Thin composite shells are increasingly being used to support large area space systems due to their high strength to mass ratio and ability to withstand tight packaging for launch and then deploy once in space. This thesis specifically focuses on long, slender composite shells (longerons) with an open cross-section consisting of two circular arc flanges bonded along one edge. They are useful components of lightweight space structures, including the Caltech Space Solar Power Project spacecraft. The flanges of the longerons are less than 100 \u00b5m thick, and these extremely thin composite shells are prone to manufacturing imperfections and local buckling. Therefore, the primary objective of this thesis is to better understand the buckling behavior of open cross-section, ultra-thin composite shells that contain manufacturing imperfections, with the goal of informing the design of future space structures that are more resistant to buckling.
\r\n\r\nThe first step towards understanding a structure\u2019s imperfection sensitivity is to measure its imperfections. Thus, the thesis begins by characterizing the geometric imperfections present in experimental composite longerons. A method to measure and quantify the parameters of both local and global imperfections in thin shells is developed and applied. Results show that the longerons contain local imperfections as large as five to ten times the shell thickness, which can have serious implications for local buckling.
\r\n\r\nOnce the imperfections were measured, both numerical and experimental studies are performed to study the effects of these imperfections on the buckling behavior and knockdown factor of longerons loaded in pure bending. In the numerical study, a finite element analysis is used to characterize the effect of a single imperfection, created using a simplified model based on the shape of the experimentally measured imperfections, with a wide range of geometric parameters. Then, experiments are performed to measure the buckling behavior of actual longerons, whose random manufacturing imperfections were characterized. The results of these studies show that imperfections, especially ones with large amplitudes, significantly reduce the longeron\u2019s critical buckling load and bending stiffness. Good agreement between the experimental and numerical results was achieved, particularly for higher quality longerons with a single, dominant imperfection.
\r\n\r\nMotivated by the imperfections and their detrimental effects found in the earlier parts of the thesis, key parameters of the longeron's cross-section are varied with the goal of increasing its stability. The subtended angle of the longeron's flanges is varied in both experiments and numerical simulations of longerons loaded in bending. The results show good agreement between the experiments and simulations, with both showing a trend of increasing critical buckling load and bending stiffness with increasing flange subtended angle. Then, based on these promising results, the radius at the edge of the flange is decreased, which is shown to significant improve the longeron's stability and imperfection sensitivity without increasing its mass.
\r\n\r\nFinally, the effect of length on the buckling behavior of longerons loaded in bending is studied numerically with the goal of extending the current work to longer longerons. For lengths varying from 0.5 m to 5 m, both perfect and imperfect longerons with realistic geometric imperfections are studied. It is shown that for longer longerons, the critical buckling moment and imperfection sensitivity remain almost constant with increasing length, which is promising for future large space structures.
", "doi": "10.7907/ffpp-gr08", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18495", "collection": "thesis", "collection_id": "18495", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04162026-011414705", "primary_object_url": { "basename": "Caltech_Thesis.pdf", "content": "final", "filesize": 35879887, "license": "other", "mime_type": "application/pdf", "url": "/18495/1/Caltech_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Bridging Length and Time Scales of Plate Motions and Great Earthquakes", "author": [ { "family_name": "Fang", "given_name": "Jiaqi", "orcid": "0000-0001-6369-4802", "clpid": "Fang-Jiaqi" } ], "thesis_advisor": [ { "family_name": "Gurnis", "given_name": "Michael C.", "orcid": "0000-0003-1704-597X", "clpid": "Gurnis-M-C" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Jackson", "given_name": "Jennifer M.", "orcid": "0000-0002-8256-6336", "clpid": "Jackson-J-M" }, { "family_name": "Zhan", "given_name": "Zhongwen", "orcid": "0000-0002-5586-2607", "clpid": "Zhan-Zhongwen" }, { "family_name": "Gurnis", "given_name": "Michael C.", "orcid": "0000-0003-1704-597X", "clpid": "Gurnis-M-C" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Tectonic plates move at a steady velocity of several centimeters per year, which is intermittently interrupted by great megathrust earthquakes with rapid slip up to tens of meters. These large rupture events are driven by the slow tectonic loading and can substantially alter the deformation rate and state of stress in adjacent regions. With advances in computational algorithms, we develop cross-scale finite-element models that self-consistently integrate long-term motion of entire plates and the intervening space-time evolution associated with great earthquakes. The objectives are two-fold: firstly, to gain insight into the occurrence and magnitude of great earthquakes and their relationship with large-scale tectonic processes, and secondly, to constrain rheological properties of the solid Earth with multi-scale geophysical observations.
\r\n\r\nWe begin with formulating a generic subduction model that simultaneously resolves the dynamics of plate motions and megathrust seismic cycles (Chapter 2). Driven by internal buoyancy forces and governed by a nonlinear visco-elasto-plastic rheology, the predicted plate convergence and seismic cycle behavior align with observations in subduction zones. Using an efficient 2.5-dimensional approach, we show that the along-strike resistance arising from slip variations plays a key role in modulating the earthquake magnitude. In Chapter 3, we vary the rupture dimensions, rheological parameters and subduction characteristics to examine their influence on the plate velocity and coseismic slip. In Chapter 4, we build a three-dimensional model tailored to the Chilean Subduction Zone, and reproduce both the long-term motion of the Nazca Plate and post-seismic deformation in the adjacent non-ruptured segment after the 2010 Maule earthquake (Mw = 8.8). Combining these multi-scale geodetic observations as constraints significantly improves the uniqueness of inferred mantle viscosity structure. Following the previous work, we implement a margin-resolving global model of plate motions and earthquakes using the highly scalable finite-element code Rhea (Chapter 5). Constrained by both background plate motions and transient earthquake-related deformations, the model quantifies the sensitivity of different geodetic observations to the rheology of the megathrust, lithosphere and underlying mantle. In Chapter 6, we incorporate a true free surface and link plate motions, surface topography and off-megathrust stress state to Earth's nonlinear rheology within a unified cross-scale model. Our work demonstrates the potential for assimilating multi-scale geophysical observations in unified, physics-based models to better characterize Earth's internal structure and assess seismic hazards.
", "doi": "10.7907/c31c-fg36", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:16637", "collection": "thesis", "collection_id": "16637", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08142024-141724425", "primary_object_url": { "basename": "Shengduo_Liu_Thesis_Final.pdf", "content": "final", "filesize": 68545231, "license": "other", "mime_type": "application/pdf", "url": "/16637/1/Shengduo_Liu_Thesis_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Numerical Analyses of Frictional Sliding on Rate-and-State Interfaces: Fluid Effects, Dynamic Weakening, and Potential-Based Formulation Through Machine Learning", "author": [ { "family_name": "Liu", "given_name": "Shengduo", "orcid": "0009-0009-5259-0966", "clpid": "Liu-Shengduo" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Rate-and-state friction formulations have been widely used to reproduce a number of observations on faulting in the earth's crust, including earthquake nucleation, creeping fault segments, dynamic earthquake rupture, aftershock sequences, and episodic slow slip events. The formulations have also been used to explain the motion of landslides and glaciers. In this thesis, we use numerical simulations to study various factors that can affect the stability of fault slip with rate-and-state friction, including poroelastic bulk properties and dilatation/compaction of the fault material in the presence of fluids, fault healing, injection rate when there is fluid injected into the fault, as well as dynamic weakening of the fault gouge. We also seek to optimize simulations with rate-and-state friction by developing a potential-based formulation using machine learning.
\r\n\r\nFirst, we study the stability of frictional fault slip in the presence of fluids, with a focus on fault loading due to fluid injection into the fault as done in many field and laboratory experiments. In Chapter 2, we present a boundary-integral approach on simulating frictional fault slip in a permeable shear layer surrounded by poroelastic bulk. The approach is then used to explore the effects of poroelasticity and inelastic dilatancy on the stability of frictional fault slip in a fluid-injection problem. We find that the diffusion into and poroelastic properties of the bulk can significantly stabilize fault slip, with the stabilization by bulk diffusion and poroelastic properties comparable to the well-known stabilizing effects of the dilatancy mechanism.
\r\n \r\nIn Chapter 3, we further develop the boundary integral code to allow for purely elastic bulk with the same fluid transport properties as the poroelastic bulk material and consider the effect of fault healing and fluid injection rate on fault slip. We show that the poroelastic bulk effects can be very closely captured by using the undrained value of Poisson\u2019s ratio in an elastic bulk model with the same fluid mass diffusivity of the bulk. We find that fault healing significantly delays the onset of dynamic slip events and restricts their spatial extent, making the initial response of the fault to fluid injection much different than its longer-term response. While this is an expected conclusion, fault healing is not typically accounted for in fluid injection modeling which often uses simpler slip-dependent friction laws. We also find that faster or intermittent injection rates lead to more frequent but more spatially constrained dynamic slip events, for the same injected fluid mass, motivating further investigations into injection strategies that would optimize fault stability.
\r\n \r\nSecond, in Chapter 4, we numerically simulate a laboratory experiment of spontaneous dynamic rupture by developing a 3D finite-element model of the experiment with rate-and-state friction. In the experiment, a dynamic rupture is initiated on a Homalite-100 interface and then produces an intermittent slip in the rock gouge embedded into a part of the interface. Our simulations show that the laboratory findings are consistent with rock gouge which is rate-strengthening at low slip rates but dynamically weakening at high slip rates through the mechanism similar to flash heating. However, to fit the experimental results, the traditional flash-heating formulation needs to be substantially modified, potentially due to effects of localization and delocalization of slip in the rock gouge.
\r\n\r\nThe third part of the thesis focuses on identifying a potential-based formulation for the rate-and-state friction laws. Due to their empirical derivation, the rate-and-state friction laws cannot be written as the gradients of a potential, which leads to difficulties in implicit solution of dynamic frictional problems. In Chapter 5, we present a potential-based formulation for the rate-and-state friction law through Neural Network approximation and training on datasets generated by a one-degree of-freedom spring-slider system with the rate-and-state friction law. The learnt potential is able to reproduce the results with rate-and-state friction law, and indeed facilitates an implicit solution of dynamic problems. However, the training of the potential requires a much larger dataset than fitting the original rate-and-state friction law.
\r\n\r\nOverall, our modeling significantly advances our understanding of the factors that control stability of frictional sliding on natural faults and suggests promising machine-learning directions in replacing the empirical rate-and-state formulations with the ones based on thermodynamic potentials.
", "doi": "10.7907/1tzb-pn69", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16630", "collection": "thesis", "collection_id": "16630", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08132024-035518437", "primary_object_url": { "basename": "MyCaltechThesis_TaehoKim.pdf", "content": "final", "filesize": 102966987, "license": "other", "mime_type": "application/pdf", "url": "/16630/2/MyCaltechThesis_TaehoKim.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Modeling Frictional Processes in the Presence of Fluids: From Earthquakes in the Laboratory to Induced Seismicity in Geothermal Reservoirs", "author": [ { "family_name": "Kim", "given_name": "Taeho", "orcid": "0000-0002-2560-7728", "clpid": "Kim-Taeho" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" } ], "thesis_committee": [ { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Faulkner", "given_name": "Daniel R.", "orcid": "0000-0002-6750-3775", "clpid": "Faulkner-Daniel-R" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Induced seismicity - earthquakes driven by injections of fluids into the subsurface - is of growing societal importance in its impact on clean energy technology. Advancements central to the world\u2019s transition to a greener economy such as geothermal energy and long-term geologic storage of CO2 are hampered by a lack of understanding and control of the associated seismic hazards. In its mechanics, frictional processes in the presence of fluids is a difficult problem to model given the challenges of studying frictionally unstable material in a controlled environment. Unstable gouge material is commonly found along faults in nature, due to pulverization of brittle rock in to granular layers called `gouge.' This thesis approaches the challenge at two different scales: 1. at the scale of the localized shear layer along the interface between two faults where we model laboratory earthquakes in the presence of pressurized fluids, and 2. at the scale of a reservoir where we model the rate of earthquakes given the injection/extraction schedule.
\r\n\r\nIn order to infer the frictional properties of unstable gouge material from laboratory experiments, we develop a probabilistic model based on a spring-slider representation of the experiment along with the rate-and-state friction law. Inversions indicate that the presence of pressurized pore fluids stabilizes the gouge - by an increase in the strength of the contacts and a lesser decrease in the grain size with slip - even under the same effective normal stress. Assuming purely slip-dependent healing of friction leads to an evolution of parameters with slip that is consistent with previously established interpretations of rate-and-state parameters. The best fitting spring-slider model still shows significant discrepancies to the experiment in the evolution of creep and in the dependence on loading rate. A quasi-static finite-element model with the same rate-and-state properties suggests that the gouge in the sample likely slides in a spatially uniform manner. Thus, the discrepancies between the spring-slider model and the experiment can likely be attributed to flaws in the rate-and-state formalism and the slip law rather than the idealization of a finite geometry to a single-degree-of-freedom system. The results prove that quantitative analysis of frictional processes of gouge in the unstable regime is possible, and that future development of constitutive relationships for friction should aim to reproduce key features of stick-slip in detail.
\r\n\r\nTo model seismicity induced by a geothermal well stimulation, we develop physical and statistical models of the seismicity rate. The physical models are based on rate-and-state friction and stress changes due to pore-pressure diffusion. The statistical model performs a convolution of a kernel function inspired by Omori law decay with the injection rate. Both models successfully reproduce the seismicity observed during the 2018 enhanced geothermal system (EGS) simulation in Otaniemi, Finland. We find that the effect of time-dependent nucleation from rate-and-state friction is crucial in reproducing the temporal and spatial patterns of the observed seismicity. We also find that the effect of finite nucleation cannot be approximated well by introducing a stress threshold in the standard Coulomb friction model, at least in the context of rapid variations of injection rates common in EGS operations.
\r\n\r\nWe highlight the major assumptions of the Dieterich seismicity rate model and examine how they may bias interpretations of induced seismicity observed in real reservoirs by comparing it directly to a Discrete Fault Network (DFN) model. The spatio-temporal pattern of seismicity in the finite setting is not only dependent on fluid transport properties and its combination with nucleation characteristics but also the distribution of initial conditions of the fault network. The back-propagation front, in particular, occurs co-injection if the time to instability for the minimum slip rate is shorter than the injection duration. The relocated catalogue of the 1993 GPK1 stimulation in Soultz-Sous-Forets shows such a back-front which can be fit qualitatively using the time to instability measure. A simple model for the rate of magnitudes that accounts for the evolution of frictional stability reproduces the apparent increase in the source radius of induced events in Soultz-Sous-Forets. The rate of larger events is overestimated by the model, possibly due to an overestimation of maximum magnitudes by the volume of stimulation. The comparisons reveal that parameters of the Dieterich model lack clear physical meaning in the finite analogue and highlight the importance of using realistic physics, especially in models at large scales where uncertainty due to assumptions at smaller scales may be amplified.
\r\n \r\nWe end the thesis with the application of rate-and-state friction to dynamic rupture modeling of seismic data from distributed acoustic sensing (DAS). The modeling of the high-frequency DAS recordings of a Magnitude 6.0 earthquake suggests a highly heterogeneous underlying fault with several prominent asperities and barriers that may control rupture dynamics. The model demonstrates how the high-stress patches both inhibit and promote the overall rupture, while also contributing to a significant amount of the energy release themselves. The successful interpretations of modern seismological data encourage future development efficient models that can be used for dynamic inversions.
", "doi": "10.7907/pp3a-2609", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16822", "collection": "thesis", "collection_id": "16822", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10302024-162816237", "primary_object_url": { "basename": "KoehneTobias2024Thesis.pdf", "content": "final", "filesize": 29555398, "license": "other", "mime_type": "application/pdf", "url": "/16822/1/KoehneTobias2024Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "From Daily Deformation to Millennial Mechanics: Insights from Subduction Zone Earthquake Cycle Models", "author": [ { "family_name": "K\u00f6hne", "given_name": "Tobias", "orcid": "0000-0002-8400-7255", "clpid": "K\u00f6hne-Tobias" } ], "thesis_advisor": [ { "family_name": "Simons", "given_name": "Mark", "orcid": "0000-0003-1412-6395", "clpid": "Simons-M" } ], "thesis_committee": [ { "family_name": "Ross", "given_name": "Zachary E.", "orcid": "0000-0002-6343-8400", "clpid": "Ross-Z-E" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Simons", "given_name": "Mark", "orcid": "0000-0003-1412-6395", "clpid": "Simons-M" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Subduction zones have hosted all the largest five earthquakes in the last one hundred years, including the 2011 Mw 9.1 Tohoku-oki earthquake on 11 March 2011, one of the largest natural disasters in history. While the general mechanism of these thrust-style earthquakes is well described by stress accumulation due to the locking between the incoming and the overriding tectonic plates, many questions remain as to the size and longevity of the asperities which host the coseismic rupture, the rheological models best describing the rock in and around the fault zone, and the effects of stress shadows and interactions between different asperities on the same plate interface. These questions are addressed by using large earthquakes as natural experiments, which we can observe using geodetic, seismic, and other techniques. However, ambiguities in the modeling results point to the inherent problem of non-uniqueness when interpreting surface observations of single events to infer complex processes at depth.
\r\n\r\nThis dissertation presents a new framework to study subduction zones and their rheological properties by extending both the time period modeled and the observations considered to all phases of the seismic cycle, on a fault interface that experiences earthquakes at multiple points in space and time. The motivating concept is that the recovery of rheological parameters could be greatly improved when considering that the constitutive laws for fault material must be able to reproduce all phases of the earthquake cycle, since it is the same physical material. Here, we (1) develop a timeseries analysis software that enables the efficient processing of large geodetic networks with long timeseries, allowing us to extract the relevant subduction-zone related signal in the observations, (2) formulate a forward model that, based on ancillary historical and seismic datasets as well as a candidate rheological model, simulates surface motion over multiple earthquake cycles, and (3) use a probabilistic inverse method to estimate the best-fitting rheological parameters given the postprocessed surface deformation timeseries and model uncertainties.
\r\n\r\nWe validate the timeseries analysis software on the transient volcanic deformation of Long Valley Caldera, California, USA, before extracting the megathrust component of the surface observations on Northern Honshu Island, Japan. We then estimate the rheological properties of the Northern Japanese subduction zone using our inversion method, simultaneously producing time-varying estimates of kinematic coupling, slip deficit, and surface deformation. Our model predictions match the pre- and postseismic displacement timeseries of the 2011 Tohoku-oki earthquake well. On the steadily creeping part of the plate interface, we infer rate-dependent frictional parameters generally increasing with depth, but with second-order along-strike variation. Finally, we discuss the potential impact of our cycle-spanning, probabilistic inversion method on the field of subduction zone studies, and present possible avenues for further improvements to our framework.
", "doi": "10.7907/sn24-zn74", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16133", "collection": "thesis", "collection_id": "16133", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07032023-235459528", "primary_object_url": { "basename": "Moncada_Rigoberto_2024_revised.pdf", "content": "final", "filesize": 62933757, "license": "other", "mime_type": "application/pdf", "url": "/16133/1/Moncada_Rigoberto_2024_revised.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Sea Ice Discrete Element Modeling: Melt and Fracture of Floes and Sheets", "author": [ { "family_name": "Moncada Lopez", "given_name": "Rigoberto", "orcid": "0000-0001-7655-5406", "clpid": "Moncada Lopez-Rigoberto" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "orcid": "0000-0003-3741-0364", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" }, { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Andrade", "given_name": "Jose E.", "orcid": "0000-0003-3741-0364", "clpid": "Andrade-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Over the last 40 years, the Arctic Ocean has experienced a significant reduction in surface area and thickness of sea ice for its minimum summer and year-round values. Sea ice, existing both as continuous ice sheets and distinct broken floes or blocks, is disappearing earlier and faster over time. These changes are largely occurring within marginal ice zones, where ice is most vulnerable to thermal forcings from the sun, oceans, and atmosphere and wind and ocean currents. Given that sea ice plays a vital role in regulating climate by delaying global energy exchanges, its loss is a vital factor in increasing global temperatures and the frequency of extreme weather events. Understanding and projecting seasonal variations in sea ice is imperative to improve climate predictions. However, many of the processes in sea ice are not fully described by most existing models, due to the limitations of continuum sea ice approaches. As a result the use of discontinuum techniques on sea ice is a very active field. In this work, we combine discrete element methods with satellite image analysis to study changes in sea ice concentration and floe size distribution during the summer melt transition for ensembles of distinct floes decaying into open waters and continuous fast sea ice sheets breaking into multiple floes. For the pure floe-like behavior, we use the `Level Set Discrete Element Method for Sea Ice' or LS-ICE method. This model can resolve individual sea ice floes with realistic shapes, and represent their physical interactions by leveraging level-set functions to detect contacts. LS-ICE can also be coupled to atmospheric and oceanic heat and momentum forcings and simulate associated melt and breakage processes. With it, we are able to reproduce sea ice concentration decline for the summers of 2018 and 2020 at Baffin Bay. Using LS-ICE we also unveil the sensitivity of sea ice loss and floe size distribution to different intensities of fracturing and ocean/solar melt and how sea ice floe size determines which is more dominant. For monolithic landfast sea ice sheets, we use a bonded particle method within the level set discrete element model called LS-DEM-BPM. We explore the relationship between landfast sea ice breakage and area decline, ocean currents and floe size distribution for a region in Fram Strait in 2023. We also replicate its fracture characteristics, using idealized pulses and arbitrary eddying ocean currents, and unveil particular combinations of wavelengths and wave speeds that facilitate breakage. Our results give new insight on sea ice melt and breakage interactions and provide a numerical framework for simulating the complete transition of sea ice from intact sheets to open oceans.", "doi": "10.7907/erqr-cr51", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16392", "collection": "thesis", "collection_id": "16392", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05202024-162817936", "primary_object_url": { "basename": "Sirorattanakul_PhDThesis_2024.pdf", "content": "final", "filesize": 188097142, "license": "other", "mime_type": "application/pdf", "url": "/16392/1/Sirorattanakul_PhDThesis_2024.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Response of Earthquakes to Transient Stresses, in Laboratory and Nature", "author": [ { "family_name": "Sirorattanakul", "given_name": "Krittanon", "orcid": "0000-0003-2310-8447", "clpid": "Sirorattanakul-Krittanon" } ], "thesis_advisor": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Zhan", "given_name": "Zhongwen", "orcid": "0000-0002-5586-2607", "clpid": "Zhan-Zhongwen" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Ross", "given_name": "Zachary E.", "orcid": "0000-0002-6343-8400", "clpid": "Ross-Z-E" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" } ], "local_group": [ { "literal": "Center for Geomechanics and Mitigation of Geohazards (GMG)" }, { "literal": "div_gps" } ], "abstract": "Earthquake rates are known to fluctuate with time according to the changing state of stress in the Earth\u2019s crust. Studying the response of earthquakes to transient stresses provides a unique insight into the mechanisms controlling the earthquake nucleation process. Common sources of transient stresses include stress changes from fault slip during large earthquakes, spontaneous slow fault slip, fluid pressure diffusion, seasonal changes of water mass and snowpacks related to hydrological cycles, tidal stresses from changes of gravitational forces of the Sun and the Moon, and anthropogenic fluid injection and extraction related to geoenergy production. In this\r\nthesis, we first start in the laboratory-scale fault and conduct friction experiments to enhance our understanding of the underlying friction laws used for modeling earthquakes. We find that the traditional view of Coulomb friction, which postulates that there exists a threshold shear force called \u201cstatic friction,\u201d below which the frictional interface remains stationary, is incorrect. Our measurements have shown that such an interface is still sliding, albeit with extremely small decaying slip rates down to 10^{\u221212} m/s. This is consistent with a more recently developed friction law, which describes friction as dependent on slip rate and the state of the interface, e.g., time since the last earthquake. Next, we move beyond the laboratory and study natural faults. In one example, we study the response of earthquakes to transient stress induced by a spontaneous slow fault slip event that preceded the earthquake swarm\r\nsequence by approximately half a day. In another example, we study the response of earthquakes to seasonal stress perturbations as a result of seasonal changes in groundwater mass and snowpack between wet and dry seasons, using California as a case study. In both examples, we find that earthquake nucleation is not an instantaneous process. Rather the earthquake rates lag after the stress rates. Such behavior cannot be described by Coulomb friction but can be quantitatively explained by the rate- and state-dependent friction. In the final example, we document bursts of fast propagating swarms of induced earthquakes at the Groningen gas field in the Netherlands. While transient stress must exist to drive the sequence, we cannot explicitly quantify the sources. Overall, our work provides key insights into the earthquake nucleation process, allowing us to better understand how to model the response of earthquakes to transient stress, including earthquakes that are induced by anthropogenic activities related to geoenergy production.", "doi": "10.7907/2fgg-0m89", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16265", "collection": "thesis", "collection_id": "16265", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12132023-225911259", "type": "thesis", "title": "Vibration Damping of Coiled Structures Through Frictional Slip", "author": [ { "family_name": "Wen", "given_name": "Alexander Huai-Cheng", "orcid": "0009-0008-9038-3039", "clpid": "Wen-Alexander-Huai-Cheng" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" }, { "family_name": "Watkins", "given_name": "Michael M.", "clpid": "Watkins-M-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Vibration management is important for the survivability of structures. The response of a structure under vibration is dependent upon interaction between the excitation environment and the properties of the structure. If the input excitation cannot be adjusted, then the structure must be engineered to survive. One approach to engineering structures to reduce vibration response is through damping, which is achieved by adding damping devices or materials to covert kinetic energy into heat, where removing energy from the system reduces the amplitude of response. There are a variety of existing vibration damping concepts and techniques, however, conventional methods of these approaches are subject to limitations such as compromising stiffness for increased damping and performance that is excitation profile dependent.
