Browsing by Author "Li, Ling"

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    Analytical and Spectro-Spatial Analyses of Nonlinear Metamaterials for Vibration Control, Energy Harvesting, and Acoustic Non-Reciprocity
    Bukhari, Mohammad Abdulbaqi (Virginia Tech, 2021-06-23)
    This dissertation investigates the nonlinear wave propagation phenomena in nonlinear metamaterials with nonlinear chains and nonlinear resonators using analytical and spectro-spatial analyses. In the first part of the thesis, the nonlinear metamaterials are modeled as a chain of masses with multiple local resonators attached to each cell. The nonlinearity stems from the chain's stiffness in one case and the local resonator's stiffness in another. Analytical approximates solutions are obtained for each case using perturbation techniques. These results are validated through numerical simulations and the results show good agreement. To further demonstrate the nonlinear wave propagation characteristics, spectro-spatial analyses are conducted on the numerical integration data sets. The wave profiles, short-term Fourier transform spectrograms, and contour plots of 2D Fourier transform show the presence of solitary waves for both sources of nonlinearity. In addition, spectro-spatial features demonstrate the presence of significant frequency shifts at different wavelength limits. indent The second part of the thesis studies a nonlinear electromechanical metamaterial and examines how the electromechanical coupling in the local resonator affects the wave propagation. Numerical examples indicate that the system can be used for simultaneous energy harvesting and vibration attenuation without any degradation in the size of bandgaps. Spectro-spatial analyses conducted on the electromechanical metamaterial also reveal the presence of solitons and frequency shifts. The presence of solitary wave in the electromechanical metamaterial suggests a significant improvement in energy harvesting and sensing techniques. The obtained significant frequency shift is employed to design an electromechanical diode, allowing voltage to be sensed and harvested only in one direction. Design guidelines and the role of different key parameters are presented to help designers to select the type of nonlinearity and the system parameters to improve the performance of acoustic diodes. indent The last part of this thesis studies the passive self-tuning of a metastructure via a beam-sliding mass concept. The governing equations of motions of the holding structure, resonator, and sliding mass are presented and discretized into a system of ODEs using Galerkin's projection. Given that the spatial parameters of the system continuously change over time (i.e., mode shapes and frequencies), instantaneous exact mode shapes and frequencies are determined for all possible slider positions. The numerical integration is conducted by continuously updating the spatial state of the system. The obtained exact mode shapes demonstrate that the resonance frequency of the resonator stretches over a wide frequency band. This observation indicates that the resonator can attenuates vibrations at a wide frequency range. Experiments are also conducted to demonstrate the passive self-tunability of the metastructure and the findings colloborate the analytical results.
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    Bioinspired design of flexible armor based on chiton scales
    Connors, Matthew; Yang, Ting; Hosny, Ahmed; Deng, Zhifei; Yazdandoost, Fatemeh; Massaadi, Hajar; Eernisse, Douglas; Mirzaeifar, Reza; Dean, Mason N.; Weaver, James C.; Ortiz, Christine; Li, Ling (Springer Nature, 2019-12-10)
    Man-made armors often rely on rigid structures for mechanical protection, which typically results in a trade-off with flexibility and maneuverability. Chitons, a group of marine mollusks, evolved scaled armors that address similar challenges. Many chiton species possess hundreds of small, mineralized scales arrayed on the soft girdle that surrounds their overlapping shell plates. Ensuring both flexibility for locomotion and protection of the underlying soft body, the scaled girdle is an excellent model for multifunctional armor design. Here we conduct a systematic study of the material composition, nanomechanical properties, three-dimensional geometry, and interspecific structural diversity of chiton girdle scales. Moreover, inspired by the tessellated organization of chiton scales, we fabricate a synthetic flexible scaled armor analogue using parametric computational modeling and multi-material 3D printing. This approach allows us to conduct a quantitative evaluation of our chiton-inspired armor to assess its orientation-dependent flexibility and protection capabilities.
