Shape-morphing and self-propelled locomotion are examples of mechanical behaviors that can be “programmed” in structured media by designing geometric features at micro- and mesostructural length scales. This programmability is possible because the small-scale geometry often imposes local kinematic modes that are strongly favored over other deformations. In turn, global behaviors are influenced by local kinematic preferences over the extent of the structured medium and by the kinematic compatibility (or incompatibility) between neighboring regions of the domain. This considerably expands the design space for effective mechanical properties, since objects made of the same bulk material but with different internal geometry will generally display very different behaviors. This motivates pursuing a mechanistic understanding of the connection between small-scale geometry and global kinematic behaviors. This thesis addresses challenges pertaining to the modeling and design of structured media that undergo large deformations.
The first part of the thesis focuses on the relation between micro- or mesoscale patterning and energetically favored modes of deformation. This is first discussed within the context of twisted bulk metallic glass ribbons whose edges display periodic undulations. The undulations cause twist concentrations in the narrower regions of the structural element, delaying the onset of material failure and permitting the design of structures whose deployment and compaction emerge from the ribbons’ chirality. Following this discussion of a periodic system, we study sheets with non-uniform cut patterns that buckle out-of-plane. Motivated by computational challenges associated with the presence of geometric features at disparate length scales, we construct an effective continuum model for these non-periodic systems, allowing us to simulate their post-buckling behavior efficiently and with good accuracy.
The second part of the thesis discusses ways to leverage the connection between micro/mesoscale geometry and energetically favorable local kinematics to create “programmable matter” that undergo prescribed shape changes or self-propelled locomotion when exposed to an environmental stimulus. We first demonstrate the capabilities of an inverse design method that automates the design of structured plates that morph into target 3D geometries over time-dependent actuation paths. Finally, we present devices made of 3D-printed liquid crystal elastomer (LCE) hinges that change shape and self-propel when heated.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/0hf9-8m62, author = {Costanza, Vincenzo}, title = {Thermally Responsive Polymers for Wearable Calorimeters}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/0hf9-8m62}, url = {https://resolver.caltech.edu/CaltechTHESIS:05272022-085102678}, abstract = {The measurement of the body core temperature (BCT) can provide insightful health information spanning from hypothermia and heat stroke to inflammations and infections. In addition, the continuous monitoring of the BCT can unlock new possibilities for people’s well-being such counting of burnt calories, prediction of the ovulation period in the female population, and for the assessment of mental health issues. However, the integration of a BCT sensor in wearable devices is extremely challenging, since standard methods cannot combine minimal invasiveness with high measurement accuracy. Dual heat flux (DHF) thermometry is a novel technique that allows the precise estimation of BCT from the measurement of skin temperature. Nevertheless, the limited precision of currently available temperature sensors has not favored the wide spread of devices based on this architecture. In this thesis, we present the fabrication of a fully wearable DHF thermometer realized by integrating new polymers with a remarkable temperature sensitivity. In these particular polymers, an increase in temperature results in a change of the ionic conductivity. In the first part of this work, we focus on the understanding of the ion transport mechanism in these polymers and, in particular, on the nature of the interaction between the functional groups present on the polymer backbone and the conducting species (i.e. metal cations and water molecules). We show that the ion’s coordinating environment is the key to make these materials highly sensitive to temperature. The second part of the thesis tackles the fabrication of a BCT sensor, integrating these temperature responsive polymers in an ultrathin DHF thermometer. Building on the understanding of the nature of the temperature response, we optimize the polymer’s composition to obtain a thermal sensitivity that allows a good precision when measuring the BCT. Finally, we characterize the performance of the fabricated DHF thermometer in different conditions, assessing the sensor’s accuracy and response time.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/3rfm-1x78, author = {Injeti, Sai Sharan}, title = {Multi-Functional Metamaterials}, school = {California Institute of Technology}, year = {2021}, doi = {10.7907/3rfm-1x78}, url = {https://resolver.caltech.edu/CaltechTHESIS:05062021-023201965}, abstract = {Optimally designing interdependent mechanical properties in a structure allows for it to be used in application where an arbitrary combination of properties is desired. Architected materials have proven to be an effective way of attaining mechanical behaviors that are unattainable using their constituent materials alone, such as unusual static mechanical properties, unusual wave propagation behavior, and shape morphing. The advent of 3-D printing has allowed for fabricating metamaterials with complex topologies that display engineered mechanics. However, much of the current efforts have focused on optimally designing simple mechanical behaviors such as designing for stiffness and weight, particular frequency bandgaps, or bi-stability. In this work, we study two metamaterial systems where we control and optimize a wide set of static and dynamic properties, and one complex multi-stable structure.
