaction from the elemental powder mixture (i.e., 5 Ti + 3 Si). The initial powder porosity is varied from 40% to 49% of the theoretical density for two different size powders. The threshold shock energy necessary for complete silicide reaction is established. The powders are consolidated with shock energies up to 671 J/g and shock pressures up to 7.3 GPa. The threshold shock energy for the large powder mixture is found to be approximately 80% higher than that for the smaller powder mixture. For both sized powders, an increase in the threshold shock energy of 75% is observed in decreasing the initial porosity of the powders from 49% to 40%. Evidence for the reaction of solid Ti and liquid Si is observed in isolated regions at shock energies slightly below the threshold energy.\r\n\r\nMechanical alloying and shock wave consolidation are examined as viable alternatives for the synthesis and consolidation of MoSi [subscript 2]. Mechanic alalloying of Mo + 2Si is monitored with X-ray diffraction and differential scanning calorimetry (DSC). The milling time is varied from two hours to one hundred forty-four hours. Nanocrystalline MoSi [subscript 2] is observed after sixteen hours of ball milling. X-ray diffraction is used to follow the extent of alloying and average grain size as a function of ball milling time. DSC is utilized to determine the onset endothermic and exothermic reactions in the ball milled powder. MoSi [subscript 2] is produced from the elemental powder mixture by shock wave consolidation.", "doi": "10.7907/9g63-5c59", "publication_date": "1998", "thesis_type": "phd", "thesis_year": "1998" }, { "id": "thesis:279", "collection": "thesis", "collection_id": "279", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01222008-140642", "primary_object_url": { "basename": "Lee_s_1998.pdf", "content": "final", "filesize": 7347042, "license": "other", "mime_type": "application/pdf", "url": "/279/1/Lee_s_1998.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Failure of laminated composites at thickness discontinuities under complex loading and elevated temperatures", "author": [ { "family_name": "Lee", "given_name": "Sangwook", "clpid": "Lee-Sang" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Failure initiation of laminated composites with discontinuous thickness is examined in terms of typical structural load description (tension, shear force and bending moment) rather than in terms of micromechanics considerations. Because transverse shear produced relatively small effects in failure initiation, results are presented as tension-bending interactions. Two loading frames were designed to apply moments and tension simultaneously. Four types of specimens of different stacking sequence were examined to determine failure initiation, and analyzed subsequently via a finite element analysis (ABAQUS). Depending on the stacking sequence across the interface of the step, two different failure modes are identified: For uni-directional fiber orientation across the interface in the tension direction, failure occurs through cracking and delamination which is governed by a fracture mechanics criterion. While the initiation strength for this failure mode is higher than for the cross-ply configurations, the residual strength after initiation is only marginally higher, providing virtually no margin of safety (10%). For cases involving cross-plies on either side of the interface, failure initiation occurs by matrix cracking, with a critical strain across the fibers providing a universal failure criterion. In these cases the residual load bearing capability was 30 to 45% higher than the failure initiation loads. The interaction between moment and tension at failure initiation is linear, an observation that does not hold for the delamination failure driven by crack propagation. It is found that all failures can be described in terms of a common fracture principle; the stress or strain criteria are interchangeable with the fracture energy computations, provided one allows for a range of values of associated fracture energies. Assuming that time dependent aspects of the failure process are not dominant, elevated temperatures did not change the general results of how bending and tension loads interact, provided one accounts for residual thermal stresses; however, the stress magnitude at which the failure initiation occurs decreases with increasing temperature.\r\n", "doi": "10.7907/9p36-ht49", "publication_date": "1998", "thesis_type": "phd", "thesis_year": "1998" }, { "id": "thesis:496", "collection": "thesis", "collection_id": "496", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02042008-151022", "primary_object_url": { "basename": "Turteltaub_sr_1997.pdf", "content": "final", "filesize": 4696029, "license": "other", "mime_type": "application/pdf", "url": "/496/1/Turteltaub_sr_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Dynamics of phase transformations in thermoelastic solids", "author": [ { "family_name": "Turteltaub", "given_name": "Sergio Ricardo", "clpid": "Turteltaub-S-R" } ], "thesis_advisor": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The dynamical aspects of solid-solid phase transformations are studied within the framework of the theory of thermoelasticity. The main purpose is to analyze the role of temperature in the theory of phase transitions. This investigation consists of two parts: first, it is shown that by imposing a kinetic relation and a nucleation criterion it is possible to single out a unique solution to the Riemann problem for an adiabatic process. This extends to the thermomechanical case results previously found in a purely mechanical context. Secondly, based on an admissibility criterion for traveling wave solutions within the context of an augmented theory that includes viscosity, strain gradient and heat conduction effects, a special kinetic relation is derived using singular perturbation techniques.\n", "doi": "10.7907/gk0c-8r75", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:5096", "collection": "thesis", "collection_id": "5096", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12212004-164817", "primary_object_url": { "basename": "Gonzalez_j_1997.pdf", "content": "final", "filesize": 20079935, "license": "other", "mime_type": "application/pdf", "url": "/5096/1/Gonzalez_j_1997.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Full Field Study of Strain Distribution Near the Crack Tip in the Fracture of Solid Propellants Via Large Strain Digital Image Correlation and Optical Microscopy", "author": [ { "family_name": "Gonzalez", "given_name": "Javier Gonzalez", "clpid": "Gonzalez-Javier-Gonzalez" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A full field method for visualizing deformation around the crack tip in a fracture process with large strains is developed. A digital image correlation program (DIC) is used to incrementally compute strains and displacements between two consecutive images of a deformation process. Values of strain and displacements for consecutive deformations are added, this way solving convergence problems in the DIC algorithm when large deformations are investigated. The method developed is used to investigate the strain distribution within 1 mm of the crack tip in a particulate composite solid (propellant) using microscopic visualization of the deformation process.", "doi": "10.7907/HRM1-RJ74", "publication_date": "1997", "thesis_type": "engd", "thesis_year": "1997" }, { "id": "thesis:71", "collection": "thesis", "collection_id": "71", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01082008-112920", "primary_object_url": { "basename": "Deng_th_1997.pdf", "content": "final", "filesize": 5170673, "license": "other", "mime_type": "application/pdf", "url": "/71/1/Deng_th_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Measurement of the dynamic bulk compliance of polymers", "author": [ { "family_name": "Deng", "given_name": "Tony H.", "clpid": "Deng-T-H" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Measurements are described and analyzed for the determination of the dynamic bulk compliance for polyvinyl acetate) [PVAc] as a function of frequency and temperature. The real and imaginary parts of the dynamic bulk compliance over the frequency range from 10 Hz to 1,000 Hz have been measured at different temperatures by determining the compressibility of a specimen confined to an oil-filled cavity via pressurization by a piezoelectric driver and response of a piezoelectric sensor. The wavelength of the compressional wave generated by the piezoelectric transducer over the frequency range used is much larger than the size of the cavity so that the pressure can be considered uniform inside the cavity. The complex compliances of the specimen, confining liquid, and the cavity, are additive upon the pressure variations due to the piezoelectric transducer expansion and contraction. All deformations are considered to be purely dilitational.\n\nA master compliance curve over a total frequency range of about 12 decades is generated by the method of time-temperature superposition. Experimental problems addressing limitations in resolution capability are discussed. The results are compared with the classical measurements obtained by McKinney and Belcher over thirty years ago. Further comparison of the bulk with shear compliance data shows that the extent of the transition ranges for the shear and for the bulk functions are comparable, but the two transitions belong to different time scales: That of the bulk response falls mostly into the glassy domain of the shear behavior. One concludes thus that for linearly viscoelastic response the molecular mechanisms contributing to shear and bulk deformations have different conformational sources.\n", "doi": "10.7907/vxb1-4950", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:4529", "collection": "thesis", "collection_id": "4529", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11122003-181337", "primary_object_url": { "basename": "krok_jc_1997.pdf", "content": "final", "filesize": 13071644, "license": "other", "mime_type": "application/pdf", "url": "/4529/1/krok_jc_1997.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Jet Initiation of Deflagration and Detonation", "author": [ { "family_name": "Krok", "given_name": "James Christopher", "clpid": "Krok-James-Christopher" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Kirschvink", "given_name": "Joseph L.", "orcid": "0000-0001-9486-6689", "clpid": "Kirschvink-J-L" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "abstract": "We have constructed a facility for the study of jet-initiated deflagration and detonation in hydrogen-air-steam mixtures. The facility is built around two pressure vessels. Mixtures of hydrogen, oxygen and nitrogen are spark-ignited in the driver vessel, generating a hot mixture of combustion products. The pressure rise ruptures a diaphragm, venting the products into the receiver vessel through nozzles of 12.7-92 mm diameter. The receiver is filled with hydrogen-air and hydrogen-oxygen mixtures diluted with either nitrogen or steam.