\r\n\r\nThis research proposes a novel, passive vibration damping concept which is motivated by recent deployable structures for space that use coiling as a packaging architecture. The proposed concept, referred to as \"wound roll damping\", is a friction-based damping scheme for coiled structures, where the structure is wound around a mandrel with tension that allows interlayer slip during vibration. The friction between slipping layers provides an energy dissipation mechanism, which reduces the overall level of response. The concept was developed with the challenges of mitigating spacecraft launch vibration and the limitations of conventional damping techniques in mind.
\r\n\r\nUnderstanding of the working principle and performance of this damping concept is achieved using a combination of experiments, analysis, and FEA. A method for determining the locations of slip within a wound roll under vibration is presented. This consists of modeling the interlayer friction forces, using analytical expressions for the stress fields that arise during tension winding of wound rolls, and comparing these values against loading estimates obtained from analysis and FEA. The locations of slip for wound rolls supported by a cantilevered mandrel with bending vibration modes are towards the root of the wound roll structure, near the inner layers.
\r\n\r\nExperimental studies that demonstrate the performance and properties of this damping concept are presented in this work. A wound roll test sample is subjected to a range of excitation profiles including: sine sweep, sine dwell, random, and shock with varying levels of sample winding tension and excitation amplitude. Using these experiments, this concept is demonstrated to not be subject to the limitations of conventional damping schemes. This scheme is observed to be capable of significantly increasing the overall stiffness while providing elevated damping levels, with a performance that is tunable with winding tension, independent of excitation profile, and scales with excitation amplitude. The locations of slip are observed to be consistent with predictions from FEA and analysis.
\r\n\r\nTwo approaches to simulate and model the wound roll damper are developed to both better understand the physical mechanism of this concept and provide analysis tools. The first method is an FEA model, consisting of the base vibration of concentric shells and solids that have frictional contact interactions. The second method is a 2-DoF reduced order model that simulates the frictional contact between two mass-spring-damper systems. Both methods are demonstrated to have good correlation with experimental measurements.\r\n
\r\n\r\nA majority of this work demonstrates the performance of this concept, using both experiments and simulation at lab scales. This work also presents simulation studies that demonstrate the viability of this concept at realistic scales. Using simulations scaled to recent coilable space structures, both implemented and proposed, the wound roll damping concept is demonstrated to provide significant stiffness and damping.
", "doi": "10.7907/gvps-8x65", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:14998", "collection": "thesis", "collection_id": "14998", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08082022-055217161", "primary_object_url": { "basename": "Oliver_Stephenson_Thesis_2023.pdf", "content": "final", "filesize": 119182132, "license": "other", "mime_type": "application/pdf", "url": "/14998/1/Oliver_Stephenson_Thesis_2023.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Investigating the Earthquake Cycle on Multiple Temporal and Spatial Scales Using Satellites and Simulations", "author": [ { "family_name": "Stephenson", "given_name": "Oliver Laurent", "orcid": "0000-0002-5509-090X", "clpid": "Stephenson-Oliver-Laurent" } ], "thesis_advisor": [ { "family_name": "Simons", "given_name": "Mark", "orcid": "0000-0003-1412-6395", "clpid": "Simons-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Ross", "given_name": "Zachary E.", "orcid": "0000-0002-6343-8400", "clpid": "Ross-Z-E" }, { "family_name": "Simons", "given_name": "Mark", "orcid": "0000-0003-1412-6395", "clpid": "Simons-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Clayton", "given_name": "Robert W.", "orcid": "0000-0003-3323-3508", "clpid": "Clayton-R-W" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The motion of the Earth's tectonic plates creates a gradual accumulation of stress at their boundaries, followed by a rapid release in earthquakes, a process known as the earthquake cycle. Studying this process is important because of the hazards earthquakes pose, but presents challenges due to the multi-scale nature of the problem\u2014stresses build up over hundreds to thousands of years, while earthquakes break narrow fault zones in a matter of seconds. In this thesis, we combine a variety of techniques to study the earthquake cycle on multiple temporal and spatial scales, including satellite-based interferometric synthetic aperture radar (InSAR) to observe the slow deformation of the Earth over wide areas, and high-performance computational simulations to model faults during earthquakes. We begin by presenting a method for removing the signal of plate-tectonic motion in large-scale InSAR measurements, allowing for better observation of small ground deformations. We then use these corrections to study the Makran subduction zone, on the Iran-Pakistan border. Our InSAR-derived ground velocity map can resolve motions at the level of millimeters per year over an area of nearly one million square kilometers, and we use it to place constraints on the degree of coupling on the subduction megathrust. Next, we show how InSAR can be combined with deep learning techniques to rapidly map earthquake damage in all weather conditions, day and night. Such products will hopefully prove useful in future disaster response. Finally, we present computational simulations of dynamic earthquake ruptures with enhanced dynamic weakening due to thermal pressurization. We apply our simplified model to the creeping section of the San Andreas Fault, which is generally thought to be a barrier to earthquake rupture. Our results show how thermal pressurization can allow earthquakes to propagate partially or completely through the creeping section for a range of physically reasonable parameters. Our work illustrates how results from multiple fields can be combined to deliver new insights into the earthquake cycle and the hazards that it poses.", "doi": "10.7907/ha9m-4p17", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14984", "collection": "thesis", "collection_id": "14984", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07202022-212008345", "primary_object_url": { "basename": "korner_kevin_2022_thesis_full.pdf", "content": "final", "filesize": 23760871, "license": "other", "mime_type": "application/pdf", "url": "/14984/3/korner_kevin_2022_thesis_full.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Modeling Deformations of Active Rods, Ribbons, and Plates", "author": [ { "family_name": "Korner", "given_name": "Kevin Andreas", "orcid": "0000-0002-2967-9657", "clpid": "Korner-Kevin-Andreas" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Slender structures are mechanical components which have at least one spatial dimension much smaller than another. Some canonical examples are beams, rods, ribbons, plates, and shells. Although these systems have been studied for many centuries, the focus of development has generally been limited to small strains and the onset of buckling modes. Outside of this regime, both geometric and material non-linearities contribute significant complexity to the analytical and computational techniques which can be applied to these problems. Despite this, large deformations demonstrate tremendous potential in engineering applications, particularly with soft materials. This thesis examines various methods of modeling slender structures. We focus on large strain behaviors, often accentuated by spontaneous strains generated with active materials. These systems demonstrate a wide range of interesting and useful behaviors, such as bifurcations, snap-through, and cyclic deformations.
", "doi": "10.7907/2zb0-m166", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15223", "collection": "thesis", "collection_id": "15223", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292023-160132013", "primary_object_url": { "basename": "Reddy_Narravula_PhD_thesis.pdf", "content": "final", "filesize": 73899207, "license": "other", "mime_type": "application/pdf", "url": "/15223/1/Reddy_Narravula_PhD_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Folding and Dynamic Deployment of Ultralight Thin-Shell Space Structures", "author": [ { "family_name": "Reddy", "given_name": "Narravula Harshavardhan", "orcid": "0000-0003-3897-8162", "clpid": "Reddy-Narravula-Harshavardhan" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Thin-shell structures are becoming increasingly popular for space missions due to their high stiffness-to-mass ratio, easy folding and coiling, and self-deployment using stored strain energy. Broadly, two deployment strategies exist: 1) controlled or deterministic, and 2) unconstrained. Controlled deployment involves carefully orchestrated events using control or guidance systems, while in unconstrained deployment, the structure is simply allowed to self-deploy with minimal guidance. Unconstrained deployment offers lighter deployment mechanisms and better packaging efficiency but the unpredictability of this process has been a significant obstacle to its adoption.
\r\n\r\nThis study focuses on demonstrating the predictability of unconstrained dynamic deployment of thin-shell structures, using the Caltech Space Solar Power Project (SSPP) structures as a case study. The Caltech SSPP uses composite triangular rollable and coilable longerons as the primary building blocks to create large bending-stiff structures. The specific objective is to improve the predictability and robustness of the unconstrained dynamic deployment of the Caltech SSPP structures. Deployment is influenced by the initial conditions and the interaction between the structure and the mechanism during the deployment. To understand these effects, high-fidelity numerical simulations are developed and validated against experiments. The study also examines the sensitivity of deployment characteristics to various design parameters and external influences to ensure the robustness of deployment.
\r\n\r\nThis research demonstrates that the interaction between the structure and the deployment mechanism must be minimal to ensure the predictability of deployment, as thin-shell structures can self-deploy using stored strain energy. This study's sensitivity analysis will inform the design of future SSPP deployment mechanisms and structures. Additionally, the numerical simulation techniques developed have broader applicability beyond this specific case study to any deployable thin-shell structure.
\r\n\r\nDue to the large aspect ratios of thin-shell structures, a very fine finite element mesh is required to model them accurately. A dense finite element mesh is also required to model the contact interactions between the structure and the rigid components of the deployment mechanism. As large spacecraft structures become increasingly complex, full-scale numerical modeling becomes impractical, necessitating the search for more computationally efficient finite element methods. In this study, NURBS-based isogeometric analysis is explored, and it is shown that it is not yet worth switching to NURBS-based elements for the analysis of thin-shell deployable structures. In addition, h-adaptive meshing for quadrilateral shell elements is investigated, and more efficient refinement indicators and solution mapping techniques for nonlinear analyses are proposed and their superior performance is demonstrated using a test case of quasi-static folding of a tape spring.
\r\n\r\nThis thesis fills a gap in the literature on unconstrained dynamic deployment of space structures, providing crucial insights and numerical modeling tools for further research. It establishes a knowledge and resource foundation to advance space structure design and promote more frequent use of unconstrained deployment, marking a pivotal contribution to the field and enabling safe and efficient space structure deployment. Furthermore, the study provides insights into more computationally efficient finite element methods, such as h-adaptive meshing. These insights are broadly applicable and can inform the design of future deployable structures beyond the tested cases.
", "doi": "10.7907/m7rd-6s86", "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: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:15246", "collection": "thesis", "collection_id": "15246", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312023-212652388", "primary_object_url": { "basename": "EricOcegueda_2023Thesis.pdf", "content": "final", "filesize": 66084671, "license": "other", "mime_type": "application/pdf", "url": "/15246/2/EricOcegueda_2023Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Physics-Based and Data-Driven Computational Models of Inelastic Deformations", "author": [ { "family_name": "Ocegueda", "given_name": "Eric", "orcid": "0000-0001-7845-6890", "clpid": "Ocegueda-Eric" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Stuart", "given_name": "Andrew M.", "orcid": "0000-0001-9091-7266", "clpid": "Stuart-A-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Crystalline materials inevitably exhibit inelastic deformation when applied to large enough loads. The behavior in this inelastic regime is a coupling of physics across several length scales: from initiating as defects at the atomic scale, interacting with crystal defects, and finally spanning multiple grains and influencing macroscopic stress behavior. These length-scale interactions make predicting material response an open challenge and an avenue for leveraging microscale physics for material design. This thesis examines developing physics-based and data-driven computational models to capture complex inelastic behavior at appropriate length scales.
\r\n\r\nFirst, we present a mesoscale model for capturing deformation twinning physics at the polycrystal scale. Mechanical twinning is a form of inelastic deformation observed in low-symmetry crystals, such as magnesium and other hexagonal close-packed (hcp) metals. Twinning, unlike slip, forms as bands collectively across grains with complex local morphology propagating into bulk behavior, drastically affecting strength and ductility. We, thus, propose a model where twinning is treated using a phase-field approach, while dislocation slip is considered using crystal plasticity. Lattice reorientation, length-scale effects, interactions between dislocations and twin boundaries, and twin and slip interactions with grain boundaries are all carefully considered. We first outline the model and its implementation using a novel approach of accelerated computational micromechanics in a two-dimensional, single twin-slip system, polycrystal case to demonstrate its capabilities. Finally, we consider multiple twin-slip systems and conduct three-dimensional simulations of polycrystalline magnesium. We summarize the insights gained from these studies and the implications on the macroscale behavior of hcp materials.
\r\n\r\nThe second part of the thesis focuses on data-driven models for capturing microscopic history-dependent phenomena for multiscale modeling applications. The multiscale modeling framework has seen increased usage over the last few decades for its ability to capture complex material behavior over a range of time/length scales by solving a macroscale problem directly with a constitutive relation defined implicitly by the solution of a microscale problem. However, this implementation is computationally expensive -- needing to solve a microscale problem at each point and time of the macroscopic calculation. In this study, we examine the use of machine learning by utilizing data generated through repeated solutions of a microscale problem to: (i) gain insights into the history dependent macroscopic internal variables that govern the response and (ii) create a computationally efficient surrogate. We do so by introducing a recurrent neural operator, which can provide accurate approximations of the stress response and insights into the physics of the macroscopic problem. We illustrate these capabilities on a laminate composite and polycrystal made of elasto-viscoplastic materials, summarize insights on the learned internal variables, and accuracy of stress predictions.
", "doi": "10.7907/3gqd-zp93", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14507", "collection": "thesis", "collection_id": "14507", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02232022-193800084", "primary_object_url": { "basename": "Kavya_thesis_2022.pdf", "content": "final", "filesize": 39218273, "license": "other", "mime_type": "application/pdf", "url": "/14507/1/Kavya_thesis_2022.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Slip Patterns on Heterogeneous Frictional Interfaces", "author": [ { "family_name": "Sudhir", "given_name": "Kavya", "orcid": "0000-0001-6673-0979", "clpid": "Sudhir-Kavya" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Understanding the implications of heterogeneity on frictional interfaces for the resulting slip patterns is a challenging, highly nonlinear, and dynamic problem with special relevance to earthquake source processes. Natural fault surfaces are rarely homogeneous and host a spectrum of slip behaviors in response to slow tectonic loading where slow steady slip and earthquake ruptures are just the end members. Understanding how heterogeneous frictional properties translate into different slip patterns would enable us to constrain the heterogeneity of natural faults and get an insight into processes that are difficult to observe in the field such as earthquake nucleation, with important implications for the assessment of seismic hazard.
\r\n\r\nIn this thesis, we advance our understanding of fault heterogeneity and its effects by conducting numerical simulations of long-term slip histories on heterogeneous frictional interfaces. We first focus on how irregular fault geometry affects the variability in repeating sequences by investigating a specific example of the SF-LA repeaters in the Parkfield segment of the San Andreas Fault (SAF) in California. We then investigate the effect of increasing heterogeneity in the effective normal stress on earthquake nucleation processes, complexity of earthquake sequences, and features of larger-scale ruptures. In both cases, we incorporate the heterogeneity in physical properties into 2D planar faults governed by rate-and-state friction and embedded into 3D homogeneous elastic bulk. Fully dynamic simulations are used to numerically solve the resulting elastodynamic problems with friction as a nonlinear boundary condition.
\r\n\r\nOur models reproduce many observations about SF-LA repeating sequences, in- cluding their mean moment, mean recurrence times, stress drops, the observed non- trivial scaling between the seismic moment and recurrence times of the repeaters, the ranges of variability in moment and recurrence time, and the ranges of triggering times between the two sequences. Multiple models produce slip behaviors com- parable to observations, indicating that the models cannot be uniquely constrained based on available observations. We also study how small-scale features of hetero- geneity affect model response. We find that smoothing the distribution over scales smaller than governing length scales in the problem, such as the nucleation size in our case, changes the specific evolution of slip, but preserves its key characteristics, such as the range of event variability and triggering times between events. However, smoothing the distribution on larger scales modifies the response qualitatively.
\r\n\r\nOur study of the earthquake initiation processes on interfaces with normal stress heterogeneity reveals that systematic increase in heterogeneity induces a continuum of behaviors, ranging from purely fault-spanning events to persistent foreshock-like events interspersed between fault-spanning mainshocks. In models with strong heterogeneity, most smaller-scale and larger-scale events initiate from scales much smaller than the nucleation size estimates calculated for uniform interfaces with equivalent average properties. While the variations in normal stress induce inversely proportional variations in the instability length scale often called nucleation size, we find that the nucleation-size variations by themselves are insufficient to cause such behavior, and that the associated strong heterogeneity in frictional strength is also required. In models with uniform friction strength but the same nucleation-size variation, the nucleation processes of larger-scale events are similar to those on uniform interfaces, with an addition of multiple triggered small-scale earthquakes. Our simulations show that several hypothesized scenarios of earthquake nucleation and foreshocks on natural faults may be viable and reflect different types and levels of heterogeneity on different faults the effects of which, in addition, vary as fault conditions evolve. For example, even with strong fault heterogeneity, some large- scale events have foreshocks and some do not, in the same simulation.
\r\n\r\nThe increasing fault heterogeneity generally leads to increasing complexity of the resulting earthquake sequences and moment-rate release (also called source-time function) of large-scale, fault-spanning events, as intuitively expected, although with some saturation at the higher heterogeneity levels. We find that, in the presence of significant normal-stress heterogeneity, source-time functions of many larger-scale events exhibit prolonged seismic initiation phases, similar to some observations, as the events nucleate from the heterogeneity scale and re-rupture the areas pres-lipped quasi-statically and in foreshocks. The source-time functions also reveal that larger-scale events in our models -- that are arrested by velocity-strengthening barriers -- have a more abrupt arrest phase than natural earthquakes, which places constraints on rupture-arresting mechanisms that should be used in modeling. The initial moment rates are similar for events of different eventual sizes on interfaces with strong heterogeneity, implying that, in those cases, large events are just small events that ran away.
", "doi": "10.7907/xkbp-ks08", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14651", "collection": "thesis", "collection_id": "14651", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302022-071239478", "primary_object_url": { "basename": "PhD_thesis_Stacy_Larochelle.pdf", "content": "final", "filesize": 94832965, "license": "other", "mime_type": "application/pdf", "url": "/14651/1/PhD_thesis_Stacy_Larochelle.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Mechanical Interactions Between Water and the Solid Earth: from Quasi-Static Geodetic Deformation to Dynamic Fault Slip", "author": [ { "family_name": "Larochelle", "given_name": "Stacy", "orcid": "0000-0001-6161-5605", "clpid": "Larochelle-Stacy" } ], "thesis_advisor": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Ross", "given_name": "Zachary E.", "orcid": "0000-0002-6343-8400", "clpid": "Ross-Z-E" }, { "family_name": "Clayton", "given_name": "Robert W.", "orcid": "0000-0003-3323-3508", "clpid": "Clayton-R-W" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "Center for Geomechanics and Mitigation of Geohazards (GMG)" }, { "literal": "div_gps" } ], "abstract": "Mechanical interactions between Earth's solid interior and its hydrosphere are central to many geophysical problems of crucial societal importance: Changing conditions in the global water cycle deform the solid Earth; the groundwater storage capacity of aquifer systems is controlled by its interaction with geological materials; and crustal water - either natural occurring or added through anthropogenic activities - affects earthquakes and fault slip processes. In this thesis, we investigate some of these interactions by harnessing recent developments in the fields of satellite geodesy, statistical data analysis and elastodynamic earthquake modelling. We start by developing a procedure to identify and extract seasonal deformation signals associated with hydrological loading of the solid Earth from geodetic time series in Chapter 1. In Chapters 2 and 3, we consider the examples of the Ozarks Plateau (central United States) and Sacramento Valley (California) to establish a methodology for characterizing poroelastic deformation arising from groundwater variations with space-based geodesy. Then, in Chapter 4, we develop a model to simulate fault slip due to crustal water injections and calibrate it against a well-instrumented field experiment on a natural fault. We conclude by deriving a theoretical understanding of these fault slip simulations by considering the simple case of a fixed-length pressurized zone in Chapter 5. Overall, our work provides key insights for extracting and using different sources of hydrogeodetic signals as well as for modeling and understanding fluid-induced fault slip processes, which is becoming increasingly important in a world faced with water scarcity, a changing climate and an increased reliance on groundwater and geoenergy resources.
", "doi": "10.7907/2r5a-9277", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14507", "collection": "thesis", "collection_id": "14507", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02232022-193800084", "primary_object_url": { "basename": "Kavya_thesis_2022.pdf", "content": "final", "filesize": 39218273, "license": "other", "mime_type": "application/pdf", "url": "/14507/1/Kavya_thesis_2022.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Slip Patterns on Heterogeneous Frictional Interfaces", "author": [ { "family_name": "Sudhir", "given_name": "Kavya", "orcid": "0000-0001-6673-0979", "clpid": "Sudhir-Kavya" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Understanding the implications of heterogeneity on frictional interfaces for the resulting slip patterns is a challenging, highly nonlinear, and dynamic problem with special relevance to earthquake source processes. Natural fault surfaces are rarely homogeneous and host a spectrum of slip behaviors in response to slow tectonic loading where slow steady slip and earthquake ruptures are just the end members. Understanding how heterogeneous frictional properties translate into different slip patterns would enable us to constrain the heterogeneity of natural faults and get an insight into processes that are difficult to observe in the field such as earthquake nucleation, with important implications for the assessment of seismic hazard.
\r\n\r\nIn this thesis, we advance our understanding of fault heterogeneity and its effects by conducting numerical simulations of long-term slip histories on heterogeneous frictional interfaces. We first focus on how irregular fault geometry affects the variability in repeating sequences by investigating a specific example of the SF-LA repeaters in the Parkfield segment of the San Andreas Fault (SAF) in California. We then investigate the effect of increasing heterogeneity in the effective normal stress on earthquake nucleation processes, complexity of earthquake sequences, and features of larger-scale ruptures. In both cases, we incorporate the heterogeneity in physical properties into 2D planar faults governed by rate-and-state friction and embedded into 3D homogeneous elastic bulk. Fully dynamic simulations are used to numerically solve the resulting elastodynamic problems with friction as a nonlinear boundary condition.
\r\n\r\nOur models reproduce many observations about SF-LA repeating sequences, in- cluding their mean moment, mean recurrence times, stress drops, the observed non- trivial scaling between the seismic moment and recurrence times of the repeaters, the ranges of variability in moment and recurrence time, and the ranges of triggering times between the two sequences. Multiple models produce slip behaviors com- parable to observations, indicating that the models cannot be uniquely constrained based on available observations. We also study how small-scale features of hetero- geneity affect model response. We find that smoothing the distribution over scales smaller than governing length scales in the problem, such as the nucleation size in our case, changes the specific evolution of slip, but preserves its key characteristics, such as the range of event variability and triggering times between events. However, smoothing the distribution on larger scales modifies the response qualitatively.
\r\n\r\nOur study of the earthquake initiation processes on interfaces with normal stress heterogeneity reveals that systematic increase in heterogeneity induces a continuum of behaviors, ranging from purely fault-spanning events to persistent foreshock-like events interspersed between fault-spanning mainshocks. In models with strong heterogeneity, most smaller-scale and larger-scale events initiate from scales much smaller than the nucleation size estimates calculated for uniform interfaces with equivalent average properties. While the variations in normal stress induce inversely proportional variations in the instability length scale often called nucleation size, we find that the nucleation-size variations by themselves are insufficient to cause such behavior, and that the associated strong heterogeneity in frictional strength is also required. In models with uniform friction strength but the same nucleation-size variation, the nucleation processes of larger-scale events are similar to those on uniform interfaces, with an addition of multiple triggered small-scale earthquakes. Our simulations show that several hypothesized scenarios of earthquake nucleation and foreshocks on natural faults may be viable and reflect different types and levels of heterogeneity on different faults the effects of which, in addition, vary as fault conditions evolve. For example, even with strong fault heterogeneity, some large- scale events have foreshocks and some do not, in the same simulation.