Most studies on the optimal design of static properties have focused on engineering stiffness and weight, and much remains unknown about ways to decouple the critical load to failure from stiffness and weight. This is the focus of the first part of our work. We show that the addition of local internal pre-stress in selected regions of architected materials enables the design of materials where the critical load to failure can be optimized independently from the density and/or quasistatic stiffness. We propose a method to optimize the specific load to failure and specific stiffness using sensitivity analysis, and derive the maximum bounds on the attainable properties. We demonstrate the method in a 2-D triangular lattice and a 3-D octahedral truss, showing excellent agreement between experimental and theoretical results. The method can be used to design materials with predetermined fracture load, failure location and fracture paths.
For the second part of our work, we focus on designing acoustically transparent structures, by engineering the acoustic impedance – a combination of wave speed and density, to match that of the surroundings. Owing to the strong correlation between acoustic wave speed and static stiffness, it is challenging to design acoustically transparent materials in a fluid, while maintaining their high structural rigidity. We provide a sensitivity analysis to optimize these properties with respect to design parameters of the structure, that include localized masses at specific positions. We demonstrate the method on five different periodic, three dimensional lattices, to calculate bounds on the longitudinal wave speed as a function of their density and stiffness. We then perform experiments on 3-D printed structures, to validate our numerical simulations. Further, using the sensitivity analysis together with a data-driven approach, we design and demonstrate a mode demultiplexer, that is capable of splitting arbitrarily mixed modes. The tools developed in this work allow for designing structures in a plethora of applications, including ultrasound imaging, wave filtering, and waveguiding.
Finally, most multi-stable structures are limited by bi-stability either at the macroscopic or the unit cell level owing to the difficulty in engineering a highly non-linear energy landscape using just elements that display convex energy landscapes. We demonstrate a method to design arbitrarily complex multi-stable shape morphing structures, by introducing rigid kinematic constraints together with disengaging energy storing elements. We present the idea on a kagome lattice configuration, producing a quadri-stable unit cell and complex stable topologies with larger tessellations, validated by demonstrations on 3-D printed structures. Most designs that use passive actuation address one-way shape morphing along the direction of least resistance. We demonstrate reversible, thermally actuated shape morphing between stable open and closed topologies using shape memory springs. The designs can be extended to non-planar structures and fabricated at vastly different length scales.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bhattacharya, Kaushik and Daraio, Chiara}, } @phdthesis{10.7907/wm2f-4013, author = {Pajunen, Kirsti Mari}, title = {Dynamics of Lightweight Tensegrity-Inspired Metamaterials Fabricated with 3D-Printing}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/wm2f-4013}, url = {https://resolver.caltech.edu/CaltechTHESIS:06012020-003455628}, abstract = {Tensegrity structures and lattices have been of interest in engineering applications for decades, with their dynamics becoming a thriving field of study. Tensegrities consist of structural members under purely axial loading, either tension or compression, and obtain their stability from prestress. They possess unique characteristics such as high strength-to-weight ratio, nonlinear behavior, and elastic response under severe deformation. Tensegrity lattices (or metamaterials) have been shown to exhibit appealing dynamic attributes such as continuous tunability with prestress, impact mitigation, energy trapping and lensing, and nonlinear wave propagation, to name a few. However, their pin-jointed and prestressed nature presents significant manufacturing limitations, especially in the formation of lattices with large numbers of tessellated unit cells. Therefore, experimental validation of the dynamics of tensegrity metamaterials has remained elusive. For lattices with tensegrity-like characteristics to be manifested for real-world applications, a method for producing tensegrity-like metamaterials at multiple length scales is needed.