\r\n\r\nThe deflagration tests studied the lean and maximum-dilution limits of hydrogen-air mixtures ignited by a hydrogen-steam jet. The lean limit of 6% hydrogen was comparable to other studies. The maximum dilution limit for steam was 60%. This is higher than the limit found in spark/glow plug ignition experiments. Shock oscillations in the receiver increased with nozzle size.
\r\n\r\nFurther tests studied the initiation of detonation in both hydrogen-air and stoichiometric hydrogen-oxygen-diluent mixtures. In terms of jet diameter, D, and receiver detonation cell size, \u03bb, we found initiation limits of 2 < D/\u03bb < 7, where other experiments required a D/\u03bb of 11 or more. We propose that the D/\u03bb model does not adequately characterize jet initiation, as it does not reflect the conditions in the driver.
\r\n\r\nThe tests indicated that shock focusing plays an important role, promoting strong secondary explosions with or without prompt initiation of detonation. Mixtures with steam dilution were prone to DDT near the detonation limit, as the slower flame speed allows shock reflection and pressurization to occur before the reactants are consumed. Tests with nitrogen dilution had no DDT regime. Because of DDT and shock focusing, peak pressures were highest in mixtures that were slightly less sensitive than the detonation threshold. Schlieren movies confirmed the formation of a detonation near the nozzle exit.
", "doi": "10.7907/PSTR-T717", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:4986", "collection": "thesis", "collection_id": "4986", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12132007-082556", "primary_object_url": { "basename": "Fey_ke_1996.pdf", "content": "final", "filesize": 4094956, "license": "other", "mime_type": "application/pdf", "url": "/4986/1/Fey_ke_1996.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Experimental and theoretical aspects of dynamic crack growth along bimaterial interfaces", "author": [ { "family_name": "Fey", "given_name": "Kate Elizabeth", "clpid": "Fey-K-E" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This work presents findings of an experimental and theoretical study of dynamic bimaterial crack growth. Bimaterial systems composed of constituents with large material mismatch were investigated under dynamic loading conditions. The materials used in this study consisted of Poly-Methylmethacrylate (PMMA) and AISI 4340 Steel, bonded together using a Methylmethacrylate monomer. One point bend loading was achieved using a drop weight tower. Dynamic crack growth, with velocities up to eighty percent of the Rayleigh wave speed of PMMA, was observed using the lateral shearing interferometric technique of Coherent Gradient Sensing (CGS) in conjunction with high speed photography. The results of these experiments are first discussed within the realm of the validity of the linear, elastodynamic asymptotic stress fields. The complex interdependency of stress intensity and mode mixity with crack tip speed is also discussed. The interpretation of |K[superscript d]| and [phi superscript d] in a dynamic bimaterial crack is clarified through the experimental observation of crack growth.\r\n\r\nComplications in analysis arising from this interdependency between the dynamic K[superscript d]-field and velocity are examined for experimentally obtained CGS fringe patterns. Improvements of existing analyzing procedures are made, resulting in increased confidence in data obtained utilizing the method of CGS in dynamic bimaterial fracture. Special attention is given to the interaction of loading and velocity in the behavior of these crack tip fields.\r\n\r\nPrevious methods of investigation have used an elastodynamic, asymptotic K[superscript d]-field to describe the deformations near a bimaterial crack tip. Attempts to develop a fracture criterion based on these results have suffered from the lack of natural length scale as the major criticism. Motivated by experimental observations, a cohesive zone model is presented in this thesis that allows an investigation of dynamic crack growth. The length of the cohesive zone is given by a combination of stress intensity and mixity, bimaterial behavior, and velocity, and emerges as a natural, time evolving length scale with which to examine the bimaterial crack problem. A fracture criterion based on critical cohesive displacements at the trailing edge of the cohesive zone is presented.\r\n\r\nThis cohesive zone model is subsequently used to examine data obtained from experiment. The model enhances our ability to extrapolate our experimental measurements to the near tip region, and to thus study the neighborhood close to the propagating crack tip. Within experimental error, predictions of the proposed fracture criterion are shown to correspond to the experimentally observed dependence of |K[superscript d]| and [phi superscript d] on the instantaneous crack tip velocity. The fracture criterion based on the cohesive model presented in this paper provides the natural next step in understanding dynamic bimaterial crack growth. It provides a criterion based on physically motivated parameters, introduces a natural length scale into the problem, and increases our understanding of dynamic bimaterial fracture mechanics.\r\n", "doi": "10.7907/ehea-x787", "publication_date": "1996", "thesis_type": "phd", "thesis_year": "1996" }, { "id": "thesis:4228", "collection": "thesis", "collection_id": "4228", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10242005-083544", "primary_object_url": { "basename": "Howard_dd_1996.pdf", "content": "final", "filesize": 17632777, "license": "other", "mime_type": "application/pdf", "url": "/4228/1/Howard_dd_1996.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Part I. Mechanisms of injury associated with extracorporeal shock wave lithotripsy; Part II. Exsolution of volatiles", "author": [ { "family_name": "Howard", "given_name": "Danny Dwayne", "clpid": "Howard-D-D" } ], "thesis_advisor": [ { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" } ], "thesis_committee": [ { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Hornung", "given_name": "Hans G.", "clpid": "Hornung-H-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nPart I - Shock waves are focused in extracorporeal shock wave lithotripsy (ESWL) machines to strengths sufficient to fracture kidney stones. Substantial side effects-most of them acute-have resulted from this procedure, including injury to soft tissue. The focusing of shock waves through various layers of tissue is a complex process which stimulates many bio-mechano-chemical responses.\n\nThis thesis presents results of an in vitro study of the initial mechanical stimulus. Planar nitrocellulose membranes of order 10 um thick were used as models of thin tissue structures. Two modes of failure were recorded: Failure due to cavitation collapsing on or near the membranes, and failure induced by altering the structure of shock waves. Tests were done in water at and around F2 to characterize the extent of cavitation damage, and was found to be confined within the focal region, 1.2 cm along the axis of focus.\n\nScattering media were used to simulate the effects of acoustic nonuniformity of tissue and to alter the structure of focusing shock waves. 40 um diameter (average) hollow glass spheres were added to ethylene glycol, glycerine and castor oil to vary the properties of the scattering media. Multiple layer samples of various types of phantom tissue were tested in degassed castor oil to gauge the validity of the scattering media. The scattering media and tissue samples increased the rise time decreased strain rate in a similar fashion. Membranes were damaged by the decreased strain rate and accumulated effects of the altered structure: After about 20 or so shocks immersed in the scattering media and after about 100 shocks behind the tissue samples. The mode of failure was tearing with multiple tears in some cases from about .1 cm to about 3 cm depending of the number of shocks and membrane thickness.\n\nPart II - This work examines the exsolution of volatiles-carbon dioxide from water-in a cylindrical test cell under different pressure conditions. Water was supersaturated with carbon dioxide under various pressures (620 to 1062 kPa), and depressurized rapidly to investigate how carbon dioxide is undissolved, exsolution, and its effects on the surrounding environment. Cavities grow as a result of convective diffusion: They move before depleting carbon dioxide in a given region. The radius of a cavity in this environment grows at a faster rate [...] than that of a cavity at rest [...]