Symposium VII-D: Technical Program

Track VII: Mechanics of Soft and Biological Materials and Flexible Structures
Symposium VIID: Non-linear Response of Highly Deformable Structures
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Wednesday July 26, 2017 – Session 1B – 10:15am to 11:55am

Location: 433 Curry Student Center

Session Chair: Teng Zhang, Syracuse University

Time Authors Talk Title
10:15am H. Jin* | A. Landauer | K-S. Kim Ruga Mechanics of Cylindrical Cavity Bilayer: Post-Bifurcation of Surface-film Wrinkling Versus Ring Buckling
Abstract: A cylindrical cavity bilayer (CCB) system consisting of a stiff film on a cavity surface in an infinite medium has potential in both biological and soft materials engineering applications. When the CCB system is subjected to far-field pressure, the surface morphology bifurcates, subsequently evolving into a series of ruga structures. Unlike the primary bilayer (PB) system, the critical pressures of the CCB system for creasing, wrinkling, period-doubling, and other postbifurcation modes depend not only on the stiffness mismatch between the film and the medium, but also on the film thickness to cavity radius ratio (t = t/r). Finite element method (FEM) is used to construct the ruga-phase diagrams of CCB systems (CCB-RPDs) on the k − p plane where k is the wavenumber normalized by film thickness, and p is the far-field pressure normalized by the shear stiffness of the medium. We considered the stiffness ratios (μ) ranging from the homogenous (μ = 1) case to the extremely stiff-film (μ = 10^4) case, for four ratios of t. We found two buckling mechanisms of the CCB system: “surface-film wrinkling” and “ring buckling” mechanisms. These two mechanisms additively contribute to the critical wrinkling pressure. Experiments were also performed to measure the criticality and the modes of the CCB system instability with or without a stiff film. We believe that the CCB-RPDs will be useful in understanding the characteristics of the collapse modes of cylindrical cavities, e.g. blood vessels, and in designing various pressure-controlled soft-material orifice valves.
10:35am P. Plucinsky* | K. Bhattacharya Microstructure-induced Suppression of Wrinkling in Nematic Elastomer Sheets
Abstract: Nematic elastomers are rubbery solids which have liquid crystals incorporated into their polymer chains. These materials display many unusual mechanical properties, one such being the ability to form fine-scale microstructure. In this talk, we explore the response of taut and appreciably stressed sheets made of nematic elastomer. Such sheets feature two potential instabilities — the formation of fine-scale material microstructure and the formation of fine-scale wrinkles. We develop a theoretical framework to study these sheets that accounts for both instabilities, and we implement this framework numerically. Using both analytical and numerical studies, we show that nematic elastomer sheets can suppress wrinkling by modifying the expected state of stress through the formation of microstructure.
10:55am M. Pezzulla | M. Steranka | A. Bade | D. Holmes* Instability Reversal in Growing Shells
Abstract: Many fundamental biological structures, including lipids, vesicles, and cells, experience an evolving natural curvature. This stimulus causes bending and buckling in drying pollen grains, and a snap-through inversion in the development of a Volvox embryo.  A spherical shell under external pressure will eventually buckle locally through the development of a dimple. However, when a free spherical shell is subject to variations in natural curvature, it will either buckle globally or snap towards a buckled configuration. We study the similarities and differences between pressure and curvature instabilities in spherical shells, and develop a direct analogy between a curvature potential and a pressure potential. We show how the critical buckling natural curvature is largely independent of the thinness and half-angle of the shell, while the critical snapping natural curvature grows linearly with the half-angle. As a result, we demonstrate how a critical half-angle, depending only on the thinness of the shell, sets the threshold between two different kinds of snapping: as a rule of thumb, shallow shells snap into everted shells, while deep shells snap into buckled shells. Finally, we show that the presence of a boundary on a growing shell can cause the curvature-driven instability to switch from snapping to buckling, and vice versa. As the developed models are purely geometrical, the results are applicable to a large variety of stimuli and scales.
11:15am T. Zhang* A Novel Lattice Model for Tunable Adhesion Through Surface Wrinkles 
Abstract: Surface wrinkles have been utilized to dynamically control adhesion for smart surfaces. The roughened surface topology and non-uniform stress inside the materials are challenging to describe in theory and simulate with numerical methods. Here we show a novel lattice model that is capable of accurately capturing the highly nonlinear deformation of solids (i.e., Neo-Hookean solids) and incorporating the adhesion through interaction potentials, such as Lennard-Jones potential. The novel lattice can successfully reproduce the classical JKR solution for adhesion. The predictions of adhesion on wrinkled bi-layer structure are also consistent with previous experiments. Our theory provides a promising tool for rational design of smart adhesion and friction through wrinkles.


