Student Poster Showcase: Technical Program

SES Graduate Student Poster Showcase & Competition
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Tuesday July 25, 2017 – 6:30pm to 8:30pm

Session Chair: Andrew Gouldstone, Northeastern University and Marilyn L. Minus, Northeastern University

Poster # Authors Talk Title
TI-01 L. Liu* | Y. Li Hyperelastic Softening Model on the Mixed-Mode I/II Damage Evolution of 3D-Printed Soft Interfacial Layer
Abstract: Based on the concept of virtual internal bond (VIB) theory, a hyperelastic softening model was developed to model the constitutive behavior and damage evolution of 3D-printed soft interfacial layer under mixed mode I/II loading. User subroutine were developed for numerical implementation. To characterize the material parameters of the 3D-printed soft adhesive layer, a series of modified scarf-joint specimens were designed, which enables systematic variation of the mixed mode loading condition via a single geometric parameter, the slant angle. Beaks and butterfly geometry were introduced to effectively reduce the stress concentration. To further verify the material model, specimens of sinusoidal wavy interfacial layer with different waviness were designed and fabricated via a multi-material 3D printer (Objet Connex 260). Finite element simulations with the subroutine were conducted to predict the damage evolution of the material within these wavy interfacial layer. To verify the model developed, compact tension specimens for sinusoidal wavy interfacial layer were designed and fabricated via the multi-material 3D printer. FE simulations with the hyperelastic softening model were used to predict damage evolution and crack propagation of the specimens with different waviness. To compare with FE simulations, compact tension experiments were performed on the 3D-printed specimens.
TI-02 L. Cao* A Review of Work Hardening Models for Axial Cyclic Plasticity
Abstract: A review of plastic work hardening models is developed with numerical experiments to study the behavior of metallic materials. It mainly tests the Frederick and Armstrong model and the Chaboche model. The stress-strain state is investigated with selected loading paths. Results of simulation are compared through each model, to evaluate its validity and understand the prediction of fatigue life subjected to the non-proportional effect. The objective is to discuss how the non-proportional loading reduces the fatigue life, and how the cyclic stress-strain state varies associated with the loading paths.
TI-03 G. Chyr* Crack Propagation Prevention by Implementation of Spiral Shapes in Composite Materials
Abstract: Cracks are prevalent in all kinds of infrastructure used today. The propagation of cracks in buildings and bridges can cause severe damage to the integrity of these structures over extended periods of time and potentially endanger the lives of many people.

Nature has already found an effective way to deal with high stress in materials. Spirals, which are found in nature, demonstrate a good way of distributing stress to prevent breakage. They can be found in structures that experience high tensile and compressive stresses, such as shells, spider webs, and bone osteons. The goal of this project is to slow and trap the propagation of cracks by using spiral designs inspired by nature. Composite materials with a spiral-like pattern are designed with CAD software, manufactured with 3D printing, and tested with a tensile testing machine.

We demonstrate that, by using a combination of stiff and soft materials, cracks under tensile stress can be diverted. A soft spiral in a stiff matrix and vice-versa can make a crack follow a spiral path to prevent the crack from rapidly propagating through the entire structure in a straight path. This allows the material to remain connected even after reaching its failure point, slowly releasing the fracture energy and making this material concept more safe for engineering applications.

TII-01 B. N. Patel* | D. Pandit | S. M. Srinivasan Post–Buckling Analysis of Micro-beams based on Consistent Couple Stress Theory using a Semi-Analytical Technique
Abstract: Developments in the field of micro-electro-mechanical systems (MEMS) has led to an increase in usage of micro-scale structural elements like nozzles, beams, bars, etc. Thus encouraging researchers to study the mechanical behaviour of micro-scaled structures subjected to various loading conditions. Further, it is experimentally observed that material stiffens with decrease in characteristic length of beam in micro-scale range thus indicating size-dependency. Also, classical continuum theory fails to predict the observed experimental behaviour. This led to the development of non-classical models like micro-polar theory, strain gradient theory, couple stress theory with additional material length scale parameters apart from classical Lame’s constants. Out of all the existing models, consistent couple stress theory (CCST) is perceived to be the simplest one with just one material length scale parameter. Also, ample literature is available which discusses beam-bending, free-vibration and buckling studies using various developed models. However, in present work an attempt is made to investigate post-buckling behaviour of micro-cantilevers involving large elastic deflections by developing a simple moment-curvature based constitutive law using CCST along with the introduction of a semi-analytical solution methodology.