. Bubble growth rates were inferred by measuring the bulk liquid using high speed motion pictures. Water in the test-cell is accelerated as a result of buoyancy induced by cavity growth. Cavities are elliptical in shape and grow until mutual interaction causes them to fragment. Accelerations range from 10 to 100 g were measured with velocities ranging from 7 to 13 m/s.\n", "doi": "10.7907/995X-8517", "publication_date": "1996", "thesis_type": "phd", "thesis_year": "1996" }, { "id": "thesis:58", "collection": "thesis", "collection_id": "58", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01072008-112449", "primary_object_url": { "basename": "Walter_me_1996.pdf", "content": "final", "filesize": 13790400, "license": "other", "mime_type": "application/pdf", "url": "/58/1/Walter_me_1996.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The evolution of damage in ceramic matrix composites", "author": [ { "family_name": "Walter", "given_name": "Mark E.", "clpid": "Walter-M-E" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In an effort to better understand the evolution of damage in brittle matrix composites, the mechanical behavior of a ceramic matrix composite, unidirectional SiC/CAS (SiC fibers reinforcing a calcium aluminosilicate matrix), was studied. The presented results are based on uniaxial tension experiments for specimens with the fibers aligned in the loading direction. Post-test optical and scanning electron microscopy was also used to identify the various micromechanisms of damage; axial and transverse strain gauges on all four gage section surfaces and in situ acoustic emission and ultrasonic wave speed measurements were used to monitor the evolution of damage. The experimental results demonstrate the existence of \"zones of deformation\" which are associated with the onset of different damage mechanisms. The energy dissipated in each of these zones was calculated. It is shown that the observed stress-strain behavior can be qualitatively explained in terms of the material properties of the matrix and the fiber, the material processing, and the postulated zones of deformation.\r\n\r\nThe experimental results for SiC/CAS were compared with an existing shear-lag model, and the shortcomings of the model are discussed. By approximating matrix cracks as penny shaped cracks, a micromechanical model was used to estimate the change in the axial modulus of the composite. These results also present another way to interpret the acoustic emission data.\r\n\r\nThe evolution of damage in the SiC/CAS experiments was found to be strain rate dependent even within the quasi-static strain rate regime. For higher rate experiments, the transition from elastic to matrix cracked occurred at a stress level that was nearly twice that of the same transition in the lower rate experiments. This phenomenon and the mechanisms which cause it was further investigated with a model material system (a brittle epoxy resin sandwiched between aluminum strips). In situ quantification of the stress during damage initiation and propagation was realized by the optical method of Coherent Gradient Sensing. Based on these results, the reasons for strain rate dependence of the composite are postulated.\r\n\r\nDetailed understanding of aspects of the evolution of in brittle matrix composites was achieved with finite element simulations. This modeling was based on an axisymmetric unit cell composed of a fiber and its surrounding matrix. The unit cell was discretized into linearly elastic elements for the fiber and the matrix and cohesive elements which allow cracking in the matrix, fiber-matrix interface, and fiber. The cohesive elements failed according to critical stress and critical energy release rate criteria (in shear and/or in tension). After failing, the cohesive elements could slide with Coulomb friction. The tension and shear aspects of failure were uncoupled. The cohesive elements were used to simulate a Dugdale penny shaped crack in a homogeneous cylinder; results compared well to the analytical solution. In order to solve the composite axisymmetric unit cell problem, inertia and viscous damping were added to the formulation. The resulting dynamic problem was solved implicitly using the Newmark Method. Results were compared to the experiment by assuming that only a given number of unit cells were active at any point during the simulation. The effects of changing material properties (e.g., interface strength and toughness and matrix toughness) and loading rate are discussed. Several aspects of the experimentally observed material response of SiC/CAS composite were reproduced by the numerical simulations.\r\n", "doi": "10.7907/w4b4-dx66", "publication_date": "1996", "thesis_type": "phd", "thesis_year": "1996" }, { "id": "thesis:4379", "collection": "thesis", "collection_id": "4379", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-101839", "primary_object_url": { "basename": "Chen_w_1995.pdf", "content": "final", "filesize": 14729328, "license": "other", "mime_type": "application/pdf", "url": "/4379/1/Chen_w_1995.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Dynamic failure behavior of ceramics under multiaxial compression", "author": [ { "family_name": "Chen", "given_name": "Weinong", "clpid": "Chen-Weinong" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ahrens", "given_name": "Thomas J.", "clpid": "Ahrens-T-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "An experimental technique has been developed that is capable of (1) dynamically loading the specimen in multiaxial compression; (2) controlling the stress state in the specimen in the range from uniaxial stress to uniaxial strain; and (3) allowing the recovery of the sample after loaded by a single, well defined pulse for the characterization of the failure mode. In this technique, cylindrical ceramic specimens were loaded in the axial direction using a split Hopkinson pressure bar modified to apply a single loading pulse, and were confined laterally either by shrink fit sleeves, or by eletro-magnetic force.\r\n\r\nQuasi-static and dynamic multiaxial compression experiments have been performed on a machinable glass ceramic, Macor, and a monolithic engineering ceramic, sintered aluminum nitride (A1N). The cylindrical ceramic specimens were confned laterally by shrink fit sleeves: the amount of confining pressure (0-230 MPa) was varied by using different sleeve materials. The quasi-static axial load was applied by a hydraulic driven Material Test System (MTS), whereas the dynamic axial load was provided by a modified split Hopkinson (Kolsky) pressure bar (SHPB). Under both quasi-static and dynamic loading conditions, the experimental results for both materials showed that the failure mode changed from fragmentation by axial splitting under conditions of uniaxial stress (without lateral confinement) to localized deformation on faults under moderate lateral confinement. The fault initiation process was studied experimentally in detail. Based on the experimental results, a compressive brittle failure process was summarized. A transition from brittle to ductile behavior was observed in Macor under high confinement pressure which was achieved using a second sleeve around the inner sleeve. The compressive failure strengths of both materials increased with increasing confinement pressure under both quasi-static and dynamic loading conditions. The highest dynamic compressive strengths of Macor and A1N measured in the experiments were 1.35 GPa and 5.40 GPa, respectively, whereas their quasi-static compressive strength were measured to be 0.43 GPa and 2.5 GPa, respectively.\r\n\r\nBased on the experimental results on A1N together with available data in the literature, a failure/flow criterion was developed for ceramic materials under multiaxial loading. A Mohr-Coulomb criterion and an improved Johnson-Holmquist model were found to fit the experimental data for brittle failure, whereas the materials exhibited pressure insensitive plastic flow at high pressures. Observations made in other types of dynamic experiments (e.g., shock wave loading) were rationalized based on the postulated failure mechanisms and the possibility of plastic flow beyond the Hugoniot elastic limit (HEL). The effect of various material properties on the failure behavior was investigated using the proposed failure criterion. The applicability of the present model to a range of ceramics was also explored and the limitations of the model were outlined.