\r\n\r\nThe increasing fault heterogeneity generally leads to increasing complexity of the resulting earthquake sequences and moment-rate release (also called source-time function) of large-scale, fault-spanning events, as intuitively expected, although with some saturation at the higher heterogeneity levels. We find that, in the presence of significant normal-stress heterogeneity, source-time functions of many larger-scale events exhibit prolonged seismic initiation phases, similar to some observations, as the events nucleate from the heterogeneity scale and re-rupture the areas pres-lipped quasi-statically and in foreshocks. The source-time functions also reveal that larger-scale events in our models -- that are arrested by velocity-strengthening barriers -- have a more abrupt arrest phase than natural earthquakes, which places constraints on rupture-arresting mechanisms that should be used in modeling. The initial moment rates are similar for events of different eventual sizes on interfaces with strong heterogeneity, implying that, in those cases, large events are just small events that ran away.
", "doi": "10.7907/xkbp-ks08", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14168", "collection": "thesis", "collection_id": "14168", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05202021-190145895", "primary_object_url": { "basename": "Thesis-ValereLambert.pdf", "content": "final", "filesize": 170625558, "license": "other", "mime_type": "application/pdf", "url": "/14168/1/Thesis-ValereLambert.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Constraining Earthquake Source Processes Through Physics-Based Modeling", "author": [ { "family_name": "Lambert", "given_name": "Val\u00e8re R\u00e9gis Westbrooke", "orcid": "0000-0002-6174-9651", "clpid": "Lambert-Valere-Regis-Westbrooke" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Simons", "given_name": "Mark", "orcid": "0000-0003-1412-6395", "clpid": "Simons-M" }, { "family_name": "Zhan", "given_name": "Zhongwen", "orcid": "0000-0002-5586-2607", "clpid": "Zhan-Zhongwen" } ], "local_group": [ { "literal": "Center for Geomechanics and Mitigation of Geohazards (GMG)" }, { "literal": "div_gps" } ], "abstract": "Determining principles and conditions governing motion along faults is crucial for assessing how earthquake ruptures start and how large they may ultimately become. This thesis aims to shed light on the physics governing earthquake source processes by (i) developing physics-based numerical models that combine geological observations and laboratory insight with theoretical developments, and (ii) using these models to examine how different physical mechanisms and conditions are reflected in a range of geophysical observations taken together, from heat-flow constraints and seismologically determined properties of earthquakes to geodetic inferences and earthquake frequency-magnitude statistics.
\r\n\r\nWe examine the behavior and observable characteristics of numerically simulated sequences of earthquakes and aseismic slip in fault models designed to reproduce well-known features of mature faults that produce large destructive earthquakes. In part, the models are consistent with the inferred low-stress, low-heat operation of mature faults, which host large earthquakes at much lower levels of stress than their expected static strength. We explore two potential explanations for such behavior, one that faults are indeed quasi-statically strong but experience dramatic weakening during earthquakes, or that faults are persistently weak, e.g., due to fluid overpressure. We find that the two classes of fault models can, in principle, be distinguished based on the amount of seismic energy radiated from earthquake ruptures. Dynamic ruptures in the form of self-healing pulses, which occur on quasi-statically strong but dynamically weak faults, result in much larger radiated energy than inferred teleseismically for megathrust events, whereas crack-like ruptures on persistently weak faults are consistent with the seismological observations. The larger radiated energy of self-healing pulses is similar to limited regional inferences for crustal strike-slip faults. Our results suggest that re-evaluating estimates of radiated energy and static stress drop would provide substantial insight into the driving physics of large earthquakes and the absolute stress conditions on faults, with potential differences between tectonic settings.
\r\n\r\nThe results also have significant implications for seismic hazard, since our modeling shows that fault models that experience efficient dynamic weakening during ruptures tend to predominantly produce large earthquakes, at the expense of smaller earthquakes. Such behavior is consistent with some mature fault segments, such as several segments of the San Andreas Fault in California that have hosted large earthquakes but are currently nearly seismically quiescent. These considerations can provide physical basis for improving earthquake early warning systems. If mature faults in California are indeed governed by enhanced dynamic weakening, then our results suggest that the likelihood of an earthquake on these faults becoming substantially larger is much higher than typical expectations based on Gutenberg-Richter statistics.
\r\n\r\nBy considering average fault stress before simulated earthquake ruptures, we find that critical stress conditions for earthquake occurrence depend on the size and style of motion (e.g. the degree of slip acceleration at the rupture front) during individual ruptures. In particular, the stress conditions required to propagate large earthquake ruptures can be considerably lower than those required for rupture nucleation, and standard notions of quasi-static fault strength based on laboratory studies. Our results demonstrate that the critical stress for earthquake occurrence is not governed by a simple condition such as a certain level of Coloumb stress, as commonly used in studies of stress interactions among faults and earthquake aftershocks patterns. More robust criteria for critical stress conditions would depend on the strength evolution during dynamic rupture and can be explored in numerical simulations.
\r\n\r\nFinally, evaluating the predictive power of numerical earthquake models for future hazards is a topic of great importance for physics-based seismic hazard assessment. Towards that end, we investigate the sensitivity of outcomes from numerical simulations of sequences of earthquakes and aseismic slip, including the long-term interaction of fault segments, to choices in numerical discretization and treatment of inertial, wave-mediated effects. In particular, we find that the rate of earthquake ruptures that manage to jump between two fault segments, a parameter routinely used in seismic hazard studies, is highly sensitive to numerical and physical modeling choices. These results suggest the need for developing different parameterization of seismic hazard than currently used, a task for which numerical modeling is well-suited.
", "doi": "10.7907/7s93-k485", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14213", "collection": "thesis", "collection_id": "14213", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012021-002457442", "primary_object_url": { "basename": "Antonio_Pedivellano-Thesis_2020.pdf", "content": "final", "filesize": 45771850, "license": "other", "mime_type": "application/pdf", "url": "/14213/1/Antonio_Pedivellano-Thesis_2020.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Deployment Dynamics of Thin-Shell Space Structures", "author": [ { "family_name": "Pedivellano", "given_name": "Antonio", "orcid": "0000-0003-2321-7301", "clpid": "Pedivellano-Antonio" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Thin-shell structures provide a lightweight solution for deployable structure applications. Despite being only few tens of microns thick, these structures provide excellent bending stiffness, thanks to their curved cross-section. Their thinness also allows them to be elastically packaged into small volumes to fit into a launch vehicle; once in space, they can be self-deployed by releasing their stored elastic energy.
\r\n\r\nMost space applications use thin-shell structures to deploy and tension thin membranes, such as solar sails, drag sails, and solar arrays. Recently, a novel space solar power architecture has been developed at Caltech, and it relies on distributed thin-shell components, connected in a space frame, to create large-area deployable structures. Thanks to the unique properties of thin shells, these structure provide superior stiffness-to-mass ratio and self-deployment capabilities. However, to demonstrate their reliability and enable their use on space missions, their deployment dynamics must be understood and predicted.
\r\n\r\nGround testing is the established approach to verify a structure throughout its design and qualification process. However, replicating the space environment in a laboratory setting is generally not possible, especially for lightweight structures, which are very sensitive to the effects of gravity and air. Numerical models are therefore the only tool to predict the behavior of a structure in space. However, validation with ground experiments is necessary to build confidence in the models, which must be able to capture the complexity of the interaction with air, gravity, and the suspension system that supports the weight of the structure.
\r\n\r\nThe goal of this thesis is to develop high-fidelity models for large space structures, where multiple thin-shell components are folded together and deploy by releasing their strain energy. This overall objective is achieved in 3 steps. First, a ladder-type rectangular strip is introduced, as a building block for more complex architectures. The strip is composed by two thin-shell longerons, symmetrically folded at two locations. The deployment dynamics of this structure is investigated through experiments on 1 m-scale prototypes, both in air and in vacuum. A detailed analysis of its elastic folds is performed using full-field displacement measurements from Digital Image Correlation. A finite element model of this strip is presented, and it is shown to accurately capture the dynamics of the strip for all tested conditions. Then, the implementation of the packaging and deployment scheme of a space solar power spacecraft, composed of multiple strips, is discussed. A kinematic model of the structure is proposed as a design tool to achieve systematic folding. A novel concept of a deployment mechanism to coil the structure in a robust and reliable way is proposed. Also, a staged deployment scheme is demonstrated, to reduce the uncertainty of strain-energy deployment for large space structures. Finally, the deployment dynamics of a 2 m-scale space structural prototype, based on the space solar power architecture, is investigated. A full-scale finite element model of the structure is implemented to replicate its complex folding scheme and capture the deployment process, including the interaction with the deployment mechanism and the suspension system. The simulations predict well the behavior of the structure observed in experiments through motion capture techniques.
\r\n\r\nThe work presented in this thesis advances previous studies on the deployment dynamics of simple thin-shell components, and demonstrates that even complex thin-shell architectures can be packaged and deployed in a controlled and predictable way. The solutions proposed in this thesis have guided the packaging process and the design of the deployment mechanism for DOLCE, an upcoming flight demonstration of the space solar power architecture described in this work. However, this research has much broader implications, as the experimental and numerical framework presented herein can be generalized to different shell-based architectures, and contributes to enabling a new generation of lightweight deployable structures for future space applications.
", "doi": "10.7907/4zbq-g037", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14036", "collection": "thesis", "collection_id": "14036", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12182020-181342301", "type": "thesis", "title": "Multiscale, Data-Driven and Nonlocal Modeling of Granular Materials", "author": [ { "family_name": "Karapiperis", "given_name": "Konstantinos", "orcid": "0000-0002-6796-8900", "clpid": "Karapiperis-Konstantinos" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Granular materials are ubiquitous in both nature and technology. They play a key role in many applications ranging from storing food and energy to building reusable habitats and soft robots. Yet, predicting the continuum mechanical response of granular materials continues to present extraordinary challenges, despite the apparently simple laws that govern particle-scale interactions. This is largely due to the complex history dependence arising from the continuous rearrangement of their internal structure, and the nonlocality emerging from their self-organization. There is clearly an urge to develop methods that adequately address these two aspects, while bridging the long-standing divide between the grain- and the continuum scale.
\r\n\r\nThis dissertation introduces novel theoretical and computational approaches for behavior prediction in granular solids. To begin with, we develop a framework for investigating their incremental behavior from the perspective of plasticity theory. It relies on systematically probing, through level-set discrete element calculations, the response of granular assemblies from the same initial state to multiple directions is stress space. We then extract the state- and history-dependent elasticity and plastic flow, and investigate the evolution of pertinent internal variables. We specifically study assemblies of sand particles characterized by X-ray computed tomography, as well as morphologically simpler counterparts of the same systems. Naturally arising from this investigation is the concept of a granular genome. Next, inspired by the abundance of generated high-fidelity micromechanical data, we develop an alternative data-driven approach for behavior prediction. This new multiscale modeling paradigm completely bypasses the need to define a constitutive law. Instead, the problem is directly formulated on a material data set, generated by grain-scale calculations, while pertinent constraints and conservation laws are enforced. We particularly focus on the sampling of the mechanical phase space, and develop two methods for parametrizing material history, one thermodynamically motivated and one statistically inspired. In the remainder of the thesis, we direct our attention to the understanding and modeling of nonlocality. We base our investigation on data derived from a discrete element simulation of a sample of sand subjected to triaxial compression and undergoing shear banding. By representing the granular system as a complex network, we study the self-organized and cooperative evolution of topology, kinematics and kinetics within the shear band. We specifically characterize the evolution of fundamental topological structures called force cycles, and propose a novel order parameter for the system, the minimal cycle coefficient. We find that this coefficient governs the stability of force chains, which succumb to buckling as they grow beyond a characteristic maximum length. We also analyze the statistics of nonaffine kinematics, which involve rotational and vortical particle motion. Finally, inspired by these findings, we extend the previously introduced data-driven paradigm to include nonaffine kinematics within a weakly nonlocal micropolar continuum description. By formulating the problem on a phase space augmented by higher-order kinematics and their conjugate kinetics, we bypass for the first time the need to define an internal length scale, which is instead discovered from the data. By carrying out a data-driven prediction of shear banding, we find that this nonlocal extension of the framework resolves the ill-posedness inherent to the classical continuum description. Finally, by comparing with available experimental data on the same problem, we are able to validate our theoretical developments.
", "doi": "10.7907/7rtg-x780", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:11744", "collection": "thesis", "collection_id": "11744", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07202019-135213721", "primary_object_url": { "basename": "main-bibtex -schill-7-22-2019-final.pdf", "content": "final", "filesize": 19557065, "license": "other", "mime_type": "application/pdf", "url": "/11744/2/main-bibtex -schill-7-22-2019-final.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Variational and Multiscale Modeling of Amorphous Silica Glass", "author": [ { "family_name": "Schill", "given_name": "William Joseph", "orcid": "0000-0003-0950-7433", "clpid": "Schill-William-Joseph" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Stainier", "given_name": "Laurent F.", "clpid": "Stainier-Laurent-F" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "We develop a critical-state model of fused silica plasticity on the basis of data mined from molecular dynamics (MD) calculations. The MD data is suggestive of an irreversible densification transition in volumetric compression resulting in permanent, or plastic, densification upon unloading. Moreover, this data exhibits dependence on temperature and the rate of deformation. We show that these characteristic behaviors are well-captured by a critical state model of plasticity, where the densification law for glass takes the place of the classical consolidation law of granular media and the locus of constant volume states denotes the critical-state line. A salient feature of the critical-state line of fused silica, as identified from the MD data, that renders its yield behavior anomalous is that it is strongly non-convex, owing to the existence of two well-differentiated phases at low and high pressures. We argue that this strong non-convexity of yield explains the patterning that is observed in molecular dynamics calculations of amorphous solids deforming in shear. We employ an explicit and exact rank-2 envelope construction to upscale the microscopic critical-state model to the macroscale. Remarkably, owing to the equilibrium constraint the resulting effective macroscopic behavior is still characterized by a non-convex critical-state line. Despite this lack of convexity, the effective macroscopic model is stable against microstructure formation and defines well-posed boundary-value problems. We present examples of ballistic impact of silica glass rods by way of the optimal transport meshfree method. We extend the study of the inelastic behavior of silica glass to include the effect of many different temperatures, pressures, and strain rates using MD and maximum entropy atomistics (MXE) calculations. Owing to the temperature dependence of the model, the macroscopic model becomes unstable against adiabatic shear localization. Thus, the material adopts small inter-facial regions where the shear strain is extremely high. We characterize the shear band size, thereby predicting a yield knockdown factor at the macroscale, and compare the results to behavior reported in flyer plate impact experiments.
", "doi": "10.7907/B2A9-RQ38", "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:11571", "collection": "thesis", "collection_id": "11571", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302019-150220368", "type": "thesis", "title": "Improving Site Response Analysis for Earthquake Ground Motion Modeling", "author": [ { "family_name": "Shi", "given_name": "Jian", "orcid": "0000-0002-1969-7579", "clpid": "Shi-Jian" } ], "thesis_advisor": [ { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Graves", "given_name": "Robert W.", "clpid": "Graves-R-W" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The modeling of earthquake-induced ground motions plays an important role in the quantification of seismic hazards, which contributes to the ultimate goal of saving lives and reducing economic loss. Site response is a natural phenomenon in which soils in the earth\u2019s shallow crust alter the amplitude, frequency content, and duration of earthquake-induced ground motions. Therefore, improvements in the research of site response directly contribute to ground motion modeling, and eventually to seismic hazard quantification.
\r\n\r\nThis thesis presents two models that advance the current research in site response.
\r\n\r\nThe first model provides a tool to predict near-surface shear-wave velocity profiles from Vs30 (a proxy that represents the general stiffness of a site). This model bridges the gap between the lack of information about near-surface soil properties and the need to model site response on a regional scale (city, county, or above).
\r\n\r\nThe second model is a stress-strain model for describing 1D shearing behaviors of soils. It is capable of capturing both the small-strain and the large-strain behaviors, which makes it suitable for modeling very strong ground motions. More importantly, this model enables seismologists to construct stress-strain curves from only shear-wave velocity information, again improving our ability to model site response on a regional scale. Our validation study shows that this model outperforms the prevalent stress-strain model (namely, the MKZ model) by a considerable margin.
\r\n\r\nLastly, we demonstrate how the two models above can improve earthquake ground motion modeling: we develop an improved version of site factors for the Western United States. These site factors are provided as Fourier spectral ratios, and phase factors are provided for the first time, which enables the time delay of earthquake waves to be modeled. They can be used for incorporating site response in earthquake ground motion simulations, as well as for improving seismic hazard maps for the Western United States.
", "doi": "10.7907/X5NZ-DQ21", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:10415", "collection": "thesis", "collection_id": "10415", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09062017-132833234", "type": "thesis", "title": "Earthquake Moment-Area Scaling Relations and the Effect of Fault Heterogeneity on Slow to Fast Earthquake Slip", "author": [ { "family_name": "Luo", "given_name": "Yingdi", "orcid": "0000-0002-1165-6107", "clpid": "Luo-Yingdi" } ], "thesis_advisor": [ { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" } ], "thesis_committee": [ { "family_name": "Clayton", "given_name": "Robert W.", "clpid": "Clayton-R-W" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Earthquake moment-area scaling relations play a key role in both earthquake physics studies and earthquake hazard assessment. A three-stage moment-area relation, based on advances in earthquake source inversion, is currently in use in Japan. The second stage has a scaling exponent outside the range of commonly accepted models of small and very large earthquakes. We develop theoretical insight on the mechanical origin of this second-stage scaling. We utilize an analytical dislocation model, a numerical crack model and multi-cycle rate-and-state simulations of strike-slip faults with heterogeneous friction properties. We find that the second stage in earthquake moment-area scaling results from a combination of surface rupture effects, comprising an effective rupture elongation along-dip due to a mirror effect and systematic changes of the shape factor relating slip to stress drop. Based on this physical insight, we propose a simplified formula to account for these effects in moment-area scaling relations.
\r\n\r\nGeological, seismological, geodetic and experimental studies provide evidence of the heterogeneous structure of natural faults. To advance our understanding of the mechanical role of fault heterogeneity on the diversity of earthquake slip behavior, we conduct a theoretical and computational study of heterogeneous fault models. We consider faults with a mixture of frictionally stable and unstable materials and spatial contrasts of fault zone pore fluid pressure, akin to hydraulically sealed brittle asperities embedded in a ductile fault zone matrix. We first study faults with a regular alternation of materials, using linear stability analysis and quasi-dynamic rate-and-state simulations. We find transitions in fault behavior from fast to slow earthquakes to steady slip, and determine how these transitions depend on the composition and strength contrast of the material mixture. Based on these results, we develop rate-and-state models with stochastic distributions of brittle asperities in a ductile matrix to study slow slip and tremor phenomena. We focus on the hierarchical patterns of tremor migration observed in subduction zones, which feature distinct tremor propagation speeds in different directions. Our models are in quantitative agreement with observations of episodic slow slip and tremor events in Cascadia. We discovered that, in contrast to a common view, slow slip might well be a result of tremor activity rather than its cause. The collective interaction of asperities with a broad range of material properties, mediated by creep, is a novel and robust mechanism for the generation of slow slip events. We find that the hierarchical patterns of tremor migration and the nucleation locations of tremor swarms provide constraints on fault rheology. Our study also shows that, despite multiple asperity interactions, there is a close relation between tremor rate and the underlying slip rate which supports an approach to constrain slow slip rate via observed tremor rates.
", "doi": "10.7907/Z9SQ8XMV", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:11027", "collection": "thesis", "collection_id": "11027", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06062018-071106334", "type": "thesis", "title": "Analyzing Stress Change and Energy Budget of Earthquakes Through Physics-Based Modeling", "author": [ { "family_name": "Perry", "given_name": "Stephen Michael", "orcid": "0000-0003-1748-1827", "clpid": "Perry-Stephen-Michael" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Tsai", "given_name": "Victor C.", "clpid": "Tsai-V-C" }, { "family_name": "Zhan", "given_name": "Zhongwen", "clpid": "Zhan-Zhongwen" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Researchers use concepts such as stress drop, breakdown energy, and available energy to describe earthquakes sources and study earthquake physics. These quantities represent the spatially and temporally varying dynamic events by single, event-averaged values. They are inferred indirectly from observations, often based on simplified models. Thus, their relationship to fault constitutive properties, which are local on the fault, is not straightforward.
\r\n\r\nHere, we use simulations of earthquake sequences in fault models with friction laws motivated by laboratory experiments to examine how the event-averaged observables arise from spatially and temporally varying earthquake rupture. In particular, we consider whether several typically used fault mechanisms, such as rate-and-state friction, thermal pressurization of pore fluids, and flash heating, are consistent with common observations such as magnitude-invariant stress drop, increasing breakdown energy with the event size, and radiation efficiencies of ~0.5.
\r\n \r\nStress drops, observed to be magnitude invariant, are a key characteristic used to describe natural earthquakes. Theoretical studies and lab experiments indicate that dynamic weakening, such as thermal pressurization of pore fluids, may be present on natural faults. At first glance, these two observations seem incompatible, since larger events may experience greater weakening and should thus have lower final stresses. We hypothesize that dynamic weakening can be reconciled with magnitude-invariant stress drops due to larger events having lower average prestress when compared to smaller events. The additional weakening would allow the final stresses to also be lower, but the stress drops may be similar.
\r\n\r\nTo explore this hypothesis, we study long-term earthquake sequences on a rate-and-state fault segment with enhanced dynamic weakening due to thermal pressurization using a fully dynamic simulation approach with a seismogenic segment that has uniform friction properties. Our results show, for a range of event sizes, that such models can explain both observationally inferred stress drop invariance and breakdown energy increase with event magnitude. Smaller events indeed have larger average initial stresses than medium-sized events, and we get nearly constant stress drops for events spanning up to five orders of magnitude in seismic moment. Segment-spanning events have more complex behavior, which is dependent on the properties of the velocity-strengthening (VS) region at the edges of the fault. Models with large values of velocity strengthening in their boundary regions do not allow ruptures to propagate much into the velocity-strengthening region, thus containing the rupture area and leading to higher stress drops for a larger amount of slip. Decreasing the velocity strengthening of the boundaries leads to farther rupture propagation into the velocity-strengthening region and thus lower stress drops.
\r\n\r\nIn all models with the thermal pressurization of pore fluids that we have examined, both the smaller and segment-spanning events exhibit increases in breakdown energy consistent with observations. The breakdown energy is the portion of the dissipated energy that governs the event dynamics, analogous to the fracture energy concept of fracture mechanics. The increase in the breakdown energy is due to continuous weakening of the fault with slip, as hypothesized in previous analytical studies.
\r\n\r\nWe also examine the accuracy of seismically estimated breakdown energies GSE for a range of models, by comparing the values computed directly from our fault models and indirectly from seismically available observations. Observationally, GSE is typically obtained as the difference between the seismically estimated available energy \u0394W0 per unit area and radiated energy ER. This defines the available energy \u0394WA as the sum of the breakdown energy and radiated energy. However, the seismically estimated available energy \u0394W0 is obtained as one-half of the product of the (average) stress drop and (average) final slip, based on a simplified model. As such, we examine the relation between the actual available energy \u0394WA and its seismic estimate \u0394W0 in our models. We find that, as rupture mode changes from crack-like to pulse-like, the actual available energy \u0394WA, becomes increasingly larger that the seismically estimated available energy \u0394W0, due to significant and increasing stress undershoot characteristic of pulse-like ruptures. The extra available energy for more pulse-like ruptures either makes the breakdown energy much larger than its seismically estimated value, or makes the radiated energy much larger than the seismically estimated available energy \u0394W0, or both. In the two latter cases, the radiation ratio \u03b7 (sometimes called radiation efficiency) between the radiated energy and seismically estimated available energy increases beyond 1, consistent with some observations that were previously thought to be aphysical.
\r\n\r\nOverall, we find that models with rate-and-state friction and thermal pressurization of pore fluids, when resulting in continuous weakening of fault with slip and crack-like ruptures, produce events with magnitude-invariant stress drops, increases in breakdown energies with the event sizes consistent with observations, radiation ratios consistent with observations, and available energies similar to the ones inferred seismically. More pulse-like ruptures, which result occasionally in such models and reliably in models that incorporate more severe enhanced weakening motivated by flash heating, have increasingly more significant undershoot and hence extra energy available for breakdown and radiation compared with the seismically estimated available energy. Therefore, current seismic estimates of their breakdown energy and radiation ratio are not reliable. More work is needed to understand the energy budget of pulse-like events obtained in realistic fault models, especially since one of the common paradigms in earthquake physics is that many large events occur as pulse-like ruptures.
