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: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: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: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: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: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: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: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: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: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: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:10243", "collection": "thesis", "collection_id": "10243", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012017-131024630", "type": "thesis", "title": "Transverse Photonic Doppler Velocimetry for Plate Impact Experiments", "author": [ { "family_name": "Bischann", "given_name": "Moriah Nicole", "orcid": "0000-0002-6674-5158", "clpid": "Bischann-Moriah-Nicole" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "Senior Undergraduate Thesis Prize" }, { "literal": "div_eng" } ], "abstract": "High-pressure shock waves propagate in materials when automobiles collide, projectiles impact against bunkers, and mining charges detonate beneath Earth's surface. It is difficult to quantify the behavior of materials experiencing such sudden and high pressures. Specialized interferometry techniques have enabled the study of rapid, high-strain-rate deformation during controlled plate impact testing.
\r\n\r\nTransverse motion during plate impact experiments is currently measured with the transverse displacement interferometer (TDI). However, the TDI does not take advantage of modern telecommunications technology and data analysis techniques that allow for the measurement of high velocities (in the km/s range) with fine time resolution. We designed and developed a transverse photonic Doppler velocimetry (TPDV) technique for plate impact experiments based entirely on fiber optic components. The TPDV system uses light diffracted from a grating to capture transverse motion. This signal is frequency upshifted to achieve more fringes per unit time. Data is analyzed using spectral analysis techniques to detect micrometer displacements on a nanosecond to microsecond timescale.
\r\n\r\nWe demonstrated the TPDV technique's capabilities with normal impact of single crystalline y-cut alpha-quartz against borosilicate. We simultaneously collected photonic Doppler velocimetry (PDV) measurements of longitudinal displacements. Finally, we compared our longitudinal and transverse experimental results to theoretical calculations. Our data's orthogonal velocity jumps showed that the TPDV technique accurately detected transient velocities and the magnitudes of longitudinal and shear waves.
\r\n\r\nOur TPDV technique will facilitate the study of deformation and failure of materials during normal and pressure-shear plate impact. Comprehending shock-wave dominated deformation in materials is important for the design of aerospace structures, understanding planetary impact, and creating shock-mitigating materials.
", "doi": "10.7907/Z9GB224C", "publication_date": "2017", "thesis_type": "senior_major", "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: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: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: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: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: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: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: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: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: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: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: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: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: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: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: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: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: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: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: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: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:3093", "collection": "thesis", "collection_id": "3093", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08112005-103246", "primary_object_url": { "basename": "Chow_B_2001.pdf", "content": "final", "filesize": 8372107, "license": "other", "mime_type": "application/pdf", "url": "/3093/1/Chow_B_2001.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Application of dynamic fracture mechanics to the investigation of catastrophic failure in aircraft structures", "author": [ { "family_name": "Chow", "given_name": "Benjamin Bin", "clpid": "Chow-B-B" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "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\nA dynamic fracture mechanics approach to the estimation of the residual strength of aircraft structures is presented. The dependence of the dynamic crack initiation toughness of aluminum 2024-T3 on loading rate is first studied experimentally. A drop of up to 40% in the value of dynamic initiation toughness, [...], is discovered for loading rates in the range of [...]. This range of loading rate corresponds to the typical rates found in an aircraft fuselage experiencing explosive loading conditions. A dramatic increase in the value of dynamic crack initiation toughness is also found for loading rates above [...]. Based on these results and on established dynamic fracture mechanic concepts, a fracture mechanics based failure model is established and is used to estimate the residual strength of aircraft structures.\r\n\r\nA methodology to determine residual strength of dynamically loaded structures based on global structural analysis coupled with local finite element analysis is introduced. Local finite element calculations were performed for different loading rates, [...], ranging from [...] to [...], to simulate the conditions encountered in an explosively loaded aircraft fuselage. Simulations were conducted at a number of loading rates for the following cases of relevance to aircraft fuselage: (i) center cracked panels, (ii) rivet holes with wing cracks, (iii) biaxially loaded panels and (iv) panels prestressed to simulate pressurization. The results from the analyses were then used in conjunction with the experimental results for the dynamic fracture toughness of a 2024-T3 aluminum alloy as a function of loading rate, [...], to determine the time to failure, [...], for a given loading rate. A failure envelope, [...], based on the failure model and finite element analysis, is presented for the different cases and the implications for the residual strength of aircraft structures is discussed.