\r\n", "doi": "10.7907/0NNE-JD20", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" }, { "id": "thesis:4215", "collection": "thesis", "collection_id": "4215", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10222007-135103", "primary_object_url": { "basename": "Zhong_x_1995.pdf", "content": "final", "filesize": 4093458, "license": "other", "mime_type": "application/pdf", "url": "/4215/1/Zhong_x_1995.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Continuum dynamics of solid-solid phase transitions", "author": [ { "family_name": "Zhong", "given_name": "Xiaoguang Allan", "clpid": "Zhong-Xiaoguang-Allan" } ], "thesis_advisor": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Hou", "given_name": "Thomas Y.", "clpid": "Hou-T-Y" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This work focuses on the applications in dynamics of recently developed continuum-mechanical models of solid-solid phase transitions. The dynamical problems considered here involve only one space coordinate, and attention is limited to hyperelastic materials that involve two phases. This investigation has two purposes. The first is to determine the predictions of the models in complicated situations. Secondly, the present study attempts to develop analytical and numerical approaches to problems that may be relevant to the interpretation and understanding of experiments involving phase transitions under dynamical conditions.
\r\n\r\nThe first problem studied involves the study of a semi-infinite bar initially in an equilibrium state that involves two material phases separated by a phase boundary at a given location. The end of the bar is suddenly subject to a constant impact velocity that persists for a finite time and is then removed. Interaction between the phase boundary and the elastic waves generated by the impact and subsequent reflections are studied in detail, and the trajectory of the phase boundary is determined exactly. The second task addressed involves the development of a Riemann solver to be applied to the numerical solution of Riemann problems for two-phase elastic materials. Riemann problems for such materials involve complications not present in the corresponding problems that arise, for example, in classical gas dynamics. Finally, a finite-difference method of Godunov type is developed for the numerical treatment of boundary-initial-value problems arising in the model of Abeyaratne and Knowles. The method is applied to specific problems.
", "doi": "10.7907/PW4M-9B73", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" }, { "id": "thesis:4037", "collection": "thesis", "collection_id": "4037", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10112007-090033", "primary_object_url": { "basename": "Jain_mk_1995.pdf", "content": "final", "filesize": 30732014, "license": "other", "mime_type": "application/pdf", "url": "/4037/1/Jain_mk_1995.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Processing and mechanical behavior of ultrafine grain materials", "author": [ { "family_name": "Jain", "given_name": "Mohit Kumar", "clpid": "Jain-M-K" } ], "thesis_advisor": [ { "family_name": "Christman", "given_name": "Tom", "clpid": "Christman-T" } ], "thesis_committee": [ { "family_name": "Christman", "given_name": "Tom", "clpid": "Christman-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Vreeland", "given_name": "Thad", "clpid": "Vreeland-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nThe mechanical behavior of ultrafine grain Fe-28A1-2Cr and 304 stainless steel was \nexamined by conducting conventional mechanical testing and pre- and post-deformation \nmicrostructural characterization on bulk samples.\n\nShock wave consolidation was used to produce a fully-dense nanophase Fe-28A1-2Cr \n(grain size = 80 nm) intermetallic compound. In tension, the nanophase intermetallic \nfailed in a brittle fashion with failure strength comparable to the coarse grain intermetallic \nof similar composition. However, the nanophase intermetallic yielded at 2.1 GPa during \nquasi-static compressive deformation and deformed to true strains greater than 1.4 without \nwork hardening. The elastic-perfectly plastic behavior of nanophase Fe-28A1-2Cr is \nsignificantly different from that of coarse-grained intermetallic of the same composition, \nwhich yielded at 0.25 GPa and works hardened to 1.5 GPa before failure (at true strain \nof about 0.37). Microstructural examination before and after compressive deformation \nrevealed that a significant portion of the microstructure refined to 10 nm grains surrounded \nby amorphous material. A similar grain refinement process was observed in 80 urn Fe-\n28A1-2Cr produced by ingot metallurgical technique.\n\nA novel thermo-mechanical processing technique was developed for the production of a \nultrafine grain 304 stainless steel (grain size = 200 nm). The key steps to this processing \ntechnique involved (1) formation of ultrafine dislocation cell structure, and (2) the conversion of dislocation cells into grains with medium to high misorientation by initiating \ngrain boundary sliding in the microstructure. The ultrafine grain steel (grain size = 200\nnm) was about six times stronger ([...] = 1700 MPa) than coarse-grained steel of the same \ncomposition. Grain size hardening behavior of 304 stainless steel was also investigated \nover a broad range of grain sizes (200 nm to 200[...]).\n", "doi": "10.7907/8fty-t229", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" }, { "id": "thesis:3470", "collection": "thesis", "collection_id": "3470", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09112007-130646", "primary_object_url": { "basename": "Bruck_ha_1995.pdf", "content": "final", "filesize": 5215883, "license": "other", "mime_type": "application/pdf", "url": "/3470/1/Bruck_ha_1995.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Quasi-static and dynamic constitutive characterization of beryllium bearing bulk metallic glasses", "author": [ { "family_name": "Bruck", "given_name": "Hugh Alan", "clpid": "Bruck-H-A" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nMetallic glasses were first discovered by Pol Duwez in 1960 using the fabrication technique of splat quenching. The mechanical behavior of metallic glasses were first characterized in 1969 from tensile tests conducted on thin ribbons. From these tests it was apparent that metallic glasses possessed tensile fracture strengths of approximately [...], which approach theoretical limits. Compressive mechanical data became available in 1974 with the fabrication of small cylindrical rods of [...]. This data indicated that the quasi-static yield behavior of metallic glasses may obey a pressure insensitive von Mises yield criterion. In 1983, Mechanical tests were conducted on [...] in multi-axial stress states which further confirmed the von Mises yield behavior. However, in 1988, mechanical tests performed on [...] indicated that metallic glasses may instead obey a pressure sensitive Mohr-Coulomb criterion.\n\nThere is some ambiguity in interpreting the results of mechanical tests performed on metallic glasses. The data from these tests were obtained by testing specimens whose sizes do not guarantee a well-defined stress state. Furthermore, the mechanical behavior of metallic glasses may depend on composition. In order to properly determine the yield behavior of metallic glasses in multi-axial stress states, it was necessary to fabricate specimens with geometries suitable for generating well-defined stress states.\n\nIn 1993, a new beryllium bearing bulk metallic glass with the nominal composition of [...] was discovered at Caltech. This metallic glass can be cast as cylindrical rods as large as 16 mm in diameter. Specimens could then be fabricated with geometries that conformed to ASTM testing standards. These specimens were then tested in quasi-static compressive, tensile, and torsional stress states at strain rates of [...] to [...] in order to properly characterize the yield behavior of the metallic glass. From these tests it was determined that the beryllium bearing bulk metallic glass obeys a von Mises yield criterion. In addition it was discovered that the ductility of this metallic glass could be altered by adding Boron and varying the quench rate.