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    Biomineralized Composites: Material Design Strategies at Building-Block and Composite Levels
    Deng, Zhifei (Virginia Tech, 2023-01-12)
    Biomineral composites, consisting of intercrystalline organics and biogenic minerals, have evolved unique structural designs to fulfill mechanical and other biological functionalities. Aside from the intricate architectures at the composite level and 3D assemblies of the biomineral building blocks, the individual mineral blocks enclose intracrystalline structural features that contribute to the strengthening and toughening at the intrinsic material level. Therefore, the design strategies of biomineralized composites can be categorized into two structural levels, the individual building block level and the composite level, respectively. This dissertation aims at revealing the material design strategies at both levels for the bioinspired designs of advanced structural ceramics. At the building block level, there is a lack of comparative quantification of the mechanical properties between geological and biogenic minerals. Correspondingly, I first benchmark the mechanical property difference between biogenic and geological calcite through nanoindentation techniques. The selected biogenic calcite includes Atrina rigida prisms and Placuna placenta laths, corresponding to calcite {0001}, and {101 ̅8} planes. The natural cleavage plane {101 ̅4} of geological calcite was added to the comparative study. Under indentation load, geological calcite deforms plastically via twinning and slips under low loads, and shifts to cleavage fracture under high loads. In comparison, the P. placenta composites, composed of micro-sized single-crystal laths and extensive intercrystalline organic interfaces, exhibit better crack resistance. In contrast, the single-crystal A. rigida prisms show brittle fracture with no obvious plastic deformation. Secondly, how the internal microstructures and loading types affect the mechanical properties of individual building blocks is investigated. The prismatic building blocks are obtained from the bivalves A. rigida and Sinanodonta woodiana, where the former consists of single-crystal calcite and the latter consists of polycrystalline aragonite. The comparative investigation under different loading conditions is conducted through micro-bending and nanoindentation. The continuous mineral matrix in A. rigida prisms leads to comparable modulus under tensile and compressive loadings in the elastic regime, while the high-density intracrystalline nanoinclusions contribute to the conchoidal fracture behaviors (instead of brittle cleavage). In comparison, the interlocking grain boundaries in S. woodiana prisms correlate with easier tensile deformation (smaller tensile modulus) than compression, as well as the intergranular fracture morphologies. The third topic in the biomineral-level investigation focuses on how biomineral utilizes residual stress at the macroscopic scale. The selected model system is the spine from the sea urchin Heterocentrotus mamillatus, which has a bicontinuous porous structure and mesocrystalline texture. It is confirmed that the spine has a macroscopic stress field with residual tension in the central medulla and compression in the radiating layers. The multimodal characterizations on the spine conclude that the structural origins are not associated with the gradient distribution of the intracrystalline defects, including Mg substitution in the calcite matrix, intracrystalline organics, and amorphous calcium carbonates (ACC). It is hypothesized that the residual stress is generated due to the volume expansion during ACC crystallization at the compacted growth front. At the composite level, even though enhanced crack resistance is expected in biomineralized composites due to their hierarchical structures, the correlation between their 3D composite structures and damage/crack evolution is quite limited in the literature. I developed in-situ testing devices integrated with synchrotron-based X-ray tomography to capture the crack propagation in the materials, including the four-point bending and compression/indentation configurations. Two representative models are chosen to demonstrate the deformation of biomineralized composites under bending and compression, respectively, including the calcium carbonate-based gastropod shell (Melo diadema) and the hydroxyapatite-based fish teeth (Pogonias cromis). Also, the two composites are designed to achieve different functional requirements, i.e., enhanced fracture toughness vs. wear resistance. The comprehensive characterizations of these two composites revealed how biological structural composites are designed accordingly to their functional needs. For the crossed-lamellar M. diadema shell, directional dependence of the shell property was revealed, where the transversal direction (perpendicular to the growth line) represents both the stronger and tougher direction, but the longitudinal direction is more resistant to notches and defects. For the P. cromis teeth, the enhanced wear resistance of the near-surface enameloid originates from the intricate designs at the microscale, with c-axes of hydroxyapatite crystals and micro-sized enameloid rods coaligned with biting direction and F and Zn doping. In addition, the fracture morphologies of the fish teeth correlate with the microstructures; the enameloid exhibits corrugated fracture paths due to the interwoven fibrous building blocks, and the dentin exhibits clean planar fracture surfaces.
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    Comprehensive Modeling of Novel Thermal Systems: Investigation of Cascaded Thermoelectrics and Bio-Inspired Thermal Protection Systems Performance
    Kanimba, Eurydice (Virginia Tech, 2019-12-04)
    Thermal systems involve multiple components assembled to store or transfer heat for power, cooling, or insulation purpose, and this research focuses on modeling the performance of two novel thermal systems that are capable of functioning in environments subjected to high heat fluxes. The first investigated thermal system is a cascaded thermoelectric generator (TEG) that directly converts heat into electricity and offers a green option for renewable energy generation. The presented cascaded TEG allows harvesting energy in high temperatures ranging from 473K to 973K, and being a solid-state device with no moving parts constitutes an excellent feature for increase device life cycle and minimum maintenance in harsh, remote environments. Two cascaded TEG designs are analyzed in this research: the two-stage and three-stage cascaded TEGs, and based on the findings, the two-stage cascaded TEG produces a power output of 42 W with an efficiency of 8.3% while the three-cascaded TEG produces 51 W with an efficiency of 10.2%. The second investigated novel thermal system is a thermal protection system inspired by the porous internal skeleton of the cuttlefish also known as cuttlebone. The presented bio- inspired thermal protection has excellent features to serve as an integrated thermal protection system for spacecraft vehicles including being lightweight (93% porosity) and possessing high compressive strength. A large amount of heat flux is generated from friction between air and spacecraft vehicle exterior, especially during reentry into the atmosphere, and part of the herein presented research involves a thermomechanical modeling analysis of the cuttlebone bio-inspired integrated thermal protection system along with comparing its performance with three conventional structures such as the wavy, the pyramid, and cylindrical pin structures. The results suggest that the cuttlebone integrated thermal protection system excels the best at resisting deformation caused by thermal expansion when subjected to aerodynamic heat fluxes.