Post-buckling problems involve geometric non-linearity making the governing differential equation non-linear. Analytical solution to such problems are limited to evaluation of elliptic integrals. Moreover, evaluation of elliptic integrals using engineering handbook has become less frequent in modern times. On the other hand direct numerical integration methods applied to elliptic integrals are found to be extremely expensive. This is primarily because elliptic integrals are singular at boundary of integration. And so impractically large number of discretization points are necessary near the singularity. Thus, a semi-analytical technique named as Singularity Removal Method is introduced. Wherein the elliptic integral is replaced by a corresponding singularity free integral (SFI) and a singularity causing function (SCF). The SCF is obtained by applying limit to elliptic integrand at the point of singularity. Further the SFI is numerically integrated while SCF possesses a simple analytical expression using definite integration. The developed method is found to be several times more efficient than conventional numerical integration for solving elliptic integrals. Moreover results pertaining to post-buckling of micro-cantilevers are presented which can be used in designing micro-cantilever based applications.

TII-02 D. Sun* | M. Ponga | K. Bhattacharya | M. Ortiz Proliferation of Twinning in HCP Metals: Application to Magnesium Alloys
Abstract: Hexagonal close-packed (HCP) materials, such as magnesium alloys, are highly promising for the design of the next generation of lightweight, strong alloys. Because of the crystal structure, the mechanisms by which HCP materials accommodate deformation are not particularly well understood. In particular, twinning – a symmetric reorientation of the material lattice about a planar discontinuity – has been a subject of contention, as there have been many observations of so-called “anomalous” modes that do not necessarily agree with classical theory. In order to address the existence of these anomalous modes and attempt to predict new modes in these materials, we develop a three-step framework for twinning. We begin by kinematically predicting all of the possible twin modes, after which we study the energetics of all of these modes and then estimate the stress necessary to observe these modes. Applying this in particular to magnesium, we show that there are a significant number of twin modes which actually participate in governing the yield behavior of magnesium; many of these modes were not previously predicted and some of these modes match experimentally-observed anomalies. We also discuss generalizations of this framework to additional materials – both HCP and non-HCP.
TII-03 H. Chi*| L. Beirao da Veiga | G. Paulino Nonlinear Elements without Explicit Shape Functions using a Mimetic-inspired Approach
Abstract: The virtual element method (VEM), based on mimetic technology, is extended to finite deformations in the present work. Specifically, we present a projection-based VEM framework for finite elasticity considering a two-field mixed formulation, which emphasizes two issues: stabilization and element-level volume change (volume average of the determinant of the deformation gradient). To address the former issue, we introduce a novel stabiliza-tion scheme that evolves with the deformation level, which better captures highly heterogeneous and localized deformations. For the later issue, we provide exact evaluations of the average volume change in both 2D and 3D on properly constructed local displacement spaces. Convergence and accuracy of the proposed mixed VEM are verified by means of numerical examples using unique element shapes inspired by Escher (the artist), and an engineering application is demonstrated.
TIII-01 X. Dong* | Z. Li   SPH Simulation of Large Deformation and Chip Separation Caused by Impact(s) of Single Angular-Type Particle on a Viscoplastic Material
Abstract: This paper presents an implementation of a (mesh-free) three-dimensional smoothed particle hydrodynamics (SPH) model to simulate surface damage on an elasto-viscoplastic material caused by impact of single angular-type particle. The target materials are discretized with a set of SPH nodes, and the angular particle is modeled as a rigid polyhedron. Elasto-viscoplastic material properties are modeled by using the Mie–Gruneisen equation of state (for hydrostatic behavior) and the Johnson–Cook model (for deviatoric behavior). Based on the basic SPH theory, SPH formulations are generated to solve the governing equation of continuum mechanics. Two modified schemes in terms of artificial density and kernel gradient correction (KGC) are adopted to improve the numerical stability and approximation accuracy of the SPH algorithm. The improved SPH model is used to simulate single rhombic particle impact on annealed oxygen-free high conductivity copper. The results are compared with available experimental data, and good agreement has been achieved in terms of the crater profiles and rebound parameters. Through adjusting the incident parameters of impacting particle, common erosion phenomenon, such as large deformation and chip separation, are successfully reproduced by the SPH model. It is demonstrated that the present SPH model is superior to the conventional numerical methods in treating problems of extremely large deformations and with breakages, which usually occurs in the surface erosion process by angular particles.