In this thesis, we present a design for a 3D-printable tensegrity-inspired structure with the equivalent strain energy capacity and stress-strain response as a pin-jointed tensegrity. Using this structure as a building block for multidimensional lattices, we subject them to a range of dynamic loading conditions to study their response. First, we perform experiments and simulations to obtain the dispersion relations for 1D and 3D lattices. We demonstrate the lattices’ ability to continuously tune the dispersion characteristics (e.g., band gap and wave speed) under precompression. This trait shows potential for acoustic lensing and dispersive wave propagation. In 3D, we show that the lattice shows the same type of unique properties, such as faster shear speed than longitudinal speed, as pin-jointed tensegrity lattices. Next, we study the lattices under impact loading. Long-duration impact experiments on baseline unit cells and 1D lattices show their resilience to repeated deformation, elasticity, and load limitation behaviors. Short-duration impulse experiments and simulations exhibit a wealth of desirable properties, such as high force transmission reduction, highly dispersive wave propagation, tunable wave speeds, energy trapping, and redirection of energy. We demonstrate that these tensegrity-inspired metamaterials not only exhibit and experimentally demonstrate tensegrity-like characteristics, but open a new range of lightweight metamaterials with unprecedented dynamic properties.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/Z9DR2SG2, author = {Nadkarni, Neel P.}, title = {Nonlinear Dynamics of Transition Waves in Multi-Stable Discrete and Continuous Media}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z9DR2SG2}, url = {https://resolver.caltech.edu/CaltechTHESIS:01132017-035026894}, abstract = {The concept of phase transitions, i.e., switching between two or more different equilibrium states of a system, is commonly encountered in many physical, chemical and biological phenomena. The exact mechanism of this switching is a highly nonlinear dynamical process that is accommodated by the propagation of a localized wave. The characteristics of the nonlinear wave such as its profile, velocity, energy, and width of transition are governed by the type and specifics of the system that it is propagating through which may be conservative, dissipative, or diffusive in nature. The goal of this thesis is to develop a fundamental understanding of the dynamics of such processes in general nonlinear systems capable of undergoing phase transitions and the application of new theories to elucidate the kinetic and energetic properties of transition waves in different scenarios. In conservative systems, we show that there are three different modes of stable wave propagation that we analytically solve for and validate computationally. In contrast, dissipative and diffusive systems allow the stable propagation of only the strongly nonlinear kink mode whose kinetic energy and propagation velocity are linked through a linear relation. We further validate our results in dissipative systems experimentally by fabricating and testing a strongly nonlinear lattice and show that transition waves are unidirectional in nature, as predicted by theory. Finally, as an application, we devise a strategy of using the physics of dissipative phase transitions to propagate stable mechanical signals in highly dissipative media such as soft polymers which effectively damp out small-amplitude linear waves.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Kochmann, Dennis M. and Daraio, Chiara}, } @phdthesis{10.7907/Z9D798BJ, author = {Lin, Wei-Hsun}, title = {Dynamic Characterization of Micro-Particle Systems}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9D798BJ}, url = {https://resolver.caltech.edu/CaltechTHESIS:07082015-183754265}, abstract = {Ordered granular systems have been a subject of active research for decades. Due to their rich dynamic response and nonlinearity, ordered granular systems have been suggested for several applications, such as solitary wave focusing, acoustic signals manipulation, and vibration absorption. Most of the fundamental research performed on ordered granular systems has focused on macro-scale examples. However, most engineering applications require these systems to operate at much smaller scales. Very little is known about the response of micro-scale granular systems, primarily because of the difficulties in realizing reliable and quantitative experiments, which originate from the discrete nature of granular materials and their highly nonlinear inter-particle contact forces.
In this work, we investigate the physics of ordered micro-granular systems by designing an innovative experimental platform that allows us to assemble, excite, and characterize ordered micro-granular systems. This new experimental platform employs a laser system to deliver impulses with controlled momentum and incorporates non-contact measurement apparatuses to detect the particles’ displacement and velocity. We demonstrated the capability of the laser system to excite systems of dry (stainless steel particles of radius 150 micrometers) and wet (silica particles of radius 3.69 micrometers, immersed in fluid) micro-particles, after which we analyzed the stress propagation through these systems.