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    Data-driven X-ray Tomographic Imaging and Applications to 4D Material Characterization
    Wu, Ziling (Virginia Tech, 2021-01-05)
    X-ray tomography is an imaging technique to inspect objects' internal structures with externally measured data by X-ray radiation non-destructively. However, there are concerns about X-ray radiation damage and tomographic acquisition speed in real-life applications. Strategies with insufficient measurements, such as measurements with insufficient dosage (low-dose) and measurements with insufficient projection angles (sparse-view), have been proposed to relieve these problems but are generally compromising imaging quality. Such a dilemma inspires the development of advanced tomographic imaging techniques, in particular, deep learning algorithms to improve reconstruction results with insufficient measurements. The overall aim of this thesis is to design efficient and robust data-driven algorithms with the help of prior knowledge from physics insights and measurement models. We first introduce a hierarchical synthesis CNN (HSCNN), which is a knowledge-incorporated data-driven tomographic reconstruction method for sparse-view and low-dose tomography with a split-and-synthesis approach. This proposed learning-based method informs the forward model biases based on data-driven learning but with reduced training data. The learning scheme is robust against sampling bias and aberrations introduced in the forward modeling. High-fidelity X-ray tomographic imaging reconstruction results are obtained with a very sparse number of projection angles for both numerical simulated and physics experiments. Comparison with both conventional non-learning-based algorithms and advanced learning-based approaches shows improved accuracy and reduced training data size. As a result of the split-and-synthesis strategy, the trained network could be transferable to new cases. We then present a deep learning-based enhancement method, HDrec (hybrid-dose reconstruction algorithm), for low-dose tomography reconstruction via a hybrid-dose acquisition strategy composed of textit{extremely sparse-view normal-dose measurements} and textit{full-view low-dose measurements}. The training is applied for each individual sample without the need of transferring the trained models for other samples. Evaluation of two experimental datasets under different hybrid-dose acquisition conditions shows significantly improved structural details and reduced noise levels compared to results with traditional analytical and regularization-based iterative reconstruction methods from uniform acquisitions under the same amount of total dosage. Our proposed approach is also more efficient in terms of single projection denoising and single image reconstruction. In addition, we provide a strategy to distribute dosage smartly with improved reconstruction quality. When the total dosage is limited, the strategy of combining a very few numbers of normal-dose projections and with not-too-low full-view low-dose measurements greatly outperforms the uniform distribution of the dosage throughout all projections. We finally apply the proposed data-driven X-ray tomographic imaging reconstruction techniques, HSCNN and HDrec, to the dynamic damage/defect characterization applications for the cellular materials and binder jetting additive manufacturing. These proposed algorithms improve data acquisition speeds to record internal dynamic structure changes. A quantitative comprehensive framework is proposed to study the dynamic internal behaviors of cellular structure, which contains four modules: (i) In-situ fast synchrotron X-ray tomography, which enables collection of 3D microstructure in a macroscopic volume; (ii) Automated 3D damage features detection to recognize damage behaviors in different scales; (iii) Quantitative 3D structural analysis of the cellular microstructure, by which key morphological descriptors of the structure are extracted and quantified; (iv) Automated multi-scale damage structure analysis, which provides a quantitative understanding of damage behaviors. In terms of binder jetting materials, we show a pathway toward the efficient acquisition of holistic defect information and robust morphological representation through the integration of (i) fast tomography algorithms, (ii) 3D morphological analysis, and (iii) machine learning-based big data analysis. The applications to two different 4D material characterization demonstrate the advantages of these proposed tomographic imaging techniques and provide quantitative insights into the global evolution of damage/defect beyond qualitative human observation.
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    Development of A Micro-Scale Impact Tester for Characterizing Dynamic Properties of Biological Structural Materials
    Roth, Nicklas (Virginia Tech, 2023-06-28)
    This thesis presents the design and construction of a micro-scale, air powered, impact testing device for use in Virginia Tech's Biological and Bio-inspired Materials Laboratory. A brief overview of current projectile impact testers is presented along with motivation for the fabrication of a new testing system capable of firing a projectile with a maximum diameter of 0.5 mm at velocities ranging from 20 to 50 m/s. Initial design calculations and analysis were performed to optimize barrel length, projectile size, and air pressure for desired velocity ranges. Computer aided design was then utilized to create a digital model of the entire system before production began on the device. Within the scope of this project was the development of a large-scale projectile impact tester as a proof of concept of the system's design that would later be utilized by other researchers as well as the micro-scale tester which carried over the lessons learned and design improvements from the larger device. The culmination of the project was the testing of biological samples (sea urchin spine cross sections) to prove the viability of the device and highlight its research niche. Future use cases and design improvements of the small-scale impact tester were also investigated as part of this thesis work.
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    Fabrication and Characterization of Multifunctional Soft Composites for Hybrid Electronic Systems
    Pozarycki, Tyler Anthony (Virginia Tech, 2023-07-17)
    There has been an ever-increasing need for soft, functional materials within areas of research such as soft robotics, flexible electronics, and wearable devices. These materials must be stretchable and/or flexible, thermally and electrically conductive, and robustly adhesive to a wide variety of substrates and surfaces. Over the past several decades, soft composites consisting of functional solid particles within an elastic matrix have been developed with the aim of achieving these properties. However, solid particulate fillers in elastomeric materials have various limitations which hinders the ability to achieve the aforementioned properties simultaneously. In this work, two novel approaches to developing soft conductive adhesives are introduced in an effort to solve mechanical, thermal, electrical, and adhesive trade-offs. The composites developed herein utilize liquid metal (LM) inclusions and a combination of LM with solid silver (Ag) flakes within deformable polymer matrices to maintain mechanical compliance while also achieving thermal and electrical functionality. Furthermore, adhesive properties of LM composites are enhanced through a chemical anchoring technique, while the composition and microstructure of LM-Ag composites are designed to control functional and adhesive properties. There are several demonstrations throughout which show the ability to robustly integrate the novel soft composites with rigid materials and electronic components for the creation of resilient and functional hybrid electronic systems.