", "doi": "10.7907/ryht-eb75", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:11042", "collection": "thesis", "collection_id": "11042", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072018-230955387", "type": "thesis", "title": "The Avatar Paradigm in Granular Materials", "author": [ { "family_name": "Kawamoto", "given_name": "Reid Yoshio", "orcid": "0000-0002-4936-5321", "clpid": "Kawamoto-Reid-Yoshio" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Granular materials are ubiquitous in both everyday life and various engineering and industrial applications, ranging from breakfast cereal to sand to rice to medical pills. However, despite the familiarity of granular materials, their behavior is complex and efforts to characterize them are currently broad research areas in physics and engineering. Research of granular materials, as is the case with the research of other engineering materials such as rocks and metals, is beset with two gaps: the gap between reconciling macroscopic behavior with microscale (particle-scale, in the case of granular materials) behavior, and the gap between reconciling experimental and computational results. In this dissertation, we bridge these gaps through the \"avatar paradigm.\" The avatar paradigm is a two-step process that numerically characterizes (from experimental images) and simulates the shapes and behavior of individual particles, which we call avatars. First, we validate that our avatars are indeed capable of faithfully capturing particle kinematics and interparticle contact, then apply the characterization process, level set imaging (LS-imaging), to two experimental specimens to compute particle kinematics and contact statistics. We then detail a computational method, the level set discrete element method (LS-DEM), that is able to simulate the behavior of avatars, and apply it (and LS-imaging) to two other experimental specimens, calibrating the model to one specimen and using the results to predict the behavior of the other, thus providing some reconciliation between experimental and computational results. Finally, we use the avatar process to characterize and simulate yet another experimental specimen, this time analyzing the results at length scales ranging from particle behavior to local behavior to macroscopic behavior, further validating the ability of the avatar paradigm to bridge experiments and computations and showing its power to reconcile different length scales.", "doi": "10.7907/4fr8-bn91", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10988", "collection": "thesis", "collection_id": "10988", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312018-150338327", "type": "thesis", "title": "Laboratory Investigation of Shear Ruptures: Supersonic Propagation and Nucleation by Fluid Injection", "author": [ { "family_name": "Gori", "given_name": "Marcello", "orcid": "0000-0002-7380-3723", "clpid": "Gori-Marcello" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Shear rupture nucleation and dynamic propagation is a challenging, non- linear, highly interactive process with important practical implications. Here we focus on two aspects of this problem: propagation speeds and shock front radiation from the dynamic crack tip as well as nucleation of dynamic rupture due to fluid injection.
\r\n\r\nSpontaneously propagating cracks in solids emit pressure and shear waves and are, in part, driven by energy transfer due to them. When a shear crack propagates faster than the shear wave speed of the material, the coalescence of the shear wavelets emitted by the near-crack-tip region forms a shock front that significantly concentrates particle motion. The equivalent scenario involving a pressure shock front should not be possible, since cracks should not be able to exceed the pressure wave speed, at least in an isotropic linear-elastic solid. Here we present full-field experimental evidence of dynamic shear cracks in viscoelastic polymers that result in the formation of a pressure shock front, in addition to the shear one. In that sense, the crack appears to be supersonic. The apparent violation of classic theories is explained by the strain-rate-dependent material behavior of polymers: the increased wave speeds within the highly- strained region around the crack tip allow for supersonic crack propagation with respect to the (lower) wave speeds at short distances away from the interface, resulting in the formation of the pressure shock front. The crack speed remains below the pressure wave speed prevailing locally, about its tip, in agreement with basic physics and energy considerations of linear-elastic theories.
\r\n\r\nWe find that the shock fronts emitted by the shear cracks in the viscoelastic materials are curved and propose a novel method to quantify the viscoelastic wave speeds of the solids in the dynamic range of strain rates based on the curvature. Only kinematic relationships are used in the method, without the need for the constitutive relationship of the material. Measuring or inferring the material properties at elevated strain rates in viscoelastic solids is a difficult task, because of practical limitations of obtaining accurate measurements in that regime. Under the quasi-elastic solid approximation, in which the strain-rate history is neglected, we use the pressure-wave speed measurements to infer the associated value of the Young\u2019s modulus, estimated by assuming a constant value of the Poisson\u2019s ratio. We complement these results with the characterization of the Young\u2019s modulus at lower strain rates via canonical compressive tests. Our results not only confirm previous findings that the Young\u2019s modulus dependence on the strain rate in PMMA is significant but also demonstrate that its variation is more pronounced in the dynamic strain-rate range, with important consequences for the design of structures employing viscoelastic materials that are required to withstand elevated strain rates.
\r\n\r\nThe second part of the study concentrates on the nucleation of shear dynamic rupture due to fluid injection or, more broadly, on the interaction of frictional faulting with fluids. Fluid overpressure is recognized to play a fundamental role in promoting fault motion. A large number of observations has shed light on the interplay between fluids and faulting, both in natural events and in earth-quakes induced by human activities, such as wastewater disposal associated with oil and gas extraction. Fluids can induce a variety of earthquake source behaviors ranging from unstable, dynamic motions to stable, quasi-static ones, which a number of field studies suggests that can coexist on the same fault areas at different times, depending on the local conditions. In fact, a higher pore pres-sure plays the dual role of reducing the frictional strength of the fault and of increasing the nucleation size, e.g., the critical length for a shear crack to transition from quasi-static to dynamic motions. However, due to the complexity of the frictional problem at the fault interface, the understanding of which of these two effects prevails remains elusive. The assumption of a critical nucleation length represents a powerful, yet simplified concept, which currently does not include the dependence on the rate of the pore pressure increase.
\r\n\r\nHere, we explore the effect of the rate of the pore pressure increase on the rupture nucleation. We find that elevated injection rates induce triggering of the rupture at lower pressure values and minimal volumes of the injected fluid, if compared to slow injection rates. For the slow injection rates, we experimentally observe a much larger portion of interface wetted by the fluid and a phase of accelerated slip prior to the dynamic event (quasi-dynamic nucleation process). In some cases, we record much smaller foreshock-like events at the injection site. These findings suggest the presence of a prominent quasi-static nucleation process over the interface. In cases of rapid pore pressure increase, the nucleation process is much shorter in time and much more compact in space, being highly concentrated around the injection location. The dynamic events, once initiated, are qualitatively similar across different injection rates, but quantitatively different, with the slow-injection ones experiencing higher stress drops and higher slips, perhaps due to the effect of fluids on the friction properties.\r\nThese findings suggest the need to develop nucleation size estimates that include the rate of the pore pressure increase and motivate further investigation of how friction properties depend on the presence of fluids. The details of the obtained experimental findings, once analyzed through numerical modeling, will place important constrains on the forms of the acceptable friction laws, including the effects of pore fluid pressure and its rate of change.
", "doi": "10.7907/AH9X-V905", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10951", "collection": "thesis", "collection_id": "10951", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272018-175543580", "primary_object_url": { "basename": "Schaal_Natalie_thesis_2018.pdf", "content": "final", "filesize": 26842737, "license": "other", "mime_type": "application/pdf", "url": "/10951/1/Schaal_Natalie_thesis_2018.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Modeling of Nucleation and Dynamic Rupture on Heterogeneous Frictional Interfaces with Applications to Foreshocks", "author": [ { "family_name": "Schaal", "given_name": "Natalie Sarah Ann", "orcid": "0000-0003-4825-4344", "clpid": "Schaal-Natalie-Sarah-Ann" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "While many large earthquakes are preceded by observable foreshocks, the mechanisms responsible for the occurrence of these smaller-scale seismic events remain uncertain. One physical explanation of foreshocks with growing support is that they are produced by the interaction of slow slip, due to the nucleation of the upcoming mainshock, with fault patches of different properties. Having a better understanding of how earthquakes nucleate on heterogeneous faults would increase our capacity to forecast potentially hazardous events.\r\n
\r\n\r\nWith this motivation in mind, we seek to understand what conditions produce isolated microseismicity within the nucleating region of the mainshock and to study the mechanics of the resulting events. Inspired by the suggestion from laboratory experiments that foreshocks occur on asperities, i.e., local deviations from planarity that are flattened by the overall compression, we explore the behavior of asperity-type patches of higher compressive stress embedded in the larger seismogenic region of a rate-and-state fault model by conducting 3D numerical simulations of their slip over long-term sequences of aseismic and seismic slip. Our models do produce smaller-scale seismicity during the aseismic nucleation of much larger seismic events, and we explore their properties as well as the separation in length scales needed to produce them. These foreshock-like events have stress drops that are consistent with laboratory and field observations and approximately constant, despite the highly elevated compression assigned to the source patches. Two main factors contributing to the reasonable stress drops are the significant extent of the rupture into the region surrounding the patch and the aseismic stress release just prior to the seismic event.
\r\n\r\n\r\nWe also investigate the seismologically-derived properties of the asperity-type events using the spectral analysis commonly applied to natural microseismic events. We find that the seismological methods cannot adequately capture the properties of the simulated events. In part, the seismological estimates of their stress drops are significantly different from the actual stress drops determined from the on-fault stress changes. This is because our sources have more complex features than the standard models from which the current seismological methods have been built, including heterogeneous stress change over the rupture area with much larger initial stress change, and heterogeneous rupture speed. We identify features in the far-field seismograms of the asperity-type sources that differ from the standard models and can be potentially characteristic of the asperity-type sources.\r\n
\r\n\r\n\r\nOur asperity-type models of microseismicity sources provide insight into the conditions conducive for generating foreshocks on both natural and laboratory faults and the properties of the resulting events. The conclusions provided jointly by the two perspectives in this study -- dynamically simulating the behavior of seismic sources within heterogeneous fault models and seismologically analyzing their far-field source spectra -- have important implications that warrant further study. Topics for future research include the interaction among smaller-scale seismic events and their role in the mainshock nucleation process, the effect of timing on their source properties, and relation to the so-called seismic nucleation phase of the subsequent mainshock.\r\n
\r\n", "doi": "10.7907/YCVJ-PM21", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10563", "collection": "thesis", "collection_id": "10563", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11202017-145930076", "type": "thesis", "title": "Using Heterogeneous 3D Earth Models to Constrain Interseismic and Postseismic Deformation in Southern California and Nepal", "author": [ { "family_name": "Rollins", "given_name": "John Christopher", "orcid": "0000-0002-5291-6956", "clpid": "Rollins-John-Christopher" } ], "thesis_advisor": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" } ], "thesis_committee": [ { "family_name": "Gurnis", "given_name": "Michael C.", "clpid": "Gurnis-M-C" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Clayton", "given_name": "Robert W.", "clpid": "Clayton-R-W" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "We characterize interseismic strain accumulation across the Los Angeles basin and postseismic deformation following the 2010 Mw=7.2 El Mayor-Cucapah and 2015 Mw=7.8 Nepal earthquakes using geodetic data. These settings are all characterized by strong 3D heterogeneities of elastic structure, ductile properties, fault geometries, and fault slip behavior, and we use constaints from seismology, long-term tectonic modeling, geology, and other sources to construct detailed models of these heterogeneities. Postseismic surface displacements following the 2010 El Mayor-Cucapah earthquake indicate viscoelastic relaxation in the shallow Salton Trough mantle and possibly the lower crust, a process that would have been enhanced by high heat flow induced by crustal extension at the tip of the Gulf of California. We find that a dense and prolonged aftershock sequence in the Yuha Desert may have been driven by aseismic afterslip coupled with fluid flow. Our study of interseismic strain accumulation across the Los Angeles basin shows that the soft sedimentary basin has a first-order effect on the elastostatic Green\u2019s functions mapping fault creep and locking at depth to surface deformation, and therefore on the estimation of interseismic fault creep rates and strain accumulation at depth. We infer modest interseismic coupling on the three major thrust faults underlying the Los Angeles basin, corresponding to an annual seismic moment deficit buildup rate (to be presumably released in earthquakes) of 1.7 +1.2/-0.5 x 1017 Nm/yr. We estimate the long-term seismicity model needed to balance the rate of moment deficit accumulation assuming a truncated Gutenberg-Richter magnitude-frequency distribution of earthquakes. The long-term catalog is consistent with the instrumental rates of small and moderate earthquakes and tops out at a M~6.9 earthquake every ~430 years. Finally, we characterize the postseismic deformation following the 2015 Nepal earthquake using models of the thermal structure, state of stress, and rheology that are based on the long-term evolution and topography of the Himalaya. The rheological structure based on these models predicts negligible postseismic viscoelastic deformation. Afterslip on the downdip extension of the rupture cannot realistically explain the observed displacements either. We find that the postseismic deformation is well explained by a combination of afterslip on the downdip edge of the coseismic rupture (as well as a narrow zone in between the mainshock and a large aftershock) and, more prominently, transient viscoelastic relaxation in the hot Tibetan crust. These processes contribute to the stress loading of the Main Himalayan Thrust.
", "doi": "10.7907/Z9X06572", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10431", "collection": "thesis", "collection_id": "10431", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09122017-092017294", "type": "thesis", "title": "Data Driven Computing", "author": [ { "family_name": "Kirchdoerfer", "given_name": "Trenton Thomas", "orcid": "0000-0003-2290-1857", "clpid": "Kirchdoerfer-Trenton-Thomas" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "orcid": "0000-0002-9112-6615", "clpid": "Kochmann-D-M" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Data Driven Computing is a new field of computational analysis which uses provided data to directly produce predictive outcomes. This thesis first establishes definitions of Data-Driven solvers and working examples of static mechanics problems to demonstrate efficacy. Significant extensions are then explored to both accommodate noisy data sets and apply the deveoloped methods to dynamic problems within mechanics. Possible method improvements discuss incorporation of data quality metrics and adaptive data sampling, while new applications focus on multi-scale analysis and the need for public databases to support constitutive data collaboration.\r\n", "doi": "10.7907/Z9Z899MV", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:9983", "collection": "thesis", "collection_id": "9983", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11232016-044435496", "primary_object_url": { "basename": "SKYLui_PhD_Thesis_FINAL.pdf", "content": "final", "filesize": 49646596, "license": "other", "mime_type": "application/pdf", "url": "/9983/1/SKYLui_PhD_Thesis_FINAL.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Earthquake Source Characterization Through Seismic Observations and Numerical Modeling", "author": [ { "family_name": "Lui", "given_name": "Semechah Ka Yan", "orcid": "0000-0001-7801-3635", "clpid": "Lui-Semechah-Ka-Yan" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" } ], "thesis_committee": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Clayton", "given_name": "Robert W.", "clpid": "Clayton-R-W" }, { "family_name": "Tsai", "given_name": "Victor C.", "clpid": "Tsai-V-C" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "In this thesis, I present a series of works on the characterization of source properties and physical mechanisms of various small to moderate earthquakes through both observational and numerical approaches. From the results, we find implications on a broader scheme of topics relating to larger earthquakes, shear zone structure, frictional properties of faults, and seismic hazard assessment.
\r\n\r\nPart I consists of two studies using waveform modeling. In Chapter 2, we present an in-depth study of a series of intraslab earthquakes that occurred in a localized region near the downdip edge of the 2011 Mw Tohoku-Oki megathrust earthquake. By refining source parameters of selected events, simulating their rupture properties and comparing their mechanisms to stress changes caused by the main shock in the region, we are able to identify the true rupture plane and the reactivation of a subducted normal fault, enhancing our understanding on the downdip shear zone. In Chapter 3, based on similar techniques, we further develop a systematic methodology to perform fast assessments on important source properties as an earthquake occurs. For two Mw 4.4 earthquakes in Fontana, moment magnitude and focal mechanism can be accurately estimated with 3 to 6 s after the first P-wave arrival, while focal depth can be constrained upon the arrival of S waves. Rupture directivity can also be determined with as little as 3 seconds of P waves. This study opens the opportunity to predict ground motions ahead of time and can potentially be useful for Earthquake Early Warning.
\r\n\r\nPart II involves the modeling of seismic source properties and physical mechanisms of interacting earthquakes in dynamic rupture simulations. In particular, we focus on small repeating earthquake sequences that trigger one another. In Chapter 4, we quantify the relative importance of physical mechanisms that contribute to earthquake interaction and identify that the stress change caused by post seismic slip is the dominating factor. Our findings introduce the possibility to constrain frictional properties of the fault based on earthquake interactions. We further apply this working model in Chapter 5 to reproduce the actual interacting repeating sequences in Parkfield. We are able to identify possible physical mechanisms that cause the inferred high stress drops of these repeating events, as well as reproduce their synchronized seismic cycles. Results from our simulations are consistent with the observed scaling relation between the recurrence time interval and the seismic moment of these events. Our findings indicate that the difference between the observed and the theoretical scaling relations can be explained by the significant aseismic slip in the rupture area.
", "doi": "10.7907/Z9QN64QM", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:9767", "collection": "thesis", "collection_id": "9767", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262016-131105966", "primary_object_url": { "basename": "Stevens_Victoria_20162.pdf", "content": "final", "filesize": 12986561, "license": "other", "mime_type": "application/pdf", "url": "/9767/1/Stevens_Victoria_20162.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Reconciling Geodetic Strain and Seismicity Rate with Frequency-Magnitude Relation of the Largest Earthquakes", "author": [ { "family_name": "Stevens", "given_name": "Victoria Louise", "orcid": "0000-0003-3174-9949", "clpid": "Stevens-Victoria-Louise" } ], "thesis_advisor": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" } ], "thesis_committee": [ { "family_name": "Stock", "given_name": "Joann M.", "orcid": "0000-0003-4816-7865", "clpid": "Stock-J-M" }, { "family_name": "Wernicke", "given_name": "Brian P.", "orcid": "0000-0002-7659-8358", "clpid": "Wernicke-B-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Tsai", "given_name": "Victor C.", "clpid": "Tsai-V-C" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The aim of this thesis is to study how moment buildup rate on faults can be reconciled with moment release rate. We concentrate first on the Himalaya region and go on to look at faults worldwide. We first justify the extrapolation of GPS data in the Himalayan region over the approximate timescale of an earthquake cycle. To do this we show that GPS strain rates correlate with seismicity rates, and that the principal directions of strain found from GPS data are similar to those from earthquake moment tensors, showing that GPS data has been consistent at the timescale of earthquake strain-rate build-up, roughly 100-1000 years.
\r\n\r\nWe next use geodetic data to show that the Main Himalayan Thrust (MHT) is locked from the surface to roughly 100 km north along its entire length, with no creeping patches. We also find the long-term slip rate on the fault, and these values agree with values from geomorphic studies, showing that here the tectonic regime has been stable with time, and most of the deformation is elastic. However, we also find a correspondence between the pattern of uplift rate predicted from the model and the topography, suggesting that a small amount of permanent deformation (10%) may occur, and again suggesting that the pattern of coupling has been stable with time.
\r\n\r\nWe find the moment build-up rate on the MHT to be 15.1\u00b11.0x1019 Nm/yr and compare this rate with the rate of moment release estimated from large earthquakes that have occurred on this fault in the past 1000 years. We use the conservation of moment principal to model the most likely maximum magnitude earthquake that needs to occur to balance the moment budget, and find that we need an earthquake of magnitude 9 or more with a recurrence time of roughly 800 years.
\r\n\r\nWe extend this analysis to faults with no GPS data, and no long record of large earthquakes, by developing a method to find the expected maximum magnitude earthquake on faults assuming conservation of moment, and that the earthquakes follow the Gutenberg-Richter law. Our results compare well with historical catalogs where they are available.
", "doi": "10.7907/Z9PN93K7", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9783", "collection": "thesis", "collection_id": "9783", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272016-104209268", "primary_object_url": { "basename": "Thesis2016_VinamraAgrawal - 2.pdf", "content": "final", "filesize": 7388241, "license": "other", "mime_type": "application/pdf", "url": "/9783/1/Thesis2016_VinamraAgrawal - 2.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Shock Wave Propagation in Composites and Electro-Thermomechanical Coupling of Ferroelectric Materials", "author": [ { "family_name": "Agrawal", "given_name": "Vinamra", "orcid": "0000-0002-1698-1371", "clpid": "Agrawal-Vinamra" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "How is material behavior at the macro scale influenced by its properties and structure at the micro and meso-scales? How do heterogeneities influence the properties and the response of a material? How does nonlinear coupling of electro-thermo-mechanical properties influence the behavior of a ferroelectric material? How can design at the micro-scale be exploited to obtain selective response? These questions have been topics of significant interest in the materials and mechanics community. Recently, new materials like multifunctional composites and metamaterials have been developed, targeted at selective applications. These materials find applications in areas like energy harvesting, damage mitigation, biomedical devices, and various aerospace applications. The current thesis explores these questions with two major thrusts: (i) internal reflects of shocks in composite media and (ii) shocks in ferroelectric media.
\r\n\r\nUnder the application of high-pressure, high strain rate loading, such as during high velocity impact, shock waves are generated in the material. They can cause the material to achieve very high stress states, and if transmitted without mitigation, can lead to failure of key components. An important question here is 'Can we design materials which can successfully mitigate damage due to shocks?' In a heterogeneous material, like a layered composite, the traveling waves undergo scattering due to internal reflections. In order to understand internal reflections, an idealized problem that focuses on nonlinear shocks and ignores less important elastic waves was formulated and studied in detail. The problem is studied by classifying all possible interactions in the material and then solving corresponding Riemann problems. Using dynamic programming tools, a new algorithm is designed that uses these solutions to generate a complete picture of the impact process. Different laminate designs are explored to study optimal design, by varying individual layer properties and their arrangement. Phenomena like spallation and delamination are also investigated.
\r\n\r\nUpon high strain rate loading, ferroelectric materials like lead zirconate titanate (PZT) undergo ferroelectric to anti-ferroelectric phase transition leading to large pulsed current output. These materials have thus found applications as pulsed power generators. The problem of shock induced depolarization and the associated electro-thermo-mechanical coupling of ferroelectric materials is studied in this thesis using theoretical and numerical methods. A large deformation dynamic analysis of such materials is conducted to study phase boundary propagation in the medium. The presence of high electrical fields can lead to formation of charges in the material, such as surface charge on the phase boundary. Using conservation laws and the second law of thermodynamics, a set of governing equations are formulated that dictate the phase boundary propagation in isothermal and adiabatic environments. Due to the possibility of surface charges on the phase boundary, the curvature of the phase boundary starts to play a role in the driving force acting on the phase boundary. The equations of motion and driving force see the contribution of nonlinear electro-thermomechanical coupling in the material. Using the equations derived, a canonical problem of impact on a ferroelectric material is studied. A new finite-volume, front-tracking method is developed to solve these equations. Finally, results from numerical simulations are compared to the experimental results.
", "doi": "10.7907/Z98G8HN8", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9712", "collection": "thesis", "collection_id": "9712", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05092016-151847908", "primary_object_url": { "basename": "Mital_Utkarsh_2016_Complete_Thesis.pdf", "content": "final", "filesize": 5924203, "license": "other", "mime_type": "application/pdf", "url": "/9712/1/Mital_Utkarsh_2016_Complete_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Understanding Micro- and Macro-Mechanics of Soil Liquefaction: A Necessary Step for Field-Scale Assessment", "author": [ { "family_name": "Mital", "given_name": "Utkarsh", "orcid": "0000-0001-9794-382X", "clpid": "Mital-Utkarsh" } ], "thesis_advisor": [ { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Asimaki", "given_name": "Domniki", "orcid": "0000-0002-3008-8088", "clpid": "Asimaki-D" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "orcid": "0000-0002-4827-7987", "clpid": "Ampuero-J-P" } ], "local_group": [ { "literal": "3MT Competition (Caltech)" }, { "literal": "div_eng" } ], "abstract": "Liquefaction is a devastating instability associated with saturated, loose, and cohesionless soils. It poses a significant risk to distributed infrastructure systems that are vital for the security, economy, safety, health, and welfare of societies. In order to make our cities resilient to the effects of liquefaction, it is important to be able to identify areas that are most susceptible. Some of the prevalent methodologies employed to identify susceptible areas include conventional slope stability analysis and the use of so-called liquefaction charts. However, these methodologies have some limitations, which motivate our research objectives. In this dissertation, we investigate the mechanics of origin of liquefaction in a laboratory test using grain-scale simulations, which helps (i) understand why certain soils liquefy under certain conditions, and (ii) identify a necessary precursor for onset of flow liquefaction. Furthermore, we investigate the mechanics of liquefaction charts using a continuum plasticity model; this can help in modeling the surface hazards of liquefaction following an earthquake. Finally, we also investigate the microscopic definition of soil shear wave velocity, a soil property that is used as an index to quantify liquefaction resistance of soil. We show that anisotropy in fabric, or grain arrangement can be correlated with anisotropy in shear wave velocity. This has the potential to quantify the effects of sample disturbance when a soil specimen is extracted from the field. In conclusion, by developing a more fundamental understanding of soil liquefaction, this dissertation takes necessary steps for a more physical assessment of liquefaction susceptibility at the field-scale.", "doi": "10.7907/Z9PV6HB0", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9221", "collection": "thesis", "collection_id": "9221", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10142015-142502895", "type": "thesis", "title": "Probabilistic Imaging and Dynamic Modeling of Earthquake Source Processes", "author": [ { "family_name": "Jiang", "given_name": "Junle", "orcid": "0000-0002-8796-5846", "clpid": "Jiang-Junle" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Simons", "given_name": "Mark", "clpid": "Simons-M" } ], "thesis_committee": [ { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Simons", "given_name": "Mark", "clpid": "Simons-M" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Investigation of large, destructive earthquakes is challenged by their infrequent occurrence and the remote nature of geophysical observations. This thesis sheds light on the source processes of large earthquakes from two perspectives: robust and quantitative observational constraints through Bayesian inference for earthquake source models, and physical insights on the interconnections of seismic and aseismic fault behavior from elastodynamic modeling of earthquake ruptures and aseismic processes.