We derived the equations of motion governing the dynamic response of dry and wet particles on a substrate, which we then validated in experiments. We then measured the losses in these systems and characterized the collision and friction between two micro-particles. We studied wave propagation in one-dimensional dry chains of micro-particles as well as in two-dimensional colloidal systems immersed in fluid. We investigated the influence of defects to wave propagation in the one-dimensional systems. Finally, we characterized the wave-attenuation and its relation to the viscosity of the surrounding fluid and performed computer simulations to establish a model that captures the observed response.
The findings of the study offer the first systematic experimental and numerical analysis of wave propagation through ordered systems of micro-particles. The experimental system designed in this work provides the necessary tools for further fundamental studies of wave propagation in both granular and colloidal systems.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/Z9J38QG6, author = {Burgoyne, Hayden Andrew}, title = {Dynamics of Granular Crystals with Elastic-Plastic Contacts}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9J38QG6}, url = {https://resolver.caltech.edu/CaltechTHESIS:05182016-164150213}, abstract = {We study the behavior of granular crystals subjected to impact loading that creates plastic deformation at the contacts between constituent particles. Granular crystals are highly periodic arrangements of spherical particles, arranged into densely packed structures resembling crystals. This special class of granular materials has been shown to have unique dynamics with suggested applications in impact protection. However, previous work has focused on very low amplitude impacts where every contact point can be described using the Hertzian contact law, valid only for purely elastic deformation. In this thesis, we extend previous investigation of the dynamics of granular crystals to significantly higher impact energies more suitable for the majority of applications. Additionally, we demonstrate new properties specific to elastic-plastic granular crystals and discuss their potential applications as well. We first develop a new contact law to describe the interaction between particles for large amplitude compression of elastic-plastic spherical particles including a formulation for strain-rate dependent plasticity. We numerically and experimentally demonstrate the applicability of this contact law to a variety of materials typically used in granular crystals. We then extend our investigation to one-dimensional chains of elastic-plastic particles, including chains of alternating dissimilar materials. We show that, using the new elastic-plastic contact law, we can predict the speed at which impact waves with plastic dissipation propagate based on the material properties of the constituent particles. Finally, we experimentally and numerically investigate the dynamics of two-dimensional and three-dimensional granular crystals with elastic-plastic contacts. We first show that the predicted wave speeds for 1D granular crystals can be extended to 2D and 3D materials. We then investigate the behavior of waves propagating across oblique interfaces of dissimilar particles. We show that the character of the refracted wave can be predicted using an analog to Snell’s law for elastic-plastic granular crystals and ultimately show how it can be used to design impact guiding “lenses” for mitigation applications.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/Z93J39XM, author = {Lydon, Joseph John II}, title = {Nonlinear Effects in Granular Crystals with Broken Periodicity}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z93J39XM}, url = {https://resolver.caltech.edu/CaltechTHESIS:01212015-023955427}, abstract = {When studying physical systems, it is common to make approximations: the contact interaction is linear, the crystal is periodic, the variations occurs slowly, the mass of a particle is constant with velocity, or the position of a particle is exactly known are just a few examples. These approximations help us simplify complex systems to make them more comprehensible while still demonstrating interesting physics. But what happens when these assumptions break down? This question becomes particularly interesting in the materials science community in designing new materials structures with exotic properties In this thesis, we study the mechanical response and dynamics in granular crystals, in which the approximation of linearity and infinite size break down. The system is inherently finite, and contact interaction can be tuned to access different nonlinear regimes. When the assumptions of linearity and perfect periodicity are no longer valid, a host of interesting physical phenomena presents itself. The advantage of using a granular crystal is in its experimental feasibility and its similarity to many other materials systems. This allows us to both leverage past experience in the condensed matter physics and materials science communities while also presenting results with implications beyond the narrower granular physics community. In addition, we bring tools from the nonlinear systems community to study the dynamics in finite lattices, where there are inherently more degrees of freedom. This approach leads to the major contributions of this thesis in broken periodic systems. We demonstrate the first defect mode whose spatial profile can be tuned from highly localized to completely delocalized by simply tuning an external parameter. Using the sensitive dynamics near bifurcation points, we present a completely new approach to modifying the incremental stiffness of a lattice to arbitrary values. We show how using nonlinear defect modes, the incremental stiffness can be tuned to anywhere in the force-displacement relation. Other contributions include demonstrating nonlinear breakdown of mechanical filters as a result of finite size, and the presents of frequency attenuation bands in essentially nonlinear materials. We finish by presenting two new energy harvesting systems based on our experience with instabilities in weakly nonlinear systems.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/Z9DB7ZRG, author = {Thevamaran, Ramathasan}, title = {Rate and Microstructure Effects on the Dynamics of Carbon Nanotube Foams}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9DB7ZRG}, url = {https://resolver.caltech.edu/CaltechTHESIS:10162014-104834622}, abstract = {Soft hierarchical materials often present unique functional properties that are sensitive to the geometry and organization of their micro- and nano-structural features across different lengthscales. Carbon Nanotube (CNT) foams are hierarchical materials with fibrous morphology that are known for their remarkable physical, chemical and electrical properties. Their complex microstructure has led them to exhibit intriguing mechanical responses at different length-scales and in different loading regimes. Even though these materials have been studied for mechanical behavior over the past few years, their response at high-rate finite deformations and the influence of their microstructure on bulk mechanical behavior and energy dissipative characteristics remain elusive.