Thursday July 27, 2017 – Session 1A – 10:15am to 12:15pm


Location: 348 Curry Student Center

Session Chair: Nikolaos Vasios, Harvard University

Time Authors Talk Title
10:15am K. A. Khan* | R. Umer Modeling the Compaction Response of 3D Woven Fabrics for Liquid Composite Molding Processes 
Abstract: In liquid composite molding (LCM) processes, the compaction characterization of fibrous reinforcements plays a key role in determining the thickness, fiber volume content and part shape. This study presents detailed experimental and modeling work to study the viscoelastic compaction response of three different types of 3D woven carbon fiber reinforcements, namely, orthogonal, angle interlock and layer-to-layer, each having a different weave style and z-binder yarn pattern. For all reinforcements, single-step, multi-step and cyclic compaction experiments were conducted. A nonlinear viscoelastic model is presented that accounts for large deformations and viscous effects, to capture the response of the material under various loading histories. Model verification is also presented to capture each response with separate sets of material parameters. Parametric studies are also performed to analyze the role of model parameters on the response of different types of loadings. XCT analysis showed significant permanent deformation of z-binder yarns through the thickness of the reinforcements. The comparison of modeling results with the experimental data show that the model is able to capture the stress decay after multiple compaction cycles, yet needs further investigations to predict complete cyclic hysteresis. However, model results agree reasonably well with the single and multi-step compaction loading.
10:35am Z. Liu* | Y. Wang Numerical Simulation of U-bend Tube under Harsh Operating Environment
Abstract: The tube boilers are commonly used in the steam-generating plants or in the combined cycle gas turbine plants. Since the high-pressure and high-temperature water inside the tube, the flow-accelerated corrosion occurs to induce the thinning of the internal wall; then the thinned tube wall will be deformed. It is very important to monitor structural defects in a tube boiler to guide maintenance policy for the U-bend tubes, which are include scheduled preventive maintenance and unscheduled maintenance for an undetected failure. The on-line condition monitoring is highly desired in real applications, So Fibre Bragg grating (FBG) sensors are developed for monitoring U-bend tubes of a boiler under Harsh Operating Environment. In practice, they carry out on-line monitoring for strain and temperature variations at the critical positions of U-bend tubes. For developing monitoring system, it is very important to understand the strain distribution and the temperature distribution of the perfect or one thinning area U-bend tube under working condition. This numerical investigation will help understand and optimize the position and the effective position of FBG installed. The variations of the strain and temperature distributions for a fixed thinning area with the pressure/temperature are calculated to establish a relationship between the thickness of tubes and the strain using numerical simulation.
10:55am J. Papadopoulos* | A. Dressel Nonlinear Interaction of a Slender Pneumatic Tire with the Ground – Extensions to the Rotta Model 
Abstract: Rotta’s “pressurized membrane” model of a pneumatic tire cross section is particularly applicable to slender bicycle tires because the contact patch is ‘long’ (i.e., cross-section shape varies slowly around the tire over a distance of several widths). We therefore explored its predictions for the ground reaction forces in fixed-camber rolling. For small deflection, vertical stiffness is independent of tire width, and force is aligned with the wheel plane. At finite deflection, vertical and horizontal stiffnesses exhibit sign reversal, and camber thrust displays a deficit, requiring ‘sideslip’ (yaw angle) to provide the ground reactions of a steadily turning bicycle. The nonlinear results show that classical ‘linear elastic foundations’ cannot be expected to capture important tire behaviors.The above cross-section results are applied to the entire contact patch by integration, and compared to experimental results.
11:15am N. Vasios* | M. C. Fernandes | J. T. B. Overvelde | K. Bertoldi Complex Output from a Single Input