\r\n\r\nTo constrain the shallow deformation during megathrust events, we develop semi-analytical and numerical Bayesian approaches to explore the maximum resolution of the tsunami data, with a focus on incorporating the uncertainty in the forward modeling. These methodologies are then applied to invert for the coseismic seafloor displacement field in the 2011 Mw 9.0 Tohoku-Oki earthquake using near-field tsunami waveforms and for the coseismic fault slip models in the 2010 Mw 8.8 Maule earthquake with complementary tsunami and geodetic observations. From posterior estimates of model parameters and their uncertainties, we are able to quantitatively constrain the near-trench profiles of seafloor displacement and fault slip. Similar characteristic patterns emerge during both events, featuring the peak of uplift near the edge of the accretionary wedge with a decay toward the trench axis, with implications for fault failure and tsunamigenic mechanisms of megathrust earthquakes.
\r\n\r\nTo understand the behavior of earthquakes at the base of the seismogenic zone on continental strike-slip faults, we simulate the interactions of dynamic earthquake rupture, aseismic slip, and heterogeneity in rate-and-state fault models coupled with shear heating. Our study explains the long-standing enigma of seismic quiescence on major fault segments known to have hosted large earthquakes by deeper penetration of large earthquakes below the seismogenic zone, where mature faults have well-localized creeping extensions. This conclusion is supported by the simulated relationship between seismicity and large earthquakes as well as by observations from recent large events. We also use the modeling to connect the geodetic observables of fault locking with the behavior of seismicity in numerical models, investigating how a combination of interseismic geodetic and seismological estimates could constrain the locked-creeping transition of faults and potentially their co- and post-seismic behavior.
", "doi": "10.7907/Z9639MQC", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:8199", "collection": "thesis", "collection_id": "8199", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04222014-195027916", "type": "thesis", "title": "Experimental Investigation of Thrust Fault Rupture Mechanics", "author": [ { "family_name": "Gabuchian", "given_name": "Vahe", "orcid": "0000-0001-7457-5921", "clpid": "Gabuchian-Vahe" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Bhat", "given_name": "Harsha S.", "clpid": "Bhat-H-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Thrust fault earthquakes are investigated in the laboratory by generating dynamic shear ruptures along pre-existing frictional faults in rectangular plates. A considerable body of evidence suggests that dip-slip earthquakes exhibit enhanced ground motions in the acute hanging wall wedge as an outcome of broken symmetry between hanging and foot wall plates with respect to the earth surface. To understand the physical behavior of thrust fault earthquakes, particularly ground motions near the earth surface, ruptures are nucleated in analog laboratory experiments and guided up-dip towards the simulated earth surface. The transient slip event and emitted radiation mimic a natural thrust earthquake. High-speed photography and laser velocimeters capture the rupture evolution, outputting a full-field view of photo-elastic fringe contours proportional to maximum shearing stresses as well as continuous ground motion velocity records at discrete points on the specimen. Earth surface-normal measurements validate selective enhancement of hanging wall ground motions for both sub-Rayleigh and super-shear rupture speeds. The earth surface breaks upon rupture tip arrival to the fault trace, generating prominent Rayleigh surface waves. A rupture wave is sensed in the hanging wall but is, however, absent from the foot wall plate: a direct consequence of proximity from fault to seismometer. Signatures in earth surface-normal records attenuate with distance from the fault trace. Super-shear earthquakes feature greater amplitudes of ground shaking profiles, as expected from the increased tectonic pressures required to induce super-shear transition. Paired stations measure fault parallel and fault normal ground motions at various depths, which yield slip and opening rates through direct subtraction of like components. Peak fault slip and opening rates associated with the rupture tip increase with proximity to the fault trace, a result of selective ground motion amplification in the hanging wall. Fault opening rates indicate that the hanging and foot walls detach near the earth surface, a phenomenon promoted by a decrease in magnitude of far-field tectonic loads. Subsequent shutting of the fault sends an opening pulse back down-dip. In case of a sub-Rayleigh earthquake, feedback from the reflected S wave re-ruptures the locked fault at super-shear speeds, providing another mechanism of super-shear transition.", "doi": "10.7907/Z9J96497", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8861", "collection": "thesis", "collection_id": "8861", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05172015-152006825", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 13609562, "license": "other", "mime_type": "application/pdf", "url": "/8861/1/thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Collective Behavior of Asperities as a Model for Friction and Adhesion", "author": [ { "family_name": "Hulikal Sampath Kumaran", "given_name": "Srivatsan", "clpid": "Hulikal-Sampath-Kumaran-Srivatsan" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Andrade", "given_name": "Jose E.", "clpid": "Andrade-J-E" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Understanding friction and adhesion in static and sliding contact of surfaces is important in numerous physical phenomena and technological applications. Most surfaces are rough at the microscale, and thus the real area of contact is only a fraction of the nominal area. The macroscopic frictional and adhesive response is determined by the collective behavior of the population of evolving and interacting microscopic contacts. This collective behavior can be very different from the behavior of individual contacts. It is thus important to understand how the macroscopic response emerges from the microscopic one.
\r\n\r\nIn this thesis, we develop a theoretical and computational framework to study the collective behavior. Our philosophy is to assume a simple behavior of a single asperity and study the collective response of an ensemble. Our work bridges the existing well-developed studies of single asperities with phenomenological laws that describe macroscopic rate-and-state behavior of frictional interfaces. We find that many aspects of the macroscopic behavior are robust with respect to the microscopic response. This explains why qualitatively similar frictional features are seen for a diverse range of materials.
\r\n\r\nWe first show that the collective response of an ensemble of one-dimensional independent viscoelastic elements interacting through a mean field reproduces many qualitative features of static and sliding friction evolution. The resulting macroscopic behavior is different from the microscopic one: for example, even if each contact is velocity-strengthening, the macroscopic behavior can be velocity-weakening. The framework is then extended to incorporate three-dimensional rough surfaces, long- range elastic interactions between contacts, and time-dependent material behaviors such as viscoelasticity and viscoplasticity. Interestingly, the mean field behavior dominates and the elastic interactions, though important from a quantitative perspective, do not change the qualitative macroscopic response. Finally, we examine the effect of adhesion on the frictional response as well as develop a force threshold model for adhesion and mode I interfacial cracks.
", "doi": "10.7907/Z94M92HM", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8426", "collection": "thesis", "collection_id": "8426", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292014-144526425", "primary_object_url": { "basename": "Huang_Yihe_2014_thesis.pdf", "content": "final", "filesize": 7474561, "license": "other", "mime_type": "application/pdf", "url": "/8426/1/Huang_Yihe_2014_thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Dynamic Rupture Simulation Integrated with Earthquake Observations", "author": [ { "family_name": "Huang", "given_name": "Yihe", "clpid": "Huang-Yihe" } ], "thesis_advisor": [ { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" } ], "thesis_committee": [ { "family_name": "Tsai", "given_name": "Victor C.", "clpid": "Tsai-V-C" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Dynamic rupture simulations are unique in their contributions to the study of earthquake physics. The current rapid development of dynamic rupture simulations poses several new questions: Do the simulations reflect the real world? Do the simulations have predictive power? Which one should we believe when the simulations disagree? This thesis illustrates how integration with observations can help address these questions and reduce the effects of non-uniqueness of both dynamic rupture simulations and kinematic inversion problems. Dynamic rupture simulations with observational constraints can effectively identify non-physical features inferred from observations. Moreover, the integrative technique can also provide more physical insights into the mechanisms of earthquakes. This thesis demonstrates two examples of such kinds of integration: dynamic rupture simulations of the Mw 9.0 2011 Tohoku-Oki earthquake and of earthquake ruptures in damaged fault zones:
\r\n\r\n(1) We develop simulations of the Tohoku-Oki earthquake based on a variety of observations and minimum assumptions of model parameters. The simulations provide realistic estimations of stress drop and fracture energy of the region and explain the physical mechanisms of high-frequency radiation in the deep region. We also find that the overridding subduction wedge contributes significantly to the up-dip rupture propagation and large final slip in the shallow region. Such findings are also applicable to other megathrust earthquakes.
\r\n \r\n(2) Damaged fault zones are usually found around natural faults, but their effects on earthquake ruptures have been largely unknown. We simulate earthquake ruptures in damaged fault zones with material properties constrained by seismic and geological observations. We show that reflected waves in fault zones are effective at generating pulse-like ruptures and head waves tend to accelerate and decelerate rupture speeds. These mechanisms are robust in natural fault zones with large attenuation and off-fault plasticity. Moreover, earthquakes in damaged fault zones can propagate at super-Rayleigh speeds that are unstable in homogeneous media. Supershear transitions in fault zones do not require large fault stresses. In the end, we present observations in the Big Bear region, where variability of rupture speeds of small earthquakes correlates with the laterally variable materials in a damaged fault zone.
\r\n", "doi": "10.7907/VPJW-ZF69", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:7953", "collection": "thesis", "collection_id": "7953", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09112013-133205322", "primary_object_url": { "basename": "PhD_thesis_thomas_marion_fontcor.pdf", "content": "final", "filesize": 260903591, "license": "other", "mime_type": "application/pdf", "url": "/7953/49/PhD_thesis_thomas_marion_fontcor.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Frictional Properties of Fault: From Observations on the Longitudinal Valley Fault, Taiwan, to Dynamic Simulations", "author": [ { "family_name": "Thomas", "given_name": "Marion Y.", "clpid": "Thomas-Marion-Y" } ], "thesis_advisor": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Asimow", "given_name": "Paul David", "clpid": "Asimow-P-D" }, { "family_name": "Wernicke", "given_name": "Brian P.", "clpid": "Wernicke-B-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Faults can slip either aseismically or through episodic seismic ruptures, but we still do not understand the factors which determine the partitioning between these two modes of slip. This challenge can now be addressed thanks to the dense set of geodetic and seismological networks that have been deployed in various areas with active tectonics. The data from such networks, as well as modern remote sensing techniques, indeed allow documenting of the spatial and temporal variability of slip mode and give some insight. This is the approach taken in this study, which is focused on the Longitudinal Valley Fault (LVF) in Eastern Taiwan. This fault is particularly appropriate since the very fast slip rate (about 5 cm/yr) is accommodated by both seismic and aseismic slip. Deformation of anthropogenic features shows that aseismic creep accounts for a significant fraction of fault slip near the surface, but this fault also released energy seismically, since it has produced five M_w>6.8 earthquakes in 1951 and 2003. Moreover, owing to the thrust component of slip, the fault zone is exhumed which allows investigation of deformation mechanisms. In order to put constraint on the factors that control the mode of slip, we apply a multidisciplinary approach that combines modeling of geodetic observations, structural analysis and numerical simulation of the \"seismic cycle\". Analyzing a dense set of geodetic and seismological data across the Longitudinal Valley, including campaign-mode GPS, continuous GPS (cGPS), leveling, accelerometric, and InSAR data, we document the partitioning between seismic and aseismic slip on the fault. For the time period 1992 to 2011, we found that about 80-90% of slip on the LVF in the 0-26 km seismogenic depth range is actually aseismic. The clay-rich Lichi M\\'elange is identified as the key factor promoting creep at shallow depth. Microstructural investigations show that deformation within the fault zone must have resulted from a combination of frictional sliding at grain boundaries, cataclasis and pressure solution creep. Numerical modeling of earthquake sequences have been performed to investigate the possibility of reproducing the results from the kinematic inversion of geodetic and seismological data on the LVF. We first investigate the different modeling strategy that was developed to explore the role and relative importance of different factors on the manner in which slip accumulates on faults. We compare the results of quasi dynamic simulations and fully dynamic ones, and we conclude that ignoring the transient wave-mediated stress transfers would be inappropriate. We therefore carry on fully dynamic simulations and succeed in qualitatively reproducing the wide range of observations for the southern segment of the LVF. We conclude that the spatio-temporal evolution of fault slip on the Longitudinal Valley Fault over 1997-2011 is consistent to first order with prediction from a simple model in which a velocity-weakening patch is embedded in a velocity-strengthening area.", "doi": "10.7907/0AFX-WS57", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:7851", "collection": "thesis", "collection_id": "7851", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06062013-102758695", "primary_object_url": { "basename": "thesis_Ha.pdf", "content": "final", "filesize": 5156109, "license": "other", "mime_type": "application/pdf", "url": "/7851/1/thesis_Ha.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Coupled Effects of Mechanics, Geometry, and Chemistry on Bio-Membrane Behavior", "author": [ { "family_name": "Giang", "given_name": "Ha Thanh", "clpid": "Giang-Ha-Thanh" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Lipid bilayer membranes are models for cell membranes--the structure that helps regulate cell function. Cell membranes are heterogeneous, and the coupling between composition and shape gives rise to complex behaviors that are important to regulation. This thesis seeks to systematically build and analyze complete models to understand the behavior of multi-component membranes.
\r\n\r\nWe propose a model and use it to derive the equilibrium and stability conditions for a general class of closed multi-component biological membranes. Our analysis shows that the critical modes of these membranes have high frequencies, unlike single-component vesicles, and their stability depends on system size, unlike in systems undergoing spinodal decomposition in flat space. An important implication is that small perturbations may nucleate localized but very large deformations. We compare these results with experimental observations.
\r\n\r\nWe also study open membranes to gain insight into long tubular membranes that arise for example in nerve cells. We derive a complete system of equations for open membranes by using the principle of virtual work. Our linear stability analysis predicts that the tubular membranes tend to have coiling shapes if the tension is small, cylindrical shapes if the tension is moderate, and beading shapes if the tension is large. This is consistent with experimental observations reported in the literature in nerve fibers. Further, we provide numerical solutions to the fully nonlinear equilibrium equations in some problems, and show that the observed mode shapes are consistent with those suggested by linear stability. Our work also proves that beadings of nerve fibers can appear purely as a mechanical response of the membrane.
", "doi": "10.7907/BVSK-K782", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7828", "collection": "thesis", "collection_id": "7828", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06032013-180145987", "primary_object_url": { "basename": "PhD_thesis_Ader.pdf", "content": "final", "filesize": 103680719, "license": "other", "mime_type": "application/pdf", "url": "/7828/1/PhD_thesis_Ader.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Earthquakes of the Nepal Himalaya : Towards a Physical Model of the Seismic Cycle", "author": [ { "family_name": "Ader", "given_name": "Thomas Joachim", "clpid": "Ader-Thomas-Joachim" } ], "thesis_advisor": [ { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" } ], "thesis_committee": [ { "family_name": "Simons", "given_name": "Mark", "clpid": "Simons-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Home to hundreds of millions of souls and land of excessiveness, the Himalaya is also the locus of a unique seismicity whose scope and peculiarities still remain to this day somewhat mysterious. Having claimed the lives of kings, or turned ancient timeworn cities into heaps of rubbles and ruins, earthquakes eerily inhabit Nepalese folk tales with the fatalistic message that nothing lasts forever. From a scientific point of view as much as from a human perspective, solving the mysteries of Himalayan seismicity thus represents a challenge of prime importance. Documenting geodetic strain across the Nepal Himalaya with various GPS and leveling data, we show that unlike other subduction zones that exhibit a heterogeneous and patchy coupling pattern along strike, the last hundred kilometers of the Main Himalayan Thrust fault, or MHT, appear to be uniformly locked, devoid of any of the \u201ccreeping barriers\u201d that traditionally ward off the propagation of large events. The approximately 20 mm/yr of reckoned convergence across the Himalaya matching previously established estimates of the secular deformation at the front of the arc, the slip accumulated at depth has to somehow elastically propagate all the way to the surface at some point. And yet, neither large events from the past nor currently recorded microseismicity nearly compensate for the massive moment deficit that quietly builds up under the giant mountains. Along with this large unbalanced moment deficit, the uncommonly homogeneous coupling pattern on the MHT raises the question of whether or not the locked portion of the MHT can rupture all at once in a giant earthquake. Univocally answering this question appears contingent on the still elusive estimate of the magnitude of the largest possible earthquake in the Himalaya, and requires tight constraints on local fault properties. What makes the Himalaya enigmatic also makes it the potential source of an incredible wealth of information, and we exploit some of the oddities of Himalayan seismicity in an effort to improve the understanding of earthquake physics and cipher out the properties of the MHT. Thanks to the Himalaya, the Indo-Gangetic plain is deluged each year under a tremendous amount of water during the annual summer monsoon that collects and bears down on the Indian plate enough to pull it away from the Eurasian plate slightly, temporarily relieving a small portion of the stress mounting on the MHT. As the rainwater evaporates in the dry winter season, the plate rebounds and tension is increased back on the fault. Interestingly, the mild waggle of stress induced by the monsoon rains is about the same size as that from solid-Earth tides which gently tug at the planets solid layers, but whereas changes in earthquake frequency correspond with the annually occurring monsoon, there is no such correlation with Earth tides, which oscillate back-and-forth twice a day. We therefore investigate the general response of the creeping and seismogenic parts of MHT to periodic stresses in order to link these observations to physical parameters. First, the response of the creeping part of the MHT is analyzed with a simple spring-and-slider system bearing rate-strengthening rheology, and we show that at the transition with the locked zone, where the friction becomes near velocity neutral, the response of the slip rate may be amplified at some periods, which values are analytically related to the physical parameters of the problem. Such predictions therefore hold the potential of constraining fault properties on the MHT, but still await observational counterparts to be applied, as nothing indicates that the variations of seismicity rate on the locked part of the MHT are the direct expressions of variations of the slip rate on its creeping part, and no variations of the slip rate have been singled out from the GPS measurements to this day. When shifting to the locked seismogenic part of the MHT, spring-and-slider models with rate-weakening rheology are insufficient to explain the contrasted responses of the seismicity to the periodic loads that tides and monsoon both place on the MHT. Instead, we resort to numerical simulations using the Boundary Integral CYCLes of Earthquakes algorithm and examine the response of a 2D finite fault embedded with a rate-weakening patch to harmonic stress perturbations of various periods. We show that such simulations are able to reproduce results consistent with a gradual amplification of sensitivity as the perturbing period get larger, up to a critical period corresponding to the characteristic time of evolution of the seismicity in response to a step-like perturbation of stress. This increase of sensitivity was not reproduced by simple 1D-spring-slider systems, probably because of the complexity of the nucleation process, reproduced only by 2D-fault models. When the nucleation zone is close to its critical unstable size, its growth becomes highly sensitive to any external perturbations and the timings of produced events may therefore find themselves highly affected. A fully analytical framework has yet to be developed and further work is needed to fully describe the behavior of the fault in terms of physical parameters, which will likely provide the keys to deduce constitutive properties of the MHT from seismological observations.", "doi": "10.7907/z9c8276q", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7895", "collection": "thesis", "collection_id": "7895", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06122013-030022149", "primary_object_url": { "basename": "Leonard_Thesis_Library_June.pdf", "content": "final", "filesize": 12837288, "license": "other", "mime_type": "application/pdf", "url": "/7895/1/Leonard_Thesis_Library_June.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Controlling Wave Propagation through Nonlinear Engineered Granular Systems", "author": [ { "family_name": "Leonard", "given_name": "Andrea Beth", "clpid": "Leonard-Andrea-Beth" } ], "thesis_advisor": [ { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kochmann", "given_name": "Dennis M.", "clpid": "Kochmann-D-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "We study the fundamental dynamic behavior of a special class of ordered granular systems in order to design new, structured materials with unique physical properties. The dynamic properties of granular systems are dictated by the nonlinear, Hertzian, potential in compression and zero tensile strength resulting from the discrete material structure. Engineering the underlying particle arrangement of granular systems allows for unique dynamic properties, not observed in natural, disordered granular media. While extensive studies on 1D granular crystals have suggested their usefulness for a variety of engineering applications, considerably less attention has been given to higher-dimensional systems. The extension of these studies in higher dimensions could enable the discovery of richer physical phenomena not possible in 1D, such as spatial redirection and anisotropic energy trapping. We present experiments, numerical simulation (based on a discrete particle model), and in some cases theoretical predictions for several engineered granular systems, studying the effects of particle arrangement on the highly nonlinear transient wave propagation to develop means for controlling the wave propagation pathways. The first component of this thesis studies the stress wave propagation resulting from a localized impulsive loading for three different 2D particle lattice structures: square, centered square, and hexagonal granular crystals. By varying the lattice structure, we observe a wide range of properties for the propagating stress waves: quasi-1D solitary wave propagation, fully 2D wave propagation with tunable wave front shapes, and 2D pulsed wave propagation. Additionally the effects of weak disorder, inevitably present in real granular systems, are investigated. The second half of this thesis studies the solitary wave propagation through 2D and 3D ordered networks of granular chains, reducing the effective density compared to granular crystals by selectively placing wave guiding chains to control the acoustic wave transmission. The rapid wave front amplitude decay exhibited by these granular networks makes them highly attractive for impact mitigation applications. The agreement between experiments, numerical simulations, and applicable theoretical predictions validates the wave guiding capabilities of these engineered granular crystals and networks and opens a wide range of possibilities for the realization of increasingly complex granular material design.", "doi": "10.7907/NF5J-5W42", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7780", "collection": "thesis", "collection_id": "7780", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302013-210319511", "primary_object_url": { "basename": "SURENDRA_NADH_SOMALA_thesis.pdf", "content": "final", "filesize": 17191935, "license": "other", "mime_type": "application/pdf", "url": "/7780/1/SURENDRA_NADH_SOMALA_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Source Imaging with Dense Sensor Networks: Inversions Based on Adjoint Methods", "author": [ { "family_name": "Somala", "given_name": "Surendra Nadh", "clpid": "Somala-Surendra-Nadh" } ], "thesis_advisor": [ { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Krishnan", "given_name": "Swaminathan", "clpid": "Krishnan-S" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Inversions of earthquake source slip from the recorded ground motions typically impose a number of restrictions on the source parameterization, which are needed to stabilize the inverse problem with sparse data. Such restrictions may include smoothing, causality considerations, predetermined shapes of the local source-time function, and constant rupture speed. The best regional networks have sensor spacing in the tens of kilometers range, much larger than the wavelengths relevant to key aspects of earthquake physics. Novel approaches to providing orders-of-magnitude denser sensing include low-cost sensors (Community Seismic Network) and space-based optical imaging (Geostationary Optical Seismometer). This thesis aims to understand whether the inversion results could be substantially improved, with fewer constraints, by the availability of much denser sensor networks than currently available.
\r\n\r\nInversions that involve large number of sensors and 3D crustal velocity models are intractable with the current source inversion codes. Hence we have developed a new approach that can handle thousands of sensors in heterogeneous media. It employs iterative conjugate gradient optimization based on an adjoint method and involves iterative time-reversed 3D wave propagation simulations using the spectral element method (SPECFEM3D). We have also developed a variant of this adjoint-based method for layered media that utilizes pre-computed Green\u2019s functions instead of the time-reversed wave propagation. The developed methods have been applied to two problems: impact of crustal structure uncertainties on source inversion and resolution of rise time as a function of network spacing and rupture velocity. In the first part, we show that typical uncertainties in crustal velocity models represented by a von Karman distribution of 5 km correlation length and 5% standard deviation (with Hurst exponent of zero), severely degrade the quality of source inversion. However, if the velocity uncertainties have a correlation of 500 m or a standard deviation of 1%, then source inversion has an adequate quality. In the second part we find that supershear ruptures show almost identical source recovery in terms of width of the slip pulse for network spacings ranging from few km to tens of km, even for rise times as short as 1 sec, while subshear ruptures require a network spacing finer than a penetration length that depends on rupture velocity and rise time, as their peak ground velocity decay rapidly with distance from the fault.
\r\n\r\nIn summary, we have developed scalable source inversion tools that will enable exploiting the next generation of very dense earthquake observation systems, improvements in regional scale 3D tomography models and accelerated advancements in computing capabilities. These developments will be critical in resolving the fine spatio-temporal features of earthquake sources that are pertinent to fracture mechanics and earthquake physics. With the 3D iterative time-reversal imaging, one could aspire for extracting more information from the high frequency wavefield by considering joint improvement of source and structure.