In this dissertation, we study the response of aligned CNT foams at the high strain-rate regime of 102 - 104 s-1. We investigate their bulk dynamic response and the fundamental deformation mechanisms at different lengthscales, and correlate them to the microstructural characteristics of the foams. We develop an experimental platform, with which to study the mechanics of CNT foams in high-rate deformations, that includes direct measurements of the strain and transmitted forces, and allows for a full field visualization of the sample’s deformation through high-speed microscopy.
We synthesize various CNT foams (e.g., vertically aligned CNT (VACNT) foams, helical CNT foams, micro-architectured VACNT foams and VACNT foams with microscale heterogeneities) and show that the bulk functional properties of these materials are highly tunable either by tailoring their microstructure during synthesis or by designing micro-architectures that exploit the principles of structural mechanics. We also develop numerical models to describe the bulk dynamic response using multiscale mass-spring models and identify the mechanical properties at length scales that are smaller than the sample height.
The ability to control the geometry of microstructural features, and their local interactions, allows the creation of novel hierarchical materials with desired functional properties. The fundamental understanding provided by this work on the key structure-function relations that govern the bulk response of CNT foams can be extended to other fibrous, soft and hierarchical materials. The findings can be used to design materials with tailored properties for different engineering applications, like vibration damping, impact mitigation and packaging.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/TE86-1A15, author = {Szelengowicz, Ivan Michel Nicolas}, title = {Analysis and Optimization of Stress Wave Propagation in Two-Dimensional Granular Crystals with Defects}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/TE86-1A15}, url = {https://resolver.caltech.edu/CaltechTHESIS:05082013-161911202}, abstract = {Granular crystals are compact periodic assemblies of elastic particles in Hertzian contact whose dynamic response can be tuned from strongly nonlinear to linear by the addition of a static precompression force. This unique feature allows for a wide range of studies that include the investigation of new fundamental nonlinear phenomena in discrete systems such as solitary waves, shock waves, discrete breathers and other defect modes. In the absence of precompression, a particularly interesting property of these systems is their ability to support the formation and propagation of spatially localized soliton-like waves with highly tunable properties. The wealth of parameters one can modify (particle size, geometry and material properties, periodicity of the crystal, presence of a static force, type of excitation, etc.) makes them ideal candidates for the design of new materials for practical applications. This thesis describes several ways to optimally control and tailor the propagation of stress waves in granular crystals through the use of heterogeneities (interstitial defect particles and material heterogeneities) in otherwise perfectly ordered systems. We focus on uncompressed two-dimensional granular crystals with interstitial spherical intruders and composite hexagonal packings and study their dynamic response using a combination of experimental, numerical and analytical techniques. We first investigate the interaction of defect particles with a solitary wave and utilize this fundamental knowledge in the optimal design of novel composite wave guides, shock or vibration absorbers obtained using gradient-based optimization methods.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/NF5J-5W42, author = {Leonard, Andrea Beth}, title = {Controlling Wave Propagation through Nonlinear Engineered Granular Systems}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/NF5J-5W42}, url = {https://resolver.caltech.edu/CaltechTHESIS:06122013-030022149}, 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.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/M69R-3A76, author = {Gdoutos, Eleftherios E.}, title = {Thin Metastructures with Engineered Thermal Expansion}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/M69R-3A76}, url = {https://resolver.caltech.edu/CaltechTHESIS:05292013-162505920}, abstract = {The geometry and constituent materials of metastructures can be used to engineer the thermal expansion coefficient. In this thesis, we design, fabricate, and test thin thermally stable metastructures consisting of bi-metallic unit cells and show how the coefficient of thermal expansion (CTE) of these metastructures can be finely and coarsely tuned by varying the CTE of the constituent materials and the unit cell geometry. Planar and three-dimensional finite element method modeling is used to drive the design and inform experiments, and predict the response of these metastructures. We demonstrate computationally the significance of out-of-plane