Abstract: Soft robotic systems actuated through  inflation  are great candidates for producing sophisticated motions harnessing their inherent compliance. In most applications, soft robots are able to attain complex motions through the individual actuation of their building blocks, commonly referred to as soft pneumatic actuators. The individual inputs have to be sequenced by a controller that inflates and deflates the components according to the motion that needs to be achieved, leading to a system that typically receives multiple inputs and requires just as many pumps. In this study, we introduce a new approach for the actuation of soft robotic systems. Through a combination of experiments and numerical analyses we demonstrate that by connecting the pneumatic actuators with narrow tubes, viscous effects can be exploited and complex responses can be achieved by a single and simple input.
11:35am K. A. Khan* | R. K. Abu Al Rub Modeling Time and Frequency Domain Viscoelastic Behavior of Architectured Foams
Abstract: The time-dependent behavior of architected lightweight cellular solids or foams is important to investigate for various structural applications. In this paper, we studied the linear viscoelastic properties of a novel architectured foam based on the mathematically-known Schwarz Primitive (P) triply periodic minimal surface (TPMS), referred to here as P-foam, under both time and frequency domains. Here, 3D representative volume elements (RVEs) at different relative densities (i.e., the ratio of the foam’s density to the density of its solid counterpart) were generated and studied using the finite element method. The effective time-dependent response of P-foams as a function of relative density and frequencies is investigated. For the first time, the approach similar to time-temperature superposition principle was adopted to create the master curve of the observed relative density dependent mechanical responses in both time and frequency domains. Reduced uniaxial, bulk and shear stiffness-loss map results suggested that the P-foam possesses highest bulk response while highest damping can be achieved under uniaxial responses. Depending on the applications and loading conditions variable stiffness P-foam dampers can be designed with unique and optimized dynamic mechanical properties. The comparison of relaxation response of various generic cellular architectures with the P-foam showed that the uniaxial response of P-foam is similar to Kelvin foam. However, shear relaxation and bulk responses are higher than Simple Cubic, Body Centered Cubic, Reinforced Body Centered Cubic and Gibson-Ashby foams. Based on the RVE micromechanical simulations, a macroscopic constitutive model was proposed for modeling the viscoelastic behavior of P-foam structures.
11:55am F. Zhu* | J. Lei Underbody Blast Effect on the Pelvis and Lumbar Spine: A Computational Study
Abstract: Explosion from an anti-tank landmine under a military vehicle, known as underbody blast (UBB) has been a major threat to the occupants inside the vehicle. The explosion energy and shock waves could result in large inelastic deformation and acceleration of the vehicle floor to cause injuries of lower extremity, pelvis and lumbar spine of the crew. Injury severity and patterns of these body regions subjected to UBB have been found highly related to loading conditions, i.e. the vertical acceleration pulse. A computational (finite element) human model was developed and successfully simulated the tibia fracture under UBB in a previous study. In the present work, it was further improved by including a more detailed lumbar spine and pelvis model with high biofidelity. Strain rate effect and failure criteria were considered in the constitutive model of bony materials. The newly developed pelvis and lumbar spine were validated against component level impact test data in the literature. Then the whole body model was validated with the published cadaver sled test data. Using the validated whole body model, parametric studies were conducted by adjusting the peak and time duration of acceleration pulse generated in the UBB. The critical values of these two parameters for pelvic and lumbar spine fracture were determined, and the relationship between injury pattern and loading conditions were established. With the information obtained, new protective equipment for mitigating UBB effect can be further developed.