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    Granular Composite with Addressable and Tunable Stiffness
    Elashwah, Ahmed A. (Virginia Tech, 2024-08-01)
    An integral part in the field of soft robotics is the ability to tune material stiffness. This adaptability is inspired from the natural ability of organisms to alter their stiffness to perform various tasks. The most common approach to mimic this ability is through granular jamming, where a granular material switches between fluid and solid-like states based on density alterations caused by vacuum pressure. In this thesis, a cuboid composite material is introduced, containing internal cylindrical chambers arranged in distinct matrix configurations (2x2, 3x3, and 4x4). A custom-designed pneumatic system enables precise control over this transition, allowing for selective modulation of stiffness across different regions of the material by applying differing pressures to specific regions of the composite material. This approach not only allows for rapid changes in stiffness, but enables stiffness to be adjusted uniformly throughout the material or localized to specific areas. This approach also allows for predictive modeling of granular composites to better understand its mechanical response under differential pressures.
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    Heterogeneous Distribution and Corresponding Mechanical Significance of The Mineral Phase in Fish Scales
    Tan, Yiming (Virginia Tech, 2023-03-15)
    Fish scales can be considered as a laminated composite based on collagen fibrils arranged in a cross-plywood structure. This collagen-based composite is often partially mineralized (primarily hydroxyapatite) in the scale exterior in order to resist penetration and hence to enhance protection. Together with the overlapping assembly, the fish scales offer an excellent model system for developing fiber composite materials and flexible armor systems. The primary objective of this thesis is to characterize the distribution of the mineral phase within individual scale and to investigate the corresponding mechanical consequences of the scale as a whole and its different fields through experimental and computational approaches. In this thesis, we chose the scales from the black drum (Pogonias cromis) fish as a model system. First of all, the exterior surface morphology of individual scales was systematically studied, from which several distinct structural regions are identified, including focus field (central), lateral field (dorsal and ventral), rostral field (anterior), and caudal field (posterior). In the focus field, the classic two-layer design, i.e., mineralized exterior layer and collagen-based interior layer, was observed, and nanoindentation results revealed that the high mineral exterior layer results in a much higher hardness (800 vs 450 MPa). Moreover, macroscopic tensile tests indicate that the mechanical removal of mineralized layer did not lead to reduction in strength values, whereas acid-treated demineralized scales showed reduced mechanical properties. Finally, we identified a previously unreported mineral distribution pattern in the rostral field, in which the mineral phase is segregated into long strips along the anterior-posterior direction (width, ~300 μm). In addition, towards the interior of the scale, it appears that the mineral deposition is highly correlated with the collagen orientation, resulting a unique mineralized-unmineralized collagen-based composite structure. We built finite element models to compare this unique structure to two other mineral phases in different fields at the individual scale. This unique structure demonstrates a larger deformation displacement when load was applied, indicating that it provides further flexibility in anterior end of an individual scale. The mineralized phases and structures of various fields within a single scale provide different mechanical characteristics and properties. The structural and mechanical analysis of the various regions of the fish scale can further investigate the flexibility and protective capacity of the individual scale.
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    High strength and damage-tolerance in echinoderm stereom as a natural bicontinuous ceramic cellular solid
    Yang, Ting; Jia, Zian; Wu, Ziling; Chen, Hongshun; Deng, Zhifei; Chen, Liuni; Zhu, Yunhui; Li, Ling (Nature Research, 2022-10-14)
    Due to their low damage tolerance, engineering ceramic foams are often limited to non-structural usages. In this work, we report that stereom, a bioceramic cellular solid (relative density, 0.2–0.4) commonly found in the mineralized skeletal elements of echinoderms (e.g., sea urchin spines), achieves simultaneous high relative strength which approaches the Suquet bound and remarkable energy absorption capability (ca. 17.7 kJ kg⁻¹) through its unique bicontinuous open-cell foam-likemicrostructure. The high strength is due to the ultra-low stress concentrationswithin the stereom during loading, resulted from their defect-free cellular morphologies with near-constant surface mean curvatures and negative Gaussian curvatures. Furthermore, the combination of bending-induced microfracture of branches and subsequent local jamming of fractured fragments facilitated by small throat openings in stereom leads to the progressive formation and growth of damage bands with significant microscopic densification of fragments, and consequently, contributes to stereom’s exceptionally high damage tolerance.
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    Interfacial Dynamics in Dual Channels: Inspired by Cuttlebone
    Huang, Matthew; Frohlich, Karl; Esmaili, Ehsan; Yang, Ting; Li, Ling; Jung, Sunghwan (MDPI, 2023-10-01)
    The cuttlebone, a chambered gas-filled structure found in cuttlefish, serves a crucial role in buoyancy control for the animal. This study investigates the motion of liquid-gas interfaces within cuttlebone-inspired artificial channels. The cuttlebone’s unique microstructure, characterized by chambers divided by vertical pillars, exhibits interesting fluid dynamics at small scales while pumping water in and out. Various channels were fabricated with distinct geometries, mimicking cuttlebone features, and subjected to different pressure drops. The behavior of the liquid-gas interface was explored, revealing that channels with pronounced waviness facilitated more non-uniform air-water interfaces. Here, Lyapunov exponents were employed to characterize interface separation, and they indicated more differential motions with increased pressure drops. Channels with greater waviness and amplitude exhibited higher Lyapunov exponents, while straighter channels exhibited slower separation. This is potentially aligned with cuttlefish’s natural adaptation to efficient water transport near the membrane, where more straight channels are observed in real cuttlebone.