effects in the metastructure response. We develop an experimental setup using digital image correlation and an infrared camera to experimentally measure full displacement and temperature fields during testing and accurately measure the metastructures’ CTE. We experimentally demonstrate high aspect ratio metastructures of Ti/Al and Kovar/Al which exhibit near-zero and negative CTE, respectively. We demonstrate robust fabrication procedures for thermally stable samples with high aspect ratios in thin foil and thin film scales. We investigate the lattice structure and mechanical properties of thin films comprising a near-zero CTE metastructure. The mechanics developed in this work can be used to engineer metastructures of arbitrary CTE and can be extended to three dimensions.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/25R0-JT92, author = {Raney, Jordan Robert}, title = {Hierarchical Structures of Aligned Carbon Nanotubes as Low-Density Energy-Dissipative Materials}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/25R0-JT92}, url = {https://resolver.caltech.edu/CaltechTHESIS:05072012-134317580}, abstract = {Carbon nanotubes (CNTs) are known to have remarkable properties, such as a specific strength two orders of magnitude higher than that of steel. It has remained a challenge, however, to achieve useful bulk properties from CNTs. Toward that goal, here we develop low-density bulk materials (0.1-0.4 g cm-3) entirely or nearly entirely from CNTs. These consist of nominally-aligned arrays of CNTs that display a dissipative compressive response, with a notable stress-strain hysteresis. The compressive properties of CNT arrays are examined in detail. This analysis reveals interesting features in the mechanical response, such as strain localization (resulting from a gradient in physical properties along the height), recovery after compression, non-linear viscoelasticity, and behavior under repeated compression that depends on the strain of previous cycles (similar to the Mullins effect in rubbers). We observe that in compression the energy dissipation of these materials is more than 200 times that of polymeric foams of comparable density.
Next, materials based on CNT arrays are studied as exemplary of hierarchical materials (materials with distinct structure at multiple length scales). Hierarchical materials have pushed the limits of traditional material tradeoffs (e.g., the typical trend that increased strength requires increased weight). Techniques are developed to separately vary the structure of CNT arrays at nanometer, micrometer, and millimeter length scales, and the effects on the bulk material response are examined. Structure can be modified during CNT synthesis, such as by varying the composition of the flow gas or by manipulating the input rate of chemical precursors; it can also be modified post-synthesis, e.g., by the in situ synthesis of nanoparticles in the interstices of the CNT arrays or by the assembly of multilayer structures of multiple CNT arrays connected by polymeric or metallic interlayers.
Finally, a mathematical model is applied to capture the complexities of the mechanical response. This one-dimensional, multiscale, bistable spring model is able to match the global stress-strain response as well as local effects such as strain localization and Mullins-like behavior. A technique is developed to reliably discern the model’s material parameters based on in situ optical data from the experiments.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/P3VR-Q582, author = {Khatri, Devvrath}, title = {Non-Destructive Evaluation of Material System Using Highly Nonlinear Acoustic Waves}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/P3VR-Q582}, url = {https://resolver.caltech.edu/CaltechTHESIS:05102012-091402754}, abstract = {A chain of granular particles is one of the most studied examples of highly nonlinear systems deriving its response from the nonlinear Hertzian contact interaction between particles. Interest in these systems derives from their tunable dynamic response, encompassing linear, weakly nonlinear, and strongly nonlinear regimes, controlled by varying the static and dynamic load applied. In chains with a very weak (or zero) static precompression, the system supports the formation and propagation of highly nonlinear solitary waves (HNSWs). The dual-nonlinear interaction between particles (i.e., a power-law type contact potential in compression, and zero strength in tension) combined with discreteness of the system, makes the granular system highly tunable. The propagation properties of these waves, such as traveling pulse width, wave speed, number of separated pulses (single or train of pulses), etc., can be controlled by modifying one or many of the parameters, like the particle’s dimension, material properties, static and dynamic force amplitude, the type and duration of the initial excitation applied to the system, and/or the periodicity of the chain. The ability to control the wave properties in such chains has been proposed for several different practical engineering applications.