Thursday July 27, 2017 – Session 2A – 1:30pm to 3:10pm

Location: 348 Curry Student Center

Session Chair: Zi Chen, Dartmouth College

Time Authors Talk Title
1:30pm Q. Zhang* | J. Yin Spontaneous Delamination of Crack-Free Nanoribbons in Metal and Semiconductor for Potential Extreme Stretchability
Abstract: Design of nanomaterials and structures with extreme stretchability is of great interest in flexible electronics and devices, due to their promising broad applications in wearable electronics, electronic skins, energy harvesters and storage, and bio-integrated devices for healthcare monitoring. One of the typical approaches to achieve stretchability is harnessing the coherent buckling of thin film bonded with soft elastomers to release the strain. However, the level of achieved stretchability is largely limited by the cracking fragmentation in the nanomaterials, as well as the constrained buckling with the fully bonded soft substrate. Here, we explore harnessing another mechanism mode, spontaneously buckling-driven periodic delamination of thin coatings and nanoribbons on largely pre-strained elastomers (over 100%), to achieve extreme stretchability in both metal and semiconductor. We find that globally ordered cracking fragmentations are experimentally observed in spontaneously buckling-driven delaminated thin films due to the Poisson’s effect, whereas no longitudinal cracking normal to the stretching direction is observed. The periodicity of such transverse cracking can be well predicted by a simple theoretical model. Similar transverse cracking fragmentations are also observed in its counterpart, spontaneous buckling delamination of nanoribbons on elastomers. To eliminate the unwanted transverse cracking in the nanoribbons, by manipulating the patterning of nanoribbons, we successfully fabricate crack-free nanoribbons in metal and semiconductor with potential extreme stretchability. The measurement shows that the maximum tensile strain in the spontaneously and highly delaminated nanoribbons is over 100 times smaller than the pre-strain in the elastomer substrate, accounting for the potential extreme stretchability.
1:50pm M. Pharr* | T. Pan | J. A. Rogers | Y. Huang Serpentine Interconnects on Ultrathin Elastomers for Stretchable Electronics
Abstract: Integrating deformable interconnects with inorganic functional materials has produced high-performance stretchable electronics. In a number of applications, these systems must sustain large deformation under repetitive loading. Here, we theoretically and experimentally investigate the influence of the elastomeric substrate on the stretchability of serpentine interconnects. Finite element analyses reveal a substantial increase in stretchability with reducing substrate thickness. Low-cycle fatigue tests confirm this trend by examining the stretch required to form fatigue cracks associated with plastic deformation.  To elucidate the mechanics governing this phenomenon, we examine the buckling behavior of deformed serpentine interconnects on substrates of various thickness. Our experimental and theoretical studies suggest a change in the buckling mode from local wrinkling to global buckling below a critical thickness of the substrate. The global buckling found in thin substrates accommodates large stretching prior to plastic deformation of the serpentines, thereby drastically enhancing the overall stretchability of the systems.
2:10pm Y. Xue* | J. A. Rogers | Y. Huang Mechanical Design of Soft, Wearable Microfluidic Devices
Abstract: Recent advances in materials and mechanics design of stretchable electronics is beginning to establish foundation for the next generation of wearable devices that enable real-time, continuous health care monitoring. Although many devices reported in the literature are designed to track health status through measurement of physical characteristics (e.g. temperature, motion, strain, biopotential), there is growing interest in the capture and biochemical analysis of biofluids using these same types of platforms, as a route to complementary information of high clinic values. A typical system reported recently incorporates microfluidic networks and reservoirs in soft, elastomeric materials, capable of capturing and routing sweat for the purposes of storage and chemical analysis of key biomarkers [1]. Low modulus elastomers and thin geometries provide mechanical compatibility with the