\n\nFor the first time, the dynamic compressive yield behavior of a metallic glass could be characterized at strain rates of [...] to [...] by using the split Hopkinson pressure bar. High-speed infrared thermal detectors were also used to determine if adiabatic heating occurred during dynamic deformation of the metallic glass. From these tests it appears that the yield stress of the metallic glass is insensitive to strain rate and no adiabatic heating occurs before yielding.", "doi": "10.7907/jd42-3s83", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" }, { "id": "thesis:818", "collection": "thesis", "collection_id": "818", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03012005-141633", "primary_object_url": { "basename": "Bhattacharyya_a_1994.pdf", "content": "final", "filesize": 6752918, "license": "other", "mime_type": "application/pdf", "url": "/818/1/Bhattacharyya_a_1994.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Internal flows and force matrices in axial flow inducers", "author": [ { "family_name": "Bhattacharyya", "given_name": "Abhijit", "clpid": "Bhattacharyya-A" } ], "thesis_advisor": [ { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Murray", "given_name": "Richard M.", "clpid": "Murray-R-M" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Axial flow pump runners known as inducers are subject to complex internal flows and fluid-induced lateral and rotordynamic forces. The internal flows in inducers are three dimensional and are characterized by complicated secondary flows. The current research investigates the boundary layer flows on the blades, hub and housing of unshrouded and shrouded axial flow inducers using flow visualization techniques. Rotordynamic and lateral force data on unshrouded inducers were also obtained under varying conditions of flow and whirl.\n\nStudies on the internal flows showed that the blade boundary layer flow had strong radial components at off-design conditions. The flow remains attached to the blade surface of unshrouded inducers at all flow coefficients tested. The origin of the upstream swirling backflow was found to be at the discharge plane of the inducer. In addition, flow reversal was observed at the suction side blade tip near the leading edge in a shrouded inducer. Re-entry of the hub boundary layer flow (a downstream backflow) into the blade passage area was observed at flow coefficients below design. For unshrouded inducers the radially outward flow near the blade tip mixed with the tip clearance leakage flow to form the upstream backflow. These observations provide a better understanding of the internal flows and the occurrence of upstream backflows in inducers.\n\nThe rotordynamic forces acting on an inducer due to an imposed whirl motion was also investigated. It was found that the rotordynamic force data at various whirl frequency ratios does not allow a normal quadratic fit; consequently the conventional inertial, stiffness and damping coefficients cannot be obtained and a definite whirl ratio describing the instability region does not result. Rotordynamic forces were found to be significantly dependent on the flow coefficient. At flow coefficients below design, these forces are characterized by multiple zero crossings at various whirl frequencies and large destabilizing peeks. Theoretical estimates of the tangential rotordynamic force on a non-whirling inducer using actuator disk theory were significantly different, both in magnitude and direction, from the experimentally measured forces.\n\nThe effect of upstream and downstream flow distortions on the rotordynamic and lateral forces on an inducer were studied. It was found that at flow coefficients below design, large lateral forces occurred in the presence of a downstream asymmetry. The reverse flows occurring downstream which consist of high energy fluid are the possible cause of these large forces. The imposition of a uniform downstream condition reduced these forces to near zero values. Results of inlet distortion experiments show that a strong inlet shear causes a significant increase in the lateral force. However, weak inlet shear flows and the flow asymmetry due to a 180\u00b0 upstream bend did not cause a significant lateral force. It was found that flow distortions upstream or downstream did not cause any significant effect on the rotordynamic forces. Cavitation was found to have important consequences for fluid-induced rotordynamic forces. These forces become destabilizing for both forward and reverse whirl. The magnitudes of the destabilizing forces were found to increase with decreasing cavitation numbers.\n", "doi": "10.7907/Q4ST-4X32", "publication_date": "1994", "thesis_type": "phd", "thesis_year": "1994" }, { "id": "thesis:4976", "collection": "thesis", "collection_id": "4976", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12122007-141145", "primary_object_url": { "basename": "Liu_c_1994.pdf", "content": "final", "filesize": 14303932, "license": "other", "mime_type": "application/pdf", "url": "/4976/1/Liu_c_1994.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Dynamic fracture problems involving highly transient crack growth histories : an investigation of dynamic failure in homgeneous and bimaterial systems", "author": [ { "family_name": "Liu", "given_name": "Cheng", "clpid": "Liu-C" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nHighly transient elastodynamic fracture processes in both homogeneous and bimaterial systems have been investigated. It is found that due to the wave character of the mechanical fields during transient and dynamic crack growth, the customarily assumptions of steady state and K[superscript d]-dominance may be violated. This may be particularly true during crack growth in laboratory size specimens where crack growth seldom reaches steady state conditions due to the persistence of the initiation transients and the influence of reflected stress waves from the specimen boundaries. By relaxing both restrictions of steady state and of K[superscript d]-dominance, and by permitting the crack-tip speed and the dynamic stress intensity factor to be arbitrary functions of time, the transient asymptotic elastodynamic field near the moving crack-tip was established in the form of higher order expansion for both homogeneous solids and bimaterial systems. In homogeneous solids, we considered cracks that propagated along arbitrary smooth paths, while in bimaterial systems, we only considered crack growth along a straight interface. The higher order coefficients of the asymptotic expansion were found to depend on the time derivative of crack-tip speed, the time derivative of the dynamic stress intensity factors, and for crack propagating along curved paths, on the instantaneous value of the local curvature of the crack path.\n\nThe issue of K[superscript d]-dominance during dynamic crack initiation and transient crack growth was further investigated by solving a particular transient initial/boundary value problem. This corresponds to a planar dilatational wave impinging on a semi-infinite crack in an unbounded elastic solid. The crack initiates under the influence of the wave, and then propagates dynamically. Through comparison of this full field solution and the equivalent K[superscript d]-dominant field or the field represented by the higher order transient terms, it is found that even for points which are relatively far away from the crack-tip, or for times very close to the crack initiation, the higher order transient representation provides a very good description of the actual stress field. \nThe K[superscript d]-dominant field, however, is incapable of approximating the complete stress field with any accuracy (lack of K[superscript d]-dominance).\n\nThe implications of the above observations (possible lack of K[superscript d]-dominance) on the interpretability of dynamic fracture experiments are also explored. The interpretation of experimental data in past laboratory investigations of dynamic fracture events is based on the assumption of K[superscript d]-dominance. However, as we have seen theoretically this assumption may often fail in laboratory situations. As a result, experimental measurements must be analyzed by techniques that allow for the possibility of the existence of transient higher order term effects. Several types of experiments are considered as examples. Plate impact experiments involving very high rates of loading are first analyzed by both a K[...]-dominant and a high order transient approach. The results clearly show the strong effects of transients on the interpretation of the data. As a second example, the optical method of caustics is reanalyzed. A new way of extracting the instantaneous value of the dynamic stress intensity factor K[...](t), which takes transients into account, is proposed and verified theoretically. For the bimaterial system, the issues are equivalent but much more complicated analytically. Here transient effects are found to be magnified by the material property mismatch between the constituent solids. It is shown however, that the higher order transient analysis can predict accurately the fringe patterns from actual experiment performed by means of the CGS (Coherent Gradient Sensing) technique and high speed photography.