TIV-01 H. Grover* | Z. Chen The Role of Mechanical Forces in Early Chick Brain Development (Poster Cancelled)
Abstract: The development of the neural tube (NT) is an essential part of embryonic morphogenesis which involves large-scale tissue deformation. Improper formation of the NT can lead to abnormalities in brain development known as neural tube defects (NTDs) which include: spina bifida, anencephaly, situs inverses, and heterotaxia. The NT undergoes flexure and torsion which aid in establishing the left-right asymmetry seen in developing embryos. The mechanics that drive this left-right asymmetrical event remain unclear. The complexity inherent in this process has resulted in the application of computational modeling to biological events to better understand the role of mechanical forces in NT morphogenesis. Modeling of embryonic tissue formation is non-trivial as soft tissue has been shown to undergo large elastic deformations under physiological loads where residual stress and strain remain. We employ computational modeling to further elucidate the mechanical forces that are involved in brain morphogenesis. We focus on the early brain development of the chick embryo, which is a popular model for human embryonic morphogenesis research due to the similarities in early development. In chick embryos, flexure and torsion of the NT increase during 45 to 56 hours of cultivation beginning at Hamburger-Hamilton stage 12 (HH12), where the NT is relatively straight, and concluding at HH16, where cranial, cervical, and thoracic curvatures are distinguishable. Comparatively, in a human embryo these processes do not begin until after day 17. In this study, we identify the possible sources of the mechanical forces that aid in the development of the NT. This information will provide new insights into the roles of mechanical forces behind early brain morphogenesis. The results could shed new light on the causes of NTDs in the hope that they can become more preventable or correctable.
TIV-02 Z. Shen* | Y. Li Computational Design of Core-Polyethylene Glycol-Lipid Shell (CPLS) Nanoparticles and their Potential as Drug Delivery Vehicles
Abstract: A core-polyethylene glycol-lipid shell (CPLS) nanoparticle consists of an inorganic core coated with polyethylene glycol (PEG) polymers, surrounded by a lipid bilayer shell. CPLS nanoparticle as a potential drug delivery platform is expected to inherit the advantages in liposome and proved to be more stable than liposomes and well size control. CPLS nanoparticle can be self-assembled from a PEGylated core with surface-tethered PEG chains, where all the distal ends are covalently bonded with lipid molecules. Upon adding free lipids, a complete lipid bilayer shell can be formed on the surface driven by the hydrophobic nature of lipid tails, leading to the formation of a CPLS nanoparticle. The self-assembly process is found to be sensitive to the grafting density and the amount of free lipids added under certain molecular weight of the tethered PEG chains. Basically, the CPLS nanoparticles could be only formed above a critical PEG grafting density. This critical grafting density is highly related to energy barriers during self-assembly process for the formation of small absorbed lipids vesicles. Values of the energy barrier are estimated in our simulation and their relation with the grafting density and molecular weight PEG polymer is calculated. Above the critical grafting density, a perfect CPLS nanoparticle could be formed with suitable amount of free lipids, beyond which partially-encapsulated or over-encapsulated CPLS nanoparticles will appear if the free lipids is too less or much. Under perfect CPLS nanoparticle region, the number of free lipids on the bilayer shell is determined by the size of nanoparticles, which could be estimated by the zero osmotic pressure domain. The osmotic pressure caused by the PEG polymer on the surface is estimated by the self-consistency field theory. Thus, the radius of CPLS nanoparticle could be computed and the corresponding number of the free lipids on surface are predicted. To this end, the design map of CPLS nanoparticles is obtained with the combination of simulation and theory. We expect that the design map here could act as a guideline for the fabrication of CPLS nanoparticle in experiments.
TIV-03 J. Sun* | R. Ran | K.T. Wan | A. Gu | S. Muftu A New Filtration Model on Bacterial Filtration in Porous Medium