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    Investigating Cell Viscoelastic Properties with Nanonet Force Microscopy
    Zhang, Haonan (Virginia Tech, 2022-08-04)
    Determining the mechanical properties of living cells accurately and repeatably is critical to understanding developmental, disease, and repair biology. The cellular environment is composed of fibrous proteins of a mix of diameters organized in random and aligned configurations. In the past two decades, several methods, including modified atomic force microscopy (AFM) and micro-pipette aspiration have been developed to measure cellular viscoelastic properties at single-cell resolution. We inquired if the fibrous environment affected cellular mechanobiology. Using our non-electrospinning Spinneret based Tunable Engineered Parameters (STEP) fiber manufacturing platform, we developed fused nanonets to measure single-cell forces and viscoelasticity. Using computer-controlled probes, we stretched single cells attached to two-fiber and three-fiber systems precisely and recorded the relaxation response of cells. The viscoelastic properties were determined by fitting the data to the standard linear viscoelastic solid model (SLS), which includes a spring (k0) in parallel with a spring (km)-damper (cm) series. In cases in which cells are seeded on two fibers, we tested hMSCs and BJ-5TA cells, and the viscoelastic components measurements k0, km, and cm are 26.16 ± 3.38 nN/µm, 5.81 ± 0.81 nN/µm, and 41.15 ± 5.97 nN-s/µm, respectively for hMSCs, while the k0, km, and cm, measurements of BJ-5TA cells are 20.02 ± 2.89 nN/µm, 4.62 ± 0.75 nN/µm, and 45.46 ± 6.00 nN-s/µm respectively. Transitioning to the three-fiber system resulted in an overall increase in native contractility of the cells while allowing us to understand how the viscoelastic response was distributed with an increasing number of fibers. Viscoelastic experiments were done twice. First, we pulled on the outermost fiber similar to the two-fiber case. The cell was then allowed to rest for two hours, sufficient time to regain its pre-stretching contractility. The cell was then excited by pulling on the middle fiber. The experimental results of cell seeding on three fibers proved that the viscoelastic property measurements depend on the excitation position. Overall, we present new knowledge on the cellular viscoelasticity of cells attached to ECM-mimicking fibers.
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    A Kirigami Approach for Controlling Properties of Adhesives and Composites
    Hwang, Dohgyu (Virginia Tech, 2022-02-25)
    Controlling the layout of elasticity in materials provides new opportunities for generating various functionalities such as shape-morphing capability, large stretchability, and elastic softening for aeronautics, drug delivery, soft robotics, and stretchable electronics applications. Recently, techniques building upon kirigami principles, the Japanese art of paper cutting, have been considered an effective strategy to control stiffness and deformation of materials by systemically integrating cut patterns into inextensible sheets. The performance of kirigami-inspired materials relies primarily on geometric features defined by cut patterns rather than chemistry of constituents, which can enable high compatibility with diverse material sets across a wide range of length scales. However, kirigami has been relatively unexplored to control adhesion and current challenges such as the intrinsic trade-off between high deformability and load-bearing capacity limits applications that require large shape change and structural strength. This thesis demonstrates that the kirigami approach is a powerful tool to control interfacial properties of adhesive films, and that composite approaches in kirigami-inspired material can overcome the deformation-strength trade-off. The kirigami principle is applied to adhesives to control adhesion through arrays of linear cut patterns (Chapter 2). The spatial layout of elasticity in the kirigami-inspired adhesive enhances adhesion over homogeneous adhesive systems and generates anisotropic adhesion. The utility of the proposed adhesive design criteria is further extended to complex non-linear cut patterns (Chapter 3). These non-linear patterns significantly enhance adhesion relative to linear patterns in adhesives and unpatterned films, while also enabling easy release and spatial control of adhesion across a sheet. The enhancement enabled by cut geometry remains effective in diverse adhesives, on various surfaces, and in wet and dry conditions. The adhesion dependence on cut geometry is further investigated to understand how arrays of sub-patterns adjacent to primary non-linear patterns affect adhesion performance (Chapter 4). Kirigami composites are also developed to overcome the trade-off between large deformability and load-bearing capacity (Chapter 5). A composite architecture is developed consisting of low melting point metal alloys incorporated into patterned elastomeric layers. This composite approach shows the ability to rapidly morph into complex, load-bearing shapes, while achieving reversibility and self-healing capability through phase change driven by embedded heaters. The utility of the multi-functional composite is demonstrated through a multimodal morphing drone which transforms from a ground to air vehicle and an underwater morphing machine which can be reversibly deployed to collect cargo. This thesis is then summarized by discussing key findings, contributions, and future perspectives (Chapter 6).