\n\nThe observations of this thesis suggest that a variety of conclusions made in the literature based on interpretations of experimental data on the basis of steady state or K[superscript d]-dominance may be suspect.", "doi": "10.7907/k32y-0450", "publication_date": "1994", "thesis_type": "phd", "thesis_year": "1994" }, { "id": "thesis:4779", "collection": "thesis", "collection_id": "4779", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12042007-075432", "primary_object_url": { "basename": "Lambros_j_1994.pdf", "content": "final", "filesize": 10360499, "license": "other", "mime_type": "application/pdf", "url": "/4779/1/Lambros_j_1994.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Dynamic decohesion of bimaterial interfaces", "author": [ { "family_name": "Lambros", "given_name": "John", "clpid": "Lambros-J" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nIn the present work, findings of an experimental study of dynamic decohesion of bimaterial systems composed of constituents with a large material property mismatch are presented. PMMA/steel or PMMA/aluminum bimaterial fracture specimens are used. Dynamic one point bend loading is accomplished with a drop weight tower device (for low and intermediate loading rates) or a high speed gas gun (for high loading rates). High speed interferometric measurements are made using the lateral shearing interferometer of Coherent Gradient Sensing in conjunction with high speed photography. Very high crack propagation speeds (terminal crack tip speeds up to [...], where [...] is the shear wave speed of PMMA) and high accelerations ([...], where g is the acceleration of gravity) are observed and reported. Issues regarding data analysis of the high speed interferograms are discussed. The effects of near tip three dimensionality are also analyzed. In crack propagation regions governed by large crack tip accelerations it is found that for accurate analysis of the optical data use of a transient elastodynamic crack tip field is necessary. Otherwise use of a Kd-dominant analysis is sufficient. Using the dynamic complex stress factor histories obtained by fitting the experimental data, a dynamic crack growth criterion is proposed. In the subsonic regime of crack growth it is seen that the opening and shearing displacements behind the propagating crack tip remain constant, i.e., the crack retains a self-similar profile during crack growth at any speed. This forms the basis of the proposed dynamic interfacial fracture criterion. It is also found that the process of dynamic interfacial fracture is highly unstable. This is corroborated by both the very large measured values of crack tip speed and acceleration and by the observation that the energy release rate at the propagating crack tip decreases with increasing crack tip speed. A mechanism of energy transfer from the metal to the PMMA side of the specimen is believed to be responsible for the high transient and transonic effects. An analysis and discussion of this phenomenon is also presented in this work.\n", "doi": "10.7907/36hw-c185", "publication_date": "1994", "thesis_type": "phd", "thesis_year": "1994" }, { "id": "thesis:3305", "collection": "thesis", "collection_id": "3305", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08312007-104217", "primary_object_url": { "basename": "Pulos_gc_1993.pdf", "content": "final", "filesize": 7107948, "license": "other", "mime_type": "application/pdf", "url": "/3305/1/Pulos_gc_1993.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Nonsteady crack propagation and craze behavior in PMMA", "author": [ { "family_name": "Pulos", "given_name": "Guillermo C.", "clpid": "Pulos-G-C" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This work is devoted to the study of nonsteady crack propagation under cyclic loading in polymers, specifically PMMA. The first part deals with the delineation of a precision loading facility allowing ultra-precise load or displacement control commensurate with the high resolution measurements of crack tip material response.\r\n\r\nA method of determining the advance of crack tip through combined microscope and computer-analyzed observation is presented. In particular, the experimental set up and software development is described by which these measurements are achieved. It is shown that automated crack tip location is possible with a precision of one to two microns, which is amply sufficient for present purposes to make definitive statements about the smoothness or discontinuity of crack propagation.\r\n\r\nThe craze and crack opening displacements are measured near the free surface of the specimen both under quasi-static step loading and cyclic loading. Eleven craze opening profiles for equal load increments are acquired during a single cycle under fatigue loading. A multi-linear craze stress model is used to match the opening displacements to the measurements. While the primordial thickness can be defined from the Lorentz-Lorenz equation and from the assumption of a constant index of refraction for the quasi-static loading, the effect of load history may prevent such determinations for cyclic loading. The damage accumulated through cyclic deformation reduces the strength of the fibrils in the middle of the craze and produces a drop in the middle of the stress distribution. The craze and crack opening displacement are monitored in connection with a jump-like crack/craze advance constituting 50% of the craze length. The newly-drawn craze fibrils after the jump show mechanical behavior that is different from their behavior before the jump and exhibit no deterioration in their stress-displacement relation.", "doi": "10.7907/4kqe-ec90", "publication_date": "1993", "thesis_type": "phd", "thesis_year": "1993" }, { "id": "thesis:3242", "collection": "thesis", "collection_id": "3242", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08272007-104817", "primary_object_url": { "basename": "Geubelle_ph_1993.pdf", "content": "final", "filesize": 7565792, "license": "other", "mime_type": "application/pdf", "url": "/3242/1/Geubelle_ph_1993.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Nonlinear effects in interfacial fracture", "author": [ { "family_name": "Geubelle", "given_name": "Philippe H.", "clpid": "Geubelle-P-H" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The issue of the non-coplanar quasi-static propagation of a crack in homogeneous and bimaterial sheets is investigated. Through a preliminary linear analysis, it is shown that the interface crack kinking problem is confronted, in most practical cases, with difficulties which do not arise in the homogeneous situation: the crack path as predicted by the maximum energy release rate criterion cannot be determined uniquely and an additional length parameter, absent in the homogeneous case, needs to be specified to assure uniqueness. Following that development, the assumption of small deformations is relinquished and it is shown how the size of the nonlinear zone imparts possibly the physical significance of the additional length parameter. The analysis is performed numerically in the homogeneous and bimaterial cases within the framework of the nonlinearly elastic theory of plane stress and using a \"boundary-layer\" approach. Material and geometrical nonlinearities are combined through the use of the Generalized Neo-Hookean (GNH) model. As the length of the crack extension becomes comparable to the size of the nonlinear zone, a transition is observed between the value of the energetically most favorable kink angle predicted by the linear theory and a unique \"nonlinear\" value which is found to be independent of the crack extension length and the far-field loading conditions.