\r\n", "doi": "10.7907/Z9WQ01S7", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7144", "collection": "thesis", "collection_id": "7144", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072012-032023169", "primary_object_url": { "basename": "Mello_Michael_2012_thesis.pdf", "content": "final", "filesize": 420337387, "license": "other", "mime_type": "application/pdf", "url": "/7144/1/Mello_Michael_2012_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Identifying the Unique Ground Motion Signatures of Supershear Earthquakes: Theory and Experiments", "author": [ { "family_name": "Mello", "given_name": "Michael", "orcid": "0000-0003-2129-9235", "clpid": "Mello-Michael" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The near-field ground motion signatures associated with sub-Rayleigh and supershear ruptures are investigated using the laboratory earthquake experiment originally developed by Rosakis and co-workers (Xia et al., 2004, 2005a; Lu et al., 2007; Rosakis et al., 2007). Heterodyne laser interferometers enable continuous, high-bandwidth measurements of fault-normal (FN), fault-parallel (FP), and vertical (V) particle velocity ``ground motion\" records at discrete locations on the surface of a Homalite-100 test specimen as a sub-Rayleigh or a supershear rupture sweeps along the frictional fault. Photoelastic interference fringes, acquired using high-speed digital photography, provide a synchronized, spatially resolved, whole field view of the advancing rupture tip and surrounding maximum shear stress field.
\r\n\r\nThe first phase of experimental investigations examine and verify the ground motion signatures of supershear ruptures. Experimental results demonstrate that a shear Mach front produced by a stable supershear rupture is characterized by a dominant FP velocity component. The situation is shown to reverse in the sub-Rayleigh rupture speed regime whereby the FN particle velocity component dominates the ground motion record. Additional distinguishing particle velocity signatures, consistent with theoretical and numerical predictions, and repeatedly observed in experimental records are, (1) a pronounced peak in the FP velocity record induced by the leading dilatational field, which sweeps the measurement station in advance of the shear Mach front, and (2) a pronounced velocity swing in the FN record associated with the arrival of a trailing Rayleigh sub-Rayleigh (secondary) rupture, which follows the arrival of the shear Mach front. Analysis of the particle velocity records also confirms 2D steady-state theoretical predictions pertaining to the separation, attenuation, and radiation partitioning of the shear and dilatational portions of the rupture velocity field components.
\r\n\r\nThe second phase of our experimental investigations re-examine the 2002, Mw7.9, Denali fault earthquake and the remarkable set of near-source ground motion records obtained at (PS10), located approximately 85 km east of the epicenter and just 3 km north of the fault along the Alaska pipeline. Motivated by the analysis and interpretation of these records by (Ellsworth et al., 2004; Dunham and Archuleta, 2004, 2005), we attempt to mimic the Denali strike-slip rupture scenario and replicate the PS10 ground motion signatures using a laboratory earthquake experiment. The experiments feature a left-to-right (west-to-east) propagating right lateral rupture within a Homalite-100 test specimen with particle velocity data collected at a near-field station situated just above (north of) the fault. Both sub-Rayleigh and supershear laboratory earthquake experiments are conducted using the Denali PS10 configuration in order to compare and contrast the resulting particle velocity signatures. Supershear laboratory records capture all of the prominent features displayed within the PS10 ground motion records. Noted velocity signatures are correlated to the location of the rupture fronts and their noted arrival times in the synchronized photoelastic image sequence. Scaling relationships are also presented which transform the laboratory records through six orders of magnitude in time, to match the scale of the PS10 ground motion records. The strong correlation between the scaled experimental records and the actual PS10 ground motion records support the hypothesis that the Denali strike-slip fault exhibited a supershear burst.
\r\n\r\nFinally, we present a 2D steady state, stress-velocity formulation that relates the FP and FN particle velocity records measured close to the fault, to the evolution of the stress tensor at the same location. A locally steady-state condition is assumed within a restricted time interval in order to invoke these relationships and estimate the dynamic stresses, \u03c3xx(t) and \u03c4(t), at the near-fault station. Dynamic stress measurements enable a new class of friction investigations using the laboratory earthquake configuration. Experimental findings are presented, which capture the temporal and spatial distributions of \u03c3xx and \u03c4, evolution of the dynamic friction coefficient, and velocity weakening behavior of a supershear slip-pulse.
", "doi": "10.7907/DJDD-2487", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:6870", "collection": "thesis", "collection_id": "6870", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03262012-175814763", "primary_object_url": { "basename": "TChenThesis.pdf", "content": "final", "filesize": 13489798, "license": "other", "mime_type": "application/pdf", "url": "/6870/1/TChenThesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Part I: Structure of Central and Southern Mexico from Velocity and Attenuation Tomography. Part II: Physics of Small Repeating Earthquakes ", "author": [ { "family_name": "Chen", "given_name": "Ting", "clpid": "Chen-Ting" } ], "thesis_advisor": [ { "family_name": "Clayton", "given_name": "Robert W.", "orcid": "0000-0003-3323-3508", "clpid": "Clayton-R-W" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Stock", "given_name": "Joann M.", "orcid": "0000-0003-4816-7865", "clpid": "Stock-J-M" }, { "family_name": "Clayton", "given_name": "Robert W.", "orcid": "0000-0003-3323-3508", "clpid": "Clayton-R-W" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "orcid": "0000-0002-4827-7987", "clpid": "Ampuero-J-P" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "In part I, the 3D velocity and attenuation structure of the Cocos subduction zone in Mexico is imaged using earthquakes recorded by two temporary seismic arrays and local stations. Inversion results reveal low-attenuation and high-velocity Cocos slab. The slab dip angle increases from almost flat in central Mexico near Mexico City to about 30 degrees in southern Mexico near the Isthmus of Tehuantepec. High attenuation and low velocity in the crust beneath the Trans-Mexico Volcanic Belt correlate with low resistivity, and are probably related to dehydration and melting process. The most pronounced high-attenuation, low-Vp and high-Vp/Vs anomaly is found in the crust beneath the Veracruz Basin. A high-velocity structure dipping southward from the Gulf of Mexico near the Isthmus of Tehuantepec coincides with a discontinuity from a receiver functions study, and provides an evidence for the collision between the Yucatan Block and Mexico in the Miocene.
\r\n\r\n\r\nIn part II, we show that a model of small repeating earthquakes based on laboratory-derived rate and state friction laws reproduces the observed scaling between the recurrence time and seismic moment. In the model, a small fault patch governed by velocity-weakening friction is surrounded by a much larger velocity-strengthening region. For a fixed set of friction parameters, the observed scaling is reproduced by varying the size of the velocity-weakening patch. We further investigate the behavior of small repeating earthquakes in related models under different scenarios, including several forms of the state evolution equations in rate- and state-dependent friction laws, rectangular velocity-weakening patch geometries, quasi-dynamic vs. fully dynamic representation of inertial effects, and 2D vs. 3D simulations. We find that the simulated scalings between the recurrence time and seismic moment for these different scenarios are similar while differences do exist. We propose a theoretical model for the scaling between the recurrence time and seismic moment of small repeating earthquakes. The obtained theoretical insight is used to find the combinations of fault properties that allow the model to fit the observed scaling and range of the seismic moment and recurrence time.
", "doi": "10.7907/REJW-YJ88", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:6188", "collection": "thesis", "collection_id": "6188", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11222010-114324484", "primary_object_url": { "basename": "GVenturini_thesis.pdf", "content": "final", "filesize": 6798002, "license": "other", "mime_type": "application/pdf", "url": "/6188/1/GVenturini_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Topics in Multiscale Modeling of Metals and Metallic Alloys", "author": [ { "family_name": "Venturini", "given_name": "Gabriela Natalia", "clpid": "Venturini-Gabriela-Natalia" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Marian", "given_name": "Jaime", "orcid": "0000-0001-9000-3405", "clpid": "Marian-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "In a number of areas of application, the behavior of systems depends sensitively on properties that pertain to the atomistic scale, i. e., the angstrom and femtosecond scales. However, generally the behaviors of interest are macroscopic and are characterized by slow evolution on the scale of meters and years. This broad disparity of length and time scales places extraordinary challenges in computational material science.
\r\n\r\nThe overarching objective of this dissertation is to address the problem of multiple space and time scales in atomistic systems undergoing slow macroscopic evolution while retaining full atomistic detail. Our approach may be summarized as follows:
\r\n\r\n(1) The issue of accounting for finite temperature in coarse grained systems has not been solved entirely. For finite temperature systems at equilibrium, constructing an effective free energy in terms of a reduced set of atomic degrees of freedom is still an open area of research. In particular, the thermal vibrations of the missing degrees of freedom need to be accounted for. This is specially important if the aim of the simulation is to determine the dynamic properties of a system, or to allow the transmission of dynamic information between regions of different spatial discretization. To this end, we introduce a framework to simulate (spatially) coarse dynamic systems using the Quasicontinuum method (QC). The equations of motion are strictly derived from dissipative Lagrangian mechanics, which provides a classical Langevin implementation where the characteristic time is governed by the vibrations of the finest length scale in the computational cell. In order to assess the framework's ability to transmit information across scales, we study the phonon impoverish spectra in coarse regions and the resulting underestimation of thermal equilibrium properties.
\r\n\r\n(2) Atomistic simulations have been employed for the past thirty years to determine structural and thermodynamic (equilibrium) properties of solids and their defects over a wide range of temperatures and pressures. The traditional Monte Carlo (MC) and Molecular Dynamics (MD) methods, while ideally suited to these calculations, require appreciable computational resources in order to calculate the long-time averages from which properties are obtained. In order to permit a reasonably quick, but accurate determination of the equilibrium properties of interest, we present an extension of the \u201cmaximum entropy\u201d method to build effective alloy potentials while avoiding the treatment of all the system's atomic degrees of freedom. We assess the validity of the model by testing its ability to reproduce experimental measurements.
\r\n\r\n(3) Based upon these effective potentials, we present a numerical framework capable of following the time evolution of atomistic systems over time windows currently beyond the scope of traditional atomistic methods such as Molecular Dynamics (MD) or Monte Carlo (MC). This is accomplished while retaining the underlying atomistic description of the material. We formulate a discrete variational setting in which the simulation of time-dependent phenomena is reduced to a sequence of incremental problems, each characterized by a variational principle. In this fashion we are able to study the interplay between deformation and diffusion using time steps or strain rates that are orders of magnitude larger or smaller than their MD|MC counterparts.
\r\n\r\n(4) We formulate a new class of \u201cReplica Time Integrators\u201d (RTIs) that allows for the two-way transmission of thermal phonons across mesh interfaces. This two-way transmission is accomplished by representing the state of the coarse region by a collection of identical copies or \u201creplicas\u201d of itself. Each replica runs at its own slow time step and is out-of-phase with respect to the others by one fast time step. Then, each replica is capable of absorbing from the fine region the elementary signal that is in phase with the replica. Conversely, each replica is capable of supporting --and transmitting to the fine region-- an elementary signal of a certain phase. Since fine and coarse regions evolve asynchronously in time, RTIs permit both spatial and temporal coarse graining of the system of interest. Using a combination of phase-error analysis and numerical testing we find that RTIs are convergent, and allow step waves and thermal phonons to cross mesh interfaces in both directions losslessly.
\r\n", "doi": "10.7907/D6YS-B365", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6103", "collection": "thesis", "collection_id": "6103", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10052010-230939247", "primary_object_url": { "basename": "MarcialGonzalez-Thesis.pdf", "content": "final", "filesize": 9980691, "license": "other", "mime_type": "application/pdf", "url": "/6103/1/MarcialGonzalez-Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Energy and Force Stepping Integrators in Lagrangian Mechanics", "author": [ { "family_name": "Gonzalez", "given_name": "Marcial", "clpid": "Gonzalez-Marcial" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The overarching goal of this thesis is to develop new numerical time integration schemes for Lagrangian mechanics that better cope with the challenges of understanding the dynamic behavior of materials. We specifically address the formulation of convergent time integration schemes that exhibit good long-term behavior---such as conferred by symplecticity and exact conservation properties---and that have the ability to automatically and asynchronously modulate the time step in different regions of the domain. We achieve these properties in a progression of three developments: (i) energy-stepping, (ii) force-stepping, and (iii) asynchronous energy-stepping integrators. These developments are based on a new method of approximation for Lagrangian mechanics, proposed in this thesis, that consists of replacing the Lagrangian of the system by a sequence of approximate Lagrangians that can be solved exactly. Then, energy-stepping integrators result from replacing the potential energy by a piecewise constant approximation, force-stepping integrators result from replacing the potential energy by a piecewise affine approximation, and asynchronous energy-stepping integrators result from replacing localized potential energies by piecewise constant approximations. Throughout the dissertation, the properties of these time integrators are theoretically predicted and born out by a number of selected examples of application. Furthermore, we address the challenges of understanding the propagation of solitary waves in granular crystals at low impact velocity conditions by investigating the role of energy-trapping effects with the numerical time integration schemes developed in this work.\r\n", "doi": "10.7907/SP10-A207", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6202", "collection": "thesis", "collection_id": "6202", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12122010-174517720", "primary_object_url": { "basename": "Thesis_Ahmed_Ettaf_Elbanna.pdf", "content": "final", "filesize": 8060926, "license": "other", "mime_type": "application/pdf", "url": "/6202/1/Thesis_Ahmed_Ettaf_Elbanna.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Pulselike Ruptures on Strong Velocity-Weakening Frictional Interfaces: Dynamics and Implications", "author": [ { "family_name": "Elbanna", "given_name": "Ahmed Ettaf", "clpid": "Elbanna-Ahmed-Ettaf" } ], "thesis_advisor": [ { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Greer", "given_name": "Julia R.", "clpid": "Greer-J-R" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Seismic inversion and computational models have shown that earthquake ruptures may propagate in one of two basic modes; the cracklike mode and the slip pulse mode. In this work we use analytical and numerical techniques to study the dynamics and implications of pulselike ruptures propagating on strong velocity-weakening frictional interfaces using both discrete and continuum models of fracture.
\r\n\r\nResults of the study of the discrete spring block slider model suggest that strong velocity-weakening friction might yield to the propagation of unsteady slip pulses and chaotic dynamics. The prestress in most of these systems evolves into very heterogeneous spatial distributions characterized, in general, by non-Gaussian statistics and power-law spectral properties. It is also shown that the combined effect of slip pulse propagation and strong velocity-weakening friction could yield to size effects in strength with the strength decreasing as a power law with increasing rupture length.
\r\n\r\nBy examining the energy budget of slip pulses in the discrete model, we show that it is possible to derive a nonlinear differential equation that could predict the final slip distribution in an event, given the prestress existing before that event and some information about friction and pulse dynamics. The equation is successful in replicating many of the macroscopic slip features, including the slip distribution and total rupture length, can also match many long-time statistics regarding the prestress evolution and the event size distribution.
\r\n\r\nResults from the continuum study suggest that the absence of steady pulses in previous studies could be attributed to the details of the nucleation procedure. We show that steady pulses could exist on strong velocity-weakening friction and uniform prestress if both the prestress and nucleation procedures are correctly tuned. We find that steady pulses are unstable to perturbations in the form of a step in the prestress and could arrest quickly in regions of low prestress. Steady pulses are also found to adapt well to local fluctuations in the prestress, leading to heterogeneous slip distributions. This result might have important implications for the problem of slip complexity in real earthquakes.
\r\n", "doi": "10.7907/80MV-6Y66", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6389", "collection": "thesis", "collection_id": "6389", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05122011-154526450", "type": "thesis", "title": "Damage Evolution in Composite Materials and Sandwich Structures Under Impulse Loading", "author": [ { "family_name": "Silva", "given_name": "Michael Lee", "clpid": "Silva-Michael-Lee" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Shukla", "given_name": "Arun", "clpid": "Shukla-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Damage evolution in composite materials is a rather complex phenomenon. There are numerous failure modes in composite materials stemming from the interaction of the various constituent materials and the particular loading conditions. This thesis is concerned with investigating damage evolution in sandwich structures under repeated transient loading conditions associated with impulse loading due to hull slamming of high-speed marine craft. To fully understand the complex stress interactions, a full field technique to reveal stress or strain is required. Several full field techniques exist but are limited to materials with particular optical properties. A full field technique applicable to most materials is known as thermoelastic stress analysis (TSA) and reveals the variation in sum of principal stresses of a cyclically loaded sample by correlating the stresses to a small temperature change occurring at the loading frequency. Digital image correlation (DIC) is another noncontact full field technique that reveals the deformation field by tracking the motion of subsets of a random speckle pattern during the loading cycles.
\r\n\r\nA novel experimental technique to aid in the study of damage progression that combines TSA and DIC simultaneously utilizing a single infrared camera is presented in this thesis. A technique to reliably perform DIC with an infrared (IR) camera is developed utilizing variable emissivity paint. The thermal data can then be corrected for rigid-body motion and deformation such that each pixel represents the same material point in all frames. TSA is then performed on this corrected data, reducing motion blur and increasing accuracy. This combined method with a single infrared camera has several advantages, including a straightforward experimental setup without the need to correct for geometric effects of two spatially separate cameras. Additionally, there is no need for external lighting in TSA as the measured electromagnetic radiation is emitted by the sample\u2019s thermal fields.
\r\n\r\nThe particular stress resolution of TSA will depend on properties of the material of interest but the noise floor for the temperature variation is universal to the camera utilized. For the camera system in this thesis, the noise floor was found to be fairly frequency independent with a magnitude of 0.01 oC, giving the minimum measurable stress for 2024 aluminum alloy of 3.6 MPa and for Nylon of 0.84 MPa. The average displacement range found during a static DIC test with IR images was 0.1 pixels. The maximum displacement variation at 1 Hz was 0.018 pixels. The average variation in strain at 1 Hz was 25 microstrain comparable to traditional DIC measurements in the visible optical regime.
\r\n\r\nThe combined TSA-DIC method in IR was validated with several benchmark example problems including plate structures with holes, cracks, and bimaterials. The validated technique was applied to foam-core sandwich composite beams under repeated simulated wave slamming loading. There are numerous failure modes in sandwich composite materials and the full field stress and strain from TSA and DIC, respectively, allow for improved failure analysis and prediction. Understanding damage in sandwich structures under impulse loading is a complex open area of research and the combined TSA-DIC method provides further insight into the failure process.
\r\n", "doi": "10.7907/CRX1-7D43", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6422", "collection": "thesis", "collection_id": "6422", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05242011-022845109", "primary_object_url": { "basename": "draft2.pdf", "content": "final", "filesize": 9777877, "license": "other", "mime_type": "application/pdf", "url": "/6422/1/draft2.pdf", "version": "v11.0.0" }, "type": "thesis", "title": "Repeatability of Joint-Dominated Deployable Masts", "author": [ { "family_name": "Stohlman", "given_name": "Olive Remington", "clpid": "Stohlman-Olive-Remington" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Kim", "given_name": "Yunjin", "clpid": "Kim-Yunjin" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Pellegrino", "given_name": "Sergio", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Deployable masts are a class of structure that can be stowed in a small volume and expanded into long, slender, and stable booms. Their greatest benefit as space structures is their packing ratio: masts can typically be packed to a fraction of their deployed length at a diameter only modestly wider than their deployed width. This thesis is concerned with precision deployable masts, which can be stowed and deployed with repeatability of the tip position of better than 1 mm over 60 m. The methods of investigation are experimental measurements of a sample mast and numerical modeling of the mast with specially attention to hysteretic joints.
\r\n\r\nA test article of an ADAM mast was used for the experimental work. Two categories of experi- ment were pursued: measurements of mast components as inputs to the model, and measurements of full bays as validation cases for the model. Measurements of the longeron ball end joint friction, cable preload, and latch behavior are of particular note, and were evaluated for their variability. Further measurements were made of a bay in torsion and a short two-bay mast in shear, showing that there is residual displacement in this mast after shear loading is applied and released.
\r\n\r\nThe modeling approach is described in detail, with attention to the treatment of the mast latches, which lock the structure in its deployed configuration. A user element subroutine was used within the framework of the Abaqus finite element analysis solver to model the behavior of the latches with high fidelity.
\r\n\r\nValidation cases for the model are presented in comparison with experimental observations of a two-bay mast. These cases show that the model captures a number of important and complex nonlinear effects of the hysteretic mast components. Parametric studies of the impacts of component behaviors and modeling practices are explored, emphasizing the impacts of part variability and the idealization of the mast latching mechanisms.
", "doi": "10.7907/D3AR-G573", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6473", "collection": "thesis", "collection_id": "6473", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292011-200916324", "primary_object_url": { "basename": "phanish_PhD_thesis.pdf", "content": "final", "filesize": 2123034, "license": "other", "mime_type": "application/pdf", "url": "/6473/1/phanish_PhD_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Coarse-Graining Kohn-Sham Density Functional Theory", "author": [ { "family_name": "Suryanarayana", "given_name": "Phanish", "clpid": "Suryanarayana-Phanish" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Defects, though present in relatively minute concentrations, play a significant role in determining macroscopic properties. Even vacancies, the simplest and most common type of defect, are fundamental to phenomena like creep, spall and radiation ageing. This necessitates an accurate characterization of defects at physically relevant concentrations, which is typically in parts per million. This represents a unique challenge since both the electronic structure of the defect core as well as the long range elastic field need to be resolved simultaneously. Unfortunately, accurate ab-initio electronic structure calculations are limited to a few hundred atoms, which is orders of magnitude smaller than that necessary for a complete description. Thus, defects represent a truly challenging multiscale problem.
\r\n\r\nDensity functional theory developed by Hohenberg, Kohn and Sham (DFT) is a widely accepted, reliable ab-initio method for computing a wide range of material properties. We present a real-space, non-periodic, finite-element and max-ent formulation for DFT. We transform the original variational problem into a local saddle-point problem, and show its well-posedness by proving the existence of minimizers. Further, we prove the convergence of finite-element approximations including numerical quadratures. Based on domain decomposition, we develop parallel finite-element and max-ent implementations of this formulation capable of performing both all-electron and pseudopotential calculations. We assess the accuracy of the formulation through selected test cases and demonstrate good agreement with the literature.
\r\n\r\nTraditional implementations of DFT solve for the wavefunctions, a procedure which has cubic-scaling with respect to the number of atoms. This places serious limitations on the size of the system which can be studied. Further, they are not amenable to coarse-graining since the wavefunctions need to be orthonormal, a global constraint. To overcome this, we develop a linear-scaling method for DFT where the key idea is to directly evaluate the electron density without solving for the individual wavefunctions. Based on this linear-scaling method, we develop a numerical scheme to coarse-grain DFT derived solely based on approximation theory, without the introduction of any new equations and resultant spurious physics. This allows us to study defects at a fraction of the original computational cost, without any significant loss of accuracy. We demonstrate the efficiency and efficacy of the proposed methods through examples. This work enables the study of defects like vacancies, dislocations, interfaces and crack tips using DFT to be computationally viable.
", "doi": "10.7907/GCKH-EX20", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:5909", "collection": "thesis", "collection_id": "5909", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022010-172250027", "primary_object_url": { "basename": "Thesis_Liu_2010-total.pdf", "content": "final", "filesize": 8535268, "license": "other", "mime_type": "application/pdf", "url": "/5909/17/Thesis_Liu_2010-total.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Linking Surface Evolution with Mantle Dynamic Processes Using Adjoint Models with Data Assimilation", "author": [ { "family_name": "Liu", "given_name": "Lijun", "clpid": "Liu-Lijun" } ], "thesis_advisor": [ { "family_name": "Gurnis", "given_name": "Michael C.", "orcid": "0000-0003-1704-597X", "clpid": "Gurnis-M-C" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Gurnis", "given_name": "Michael C.", "orcid": "0000-0003-1704-597X", "clpid": "Gurnis-M-C" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" }, { "family_name": "Stock", "given_name": "Joann M.", "orcid": "0000-0003-4816-7865", "clpid": "Stock-J-M" }, { "family_name": "Saleeby", "given_name": "Jason B.", "clpid": "Saleeby-J-B" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Quantifying the relationship between subsolidus mantle convection and surface evolution is a fundamental goal of geophysics. Toward this goal progress has been slow due to incomplete knowledge of the earth\u2019s internal structure and properties. While seismic tomography reveals details on internal 3D structure of the present mantle, evolution of the subsolidus mantle during the geological past remains elusive. This thesis attempts to solve the time inversion of mantle convection using the adjoint method based on present-day seismic images and geological and geophysical observations dictating the past evolution of solid earth.
