Granular materials are widely encountered in natural and engineered systems, yet their behavior under cyclic loading remains difficult to predict because bulk response emerges from complex grain-scale interactions. Repeated loading can produce irreversible deformation, history dependence, and evolving internal load-transfer mechanisms that are not fully captured by macroscopic measurements alone. Although cyclic behavior has been studied extensively at the continuum scale, direct experimental characterization of the grain-scale processes governing cyclic response remains limited.
\r\n\r\nThis thesis investigates the multiscale behavior of dry, cohesionless granular materials subjected to quasi-static cyclic simple shear loading through laboratory experiments coupled with the Granular Element Method (GEM), a mechanics-based force-inference framework for estimating interparticle contact forces from experimentally measurable grain-scale data. An experimental framework was developed by upgrading an existing simple shear apparatus to enable controlled cyclic loading and by integrating imaging, tracking, and post-processing methods for multiscale measurements.
\r\n\r\nAt the macroscale, the experiments show that cyclic simple shear produces direction-dependent and history-dependent behavior. Repeated loading generates asymmetric stress and deformation responses, incomplete recovery of the initial state, and progressive changes that depend on packing condition and confinement level. Densely packed and loosely packed assemblies exhibit qualitatively different cyclic responses, while normal confinement primarily influences the strength and persistence of those responses.
\r\n\r\nAt the grain-scale, the results show that cyclic behavior is governed largely by the reorganization of the internal force network rather than by large changes in overall contact connectivity. Force chains evolve continuously with loading direction and cycle history, while anisotropy plays a central role in linking internal structure to bulk shear resistance. Dense systems initially develop stronger, more coherent load-bearing structures that weaken with repeated cycling, whereas loose systems deform via more distributed force transmission and progressive compaction.
\r\n\r\nOverall, this work provides a mechanistic framework for understanding how granular materials accumulate history, weaken, and reorganize under repeated shear. The combined experimental and GEM results also provide benchmark data to calibrate and validate physics-based particle models for broader granular systems and relevant engineering applications.
", "doi": "10.7907/eydy-1z48", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18647", "collection": "thesis", "collection_id": "18647", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272026-045028254", "primary_object_url": { "basename": "Carmi_Meital_2026.pdf", "content": "final", "filesize": 16519350, "license": "other", "mime_type": "application/pdf", "url": "/18647/1/Carmi_Meital_2026.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Buckling of Open Cross-Section Deployable Composite Thin Shells with Manufacturing Imperfections", "author": [ { "family_name": "Carmi", "given_name": "Meital Oshrit", "orcid": "0009-0000-7837-2910", "clpid": "Carmi-Meital-Oshrit" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Shaikeea", "given_name": "Angkur", "orcid": "0000-0002-6706-0492", "clpid": "Shaikeea-Angkur-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Thin composite shells are increasingly being used to support large area space systems due to their high strength to mass ratio and ability to withstand tight packaging for launch and then deploy once in space. This thesis specifically focuses on long, slender composite shells (longerons) with an open cross-section consisting of two circular arc flanges bonded along one edge. They are useful components of lightweight space structures, including the Caltech Space Solar Power Project spacecraft. The flanges of the longerons are less than 100 \u00b5m thick, and these extremely thin composite shells are prone to manufacturing imperfections and local buckling. Therefore, the primary objective of this thesis is to better understand the buckling behavior of open cross-section, ultra-thin composite shells that contain manufacturing imperfections, with the goal of informing the design of future space structures that are more resistant to buckling.
\r\n\r\nThe first step towards understanding a structure\u2019s imperfection sensitivity is to measure its imperfections. Thus, the thesis begins by characterizing the geometric imperfections present in experimental composite longerons. A method to measure and quantify the parameters of both local and global imperfections in thin shells is developed and applied. Results show that the longerons contain local imperfections as large as five to ten times the shell thickness, which can have serious implications for local buckling.
\r\n\r\nOnce the imperfections were measured, both numerical and experimental studies are performed to study the effects of these imperfections on the buckling behavior and knockdown factor of longerons loaded in pure bending. In the numerical study, a finite element analysis is used to characterize the effect of a single imperfection, created using a simplified model based on the shape of the experimentally measured imperfections, with a wide range of geometric parameters. Then, experiments are performed to measure the buckling behavior of actual longerons, whose random manufacturing imperfections were characterized. The results of these studies show that imperfections, especially ones with large amplitudes, significantly reduce the longeron\u2019s critical buckling load and bending stiffness. Good agreement between the experimental and numerical results was achieved, particularly for higher quality longerons with a single, dominant imperfection.
\r\n\r\nMotivated by the imperfections and their detrimental effects found in the earlier parts of the thesis, key parameters of the longeron's cross-section are varied with the goal of increasing its stability. The subtended angle of the longeron's flanges is varied in both experiments and numerical simulations of longerons loaded in bending. The results show good agreement between the experiments and simulations, with both showing a trend of increasing critical buckling load and bending stiffness with increasing flange subtended angle. Then, based on these promising results, the radius at the edge of the flange is decreased, which is shown to significant improve the longeron's stability and imperfection sensitivity without increasing its mass.