\n\nThe results of the crack propagation analysis are related to those of a detailed asymptotic analysis of the structure of the near-tip stress and deformation fields for the GNU class of hyperelastic materials. The investigation addresses a) the symmetric (mode I) and non-symmetric (mixed-mode) homogeneous situations, b) the rigid substrate case and c) the general bimaterial problem which allows for an arbitrary choice, on both sides of the interface, of the three material parameters characterizing the GNH model. The asymptotic analysis allows to quantify the effect of the \"hardening\" characteristics on the blunting of the crack and the associated stress and strain singularities, and shows that the near-tip fields corresponding to a general nonsymmetric loading are, in the homogeneous situation, related to those of the symmetric (mode I) case through a rotation which depends on the material characteristics and the far-field loading conditions. A somewhat similar property is obtained in the bimaterial problem, where the existence of a non-oscillatory and \"contact-free\" solution is confirmed for all material combinations.", "doi": "10.7907/rkt0-7k90", "publication_date": "1993", "thesis_type": "phd", "thesis_year": "1993" }, { "id": "thesis:6332", "collection": "thesis", "collection_id": "6332", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04152011-095226678", "primary_object_url": { "basename": "Tsuyuki_rm_1993.pdf", "content": "final", "filesize": 11228323, "license": "other", "mime_type": "application/pdf", "url": "/6332/1/Tsuyuki_rm_1993.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Buckling of thermoviscoelastic structures under temporal and spatial temperature variation", "author": [ { "family_name": "Tsuyuki", "given_name": "Richard M.", "clpid": "Tsuyuki-R-M" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The problem of lateral instability of a viscoelastic in-plane loaded structure is considered in terms of thermorheologically simple materials. As an example of a generally in-plane loaded structure, we examine the simple column under axial load: Both cyclic loading is considered (with constant or in-phase variable temperature excursions) as well as the case of constant load in the presence of thermal gradients through the thickness of the structure.\r\nThe latter case involves a continuous movement of the neutral axis from the center to the colder side and then back to the center.\r\n\r\nIn both cases, one finds that temperature has a very strong effect on the rate at which instabilities evolve, and under in-phase thermal cycling the critical loads are reduced compared to those at constant (elevated) temperatures. The primary effect of thermal gradients beyond that of thermally-induced rate accelerations is a rate increase occasioned by the generation of an \"initial imperfection\" or \"structural bowing.\" This latter effect, which is proportional to both the temperature gradient and the coefficient of thermal expansion (presumed homogeneous in this study), can in fact be dominant. Because the coefficient of thermal expansion tends to be large for many polymeric materials, it may be necessary to take special care in lay-up design of composite structures intended for use under compressive loads in high-temperature applications. Finally, the implications for the temperature\r\nsensitivities of composites to micro-instability (fiber crimping) are also apparent from the results delineated here.\r\n", "doi": "10.7907/T54W-0694", "publication_date": "1993", "thesis_type": "engd", "thesis_year": "1993" }, { "id": "thesis:3053", "collection": "thesis", "collection_id": "3053", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08082007-100442", "primary_object_url": { "basename": "Minahen_tm_1992.pdf", "content": "final", "filesize": 5437488, "license": "other", "mime_type": "application/pdf", "url": "/3053/1/Minahen_tm_1992.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Structural instabilities involving time dependent materials : theory and experiment", "author": [ { "family_name": "Minahen", "given_name": "Timothy M.", "clpid": "Minahen-T-M" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Christman", "given_name": "Tom", "clpid": "Christman-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The creep buckling of viscoelastic structures is studied analytically and experimentally to investigate structural stability in the presence of time dependent materials. The theory of linear viscoelasticity is used to model polymeric column specimens subjected to constant compressive end loads. A strength of materials approach (Euler-Bernoulli beam theory) is employed to model the moment-curvature relation for the column. The growth of initial imperfections is calculated using the hereditary integral formulation. Solution techniques are developed for small displacements and then generalized to include the effects of large displacements and rotations. A failure criterion based on maximum deformation allows the column life to be estimated directly from the material relaxation modulus. A discussion generalizing the results to include plates and shells is presented.\n\nRectangular cross-section polymethylmethacrylate (PMMA) specimens with hinged boundary conditions are used to study viscoelastic buckling experimentally. Constant compressive end loads are applied using a servo-controlled load frame while the specimens are kept in a temperature cabinet at elevated temperatures (accelerating the creep behavior). Specimen shortening and out-of-plane deflections are monitored during the tests. The relaxation modulus of PMMA is approximated by a Prony-Dirichlet series and the model is used to simulate the laboratory experiments. Model and experimental results show good agreement during the \"glassy\" and slow growth phases of the column response. As the growth rate increases some deviations between theory and experiment are seen. It is shown that the deviations are not a result of geometric nonlinearities, but may, in part, be explained by material nonlinearities not accounted for in the model.\n", "doi": "10.7907/7h70-2p51", "publication_date": "1992", "thesis_type": "phd", "thesis_year": "1992" }, { "id": "thesis:2848", "collection": "thesis", "collection_id": "2848", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07102007-104514", "primary_object_url": { "basename": "Pfaff_rd_1991.pdf", "content": "final", "filesize": 7181768, "license": "other", "mime_type": "application/pdf", "url": "/2848/1/Pfaff_rd_1991.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Three-dimensional effects in nonlinear fracture explored with interferometry", "author": [ { "family_name": "Pfaff", "given_name": "Richard D.", "clpid": "Pfaff-R-D" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The prospects for understanding fracture mechanics in terms of a general material constitutive description are explored. The effort consists of three distinct components.\n\nFirst, optical interferometry, in its various forms (Twyman-Green, diffraction moire, etc.), can potentially be used under a wide range of conditions to very accurately measure the displacement and strain fields associated with the deformation surrounding a cracktip. To broaden the range of fracture problems to which interferometry may be applied, certain of the necessary experimental improvements have been developed:\n\n1. High speed camera designs capable of extremely high (> 10(9) frames/second) framing rates with large array sizes, (> 4000 x 4000 pixels per frame) so that the application of optical techniques to solid mechanics may be considered without limitation on the rate of deformation.\n\n2. An accurate and adaptable device for dynamic loading of fracture specimens to high load levels utilizing electromagnetic (Lorentz force) loading with ultrahigh (> 2,000,000 Amp/cm(2)) current flux densities.\n\n3. Implementation of high sensitivity (2 nm), large range (2 nm x 3,200,000) interferometry achieved with wide field array sizes of 50,000 x 50,000 and 8 bit gray scale (error restricted to 1 bit) for surface deformation measurements on fracture specimens.\n\nSecond, functional descriptions for certain aspects of the displacement fields associated with fracture specimens are developed. It is found that the fully three-dimensional crack tip field surrounding a through-thickness crack in a plate of elastic-plastic material shows a hierarchical structure of organization and that the primary aspects of the deformation field would seem to have a relatively simple form of expression if the deformation is viewed in a properly normalized form.\n\nThird, a comparison is made between interferometrically measured surface displacements for a notched 3-point-bend speciemn of a ductile heat treatment of 4340 steel and a numerical simulation of the specimen based on a material constitutive description determined from uniaxial tests performed on the same material. The small but finite notch tip radius (0.15 mm) fabricated by a wire-cutting electrical discharge machine allows one to explore the limits of applicability of standard continuum plasticity theories without involving a process zone model for the very near tip region extent in a cracked specimen geometry.