Abstract: In our previous study, the AFM measurements showed a strong correlation between micromechanical properties of different bacteria, e.g. elastic modulus, surface energy and cell dimension, and their macro-filtration behaviors. However, there are few theoretical works addressing mechanical properties of bacteria in prediction of their filtration behaviors in porous medium. In the present work, the authors collaborated the classical colloidal filtration theory (CFT theory) and established a mechanical model, taking into account mechanical properties of bacterium, ionic concentration in groundwater and fluid interaction between flow and bacteria. The new model is built relied on comparing the adhesive moment and hydrodynamic interaction applied on bacteria. A cylinder contact model based on Maugis analysis and Derjaguin, Landau and Verwey, Overbeek (DLVO) theory was adopted in calculating adhesive moment. Size, geometry and elastic modulus of bacteria are considered in this model. Hydrodynamic moment of a cylindrical particle with varying orientation angle against flow were derived using COMSOL simulation. To verify the new model, parallel column tests were conducted on two different bacterial types, Aeromonas punctate (strain Q) and Raoultella ornithinolytica (strain A), under different flow velocities and different ionic concentrations with constant temperature and bacterial concentration. The new model predicts the results agree well with column test results. The differences in mechanical properties of two strains were indicated by their different responses to hydrodynamic moment. All the bacteria in the secondary minimum were detached by flow interaction. Bacteria in the primary minimum retained until flow rate reach critical flow rate and detachment occurred afterward. The moment compare method shows a great potential to represent column test and to explain the coupled effect of flow rate and ionic concentration. The new model captured the most information provided by the column test in this study and can be used as a platform for modeling more complex filtration system in the future.
TIV-04 S. Youssefian*| N. Rahbar | C. R. Lambert | S. Van Dessel Controlling the Energy Flow using Asymmetric Lipid Bilayers: A Model for a Thermal Rectifier
Abstract: Recently, phononics, the science of manipulating heat, has attracted intense interest from fundamental and applied researches. It is demonstrated that manipulating phonons provides opportunities to control thermal properties of materials for designing thermal devices that rectify heat flow by allowing larger heat flux in one direction than the opposite direction. Given their amphiphilic nature and chemical structure, phospholipids exhibit a strong thermotropic and lyotropic phase behavior in an aqueous environment. Around the phase transition temperature, phospholipids transform from a gel-like state to a fluid crystalline structure. In this transition, many key characteristics of the lipid bilayers such as structure and thermal property alter. In this study, we employed atomistic simulation techniques to study the structure and underlying mechanisms of heat transfer across different lipid bilayers such as DPPC around their phase transformation. The results show higher temperature gradients cause an increase in the thermal conductivity of DPPC lipid bilayer. We also found that thermal conductivity of DPPC is lowest at the transition temperature that one lipid leaflet is in the gel phase and the other is in the liquid crystalline phase. This is essentially related to a growth in thermal resistance between the two leaflets of lipid at the transition temperature. These results indicate that it is feasible to construct asymmetric lipid-based thermal diodes by carefully selecting lipids with dissimilar temperature dependent thermal conductivity.
TV-01 S. Bagchi* Interfacial Load Transfer Mechanisms in Carbon-Nanotube-Polymer Based Nanocomposites (Poster Cancelled)
Abstract: Carbon nanotube (CNT) polymer nanocomposites are highly promising as next-generation light weight and ultra-high-strength materials due to the high density of CNT-polymer interfaces. However, nanomechanical experiments to-date have reported conflicting interfacial strengths between CNT and the polymer matrix. Here, we present recent results of our massively-parallel molecular dynamics (MD) simulations modeling the pull-out of CNTs from PMMA polymer matrices at length-scales comparable to nanomechanical pull-out experiments. Specifically, we consider the effects of both non-bonded and bonded interactions between CNTs and PMMA on the pull-out forces. In the case of pure van der Waals interactions between CNT and PMMA, we show that the pull-out forces are extremely low (~1-5 nN), and are independent of the tube length. We also do not observe substantial increase in the pull-out forces in the presence of Stone-Wales or vacancy defects along the CNT. In contrast, the pull-out forces increase by an order-of-magnitude when the CNT is covalently-bonded to PMMA. We demonstrate a competition between bond-by-bond breaking versus simultaneous stretching and uncoiling of the crosslinked chains depending on the crosslinked density. Implications of the crosslinked density to the strength and toughness of the composite are discussed.
TVI-01 S. Chen | J. Li | L. Fang* | Z. Zhu | S. H. Kang Simple Triple-State Polymer Actuators with Controllable Folding Characteristics for Self-Driven Folding Applications
Abstract: Driven by the interests in fabricating complex structures by self-driven material organization, there have been studies developing artificial self-folding structures at different length scales based on various actuators that can realize dual-state actuation. However, their unidirectional nature and complex fabrication/programming procedures restrict the applicability of the actuators from a wide range of multi-state self-folding behaviors. In addition, few of the existing polymer actuators can precisely control their folding characteristics (curvature and force), which are crucial factors for applications.