\r\n\r\nFinally, the effect of length on the buckling behavior of longerons loaded in bending is studied numerically with the goal of extending the current work to longer longerons. For lengths varying from 0.5 m to 5 m, both perfect and imperfect longerons with realistic geometric imperfections are studied. It is shown that for longer longerons, the critical buckling moment and imperfection sensitivity remain almost constant with increasing length, which is promising for future large space structures.
", "doi": "10.7907/ffpp-gr08", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18655", "collection": "thesis", "collection_id": "18655", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272026-222421613", "primary_object_url": { "basename": "Nakahara_Caltech_PhD_Thesis_v5.pdf", "content": "final", "filesize": 39924467, "license": "other", "mime_type": "application/pdf", "url": "/18655/2/Nakahara_Caltech_PhD_Thesis_v5.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Fabrication and Mechanical Characterization of Nano-Architected Composites Across Scales and Strain Rates", "author": [ { "family_name": "Nakahara", "given_name": "Kevin Hiroshi Keesun", "clpid": "Nakahara-Kevin-Hiroshi-Keesun" } ], "thesis_advisor": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Shaikeea", "given_name": "Angkur", "orcid": "0000-0002-6706-0492", "clpid": "Shaikeea-Angkur-J" }, { "family_name": "Kagias", "given_name": "Matias", "orcid": "0000-0003-0435-6672", "clpid": "Kagias-Matias" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "div_eng" } ], "abstract": "Transportation, infrastructure, personnel protection, and all other applications requiring dynamic impact resistance drive the demand to develop advanced manufacturing for structural materials that are simultaneously lightweight and superior at energy absorption. Polymer matrix composites (PMCs) are widely used in mechanical applications due to their high strength-to-weight ratios, high stiffness-to-weight ratios, corrosion resistance, and design flexibility. However, these composites often suffer from matrix and interfacially driven failure mechanisms under dynamic compression. Nano-architected materials are an emergent class of metamaterials capable of achieving high-stiffnesses and strengths while also exhibiting high specific energy absorptions under micro particle impacts. Most studies of nano-architected materials focus on periodic lattices geometries or other cellular solids while their use as reinforcements in composites, especially at large-scales, remains limited. Combining the works of nano-architected metamaterials and PMCs, we create nano-architected composites possessing high mechanical energy absorptions under dynamic compression without the need for dense constituent reinforcement materials.
\r\n\r\nWe first explore how nano-architected materials are fabricated at large-scales, and we demonstrate how they can be incorporated with epoxy polymer matrices to create nano-architected composites using molding methods. Subsequently, we characterize these nano-architected composites, showing how changing fabrication parameters can produce various configurations of nano-architecture within samples, and how fabrication limitations can result in defects at multiple scales. Through various quasi-static and dynamic testing methods, we study how nano-architectures deform, fail, and contribute to composite performance --- decoupling their effects from defects. Our study shows that while composite performance can be mitigated by defects, increasing the amount of nano-architectures present in composites leads to higher strengths and delayed catastrophic failure. In-situ observations of these tests allow us to directly connect deformation and failure mechanisms to enhanced stress-strain performance. We then use phenomenological modeling to relate mechanical performances across loading rates showing high nano-architected rate sensitivities and verifying the role of nano-architectures in delaying catastrophic failure. Nano-architected performance is compared against classical PMCs reported in literature demonstrating their high energy absorptions and unique capability to address shortcomings of fiber- and particle-reinforced composites.
\r\n\r\nOur nano-architected composites offer a new way to toughen polymer matrix composites, utilizing small scale architectures rather than changes to constituent material composition to delay catastrophic failures. These nano-architected composites are a unique demonstration of the capabilities of nano-architectures to perform under high rates, loads, and energies, paving the route to new possibilities for composite design. Our modeling work of nano-architected composite performance shows the potential of these materials to absorb energies through the introduction and control of nano-architected structures.
", "doi": "10.7907/gvm7-qr90", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18723", "collection": "thesis", "collection_id": "18723", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312026-214551391", "primary_object_url": { "basename": "sanagala_sathvik_2026_thesis.pdf", "content": "final", "filesize": 16041445, "license": "other", "mime_type": "application/pdf", "url": "/18723/1/sanagala_sathvik_2026_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Discrete Shell Methods for Stimuli-Responsive and Deployable Structures: Buckling, Bistability, and Topology Optimization", "author": [ { "family_name": "Sanagala", "given_name": "Sathvik R.", "orcid": "0000-0002-7490-7540", "clpid": "Sanagala-Sathvik-R" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "thesis_committee": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Watkins", "given_name": "Ryan", "orcid": "0000-0002-1883-5714", "clpid": "Watkins-Ryan" }, { "family_name": "Shaikeea", "given_name": "Angkur", "orcid": "0000-0002-6706-0492", "clpid": "Shaikeea-Angkur-J" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Shells - thin curved structures such as eggshells, insect wings, and pressure vessels - are ubiquitous in nature and engineering. They are slender yet structurally capable, supporting many times their own weight. They can also undergo dramatic geometric transitions, such as buckling, snap-through, and bistability. This combination of strength and geometric nonlinearity stems from the interplay between bending and stretching in thin geometries. As a result, simulating shells is numerically demanding. Finite-element methods are the standard tool, but thin shells require high order elements that adds substantial formulation and implementation complexity. An alternative approach to address some of these problems comes from the computer graphics community. Discrete differential geometry methods discretize the shell on a triangulated mesh using only nodal positions as degrees of freedom. This sidesteps the need for high order elements in finite element analysis. However, existing formulations are typically developed for visual realism rather than mechanical accuracy, and lack the physically meaningful energy formulations required for engineering design.