skin, to reduce the probability of interface irritation and discomfort. These same properties, however, result in systems that are vulnerable to structural instability (e.g., the self-collapse in the soft microfluidic channels) and thus bring many new challenges in the mechanical design aspect. The purpose of the presentation is to discuss the important role of mechanics modelling in the design of microfluidic systems. For the self-collapse issue in the microfluidic channels, an analytical model based on the energy balance was developed [2]. Different collapse states are identified to be governed by a single parameter, which combines the effect of geometry, stiffness of the material and work of adhesion. The established model provides a simple criterion against the unwanted collapse. The presentation will also cover some other common design tradeoffs encountered. For example, in microfluidic channels, outlets are necessary for release of backpressure (flow impedance into the channel) and the size of the outlets is optimized to ensure minimal backpressure without yielding too much sweat loss as vapor.
2:30pm R. Li* | S. Zeng | L. Sun | D. Zhang Moisture Induced Wrinkling Response in a Bio-Inspired Soft Material 
Abstract: Bio-inspired soft materials have demonstrated many unique mechanical, electrical, and optical properties, offering a wide design space for functional graded structures. Bilayer structures consisting of a thin film bonded onto a compliant substrate are extensively used in a variety of applications, such as artificial skins, flexible electronics, and soft robotics. In additions, these bilayer materials are often subjected to elevated relative humidity (RH). Since only the film swells in the wet conditions, compressive stress will develop in the film due to the constraint from the substrate. When the compressive stress exceeds a critical value, instability responses of the surface layer such as wrinkles are observed. These wrinkles can be further used to increase the surface friction and alter the light transmittance of the material.In the literature, numerous researchers have studied the mechanics of the wrinkles including the critical buckling strain, wavelength and the amplitude of the wrinkles with constant material properties. Recently, the completely reversible and irreversible wrinkle formation is reported and the key to the tunable wrinkle dynamics is to tailor the stiffness and thickness ratios between the film and substrate and the water absorption capability of the film. In this study, we will study the wrinkling response of a PVA-PDMS film-substrate bilayer material system subjected to high RH. The moisture-induced buckling and post-buckling responses are modeled using the finite element method. The focus is on the dynamics of wrinkle evolution when the material is under repeated moisture-dry conditions. The von Karman strain–displacement equations are employed to account for the large deformation in the post-buckling state. The model is able to capture the effects of film-to-substrate thickness ratio and the moisture-induced swelling and modulus change on the wrinkling behavior. The computational results will be compared against the experiment. The proposed model will be useful for designing bilayer material systems with controlled wrinkling response in moisture conditions.
2:50pm D. Wang | S. Huang | N. Hu | A. M. Nasab | M. C. Abate | X. Yu | Y. Cai | W. Shan | Z. Chen* Buckling of an Elastic Rod Embedded in a Bilayer Matrix
Abstract: The buckling behaviors of an elastic rod embedded in a bilayer elastic matrix are studied through a combined theoretical, experimental and finite element methods. Using the energy method and the variation theory, the transverse displacement is obtained as the form of the superposition of sine and cosine functions and the critical buckling load is found to depend only on the softer layer of the elastic medium. Detailed simulations show that the material inhomogeneity, geometry, and loading all have significant influences on the buckling behaviors. The explicit relation between wave number and stiffness of the elastic matrix is established. Results show that the stiffer layer can be regarded as an additional clamped boundary conditions. Experimental results and finite element simulation of superelastic nitinol wires buckling in gelatin bilayer mediums show reasonable agreement with theoretical prediction.