\r\n\r\nThe adjoint method, widely used in meteorological and oceanographic predictions, can be applied to mantle convection for the recovery of unknown initial conditions through the assimilation of present-day mantle seismic structure. We propose that an optimal first guess to the initial condition can be obtained through a simple backward integration (SBI) of the governing equations thus lessening the computational expense. By incorporating time-dependent surface dynamic topography in addition to present-day mantle structure, the adjoint method is improved so as to constrain uncertain mantle dynamic properties and initial condition simultaneously. The theory is derived from the governing equations of mantle convection and validated by synthetic experiments for a single- and two-layer viscosity mantle within regionally bounded spherical shells. For both cases, we show that the theory can constrain mantle properties with errors arising through the adjoint recovery of the initial condition. For the two-layer model, there is a trade-off between the temperature scaling and lower mantle viscosity.
\r\n\r\nBy assimilating seismic structure and plate motions in the inverse mantle convection model, we reconstruct Farallon plate subduction back to 100 Ma. We put constraints on basic mantle properties, including both the depth dependence of mantle viscosity and slab buoyancy, by predicting proxies of dynamic topography evident in the stratigraphy of the North American Cretaceous western interior seaway. Models that fit stratigraphy well require the Farallon slab to have been flat lying in the Late Cretaceous, consistent with geological reconstructions. The models predict an extensive zone of shallow-dipping subduction extending beyond the flat-lying slab farther east and north, while the limited region of subducting flat slab resembles an oceanic plateau. In order to test the hypothesis of oceanic plateau subduction and its relationship to the Laramide orogeny, we compare the inverse convection model with plate reconstructions. Two prominent seismic anomalies on the Farallon plate recovered from inverse models coincide with paleogeographically-restored positions of conjugates to the Shatsky and Hess plateaus when they subducted beneath North America. The distributed shortening of the Laramide orogeny closely tracked the passage of the Shatsky conjugate beneath North America, while the effects of Hess conjugate subduction were restricted to the northern Mexico foreland belt. We find that Laramide uplifts were consequences of the removal, rather than the emplacement, of the Shatsky conjugate, and we predict that these subducted plateaus should be detectable by the USArray seismic experiment.
\r\n\r\nThe inverse convection models predict a continuous vertical motion history of western U.S., which is further validated by constraints on the vertical motion of the Colorado Plateau since the Cretaceous. With the arrival of the flat-lying Farallon slab, dynamic subsidence swept from west to east over the western U.S., peaking at 86 Ma within the Colorado Plateau. This eastward migrating dynamic subsidence is consistent with a recently compiled backstripping study that shows a long-wavelength residual subsidence shifting to the east, coincident with the passage of the flat slab beneath North America in our inverse model. Two stages of uplift followed the removal of the Farallon slab below the Colorado Plateau: one in the latest Cretaceous, and the other in the Eocene, with a cumulative uplift of ~1.2 km; the former represents the Laramide uplift which also marks the initial uplift of the entire western U.S. Both the descent of the slab and buoyant upwellings raised the Colorado Plateau to its current elevation during the Oligocene. A locally thick lithosphere enhances coupling to the upper mantle so that the Colorado Plateau has a higher topography with sharp edges. Our models also predict that the plateau tilted downward to the northeast before the Oligocene, caused by northeast-trending subduction of the Farallon slab, and that this northeast tilting diminished and reversed to the southwest during the Miocene in response to buoyant upwellings.
\r\n\r\nOverall, this thesis shows that the adjoint models with data assimilation are useful in linking surface evolution to deep mantle processes both over North America and areas beyond. While more research is clearly needed to construct a more earth-like model, this thesis presents an important advance in data-oriented geodynamic models.
", "doi": "10.7907/6VHZ-V130", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5918", "collection": "thesis", "collection_id": "5918", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06062010-235315977", "primary_object_url": { "basename": "minson_thesis.pdf", "content": "final", "filesize": 12367870, "license": "cc_by_nc_sa", "mime_type": "application/pdf", "url": "/5918/1/minson_thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "A Bayesian Approach to Earthquake Source Studies", "author": [ { "family_name": "Minson", "given_name": "Sarah Ellen", "orcid": "0000-0001-5869-3477", "clpid": "Minson-Sarah-Ellen" } ], "thesis_advisor": [ { "family_name": "Simons", "given_name": "Mark", "clpid": "Simons-M" } ], "thesis_committee": [ { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" }, { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Simons", "given_name": "Mark", "clpid": "Simons-M" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Bayesian sampling has several advantages over conventional optimization approaches to solving inverse problems. It produces the distribution of all possible models sampled proportionally to how much each model is consistent with the data and the specified prior information, and thus images the entire solution space, revealing the uncertainties and trade-offs in the model. Bayesian sampling is applicable to both linear and non-linear modeling, and the values of the model parameters being sampled can be constrained based on the physics of the process being studied and do not have to be regularized. However, these methods are computationally challenging for high-dimensional problems.
\r\n\r\nUntil now the computational expense of Bayesian sampling has been too great for it to be practicable for most geophysical problems. I present a new parallel sampling algorithm called CATMIP for Cascading Adaptive Tempered Metropolis In Parallel. This technique, based on Transitional Markov chain Monte Carlo, makes it possible to sample distributions in many hundreds of dimensions, if the forward model is fast, or to sample computationally expensive forward models in smaller numbers of dimensions. The design of the algorithm is independent of the model being sampled, so CATMIP can be applied to many areas of research.
\r\n\r\nI use CATMIP to produce a finite fault source model for the 2007 Mw 7.7 Tocopilla, Chile earthquake. Surface displacements from the earthquake were recorded by six interferograms and twelve local high-rate GPS stations. Because of the wealth of near-fault data, the source process is well-constrained. I find that the near-field high-rate GPS data have significant resolving power above and beyond the slip distribution determined from static displacements. The location and magnitude of the maximum displacement are resolved. The rupture almost certainly propagated at sub-shear velocities. The full posterior distribution can be used not only to calculate source parameters but also to determine their uncertainties. So while kinematic source modeling and the estimation of source parameters is not new, with CATMIP I am able to use Bayesian sampling to determine which parts of the source process are well-constrained and which are not.
", "doi": "10.7907/3RT9-3215", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5883", "collection": "thesis", "collection_id": "5883", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282010-141343271", "type": "thesis", "title": "Shape Changing Transformations: Interactions with Plasticity and Electrochemical Processes", "author": [ { "family_name": "Roumi", "given_name": "Farshid", "clpid": "Roumi-Farshid" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Solids undergo phase transformations where the crystal structure changes with temperature, chemical potential, stress, applied electric fields, or other external parameters. These occur by either long-range diffusion of atoms (diffusional phase transformation) or by some form of cooperative, homogeneous movement of many atoms that results in changes in crystal structure (displacive phase transformation). In the latter case, these movements are usually less than the interatomic distances, and the atoms maintain their coordination. The most common example of displacive phase transformations is martensitic transformation. The martensitic transformation in steel is economically very important and can result in very different behavior in the product. Other examples of martensitic transformations are shape memory alloys which are lightweight, solid-state alternatives to conventional actuators such as hydraulic, pneumatic, and motor-based systems.
\r\n\r\nThe martensitic transformation usually only depends on temperature and stress and, in contrast to diffusion-based transformations, is not time dependent. In shape memory alloys the transformation is reversible. On the other hand in steel, the martensite formation from austenite by rapidly cooling carbon-steel is not reversible; so steel does not have shape memory properties.
\r\n\r\nIn Chapters 2 and 3, we study the interesting yet very complicated behavior of martensitic transformation interactions with plastic deformations. A good example here is steel, which has been known for thousands of years but still is believed to be a very complicated material. Steel can show different behavior depending on its complex microstructure. Thus understanding the formation mechanisms is crucial for the interpretation and optimization of its properties. As an example, low alloyed steels with transformation induced plasticity (TRIP), metastable austenite steels, are known for strong hardening and excellent elongation and strength. It is suggested that the strain-induced transformation of small amounts of untransformed (retained) austenite into martensite during plastic deformation is a key to this excellent behavior.
\r\n \r\nIn Chapters 4 and 5, we study the interactions of solid-solid phase transformations with electrochemical processes. It is suggested that electronic and ionic structures depends on lattice parameters, thus it is expected that structural transformations can lead to dramatic changes in material properties. These transformations can also change the energy barrier and hysteresis. It is known that compatible interfaces can reduce elastic energy and hysteresis, thus may extend the life of the system. Solid-solid transformations change the crystalline structure. These geometry changes can have long range effects and cause stresses in the whole material. The generated stress field itself changes the total free energy, due to the change in elastic energy, and thus, the electrochemical potential and processes are affected. An example is olivine phosphates which are candidates for cathode material in Li-ion batteries. These materials undergo an orthorhombic to orthorhombic phase transition. Experiments in the literature have suggested that elastic compatibility can affect rates of charge/discharge in the battery. Our theory provides some insight into this observation.
", "doi": "10.7907/P94H-4B23", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:1535", "collection": "thesis", "collection_id": "1535", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04282009-202026", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 5015961, "license": "other", "mime_type": "application/pdf", "url": "/1535/2/thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Investigations of Earthquake Source Processes Based on Fault Models with Variable Friction Rheology", "author": [ { "family_name": "Kaneko", "given_name": "Yoshihiro", "orcid": "0000-0003-2342-0131", "clpid": "Kaneko-Yoshihiro" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Clayton", "given_name": "Robert W.", "clpid": "Clayton-R-W" }, { "family_name": "Ampuero", "given_name": "Jean-Paul", "clpid": "Ampuero-J-P" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Ample experimental and observational evidence suggests that friction properties on natural faults vary spatially. In the lab, rock friction depends on temperature and confining pressure and it can be either velocity weakening or velocity strengthening, leading to either unstable or stable slip. Such variations in friction rheology can explain patterns of seismic and aseismic fault slip inferred from field observations.
\r\n\r\nThis thesis studies earthquake source processes using models with relatively simple but conceptually important patterns of velocity-weakening and velocity-strengthening friction that can arise on natural faults. Based on numerical and analytical modeling, we explore the consequences of such patterns for earthquake sequences, interseismic coupling, earthquake nucleation processes, aftershock occurrence, peak ground motion in the vicinity of active faults, and seismic slip budget at shallow depths. The velocity-dependence of friction is embedded into the framework of logarithmic rate and state friction laws.
\r\n\r\nIn addition to using existing boundary integral methods, which are accurate and efficient in simulating slip on planar faults embedded in homogeneous elastic media, the thesis develops spectral element methods to consider single dynamic ruptures and long-term histories of seismic and aseismic slip in models with layered bulk properties.
\r\n\r\nThe results of this thesis help to understand a number of observed fault slip phenomena, such as variability in earthquake patterns and its relation to interseismic coupling, seismic quiescence following decay of aftershocks at inferred rheological transitions, instances of poor correlation between static stress changes and aftershock occurrence, the lack of universally observed supershear rupture near the free surface, and coseismic slip deficit of large strike-slip earthquakes at shallow depths. The models, approaches, and numerical methods developed in the thesis motivate and enable consideration of many other earthquake source problems, such as the combined effect of two or more triggering mechanisms on aftershock rates, inferring friction properties on natural faults based on seismic and geodetic measurements, seismic hazard assessment based on observed interseismic coupling, and the effect of heterogeneous and/or nonelastic bulk properties on earthquake sequences.
\r\n\r\n", "doi": "10.7907/1WGT-6623", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:3567", "collection": "thesis", "collection_id": "3567", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09162008-023546", "type": "thesis", "title": "A Critical Appraisal of Nanoindentation with Application to Elastic-Plastic Solids and Soft Materials", "author": [ { "family_name": "Poon", "given_name": "Poh Chieh Benny", "clpid": "Poon-Poh-Chieh-Benny" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Rittel", "given_name": "Daniel", "clpid": "Rittel-D" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This study examines the accuracy of the extracted elastic properties using nanoindentation. Since the conventional method to extract these properties utilizes Sneddon\u2019s elastic solution, this study first considers indentations of linearly elastic solids for direct comparison. The study proposes a criterion for a converged specimen\u2019s geometry and modifies Sneddon\u2019s equation to account for the finite tip radius and specimen compressibility effects. A composite correction factor is derived to account for the violations of the underlying assumptions behind Sneddon\u2019s derivation. This factor is a function of indentation depth, and a critical depth is derived beyond which the finite tip radius effect will be insignificant. Techniques to identify the radius of curvature of the indenter and to decouple the elastic constants for linear elastic materials are proposed. Experimental results on nanoindentation of natural latex are reported and discussed in light of the proposed modified relation and techniques.
\r\n\r\nThe second part of the study examines the accuracy of the extracted material properties in elastic-plastic nanoindentations. The study establishes that the accurate determination of the projected area of contact, A, is crucial. However, the conventional method to determine A is largely limited to elastic materials, hence a new electrical resistance method is proposed to measure A for elastic-plastic materials. With an accurate A, the error associated with the extracted elastic material properties is reduced by more than 50% in some cases. This error remains to be a function of the material\u2019s Poisson\u2019s ratio, which is identified to influence the amount of residual stresses at the plastic imprint.
\r\n\r\nFinally, this study examines the accuracy of the extracted material properties in the nanoindentation of soft materials using an Atomic Force Microscope (AFM). The effects of cantilever stiffness, preload, and surface interaction forces are observed to influence the measurements. Three set of experiments were performed to decouple these effects. The effect of a preload resembles a shift of nanoindentation load-displacement curve, while the cantilever stiffness is observed to have significant influence on the measurement of the surface forces. Lastly, a novel technique to account for these effects is proposed, in order to accurately extract the material properties of interest.
", "doi": "10.7907/J1WM-BW36", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:5207", "collection": "thesis", "collection_id": "5207", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272009-165712", "primary_object_url": { "basename": "Ajay_Thesis.pdf", "content": "final", "filesize": 1453432, "license": "other", "mime_type": "application/pdf", "url": "/5207/1/Ajay_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Simulation of Dynamic Interface Fracture Using Spectral Boundary Integral Method", "author": [ { "family_name": "Harish", "given_name": "Ajay Bangalore", "clpid": "Harish-Ajay-Bangalore" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Simulation of three-dimensional dynamic fracture events constitutes one of the most challenging topics in the field of computational mechanics. Spontaneous dynamic fracture along the interface of two elastic solids is of great importance and interest to a number of disciplines in engineering and science. Applications include dynamic fractures in aircraft structures, earthquakes, thermal shocks in nuclear containment vessels and delamination in layered composite materials.
\r\n\r\nThis thesis presents numerical modeling of laboratory experiments on dynamic shear rupture, giving an insight into the experimental nucleation conditions. We describe a methodology of dynamic rupture simulation using spectral boundary integral method, including the theoretical background, numerical implementation and cohesive zone models relevant to the dynamic fracture problem. The developed numerical implementation is validated using the simulation of Lamb's problem of step loading on an elastic half space and mode I crack propagation along a bonded interface. Then the numerical model and its comparison with experimental measurements is used to investigate the initiation procedure of the dynamic rupture experiments. The inferred parameters of the initiation procedure can be used in future studies to model the experimental results on supershear transition and rupture models.
\r\n", "doi": "10.7907/B6P7-F707", "publication_date": "2009", "thesis_type": "engd", "thesis_year": "2009" }, { "id": "thesis:5193", "collection": "thesis", "collection_id": "5193", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05212009-173044", "primary_object_url": { "basename": "Thesis_Bo_Li.pdf", "content": "final", "filesize": 17993982, "license": "other", "mime_type": "application/pdf", "url": "/5193/1/Thesis_Bo_Li.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The Optimal Transportation Method in Solid Mechanics", "author": [ { "family_name": "Li", "given_name": "Bo", "orcid": "0000-0002-0127-8210", "clpid": "Li-Bo" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This dissertation is concerned with the development of a robust and efficient meshless method, the Optimal Transportation Method (OTM), for general solid flows involving extremely large deformation, fast, transient loading and hydrodynamic phenomena. This method is a Lagrangian particle method through an integration of optimal transportation theory with meshless interpolation and material point integrations. The theoretical framework developed in this thesis generalized the Benamou-Brenier differential formulation of optimal transportation problems and leads to a multi-field variational characterization of solid flows, including elasticity, inelasticity, equation of state, and general geometries and boundary conditions. To this end, the accuracy, robustness and versatility of OTM is assessed and demonstrated with convergence and stability test, Taylor anvil test and a series of full three-dimensional simulations of high/hyper-velocity impact examples with the aid of a novel meshless dynamic contact algorithm presented in this thesis.\r\n", "doi": "10.7907/FAT3-0247", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis: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:2282", "collection": "thesis", "collection_id": "2282", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05292009-165215", "primary_object_url": { "basename": "LeonardJosephLucasThesis.pdf", "content": "final", "filesize": 6530849, "license": "other", "mime_type": "application/pdf", "url": "/2282/1/LeonardJosephLucasThesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Uncertainty Quantification Using Concentration-of-Measure Inequalities", "author": [ { "family_name": "Lucas", "given_name": "Leonard Joseph", "clpid": "Lucas-Leonard-Joseph" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Owhadi", "given_name": "Houman", "clpid": "Owhadi-H" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This work introduces a rigorous uncertainty quantification framework that exploits concentration\u2013of\u2013measure inequalities to bound failure probabilities using a well-defined certification campaign regarding the performance of engineering systems. The framework is constructed to be used as a tool for deciding whether a system is likely to perform safely and reliably within design specifications. Concentration-of-measure inequalities rigorously bound probabilities-of-failure and thus supply conservative certification criteria, in addition to supplying unambiguous quantitative definitions of terms such as margins, epistemic and aleatoric uncertainties, verification and validation measures, and confidence factors. This methodology unveils clear procedures for computing the latter quantities by means of concerted simulation and experimental campaigns. Extensions to the theory include hierarchical uncertainty quantification, and validation with experimentally uncontrollable random variables.", "doi": "10.7907/DRAM-H941", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:4240", "collection": "thesis", "collection_id": "4240", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10242008-070701", "primary_object_url": { "basename": "LuXiao_Thesis_Oct08.pdf", "content": "final", "filesize": 7721293, "license": "other", "mime_type": "application/pdf", "url": "/4240/1/LuXiao_Thesis_Oct08.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Combined Experimental and Numerical Study of Spontaneous Dynamic Rupture on Frictional Interfaces", "author": [ { "family_name": "Lu", "given_name": "Xiao", "clpid": "Lu-Xiao" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The process of spontaneous dynamic frictional sliding along the interface of two elastic solids is of great interest to a number of disciplines in engineering and sciences. Applications include frictional rupture processes in earthquakes, delamination of layered composite materials, and sliding between soft membranes in biological systems. The transient nature of rupture dynamics presents an array of fascinating yet challenging questions, including the nucleation process, the mechanism of interface failure, and the speed and mode of rupture propagation.
\r\n\r\nThis thesis presents such a combined experimental and theoretical study aimed at understanding the conditions for selecting pulse-like vs. crack-like rupture modes and subshear vs. supershear rupture speeds. There are two major contributions in this work. The first one is high-resolution experimental study of the rupture modes on a frictional interface. The study presents first experimental observations of spontaneous pulse-like ruptures in a homogeneous linear-elastic setting that mimics crustal earthquakes, reveals how different rupture modes are selected based on the level of fault prestress, demonstrates that both rupture modes can transition to supershear speeds, and advocates, based on comparison with theoretical studies, importance of velocity-weakening friction for earthquake dynamics. The second major contribution is the numerical modeling of the rupture experiments that reveal the importance of the rupture nucleation mechanism and friction formulations. The modeling of sub-Rayleigh to supershear transition has demonstrated the influence of rupture nucleation mechanism on supershear transition distance, as well as on the mechanism of supershear transition. The modeling of pulse-like to crack-like rupture mode transition has confirmed the necessity of velocity weakening friction for producing pulse-like rupture to match the experimental observations.
\r\n", "doi": "10.7907/BGGT-MC04", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:638", "collection": "thesis", "collection_id": "638", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02142009-181805", "primary_object_url": { "basename": "Thesis_YiLiu_Sep08.pdf", "content": "final", "filesize": 7225175, "license": "other", "mime_type": "application/pdf", "url": "/638/1/Thesis_YiLiu_Sep08.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Three-Dimensional Elastodynamic Modeling of Frictional Sliding with Application to Intersonic Transition", "author": [ { "family_name": "Liu", "given_name": "Yi", "clpid": "Liu-Yi" } ], "thesis_advisor": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "thesis_committee": [ { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "clpid": "Avouac-J-P" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Spontaneous slip on frictional interfaces involves both short-lived inertially-driven events and long-term quasi-static sliding. An example of considerable practical importance is the response of faults in the Earth's crust to tectonic loading. The response combines earthquakes that cause destructive ground motions and aseismic slip. Numerical models are needed to study the physics and mechanics of such complex behavior. In part, the models can help understand the observed slip patterns and interpret them in terms of constitutive properties of rocks determined in the lab.
\r\n\r\nThis thesis contains two main contributions. The first one is the development and implementation of a 3D methodology for simulations of spontaneous long-term interface slip punctuated by rapid inertially driven ruptures. Our approach is the first one to combine long-term deformation histories and the resulting stress redistribution on faults with full inclusion of inertial effects during simulated earthquakes in the context of 3D models. It reproduces all stages of earthquake cycles, from accelerating slip before dynamic instability, to rapid inertially driven propagation of earthquake rupture, to post-seismic slip, and to interseismic creep, including aseismic transients. The second main contribution is the discovery of the potentially dominating effect of favorable heterogeneity on intersonic transition in earthquakes, in both 2D models of single dynamic ruptures and 3D models of long-term fault slip. Studies of intersonic ruptures are practically important as they have the potential to cause strong ground motion farther from the fault than subsonic ruptures. Our conclusion that rheological boundaries promote transition to intersonic speeds in 3D rupture models is completely unexpected, as the neighboring stably slipping regions inhibit fast, inertially driven slip. The result could not be established in earlier studies, as it requires the computational methodology developed here that combines inertial effects, long-term slip histories, and 3D fault models. The thesis also develops test problems for dynamic rupture propagation and evaluates simplified quasi-dynamic approaches.
\r\n\r\nThe obtained results emphasize that dynamic ruptures should be considered in the context of the entire slip history of the fault, as such approach allows dynamic ruptures to occur under stress conditions established by prior slip, which leads to characteristic stress distributions that are not considered in single-event simulations. The developed 3D methodology can be applied to a number of problems in earthquake physics and mechanics that involve interaction of seismic and aseismic slip.
\r\n", "doi": "10.7907/JWCV-8V74", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:3886", "collection": "thesis", "collection_id": "3886", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10032007-121619", "primary_object_url": { "basename": "Angelova_PhD_Thesis.pdf", "content": "final", "filesize": 8076859, "license": "other", "mime_type": "application/pdf", "url": "/3886/1/Angelova_PhD_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Visual Prediction of Rover Slip: Learning Algorithms and Field Experiments", "author": [ { "family_name": "Angelova", "given_name": "Anelia Nedelcheva", "orcid": "0000-0003-1822-7943", "clpid": "Angelova-Anelia-Nedelcheva" } ], "thesis_advisor": [ { "family_name": "Matthies", "given_name": "Larry H.", "clpid": "Matthies-L-H" }, { "family_name": "Perona", "given_name": "Pietro", "clpid": "Perona-P" } ], "thesis_committee": [ { "family_name": "Perona", "given_name": "Pietro", "clpid": "Perona-P" }, { "family_name": "Matthies", "given_name": "Larry H.", "clpid": "Matthies-L-H" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Murray", "given_name": "Richard M.", "clpid": "Murray-R-M" }, { "family_name": "Abu-Mostafa", "given_name": "Yaser S.", "clpid": "Abu-Mostafa-Y-S" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Perception of the surrounding environment is an essential tool for intelligent navigation in any autonomous vehicle. In the context of Mars exploration, there is a strong motivation to enhance the perception of the rovers beyond geometry-based obstacle avoidance, so as to be able to predict potential interactions with the terrain. In this thesis we propose to remotely predict the amount of slip, which reflects the mobility of the vehicle on future terrain. The method is based on learning from experience and uses visual information from stereo imagery as input. We test the algorithm on several robot platforms and in different terrains. We also demonstrate its usefulness in an integrated system, onboard a Mars prototype rover in the JPL Mars Yard.
\r\n\r\nAnother desirable capability for an autonomous robot is to be able to learn about its interactions with the environment in a fully automatic fashion. We propose an algorithm which uses the robot's sensors as supervision for vision-based learning of different terrain types. This algorithm can work with noisy and ambiguous signals provided from onboard sensors. To be able to cope with rich, high-dimensional visual representations we propose a novel, nonlinear dimensionality reduction technique which exploits automatic supervision. The method is the first to consider supervised nonlinear dimensionality reduction in a probabilistic framework using supervision which can be noisy or ambiguous.