\n", "doi": "10.7907/6jnw-kh62", "publication_date": "1991", "thesis_type": "phd", "thesis_year": "1991" }, { "id": "thesis:410", "collection": "thesis", "collection_id": "410", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01302007-160351", "primary_object_url": { "basename": "Fried_e_1991.pdf", "content": "final", "filesize": 10761807, "license": "other", "mime_type": "application/pdf", "url": "/410/1/Fried_e_1991.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Aspects of the morphological character and stability of two-phase states in non-elliptic solids", "author": [ { "family_name": "Fried", "given_name": "Eliot", "clpid": "Fried-E" } ], "thesis_advisor": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" }, { "family_name": "Christman", "given_name": "Tom", "clpid": "Christman-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Part I. This work focuses on the construction of equilibrated two-phase antiplane shear deformations of a non-elliptic isotropic and incompressible hyperelastic material. It is shown that this material can sustain metastable two-phase equilibria which are neither piecewise homogeneous nor axisymmetric, but, rather, involve non-planar interfaces which completely segregate inhomogeneously deformed material in distinct elliptic phases. These results are obtained by studying a constrained boundary value problem involving an interface across which the deformation gradient jumps. The boundary value problem is recast as an integral equation and conditions on the interface sufficient to guarantee the existence of a solution to this equation are obtained. The contraints, which enforce the segregation of material in the two elliptic phases, are then studied. Sufficient conditions for their satisfaction are also secured. These involve additional restrictions on the interface across which the deformation gradient jumps-which, with all restrictions satisfied, constitutes a phase boundary. An uncountably infinite number of such phase boundaries are shown to exist. It is demonstrated that, for each of these, there exists a solution - unique up to an additive constant - for the constrained boundary value problem. As an illustration, approximate solutions which correspond to a particular class of phase boundaries are then constructed. Finally, the kinetics and stability of an arbitrary element within this class of phase boundaries are analyzed in the context of a quasistatic motion.\n\nPart II. This work investigates the linear stability of an antiplane shear motion which involves a planar phase boundary in an arbitrary element of a wide class of non-elliptic generalized neo-Hookean materials which have two distinct elliptic phases. It is shown, via a normal mode analysis, that, in the absence of inertial effects, such a process is linearly unstable with respect to a large class of disturbances if and only if the kinetic response function - a constitutively supplied entity which gives the normal velocity of a phase boundary in terms of the driving traction which acts on it or vice versa - is locally decreasing as a function of the appropriate argument. An alternate analysis, in which the linear stability problem is recast as a functional equation for the interface position, allows the interface to be tracked subsequent to perturbation. A particular choice of the initial disturbance is used to show that, in the case of an unstable response, the morphological character of the phase boundary evolves to qualitatively resemble the plate-like structures which are found in displacive solid-solid phase transformations. In the presence of inertial effects a combination of normal mode and energy analyses are used to show that the condition which is necessary and sufficient for instability with respect to the relevant class of perturbations in the absence of inertia remains necessary for the entire class of perturbations and sufficient for all but a very special, and physically unrealistic, subclass of these perturbations. The linear stability of the relevant process depends, therefore, entirely upon the transformation kinetics intrinsic to the kinetic response function.\n\nPart III. This investigation is directed toward understanding the role of coupled mechanical and thermal effects in the linear stability of an isothermal antiplane shear motion which involves a single planar phase boundary in a non-elliptic thermoelastic material which has multiple elliptic phases. When the relevant process is static - so that the phase boundary does not move prior to the imposition of the disturbance - it is shown to be linearly stable. However, when the process involves a steadily propagating phase boundary it may be linearly unstable. Various conditions sufficient to guarantee the linear instability of the process are obtained. These conditions depend on the monotonicity of the kinetic response function - a constitutively supplied entity which relates the driving traction acting on a phase boundary to the local absolute temperature and the normal velocity of the phase boundary - and, in certain cases, on the spectrum of wave-numbers associated with the perturbation to which the process is subjected. Inertia is found to play an insignificant role in the qualitative features of the aforementioned sufficient conditions. It is shown, in particular, that instability can arise even when the normal velocity of the phase boundary is an increasing function of the driving traction if the temperature dependence in the kinetic response function is of a suitable nature. Significantly, the instability which is present in this setting occurs only in the long waves of the Fourier decomposition of the moving phase boundary, implying that the interface prefers to be highly wrinkled.", "doi": "10.7907/WJKM-GB39", "publication_date": "1991", "thesis_type": "phd", "thesis_year": "1991" }, { "id": "thesis:4297", "collection": "thesis", "collection_id": "4297", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-112909", "type": "thesis", "title": "Time-Temperature Response of Multi-Phase Viscoelastic Solids Through Numerical Analysis", "author": [ { "family_name": "Brinson", "given_name": "Lynda Catherine", "clpid": "Brinson-Lynda-Catherine" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Hall", "given_name": "John F.", "orcid": "0000-0002-7863-5060", "clpid": "Hall-J-F" }, { "family_name": "Tschoegl", "given_name": "Nicholas W.", "clpid": "Tschoegl-N-W" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A numerical model has been constructed for the study of the properties of multi-phase viscoelastic composites. The model utilizes the dynamic correspondence principle of viscoelasticity in a finite element program to solve boundary value problems simulating uniaxial tension or simple shear and obtains the global complex Young's or shear moduli of the composite.
\r\n\r\nEach phase of the composite is considered to be thermorheologically simple. The resulting modulus properties of the composite however, are thermorheologically complex and this investigation examines the nature of time-temperature behavior of multi-phase composite materials. The specific composite considered throughout this study contains viscoelastic inclusions embedded in a different viscoelastic matrix material. The deviation of the composite moduli from thermorheologically simple behavior of the matrix material is shown to occur at frequencies and temperatures where the glass-to-rubber transition of the included phases are reached.
\r\n\r\nProperties of polystyrene and polybutadiene are used to investigate the thermorheological complexity (non-shiftability) of a Styrene-Butadiene-Styrene (SBS) block copolymer. To achieve congruence of the results with experimental data, it is necessary to consider a transition phase of properties \"intermediate\" to those of styrene and butadiene. Using accurate physical information on the individual phase properties and on the interphase region, it is possible to utilize the numerical model to predict long term properties of multi-phase composites from short term laboratory data. Lacking detailed information on the properties of a particular phase (e.g., the interphase), but knowing the time dependent properties for the composite material at a broad range of temperatures, it is also possible to use the numerical tool to solve an inverse problem and determine the unknown properties of the phase in question.
", "doi": "10.7907/SYAS-2A50", "publication_date": "1990", "thesis_type": "phd", "thesis_year": "1990" } ]