To address these issues, we report an easy-to-fabricate triple-state actuator with controllable folding behaviors based on an analytical model. The actuator is based on bilayer polymer composites with different glass transition temperatures. Initially, the fabricated actuator is in the flat state, and it can sequentially self-fold to angled folding states of opposite directions as it is heated up. To understand the mechanics of self-folding behaviors, we investigated an analytical model that incorporated measured partial recovery effects of polymers with a bilayer beam theory. Then, we validated the

model by experiments. As the mechanics-based model enables accurate predictions of the thermally induced deformations at certain temperatures, we can precisely control the folding characteristics of actuators for rational design. To demonstrate an application of our triple-state actuator, we have developed a self-folding transformer robot, which could sequentially self-fold from a two-dimensional (2D) sheet into two three-dimensional (3D) configurations with the increase in the temperature applied to the actuator. Our findings offer a simple approach to generate multiple 3D configurations from a single system through self-folding by harnessing behaviors of polymers with rational design based on a mechanicsbased model.

TVI-02 S. Askarinejad* | N. Rahbar Organic-inorganic Interface in Nacre and Nacre-like Materials: Experiment and Theory
Abstract: Through evolution, nature has always achieved the optimized design for different purposes. Problem-solving strategies of naturally growing composites such as “Nacre” give us a fantastic vision to design and fabricate tough, stiff while strong composites. To provide the outstanding mechanical functions, nature has evolved complex and effective functionally graded interfaces. Particularly in nacre, organic-inorganic interface in which the proteins behave stiffer and stronger in proximity of calcium carbonate minerals provide an impressive role in structural integrity and mechanical deformation of the natural composite. However, further research on the toughening mechanisms and the role of the interface properties as a guide on design and synthesize new materials is essential. In this study, a micromechanical analysis of the mechanical response of “Brick-Mortar” and “Brick-Bridge-Mortar” composites is presented considering interface properties.  The closed-form solutions for the displacements in the elastic components as a function of constituent properties can be used to calculate the effective mechanical properties of composite such as elastic modulus, strength and work-to-failure. The results solve the important mysteries about nacre and emphasize on the role of organic-inorganic interface properties. The effect of mineral bridges is also studied. Our results show that the properties of proteins in mineral bridges proximity are also significant especially in increasing the elastic modulus of the structural composite. Detailed relationships are presented to identify future directions for advanced material design and development.
TVI-03 H. A. Choshali* | S. Askarinejad | J. A. Rosewitz | N. Rahbar A Study on the Effect of Bricks’ Waviness on the Mechanical Response of Nacre-inspired Composites