\r\n\r\nFinally, we consider the problem of learning to recognize different terrains, which addresses the time constraints of an onboard autonomous system. We propose a method which automatically learns a variable-length feature representation depending on the complexity of the classification task. The proposed approach achieves a good trade-off between decrease in computational time and recognition performance.
\r\n", "doi": "10.7907/F7FY-5T13", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "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:2558", "collection": "thesis", "collection_id": "2558", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06122006-161234", "primary_object_url": { "basename": "Kaushik-Dayal_PhD-Thesis.pdf", "content": "final", "filesize": 1822282, "license": "other", "mime_type": "application/pdf", "url": "/2558/2/Kaushik-Dayal_PhD-Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Nonlocal Microstructural Mechanics of Active Materials", "author": [ { "family_name": "Dayal", "given_name": "Kaushik", "orcid": "0000-0002-0516-3066", "clpid": "Dayal-Kaushik" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis deals with two aspects of the mechanics of symmetry-breaking defects such as phase boundaries, inclusions and free surfaces, and their role in the macroscopic response of active materials. We first examine the problem of kinetics using a nonlocal theory, and then study the role of geometry in active materials with fields that are not confined to the material.
\r\n\r\nClassical PDE continuum models of active materials are not closed, and require nucleation and kinetic information or regularization as additional constitutive input. We examine this problem in the peridynamic formulation, a nonlocal continuum model that uses integral equations to account for long-range forces that are important at small scales, and allows resolution of the structure of interfaces. Our analysis shows that kinetics is inherent to the theory. Viewing nucleation as a dynamic instability at small times, we obtain interesting scaling results and insight into nucleation in regularized theories. We also exploit the computational ease of this theory to study an unusual mechanism that allows a phase boundary to bypass an inclusion.
\r\n\r\nShifting focus to problems of an applied nature, we consider issues in the design of ferroelectric optical/electronic circuit elements. Free surfaces and electrodes on these devices generate electrical fields that must be resolved over all space, and not just within the body. These fields greatly enhance the importance of geometry in understanding the electromechanical response of these materials, and give rise to strong size and shape dependence. We describe a computational method that transforms this problem into a local setting in an accurate and efficient manner. We apply it to three examples: closure domains, a ferroelectric slab with segmented electrodes and a notch subjected to electro-mechanical loading.
", "doi": "10.7907/YGR6-H428", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:1822", "collection": "thesis", "collection_id": "1822", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05152007-121823", "primary_object_url": { "basename": "Vikram-Thesis-defended.pdf", "content": "final", "filesize": 3682057, "license": "other", "mime_type": "application/pdf", "url": "/1822/1/Vikram-Thesis-defended.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Electronic Structure Calculations at Macroscopic Scales", "author": [ { "family_name": "Gavini", "given_name": "Vikram", "orcid": "0000-0002-9451-2300", "clpid": "Gavini-Vikram" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knap", "given_name": "Jaroslaw", "clpid": "Knap-J" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Electronic structure calculations, especially those using density-functional theory have provided many insights into various materials properties in the recent decade. However, the computational complexity associated with electronic structure calculations has restricted these investigations to periodic geometries with small cell-sizes (computational domains) consisting of few atoms (about 200 atoms). But material properties are influenced by defects---vacancies, dopants, dislocations, cracks, free surfaces---in small concentrations (parts per million). A complete description of such defects must include both the electronic structure of the core at the fine (sub-nanometer) scale and also elastic and electrostatic interactions at the coarse (micrometer and beyond) scale. This in turn requires electronic structure calculations at macroscopic scales, involving millions of atoms, well beyond the current capability. This thesis presents the development of a seamless multi-scale scheme, Quasi-Continuum Orbital-Free Density-Functional Theory (QC-OFDFT) to address this significant issue. This multi-scale scheme has enabled for the first time a calculation of the electronic structure of multi-million atom systems using orbital-free density-functional theory, thus, paving the way to an accurate electronic structure study of defects in materials.
\r\n\r\nThe key ideas in the development of QC-OFDFT are (i) a real-space variational formulation of orbital-free density-functional theory, (ii) a nested finite-element discretization of the formulation, and (iii) a systematic means of adaptive coarse-graining retaining full resolution where necessary, and coarsening elsewhere with no patches, assumptions, or structure. The real-space formulation and the finite-element discretization gives freedom from periodicity, which is important in the study of defects in materials. More importantly, the real-space formulation and its finite-element discretization support unstructured coarse-graining of the basis functions, which is exploited to advantage in developing the QC-OFDFT method. This method has enabled for the first time a calculation of the electronic structure of samples with millions of atoms subjected to arbitrary boundary conditions. Importantly, the method is completely seamless, does not require any ad hoc assumptions, uses orbital-free density-functional theory as its only input, and enables convergence studies of its accuracy. From the viewpoint of mathematical analysis, the convergence of the finite-element approximation is established rigorously using Gamma-convergence, thus adding strength and validity to the formulation.
\r\n\r\nThe accuracy of the proposed multi-scale method under modest computational cost, and the physical insights it offers into properties of materials with defects, have been demonstrated by the study of vacancies in aluminum. One of the important results of this study is the strong cell-size effect observed on the formation energies of vacancies, where cells as large as tens of thousands of atoms were required to obtain convergence. This indicates the prevalence of long-range physics in materials with defects, and the need to calculate the electronic structure of materials at macroscopic scales, thus underscoring the importance of QC-OFDFT.
\r\n\r\nFinally, QC-OFDFT was used to study a problem of great practical importance: the embrittlement of metals subjected to radiation. The brittle nature of metals exposed to radiation is associated with the formation of prismatic dislocation loops---dislocation loops whose Burgers vector has a component normal to their plane. QC-OFDFT provides an insight into the mechanism of prismatic dislocation loop nucleation, which has remained unclear to date. This study, for the first time using electronic structure calculations, establishes vacancy clustering as an energetically favorable process. Also, from direct numerical simulations, it is demonstrated that vacancy clusters collapse to form stable prismatic dislocation loops. This establishes vacancy clustering and collapse of these clusters as a possible mechanism for prismatic dislocation loop nucleation. The study also suggests that prismatic loops as small as those formed from a 7-vacancy cluster are stable, thus shedding new light on the nucleation size of these defects which was hitherto unknown.
\r\n", "doi": "10.7907/1R69-YY30", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:1724", "collection": "thesis", "collection_id": "1724", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05112006-162905", "primary_object_url": { "basename": "Thesis_Matias_Zielonka_Final.pdf", "content": "final", "filesize": 8087385, "license": "other", "mime_type": "application/pdf", "url": "/1724/2/Thesis_Matias_Zielonka_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Configurational Forces and Variational Mesh Adaption in Solid Dynamics", "author": [ { "family_name": "Zielonka", "given_name": "Matias Gabriel", "clpid": "Zielonka-Matias-Gabriel" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis is concerned with the exploration and development of a variational finite element mesh adaption framework for non-linear solid dynamics and its conceptual links with the theory of dynamic configurational forces. The distinctive attribute of this methodology is that the underlying variational principle of the problem under study is used to supply both the discretized fields and the mesh on which the discretization is supported. To this end a mixed-multifield version of Hamilton's principle of stationary action and Lagrange-d'Alembert principle is proposed, a fresh perspective on the theory of dynamic configurational forces is presented, and a unifying variational formulation that generalizes the framework to systems with general dissipative behavior is developed. A mixed finite element formulation with independent spatial interpolations for deformations and velocities and a mixed variational integrator with independent time interpolations for the resulting nodal parameters is constructed. This discretization is supported on a continuously deforming mesh that is not prescribed at the outset but computed as part of the solution. The resulting space-time discretization satisfies exact discrete configurational force balance and exhibits excellent long term global energy stability behavior. The robustness of the mesh adaption framework is assessed and demonstrated with a set of examples and convergence tests.", "doi": "10.7907/V6RB-FR94", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1725", "collection": "thesis", "collection_id": "1725", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05112006-162948", "primary_object_url": { "basename": "Thesis_AmirSadjadpour.pdf", "content": "final", "filesize": 3787627, "license": "other", "mime_type": "application/pdf", "url": "/1725/1/Thesis_AmirSadjadpour.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Micromechanics-Inspired Three-Dimensional Constitutive Model for the Thermomechanical Response of Shape-Memory Alloys", "author": [ { "family_name": "Sadjadpour", "given_name": "Amir", "clpid": "Sadjadpour-Amir" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The goal of this thesis is to develop a full dimensional micromechanics-inspired constitutive model for polycrystalline shape-memory alloys. The model is presented in two forms: (1) The one-dimensional framework where we picture the ability of the model in capturing main properties of shape memory alloys such as superelasticity and shape-memory effect; (2) The full dimensional model where micromechanics origins of the model, the concepts emerged from those analysis and their relation to macroscopic properties in both single and polycrystals are presented.
\r\n\r\nWe use this framework to study the effects of the texture and anisotropy in the material behavior. Since phase transformation often competes with plasticity in shape-memory alloys, we incorporate that phenomenon into our model. We also demonstrate the ability of the model to predict the response of the material and track the phase transformation process for multi-axial, proportional and non-proportional loading and unloading experiments. We consider both stress-controlled and strain-controlled experiments and develop the model for isothermal, adiabatic and non-adiabatic thermal conditions. Adiabatic heating and loading rate both lead to the apparent hardening at high rates. We also visit this problem and examine the relative role of these two factors.
\r\n\r\nFinally we extend our model to study the reversible \"bcc\" to \"hcp\" martensitic phase transformation in pure iron. We consider a wide range of loading rates ranging from quasistatic to high rate dynamic loading and use our model to describe the evolution of the microstructure along with the effects of the rate hardening and thermal softening.
", "doi": "10.7907/MB1W-1V17", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:196", "collection": "thesis", "collection_id": "196", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01162006-005552", "primary_object_url": { "basename": "GLYKO_Thesis.pdf", "content": "final", "filesize": 21693932, "license": "other", "mime_type": "application/pdf", "url": "/196/1/GLYKO_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Experimental Study of Dynamic Frictional Sliding Modes along Incoherent Interfaces", "author": [ { "family_name": "Lykotrafitis", "given_name": "Georgios C.", "orcid": "0000-0003-2088-5478", "clpid": "Lykotrafitis-Georgios-C" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Tromp", "given_name": "Jeroen", "clpid": "Tromp-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Huang", "given_name": "Yonggang", "clpid": "Huang-Yonggang" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Dynamic sliding along incoherent (frictional) interfaces is investigated experimentally in a microsecond time scale. A bimaterial system comprised of Homalite and steel plates and a homogeneous system consisting of two Homalite plates are considered. The plates are held together by a uniform compressive stress while dynamic sliding is initiated by an impact-induced shear loading. The evolution of maximum shear stress contours is recorded by high-speed photography in conjunction with dynamic photoelasticity. Simultaneously with photoelasticity, a newly-developed technique based on laser interferometry is employed to locally measure the sliding speed at the interface.
\r\n\r\nThe response of the Homalite-steel bimaterial system differs according to whether the impact loading is applied to the Homalite plate or to the steel plate. In the first case, a disturbance traveling along the interface at a constant speed close to the Rayleigh wave speed of steel generates a shear Mach line crossing the P-wave front. Sliding initiates behind the P-wave front in the Homalite plate and it propagates at a supershear speed with respect to the shear wave speed of Homalite. A disturbance, traveling at constant speeds between the shear wave speed and the longitudinal wave speed of Homalite, appears behind the sliding tip. Wrinkle-like opening pulses, propagating along the bimaterial interface at a constant speed between the Rayleigh wave and the shear wave speed of Homalite, are also observed. When the impact loading is applied to the steel plate, sliding at a given point initiates with the arrival of the P-wave front there, so that the rupture is sonic with respect to steel and supersonic with respect to Homalite.
\r\n\r\nIn all the experiments performed on the bimaterial structure (Homalite-steel), sliding always occurred in a crack-like mode. In the case of a homogeneous system of Homalite plates however, direct physical evidence of different modes of sliding is recorded. Crack-like sliding, pulse-like sliding and mixed mode sliding in the form of pulses followed by a crack are discovered. Supersonic trailing pulses are also recorded. Behind the sliding tip, wrinkle-like opening pulses are developed for a wide range of impact speeds and confining stresses.
\r\n", "doi": "10.7907/0CCJ-5S66", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:2060", "collection": "thesis", "collection_id": "2060", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05252006-191203", "primary_object_url": { "basename": "Smith-D-E-2006-thesis.pdf", "content": "final", "filesize": 121747741, "license": "other", "mime_type": "application/pdf", "url": "/2060/32/Smith-D-E-2006-thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A New Paradigm for Interpreting Stress Inversions from Focal Mechanisms: How 3D Stress Heterogeneity Biases the Inversions Toward the Stress Rate", "author": [ { "family_name": "Smith", "given_name": "Deborah Elaine", "orcid": "0000-0002-8317-7762", "clpid": "Smith-Deborah-Elaine" } ], "thesis_advisor": [ { "family_name": "Heaton", "given_name": "Thomas H.", "orcid": "0000-0003-3363-2197", "clpid": "Heaton-T-H" } ], "thesis_committee": [ { "family_name": "Stock", "given_name": "Joann M.", "orcid": "0000-0003-4816-7865", "clpid": "Stock-J-M" }, { "family_name": "Avouac", "given_name": "Jean-Philippe", "orcid": "0000-0002-3060-8442", "clpid": "Avouac-J-P" }, { "family_name": "Tromp", "given_name": "Jeroen", "orcid": "0000-0002-2742-8299", "clpid": "Tromp-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Heaton", "given_name": "Thomas H.", "orcid": "0000-0003-3363-2197", "clpid": "Heaton-T-H" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Current stress studies often utilize stress inversions of earthquake focal mechanisms to estimate four parameters of the spatially uniform stress tensor, three principal stress orientations, and a ratio of the principal stresses. An implicit assumption in these studies is that earthquakes are good random samplers of stress; hence, the set of earthquake focal mechanisms within some region can be used to estimate the spatial mean stress state within the region. Numerical simulations indicate some regions, such as Southern California, have sufficient stress heterogeneity to bias the stress inversions toward the stress rate orientation and that stress studies using stress inversions need to be reinterpreted by taking this bias into account. An outline of how to subtract out this bias to yield the actual spatial mean stress is presented.
\r\n\r\nNumerical simulations demonstrate that spatially heterogeneous stress in 3D can bias stress inversions of focal mechanisms toward the stress rate tensor instead of the stress. Stochastic models of 3D spatially heterogeneous stress are created, synthetic earthquake focal mechanisms are generated using the Hencky-Mises plastic yield criterion, and results are compared with Hardebeck's Southern California earthquake catalog [Hardebeck, 2006]. The presence of 3D spatial stress heterogeneity biases which orientations are most likely to fail, a bias toward the stress rate tensor. When synthetic focal mechanisms are compared to real data, estimates of two stress heterogeneity parameters for Southern California are obtained: 1) A spatial smoothing parameter, \u03b1\u22480.8, where \u03b1 describes the spectral falloff of 1D cross sections through a 3D grid for the three principal stresses and three orientation angles. 2) A heterogeneity ratio, HR \u2248 1.25, which describes the relative amplitude of the spatial stress heterogeneity to the spatial mean stress. The estimate for \u03b1 is tentative; however, varying \u03b1 for \u03b1 \u2264 1.0 has little to no effect on the observation that spatially heterogeneous stress biases failures toward the stress rate. The estimate for HR is more robust and produces a bias toward the stress rate of approximately 40%. If the spatial mean stress and the stress rate are not aligned, the average focal mechanism failure mechanism should yield a stress estimate from stress inversions, approximately halfway between the two.
", "doi": "10.7907/42NY-WV90", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:8", "collection": "thesis", "collection_id": "8", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01032005-140446", "primary_object_url": { "basename": "Thesis_YuXiao.pdf", "content": "final", "filesize": 3483791, "license": "other", "mime_type": "application/pdf", "url": "/8/1/Thesis_YuXiao.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "The Influence of Oxygen Vacancies on Domain Patterns in Ferroelectric Perovskites", "author": [ { "family_name": "Xiao", "given_name": "Yu", "clpid": "Xiao-Yu" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis investigates the role of oxygen vacancies in determining ferroelectric properties and domain patterns of ferroelectric perovskites. Being non-polar (paraelectric) above their Curie temperature but spontaneously polarized (ferroelectric) below it, ferroelectric perovskites offer a tantalizing potential for applications: large actuation through domain switching and memory storage via switchable electric polarization. Oxygen vacancies, commonly present and mobile at high temperature, are the primary defects and thus play a central role in these applications.
\r\n\r\nWe develop a model that combines the ferroelectric and semiconducting nature of ferroelectric perovskites. Oxygen vacancies act as n-type dopants and thus affect the semiconducting properties. We show that the ferroelectric and semiconducting features interact and lead to the formation of depletion layers near the electrodes and double layers at the 90\u00b0 domain walls. We find a potential drop across 90\u00b0 domain walls even in a perfect crystal. This potential drop marks the essential difference between a 90\u00b0 and an 180\u00b0 domain wall, drives the formation of a space charge double layer in a doped crystal, promotes electronic charge injection and trapping, and leads to the redistribution of oxygen vacancies at 90\u00b0 domain walls. The rearrangement of oxygen vacancies near 90\u00b0 domain walls may form a basis for domain memory and provides a potentially new mechanism for large electrostriction.
\r\n\r\nWe also rigorously justify the continuum theory by calculating the Coulomb energy of a spontaneously polarized solid starting from a periodic distribution of charges based on the classical interpretation of ferroelectrics and with a definite choice of polarization per unit cell. We prove that in the limit where the size of the body is large compared to the unit cell, the energy of Coulombic interactions may be approximated by a sum of a local part and a nonlocal part. The local part depends on the lattice structure, but is different from the Lorentz formula for a lattice of dipoles. The nonlocal part is identical to the Lorentz formula.
", "doi": "10.7907/5QSX-9Y68", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:769", "collection": "thesis", "collection_id": "769", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02262005-161824", "primary_object_url": { "basename": "MyThesis_Xia_Text.pdf", "content": "final", "filesize": 4530246, "license": "other", "mime_type": "application/pdf", "url": "/769/1/MyThesis_Xia_Text.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Laboratory Investigations of Earthquake Dynamics", "author": [ { "family_name": "Xia", "given_name": "Kaiwen", "clpid": "Xia-Kaiwen" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Heaton", "given_name": "Thomas H.", "clpid": "Heaton-T-H" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" }, { "family_name": "Tromp", "given_name": "Jeroen", "clpid": "Tromp-J" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Earthquake represents one of most destructive geological hazards. In this thesis I will attempt to understand it through controlled laboratory experiments. The earthquake dynamic rupturing process itself is a complicated phenomenon, involving dynamic friction, wave propagation, and heat production. Because controlled experiments can produce results without assumptions needed in theoretical and numerical analysis, the experimental method is thus advantageous over theoretical and numerical methods.
\r\n\r\nOur laboratory fault is composed of carefully cut photoelastic polymer plates (Homalite-100, Polycarbonate) held together by uniaxial compression. As a unique unit of the experimental design, a controlled exploding wire technique provides the triggering mechanism of laboratory earthquakes. Three important components of real earthquakes (i.e., pre-existing fault, tectonic loading, and triggering mechanism) correspond to and are simulated by frictional contact, uniaxial compression, and the exploding wire technique. Dynamic rupturing processes are visualized using the photoelastic method and are recorded via a high-speed camera. Our experimental methodology, which is full-field, in situ, and non-intrusive, has better control and diagnostic capacity compared to other existing experimental methods.
\r\n\r\nUsing this experimental approach, we have investigated several problems: dynamics of earthquake faulting occurring along homogeneous faults separating identical materials, earthquake faulting along inhomogeneous faults separating materials with different wave speeds, and earthquake faulting along faults with a finite low wave speed fault core. We have observed supershear ruptures, rupture speed transition, directionality of rupture in faults with a material contrast, self-healing slip pulses in faults with a finite core, crack-like to pulse-like rupture transition in faults with a finite core.
", "doi": "10.7907/WQQX-6Q19", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:5084", "collection": "thesis", "collection_id": "5084", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12202004-182638", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 2056953, "license": "other", "mime_type": "application/pdf", "url": "/5084/1/Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Theory of Complex Lattice Quasicontinuum and Its Application to Ferroelectrics", "author": [ { "family_name": "Kowalewsky", "given_name": "Olga", "clpid": "Kowalewsky-Olga" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Complex lattice Quasicontinuum theory is developed and applied to the description of ferroelectric phenomena. Quasicontinuum theory is a multiscale theory that provides a unified description of materials by combining atomistic and continuum approaches. It provides a seamless transition between atomistics and continuum, but the description of the material is derived directly from the underlying atomic structure, using the computationally expensive atomistics only where needed, at the location of phenomena of atomistic origin.
\r\n\r\nComplex Lattice Quasicontinuum theory can be applied to complex lattice crystals consisting of many kinds of atoms. One highlight of it is treatment of each component lattice as separately and independently as possible. The component Quasicontinua are coupled through the microscopic forces within nodal clusters, making the complex atomistics of the heterogeneous lattice the basis of the description.
\r\n\r\nFerroelectrics are especially suited to the application of Quasicontinuum theory. The nature of defects in ferroelectric materials is atomistic, but their influence over the material is long ranged due to induced elastic fields. Many different ferroelectric phenomena involving the perovskite ferroelectrics Barium Titanate and Lead Titanate are investigated and simulated. For Barium Titanate: the 180 degree domain wall structure and quasistatic crack under load. For Lead Titanate: the 180 degree domain wall structure and a domain wall step.
\r\n\r\nThe results for the domain walls show that the domain wall thickness is atomistically small, of the order of few lattice constants, which is in agreement with recent ab initio molecular dynamics simulations, but we also observe long range effects resulting from the presence of the wall. During crack loading in the sample of Barium Titanate we observe polarization changes around the crack tip which are consistent with experimental observations of an increase of fracture toughness. The quasicontinuum study of a domain wall step gives an atomistical view into the equilibrium structure of the step.
\r\n\r\nQuasicontinuum is able to model these phenomena with atomistic precision around the defects and non-homogeneities, and also capture the influence of long-ranging effects in the samples. These studies could also give valuable modeling input for larger scale continuum approaches.
", "doi": "10.7907/rb0c-9534", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2044", "collection": "thesis", "collection_id": "2044", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05252004-131315", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 7686920, "license": "other", "mime_type": "application/pdf", "url": "/2044/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Energy-Minimizing Microstructures in Multiphase Elastic Solids", "author": [ { "family_name": "Chenchiah", "given_name": "Isaac Vikram", "orcid": "0000-0002-8618-620X", "clpid": "Chenchiah-Isaac-Vikram" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Lapusta", "given_name": "Nadia", "clpid": "Lapusta-N" }, { "family_name": "Hou", "given_name": "Thomas Y.", "clpid": "Hou-T-Y" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis concerns problems of microstructure and its macroscopic consequences in multiphase elastic solids, both single crystals and polycrystals.
\r\n\r\nThe elastic energy of a two-phase solid is a function of its microstructure. Determining the infimum of the energy of such a solid and characterizing the associated extremal microstructures is an important problem that arises in the modeling of the shape memory effect, microstructure evolution (precipitation, coarsening, etc.), homogenization of composites and optimal design. Mathematically, the problem is to determine the relaxation under fixed volume fraction of a two-well energy.
\r\n\r\nWe compute the relaxation under fixed volume fraction for a two-well linearized elastic energy in two dimensions with no restrictions on the elastic moduli and transformation strains; and show that there always exist rank-I or rank-II laminates that are extremal. By minimizing over the volume fraction we obtain the quasiconvex envelope of the energy. We relate these results to experimental observations on the equilibrium morphology and behavior under external loads of precipitates in Nickel superalloys. We also compute the relaxation under fixed volume fraction for a two-well linearized elastic energy in three dimensions when the elastic moduli are isotropic (with no restrictions on the transformation strains) and show that there always exist rank-I, rank-II or rank-III laminates that are extremal.
\r\n\r\nShape memory effect is the ability of a solid to recover on heating apparently plastic deformation sustained below a critical temperature. Since utility of shape memory alloys critically depends on their polycrystalline behavior, understanding and predicting the recoverable strains of shape memory polycrystals is a central open problem in the study of shape memory alloys. Our contributions to the solution of this problem are twofold:
\r\n\r\nWe prove a dual variational characterization of the recoverable strains of shape memory polycrystals and show that dual (stress) fields could be signed Radon measures with finite mass supported on sets with Lebesgue measure zero. We also show that for polycrystals made of materials undergoing cubic-tetragonal transformations the strains fields associated with macroscopic recoverable strains are related to the solutions of hyperbolic partial differential equations.
", "doi": "10.7907/RXE5-9A33", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis: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" } ]