The dynamic properties of these granular chains have been conventionally studied using discrete particle models (DPMs) which consider the particles in the chains as point masses connected by nonlinear Hertzian springs with the neighboring particles. Although, this is a good approximation under proper circumstances, it does not capture many features of the three dimensional elastic particles such as the elastic wave propagation within the particles, the local deformation of the particles in the vicinity of the contact point, the corresponding changes in the contact area, and the collective vibrations of the particles among others. This thesis focuses on the development of a nite element model (FEM)using the commercially available software Abaqus, which takes into account many of these characteristic features. The nite element model discretizes particles by considering them as three-dimensional deformable bodies of revolution and describes the nonlinear dynamic response of one-dimensional granular chains composed of particles with various geometries and orientations. We showed that particles’ geometries and orientations provide additional design parameters for controlling the dynamic response of the system, compared to chains composed of spherical particles. We also showed that the tunable and compact nature of these waves can be used to tailor the properties of HNSWs for specfic application, such as information carriers for actuation and sensing of mechanical properties and boundary effects of adjoining media in Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM). Using experiments and numerics, we characterized interface dynamics between granular media and adjoining linear elastic media, and found that the coupling produced temporary localization of the incident waves at the boundaries between the two media and their decomposition into reflected waves. We monitored the formation of reflected solitary waves propagating back from the interface and found that their properties are sensitive to the geometric and material properties of the adjoining media. The work done in this research enhances our understanding of the basic physics and tunability of nonlinear granular media, and further establishes a theoretical and numerical foundation in the applications of HNSWs as information carriers.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, } @phdthesis{10.7907/BAHK-WD44, author = {Boechler, Nicholas Sebastian}, title = {Granular Crystals: Controlling Mechanical Energy with Nonlinearity and Discreteness}, school = {California Institute of Technology}, year = {2011}, doi = {10.7907/BAHK-WD44}, url = {https://resolver.caltech.edu/CaltechTHESIS:05162011-131929134}, abstract = {The presence of structural discreteness and periodicity can affect the propagation of phonons, sound, and other mechanical waves. A fundamental property of many of the periodic structures and materials designed for this purpose is the presence of complete band gaps in their dispersion relation. Waves with frequencies in the band gap cannot propagate and are reflected by the material. Like the concept of a band gap, the functionality of these periodic structures has historically been based on concepts from linear dynamics. Nonlinear systems can offer increased flexibility over linear systems including new ways to localize energy, convert energy between frequencies, and tune the response of the system. Granular crystals are arrays of elastic particles that interact nonlinearly via Hertzian contact, and are a type of nonlinear periodic structure whose response to dynamic excitations can be tuned to encompass linear, weakly nonlinear, and strongly nonlinear regimes. Drawing on ideas from condensed matter physics and nonlinear science, this thesis focuses on how the nonlinearity and structural discreteness of granular crystals can be used to control mechanical energy. The dynamic response of one-dimensional granular crystals composed of compressed elastic spheres (or cylinders) is studied using a combination of experimental, numerical, and analytical techniques. The discovery of fundamental physical phenomena occurring in the linear and weakly nonlinear regimes is described, along with how such phenomena can be used to create new ways to control the propagation of mechanical wave energy. The specific mechanisms investigated include tunable frequency band gaps, discrete breathers, nonlinear localized defect modes, and bifurcations. These mechanisms are utilized to create novel devices for tunable vibration filtering, energy harvesting and conversion, and tunable acoustic rectification.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Daraio, Chiara}, }