\r\n\r\nThis thesis develops a discrete Kirchhoff-Love shell framework that combines a triangulated-mesh discretization with the mechanical rigor required for engineering design. The formulation employs a Koiter energy with standard engineering material constants, supports a spontaneous-curvature field for active materials, and provides analytic gradients and Hessians that enable Newton-Raphson equilibrium solving, adjoint sensitivity analysis, and energy-landscape path-finding. The framework is verified against four canonical shell benchmarks.
\r\n\r\nThe framework is first applied to photoactive liquid crystal elastomer shells, governed by a spontaneous-curvature evolution law that couples illumination intensity and direction to surface geometry. Simulations capture a flat sheet that bifurcates to a cylindrical configuration under uniform illumination, resulting from the Gauss curvature coupling between bending and stretching. A second example shows a thin active sheet that reorients to track a moving light source, analogous to a sunflower.
\r\n\r\nTape springs are thin curved strips used as deployable booms in spacecraft; they fold compactly via snap-through buckling and recover a stiff deployed state on release. The framework is then applied to simulate their forward bending response in opposite-sense and equal-sense loading modes. The opposite-sense results match analytical predictions for the propagation moment and localized fold geometry.
\r\n\r\nBuilding on the tape spring analysis, a multi-equilibrium topology optimization method computes adjoint sensitivities through the deployed and folded states simultaneously. A parameter sweep over volume fraction, design region, and folded-state moment threshold reveals non-intuitive topologies, including hourglass designs that single-state optimizers cannot access.
\r\n\r\nFinally, the nudged elastic band method is applied within the discrete-shell framework to compute the minimum-energy path for the eversion of a bistable spherical cap. The path passes through a non-axisymmetric saddle, illustrating the energetic favorability of asymmetric eversion, which stems from the interplay between Gauss curvature and stretch.
\r\n\r\nTogether, these applications establish the framework as a versatile, mechanically grounded platform for simulating the nonlinear mechanics of thin shells.
", "doi": "10.7907/02wt-3q12", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18793", "collection": "thesis", "collection_id": "18793", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06042026-211328312", "type": "thesis", "title": "Stable Method of Attaching Thin Films to Torsionally Compliant Space Structures", "author": [ { "family_name": "Popov", "given_name": "George Arthur", "orcid": "0000-0001-5938-0528", "clpid": "Popov-George-Arthur" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" }, { "family_name": "Shaikeea", "given_name": "Angkur", "orcid": "0000-0002-6706-0492", "clpid": "Shaikeea-Angkur-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Space Solar Power Project" }, { "literal": "div_eng" } ], "abstract": "Ultralight space structures utilize composite structural elements supporting active thin films in order to achieve deployed configurations with lower areal densities. However, as these space structures get larger, they get increasingly less stiff, leaving them susceptible to adverse effects, such as torsional buckling. Simultaneously, this conflicts with the requirements that more ambitious space missions impose on the surface accuracy to make technologies like large phased arrays viable.
\r\n\r\nThis thesis presents a novel method of continuously attaching thin films to deployable thin shell structures, allowing for materials with widely different coefficients of thermal expansion. It proposes a double s-spring border that exhibits a local post-buckling behavior and provides a tunable continuous edge attachment method that can maintain constant preload under large thermal strains. The mechanical behavior of the double s-spring under different mechanical loading conditions is studied both numerically and experimentally. Additionally, a reduced order model that greatly reduces the computational cost of optimizing double s-springs for various applications is presented.
\r\n\r\nIn parallel, the torsional buckling of elastic composite frames is studied to understand the fundamental limits of attaching thin films without incurring torsional buckling. This study analytically calculates the critical prestress of torsionally soft square frames supporting an internal thin film. The study highlights the role of the attachment scheme, which has a very significant impact on the critical prestress. It is shown that the average of the compression load on a frame caused by a prestress is an invariant buckling load. The analytical calculation is verified via numerical finite element analyses and an experiment, which characterizes the post-buckling behavior of the torsionally soft frames as well. It is concluded that distributed edge attachments, such as the double s-spring, significantly increase the stability of space structures against torsional buckling. The findings for the torsionally soft square frames are validated against a high fidelity orthotropic material model in order to justify the assumptions made in the analytical study.
\r\n\r\nIn the final section of the thesis, it is shown that the double s-spring continuous attachment scheme enables consistent deployment and compact packaging. The scheme is also shown to be versatile for additional applications, enabling novel deployment and folding schemes, such as a doubly-curved composite foldable toroid.
", "doi": "10.7907/0n4d-3v80", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" } ]