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    Mechanical Design of Selected Natural Ceramic Cellular Solids
    Yang, Ting (Virginia Tech, 2021-05-24)
    While the structure and mechanical properties of natural cellular solids such as wood and trabecular bone have been extensively studied in the past, the structural design and underlying deformation mechanisms of natural cellular solids with very high mineral contents (> 90 wt%), which we term as natural ceramic cellular solids, are largely unexplored. Many of these natural ceramic cellular solids, despite their inherent brittle constituent biominerals (e.g., calcite or aragonite), exhibit remarkable mechanical properties, such as high stiffness and damage tolerance. In this thesis, by carefully selecting three biomineralized skeletal models with distinctly different cellular morphologies, including the honeycomb-like structure in cuttlefish bone (or cuttlebone), the stochastic open-cell structure in sea urchin spines, and the periodic open-cell structure in starfish ossicles, I systematically investigate the mechanical design strategies of these natural ceramic cellular solids. The three model systems are cuttlefish Sepia officinalis, sea urchin Heterocentrotus mammillatus, and starfish Protoreaster nodosus, respectively. By investigating the relationship between their mechanical properties and structural characteristics, this thesis reveals some novel structural design strategies for developing lightweight, tough, strong, and stiff ceramic cellular solids. The internal skeleton of S. officinalis, also known as cuttlebone, has a porosity of 93 vol% (constituent material: 90 wt% aragonite), which is a multichambered structure consisting of horizontal septa and thin vertical walls with corrugated cross-sectional profiles. Through systematic ex-situ and synchrotron-based in-situ mechanical measurements and collaborative computational modeling, we reveal that the vertical walls in the cuttlebone exhibit an optimal waviness gradient, which leads to compression-dominant deformation and asymmetric wall fracture, accomplishing both high stiffness (8.4 MN∙m/kg) and high energy absorption (4.4 kJ/kg). Moreover, the distribution of walls reduces stress concentrations within the horizontal septa, facilitating a larger chamber crushing stress and more significant densification. For the stochastic open-cell foam-like structure, also known as stereom (porosity: 60-80 vol%, constituent material: 99 wt% calcite) in H. mammillatus, we first developed a computer vision-based algorithm that allows for quantitative analysis of the cellular network of these structures at both local individual branch and node level as well as the global network level. This open-source algorithm could be used for analyzing both biological and engineering open-cell foams. I further show that the smooth, highly curved branch morphology with near-constant surface curvature in stereom results in low-stress concentration, which further leads to dispersed crack formation upon loading. Combined synchrotron in-situ analysis, electron microscopic analysis, and computational modeling further reveal that the fractured branches are efficiently jammed by the small throat openings within the cellular structure. This further leads to the formation of damage bands with densely packed fracture pieces. The continuous widening of the damage bands through progressive microfracture of branches at the boundaries contributes to the observed high plateau stress during compression, thereby contributing to its high energy absorption (17.7 kJ/kg), which is comparable and even greater than many synthetic metal- and polymer-based foams. Lastly, this thesis leads to the discovery of a unique dual-scale single-crystalline porous lattice structure (porosity: 50 vol%, constituent material: 99 wt% calcite) in the ossicles of P. nodosus. At the atomic level, the ossicle is composed of single-crystal biogenic calcite. At the lattice level, the ossicle's microstructure organizes as a diamond-triply periodic minimal surface (TPMS) structure. Moreover, the crystallographic axes at atomic and lattice levels are aligned, i.e., the c-axis of calcite is aligned with the [111] direction of the diamond-TPMS lattice. This single crystallinity co-alignment at two levels mitigates the compliance of calcite in the c-axis direction by utilizing the stiff <111> direction of the diamond-TPMS lattice. Furthermore, 3D in-situ mechanical characterizations reveal that the presence of crystal defects such as 60° and screw dislocations at the lattice level suppresses slip-like fracture along the {111} planes of the calcitic diamond-TPMS lattice upon loading, achieving an enhanced energy absorption capability. Even though the skeleton of echinoderm is made up of single-crystal calcite, the structure fractures in a conchoidal manner rather than along the clipping plane, which can continuously fracture the fragments into small pieces and enhance energy dissipation.
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    Mechanics of Phase Transformation in NiTi Shape Memory Alloys at The Atomistic Scale
    Yazdandoost, Fatemeh (Virginia Tech, 2019-02-14)
    During the past decade, Shape Memory Alloys (SMAs), particularly Nickel-Titanium (NiTi) alloys, have received increasing attention mainly because of their promising role to be integrated into multifunctional systems for actuation, morphing, and sensory capabilities in a broad variety of applications including biomedical, aerospace and seismological engineering. The unique performance of all the novel devices developed by SMAs relies on either the shape memory effect or pseudoelasticity, the two distinctive properties of SMAs. Both these unique properties are based on the inherent capability of SMAs to have two stable lattice structures at different stress or temperature conditions, and the ability of changing their crystallographic structure by a displacive phase transformation between a high-symmetry austenite phase and a low-symmetry martensite phase, in response to either mechanical or thermal loading. These properties make them a superior candidate for using as damping materials under high-strain-rate loading conditions in different engineering fields. SMA materials used in the most applications are polycrystalline in nature. In polycrystalline SMAs at the bulk-level, in addition to the phase transformation at the lattice-level, the thermomechanical response is also highly sensitive to the microstructural properties. In this work, the microstructure, as well as defects, such as dislocations and the stacking faults, are studied in the NiTi crystalline structure. In addition, the performance of NiTi under shock wave loading and vibrations, and their energy dissipation capabilities are examined using computational modeling, globally and locally. The effect of graphitic and metal structures, as reinforcements, on the performance of NiTi matrix composites under static and shock stress wave loading conditions is also investigated at the atomistic scale.