\r\n\r\nMixed mode dynamic crack initiation in aluminum 2024-T3 alloy is investigated by combining experiments with numerical simulations. Pre-fatigued single edge notched specimens and three point bend specimens are subjected to dynamic symmetric and asymmetric loading to generate a range of mode mixity at the cracktip. The optical technique of coherent gradient sensing (CGS) and a strain gage method are employed to study the evolution of the mixed mode stress intensity factors. The dynamic mixed mode failure envelope is obtained using the crack initiation data from the experiments at a nominal loading rate of [...] and is compared with the static counterpart for 2024-T3 aluminum alloy. The fracture surfaces near the crack initiation site are investigated using a scanning electron microscope and reveal ductile void growth and coalescence. Numerical simulations of the experiments are conducted to both help in designing the experiments and to validate the results of the experiments. The numerical simulations show good correlation with the experimental results.", "doi": "10.7907/745f-mb29", "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: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:106", "collection": "thesis", "collection_id": "106", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01102008-074409", "primary_object_url": { "basename": "Hodowany_jn_1997.pdf", "content": "final", "filesize": 4606953, "license": "other", "mime_type": "application/pdf", "url": "/106/1/Hodowany_jn_1997.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "On the conversion of plastic work into heat", "author": [ { "family_name": "Hodowany", "given_name": "Jon", "clpid": "Hodowany-J" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "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 heat evolution in metal plasticity. Specifically, experiments were designed to measure the partition of plastic work into heat and stored energy during dynamic deformations. The fraction of plastic work converted into heat has implications in a wide range of thermomechanical phenomena, including shear bands, dynamic fracture, ballistic penetration and high speed machining.\r\n\r\nKolsky bars, in compression and torsion, were used to determine mechanical properties at strain rates between [...] and [...]. For dynamic loading, in-situ temperature changes were measured using a high-speed HgCdTe photoconductive detector. Specially designed infrared optics, configured in tandem with the HgCdTe detector and the Kolsky bar constituted a novel experimental configuration for determining the fraction of plastic work converted into heat, and thus, the amount of energy stored in metals. The temperature detection system was ideally suited for small temperature excursions from ambient conditions, and was sensitive to temperature changes as little as 0.5 \u00b0C. The emissivity of metals was found to increase above certain high levels of plastic strain due to changes in surface roughness, which can affect the validity of temperature calibration. A technique of sample recovery, rough surface layer removal, and reloading was employed to obtain large plastic strains in the Kolsky bar. A Materials Testing System (MTS) servo-hydraulic load frame was used to measure mechanical properties at lower strain rates, [...] to [...] When temperature measurement was needed within this range of strain rates, a fast E-type thin wire thermocouple, with a time response of 1 ms, was employed.\r\n\r\nThe fraction of plastic work converted into heat, [beta], was treated as a constitutive function of strain and strain rate in the heat conduction equation. 2024 aluminum alloy and commercially pure [alpha]-titanium were the metal systems used in the current study to determine the functional dependence of [beta] on strain and strain rate. The T351, T4 and T6 tempers of 2024 aluminum did not exhibit strain rate dependence in flow stress over the entire range of strain rates tested. At low levels of plastic strain, all tempers of 2024 aluminum stored more than 50% of the input plastic work. At some level of plastic strain, depending on temper, 2024 aluminum could no longer store plastic work. After this point, [beta] increased to a value near 1.0 and remained nearly constant during subsequent plastic deformation. When averaged over all strains, [beta] was 0.85-0.95 depending on the particular heat treatment. The fraction of plastic work dissipated as heat was not found to be sensitive to strain rate over a wide range of strain rates. In contrast, the flow stress of [alpha]-titanium was strongly dependent on strain rate. The initial flow stress increased by more than 15% between strain rates of [...] and [...]. In addition, the strain hardening was also observed to be rate dependent. For fixed plastic strain, the tangent modulus increased as strain rate increased. Titanium dissipated a greater proportion of energy as heat at low strains than all tempers of 2024 aluminum. The ability to store energy in titanium decreased with increasing plastic strain. For plastic strains above 0.3, titanium dissipated nearly all input plastic work as heat. The proportion of energy dissipated as heat at fixed strain increased as strain rate increased.\r\n", "doi": "10.7907/FENH-ZK36", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "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:6070", "collection": "thesis", "collection_id": "6070", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09282010-150048718", "primary_object_url": { "basename": "Srinivas_mv_1994.pdf", "content": "final", "filesize": 2079384, "license": "other", "mime_type": "application/pdf", "url": "/6070/1/Srinivas_mv_1994.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Time dependent failure of thin films", "author": [ { "family_name": "Srinivas", "given_name": "Mullahalli V.", "clpid": "Srinivas-M-V" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": " The prediction of time dependent failure at the interface of a thin film bonded to a substrate is considered in terms of a thermorheologically simple viscoelastic material. An initial investigation of stress distribution at the interface is carried out to understand the possible location and the mechanism of failure. Subsequently, a detailed analysis is developed to obtain not only the longitudinal stress component but also the lateral and shear stress components at the interface. Viscoelastic material behavior is included in the model to account for the stress relaxation behavior of the film. The proposed model satisfies both the edge free boundary conditions as well as the displacement and traction continuity at the interface. Using the model, the magnitude of the stresses and their relaxation behavior has been studied for the case of a polyimide film bonded to a silicon substrate. The analysis is capable of incorporating the material property at different temperatures. Using this capability, the stress evolution in a polyimide film during a typical curing process is obtained. With a view towards reducing the residual stress developed in the film, the effect of cooling rate and cooling cycle on final built in residual stresses during the curing process is studied.\r\n\r\n Based on the stress analysis and also from experimental observations, the most likely type of failure mechanism appears to be edge decohesion. So, the problem of edge decohesion along the interface of a thin viscoelastic film bonded to an elastic substrate under tensile residual stresses is considered in the next chapter. An analytical model is developed to predict the crack growth along the interface and its velocity. The tensile residual stress in the film is replaced by a combination of edge loads and an explicit relation of strain energy with respect to time is obtained by simple beam analysis. The strain energy function is computed at small time intervals and the energy release rate is calculated using Griffith's energy balance approach; the discretized time step is assumed to be very small so that the dissipation effects over a given time step are neglected. Crack growth along the interface is computed based on a fracture criteria. The validity of the assumptions in the analytical results is checked by performing a finite element analysis.", "doi": "10.7907/n3rb-w197", "publication_date": "1994", "thesis_type": "engd", "thesis_year": "1994" } ]