Thursday July 27, 2017 – Session 3A – 3:25pm to 5:25pm

Location: 348 Curry Student Center

Session Chair: James Hanna, Virginia Polytechnic Institute and State University

Time Authors Talk Title
3:25pm J. Hanna* Invited: Some Thoughts on Multi-stability
Abstract: I will discuss prior and ongoing work related to stability, bifurcations, defects, and stress/energy focusing in thin elastic rods, strips, and sheets. In particular, I will speculate on the roles that Gaussian singularities and topological constraints play in creating multiple stable equilibria.
4:05pm M. Moshe* | S. Shankar | M. Bowick | D. Nelson On the Mechanics of Kirigami and Geometric Charges
Abstract: Kirigami, the art of cutting and folding paper, often has dramatic effects on the nonlinear elasticity of thin sheets, thereby offering a novel and promising strategy for 2D material engineering and design. In order to elucidate the mechanical consequences of Kirigami, we study the mechanics of an isolated frame under external load, as a simple building block for more complex structures. Towards this aim we develop a technique within the geometric formalism of elasticity, for solving elastic problems of sheets punctured with holes. Our approach allows us to demonstrate the generic features of holes under stress as sources of geometric incompatibility, i.e. as strain-dependent elastic charges. This formalism allows us to translate complicated Kirigami problems into simpler ones involving interacting elastic charges. It therefore allows concrete predictions about the response of an elastic sheet interrupted by various Kirigami patterns. By studying the problem both numerically and analytically, we explore the properties of both planar and buckled configurations of frames under load, which reveals that thin isolated frames display a significant (nonlinear) softening in response to external forces, by trading stretching for bending energy.
4:25pm H. Luan* | Y. Huang | Y. Zhang | J. A. Rogers Mechanically-guided Assembly of Complex 3D Mesostructures by Compressive Buckling: Kirigami-inspired 3D Membranes and Strain Engineering of Elastomer Substrates
Abstract: Approaches capable of creating three-dimensional (3D) mesostructures in advanced materials (device-grade semiconductors, electroactive polymers, etc.) are of increasing interest in modern materials research. Previous options in 3D assembly are, however, constrained by a narrow accessible range of materials and/or 3D geometries. A versatile approach, featuring transformation of planar precursors into 3D architectures through the action of compressive forces associated with release of prestrain in a supporting elastomer substrate, is capable of forming a diverse set of 3D structures in nearly any class of material. In this presentation, recent advances overcoming two limitations, a) the 3D structures yielded are only in open-network mesh type layouts and b) the compression driving the 2D to 3D transformation lacks the ability to vary spatially, regarding the approach will be reported. Here we introduce concepts for a form of Kirigami for the precise, mechanically driven assembly of 3D mesostructures from 2D micro/nanomembranes with strategically designed geometries and patterns of cuts, [1] and ideas for elastomeric substrates with engineered distributions of thickness to yield desired strain distributions for targeted control over resultant 3D mesostructures geometries.[2] Theoretical and experimental studies demonstrate the applicability across length scales from macro to nano, in materials ranging from monocrystalline silicon to commercially available thin films, with levels of topographical complexity that significantly exceed those possible with other approaches. A broad set of examples features 3D silicon mesostructures and hybrid nanomembrane–nanoribbon systems, including heterogeneous combinations with polymers and metals, with critical dimensions ranging from 100 nm to 30 mm. The resulting engineering options in functional 3D mesostructures have important implications for construction of tunable optics and stretchable electronics, etc.
4:45pm K. A. Khan* | S. A. Mansoor | S. Z. Khan | M. A. Khan Electromechanical Behavior of Multifunctional Electroactive Cellular Materials
Abstract: Electroactive cellular materials such as piezoelectric cellular materials (PCMs) play a key role in advanced multifunctional composites industry by virtue of their unique elastic, dielectric and electromechanical coupling characteristics . The architecture-property relationship of cellular materials (CMs) can be exploited to optimize piezoelectric cellular materials for specific applications.Here, we propose three classes of novel exotic honeycomb like PCMs. These included conventional hexagonal honeycomb structure, a re-entrant feature which is known to generate auxetic behavior and a semi-re-entrant which is constructed using alternate conventional and auxetic layers. The conventional honeycomb, re-entrant and semi re-entrant show variety of deformation behavior and can produce positive, negative and even zero Poisson’s ratio in certain configurations. Passive response of such cellular materials have been studied. Here, three dimensional finite element (FE) models were developed to study the response of active cellular materials. We investigate the effect of orientation of ligament for these three class of cellular materials on the effective electromechanical properties and their suitability in specific engineering applications. Moreover, we investigated the role of anisotropic properties of cellular material constituents, the sensitivity to the poling direction and its orientation with respect to the porosity of the cellular materials on the electromechanical properties of PCMs. FE results were compared with the analytical solutions available in the literature. Numerical results showed that the excellent piezoelectric properties can be obtained and PCM exhibits unique combination of properties (low impedance and more sensitivity) couple with auxetic type deformation.
5:05pm D. Restrepo* | Y. Zhang | G. Jarrold | N. Mankame | P. Zavattieri Phase Transforming Cellular Materials for Energy Dissipation
Abstract: A phase transition is the transformation of a thermodynamic system from one phase to another. Active materials like shape memory, ferroelectric and magnetostrictive alloys obtain their characteristic properties due to phase transformations. Recently, the authors have demonstrated the posibility of extending the notion of phase transformations to periodic cellular materials by designing architectured materials with microstructures comprised of building blocks that exhibit bistable or metastable configurations. In consequence, each stable configuration of the unit cell corresponds to a phase, and transitions between these phases are interpreted as phase transformations for the material. We called these materials Transforming Cellular Materials or PXCMs.Due to the phase transformations, PXCMs are characterized by long flat loading and unloading plateaus, and hysteresis without relying on the inelastic deformation of the base material making these materials attractive for protection of humans and structures. This talk will focus on the analysis and design of PXCMs that exhibit phase transformations under multiple loading directions. Also, we will present some of our preliminary work on temperature-induced phase transformation and the analysis of the wave propagation tunability between stable phases of PXCMs.
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