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    Mechanics, Design, and Fabrication of Metal-Graphene Composites
    Agrawal, Arpit Kumar (Virginia Tech, 2023-06-01)
    In the last decade, metal-graphene composites have seen significant progress and have received increasing attention because of graphene's ability to improve the mechanical properties. The main mechanism of improvement in metal graphene composite is based on the impeding of dislocations by graphene sheets. The work includes studying the mechanisms behind the improvement caused by graphene sheets and particles using Molecular Dynamics and Density Functional Theory. Interatomic potentials that play an important role in determining the accuracy of Molecular dynamics simulations are developed for Cu-C, Ni-C, Ti-C, and Ni-Ti-C systems. Nanolayered metal-graphene composites are fabricated and the effect of graphene particles on crack's path are investigated by electron microscopy. The mechanisms behind crack's behavior is investigated by atomistic simulations and by comparing energy release rates. Metallic systems that do not deform by dislocations like metallic glasses, NiTi etc. are reinforced with graphene and are also examined by atomistic simulations. In addition, a novel metal-graphene composite in which the metal matrix undergoes a uniform large recoverable phase transformation when subjected to mechanical loading is proposed and investigated using atomistic simulations. The material has the potential to overcome the long-standing challenge of transferring the extraordinary mechanical performance of nanoscale materials to the bulk level.
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    Multiscale Structures and Mechanics of Biomineralized Lattices in Hexactinellid sponges and Echinoderms
    Chen, Hongshun (Virginia Tech, 2023-06-30)
    Biomineralized lattice materials with have high mineral contents (~ 99 wt%), usually "conceal" multiscale structural arrangements for unique mechanical or functional performance, such as the remarkable damage tolerance despite of the brittle nature of the constituents (e.g., biogenic silica and calcite). However, the quantitative explorations of the structure-mechanics relationships in multiscale of biomineralized lattices remain insufficient and hence hinder the leverage of the functional benefits to design architected cellular materials. In this dissertation, I selected two groups of marine animals (i.e., Hexactinellid sponges and Echinoderms) for systematic structural-mechanical study. Their biomineralized lattice skeletons exhibit three representative types of multiscale structures: 1) multiscale hierarchical structure: skeleton of Hexactinellid sponge such as Euplectella aspergillum; 2) multiscale functionally graded structure: spine of sea urchin Heterocentrotus mammillatus; and 3) dual-scale (atomic and microlattice scales) periodic structure: ossicle of starfish Protoreaster nodosus. This dissertation develops quantitatively the structural-mechanical/functional correlations in biomineralized cellular materials for bio-inspired material design. Four different species of Hexactinellid sponges have been studied with particular focus on the species E. aspergillum. As an example of the multiscale hierarchical biomineralized lattice, the extremely lightweight skeleton (~99% porosity) of E. aspergillum exhibits 1) amorphous nanoparticular biogenic silica; 2) micron-sized fibrous spicule with cylindrically laminated silica layers separated by organic interfaces; 3) spicule bundles where the individual spicules merged by secondary silica deposition; 4) a centimeter-sized Voronoi-like cellular dome known as sieve plate; and 5) a centimeter-sized cylindrically arranged rectangular lattice with double-diagonal reinforcement and external helical ridge. Here, we discovered a series of mechanical or functional properties or formation process of structures in different length scales: 1) for the biogenic silica in three different species of Hexactinellid sponge, consistent modulus and hardness of the biogenic silica throughout the cross section of the spicule are found via substantial correlation between the measured values and locations; 2) for the sieve plate, the Voronoi-like cellular dome constructed by porous branch with increased height achieves balance between improved mechanical stiffness and large pore opening for sponge's current pumping mechanism; 3) via microstructural study, the formation process of the sieve plate is proposed; and 4) for the cylindrical skeletal body, the double-diagonal configuration and the ridge structure are found to provide tendency to optimize torsional rigidity, and enhanced radial stiffening and improved permeability, respectively. The cellular structure in the spine of the H. mammillatus (i.e., stereom) made of ~99wt% of single-crystalline calcite shows a multiscale functionally graded structure. We developed and optimized a cellular network analysis workflow on the large-volume 3D lattice structure obtained from the synchrotron-based micro-Computed Tomography scan. The analysis provides quantitative descriptions of the branch, ring structure, and septum which reveals a functionally graded structure in multiscale from the center region to the edge region of the spine: 1) in microscale, the branch thickness and length increases, resulting in a significantly decreased porosity; and 2) in macroscale, the center region of the spine with galleried stereom of highly aligned branches transits to the edge region with laminar stereom of radially arranged septa and interconnecting branches. The multiscale structural variations lead to the mechanical variations the increased elastic modulus and mechanical isotropy from the center to the edge of the spine. This provides a biological pathway for designing the lightweight, strong, and tough beam with multiscale structural gradient. In previous work, we discovered that ossicle in starfish P. nodosus possesses a unique dual-scale periodic lattice structure, which means periodic single crystal calcite in nanoscale and diamond triply periodic minimal surface (diamond-TPMS) lattice in microscale. It has three unique structural features: 1) microlattice dislocations in ossicles similar to those found in crystals with diamond cubic lattice; 2) a diamond-TPMS microlattice with ca. 50% relative density; and 3) dual-scale crystallographic coalignment between c-axis of the single-crystalline constituent calcite and the [111] direction of the diamond-TPMS microlattice. Based on this work, this dissertation mainly reveals: 1) unique type and core structures of the dislocations in the ossicle for stiffness, strength, and toughness; 2) the 3D property compensation of dual-scale crystallographic coalignment for improved mechanical isotropy; and 3) mechanical benefits (improved mechanical isotropy and effective fragment jamming) and morphological benefits (minimal surface and highest surface area to volume ratio) for 50% relative density.
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    Nature-Inspired Hierarchical Steels
    Cao, Shan Cecilia; Liu, Jiabin; Zhu, Linli; Li, Ling; Dao, Ming; Lu, Jian; Ritchie, Robert O. (Nature Publishing Group, 2018-03-23)
    Materials can be made strong, but as such they are often brittle and prone to fracture when under stress. Inspired by the exceptionally strong and ductile structure of byssal threads found in certain mussels, we have designed and manufactured a multi-hierarchical steel, based on an inexpensive austenitic stainless steel, which defeats this “conflict” by possessing both superior strength and ductility. These excellent mechanical properties are realized by structurally introducing sandwich structures at both the macro- and nano-scales, the latter via an isometric, alternating, dual-phase crystal phases comprising nano-band austenite and nano-lamellar martensite, without change in chemical composition. Our experiments (transmission and scanning electron microscopy, electron back-scattered diffraction, nano-indentation and tensile tests) and micromechanics simulation results reveal a synergy of mechanisms underlying such exceptional properties. This synergy is key to the development of vastly superior mechanical properties, and may provide a unique strategy for the future development of new super strong and tough (damage-tolerant), lightweight and inexpensive structural materials.
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    Origins of Embrittlement of an Al-Zn-Mg-Cu Alloy Post Additive Friction Stir Deposition
    Yoder, Jake King (Virginia Tech, 2023-01-03)
    Additive Friction Stir Deposition (AFSD) is a solid state, bulk, metal additive manufacturing technology that seeks to replace certain castings and forgings wherever it is economically feasible among other applications. Critical to its deployment is an in depth understanding of how the solid state deposition process effects engineering alloys used in relevant applications. In this work, an aerospace aluminum alloy 7075 is evaluated both in the as deposited and heat treated condition via age hardening studies and tensile testing. It is found that an embrittlement phenomena occurs that is sensitive to processing parameters and quench rate during heat treatment. Through the use of SEM, TEM, and APT the embrittlement phenomena has been linked to excessive grain boundary precipitation caused by a combination of shear induced mixing and shear induced segregation which allow for the formation of phases at grain boundaries that are slow to dissolve, leaving the grain boundary in a non-equilibrium solute rich state. Critical to this process is the role of dispersoid particles, which are modified by shear processes which provide high energy spots for thermally stable precipitate nucleation. Removal of these dispersoid particles by an alloy modification had been shown to eliminate the embrittlement effect after depositing in a condition where embrittlement is expected for the unmodified 7075. Further work demonstrates the different relationships between processing conditions and the degree of embrittlement for three different tool types. Beyond the implications of the particular alloy studied, this work highlights the fundamental concepts involved when a manufacturing process operates at high strain rates and total strains which can be used for the design of alloys meant for AFSD.
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    Process-Property Characterization for Multi-Material Jetting Applications
    Bezek, Lindsey Bernadette (Virginia Tech, 2022-06-23)
    Material jetting (MJ) is an additive manufacturing (AM) process that involves the selective jetting of a liquid material into the shape of a layer and subsequent solidification, often via ultraviolet (UV) irradiation, in a layer-wise fashion. The MJ process has the potential to emerge as a robust fabrication method: the inherent, facile, multi-material capability in a high-resolution process should distinguish the technology as a competitive, multi-functional, manufacturing process. However, it is mainly constrained to prototyping use, limited by both material and process constraints. This research expands material and process knowledge by characterizing the multi-material process-structure-property relationships in photopolymer-based MJ, which provides a basis for advancing the capability of MJ to fabricate accurate and consistent multi-material parts for functional applications. One of the challenges for advancing MJ is the general lack of processable materials. For example, MJ is increasingly being used for fabricating anatomic models for use as pre-procedural planning or medical student trainee tools, but commercial MJ elastomers are unable to mimic human tissues' mechanical properties, which limits the instructional value of printed anatomic models. By combining photo-curing and non-curing materials, a cardiac tissue-mimicking material was achieved and integrated into a fully-printed heart model used to practice the transseptal puncture procedure. Several mechanical properties of this multi-material combination were evaluated to facilitate quicker screening of future tissues that would be desired to be mimicked. Also impeding technological advancement of MJ systems is a lack of understanding the effects of indiscriminate UV exposure on material properties. Depending on factors such as part design and build layout, an indiscriminate UV toolpathing strategy poses the risk for providing inconsistent UV dosing to parts and causing unintended variations in mechanical performance. Experiments were conducted to quantify these effects, and an empirical model was developed to predict the accumulated exposure parts receive. A connection was then made between accumulated exposure received by material voxels and final part properties, where it was observed that overexposure effects exist, and are largely dependent on material, build layout, and toolpathing. This work will lead to improved design guidelines and process modifications to ensure consistency of UV dosing and achieve desired mechanical performance. This knowledge will enable future photopolymer AM systems to account for potential overcuring effects toward fabricating repeatable and reproducible functional products. Finally, documented in this work are efforts toward expanding the knowledge about the use of AM to safely produce personal protective equipment during the COVID-19 pandemic. Amid prospects of large-scale, distributed production of respirators via AM, the lack of filtration efficiency testing generated concerns about the respirators' effectiveness. The goal of this work was to measure particle transmission through respirators fabricated with powder bed fusion and fused filament fabrication processes and compare their performance to that of cloth masks and standardized N95 respirators. Through systematic post-processing, the connection between printed respirator deficiencies and changes in filtration efficiency were discerned. Identifying the system-level quality control challenges responsible for the respirator failure modes highlights some the current limitations in AM for fabricating functional parts. The findings will assist future efforts toward both creating enhanced designs and optimizing printer parameters, ultimately working toward qualifiable, end-use parts.