Abstract: Nacre, the inner layer of a large number of mollusk shells, is a natural composite made up of about 95% ceramic and the rest organic matrix [1]. Its excellent properties, i.e. stiffness, strength, as well as high toughness are attributed to its unique brick and mortar structure. Understanding and duplicating the mechanisms involved in the nacre can open up new windows toward designing high performance engineered materials. Many factors have been thought to contribute to outstanding mechanical properties of nacre such as inclusion of nanoscale pillars [2] and sliding of the tablets [3]. This work explores the effect of waviness of the layers on the stiffness, strength and toughness of nacre-inspired composites in different loading conditions. The idea is that introducing curvatures and angles in the structure of bricks and mortars can result in auxetic response and interlocking which can significantly affect the material’s behavior. Inspired by nacre’s internal structure, we designed different two-phase composites (polymer-polymer and concrete-polymer) with different internal structures and studied their behavior under tensile and compressive loading. It is observed that the composite samples with engineered internal structures outperform the control specimens with equal volume fraction.
TVI-04 H. Cui* | X. Zheng Fracture Toughness of Metal – Polymer Composite Metamaterial: Combining Structural Size Effects and Material Size Effects
Abstract: Recent studies have shown the excellent performance of metamaterials architected with micron structures, which is conducive in creating lightweight, strong and stiff engineering materials. The development of additive manufacturing technologies, such as large area projection microstereolithography, would allow one to fabricate such metamaterials in bulk scale. As these miniaturized microarchitectures can be scaled up to real world applications, their fracture toughness, which is used for quantifying the resistance to fracture, will be the dominate mechanical property governing their damage tolerances, and is important when it is used in crack-bearing applications. Here, we investigated the fracture toughness of nickel-polymer composites metamaterial manufactured by a large area micro-stereolithography technique and electroless deposition. The polymer template are comprised of an isotropic network of stretch dominated, face centered cubic micro-scale octahendron tetrahedron unit cell with minimum feature size of twenty microns, while the electroless deposition thickness is controlled to vary from tens of nanometers to hundreds of nanometers. Both structural size effects and material size effects has been observed during the study. The fracture toughness of the metamaterial has a linear relationship with the square root of the unit cell size, while size-dependent strengthening of nickel shells exploit the possibility to yield a higher fracture toughness of the composites.
TVI-05 R. Hensleigh* | H. Cui | X. Zheng Ceramic Mechanical Metamaterials for Ultrastiff High-Temperature Applications (Poster Cancelled)
Abstract: Precision additive manufacturing has revolutionized the development of mechanical metamaterials. These materials have unique properties exceeding those of traditional materials due to their architecture, making structural design a powerful tool to develop advanced materials. The practical application of these materials is still a monumental challenge, but their ultimate benefits, ultralightweight with no degradation in mechanical properties, are equally monumental. Aerospace materials which are particularly constrained by weight and temperature requirements, stand to significantly benefit from mechanical metamaterials ultra-lightweight features, however; they require very high operating temperatures. The thermo-mechanical properties of mechanical metamaterials have not been previously studied. In order to further the goals of thermally robust ultra-lightweight materials for applications including aerospace materials, we fabricate and study the thermo-mechanical properties of ceramic lattices, and the effect of lattice architecture. Lattice designs are optimized to provide robust thermal resistance to maintain their ultrastiff properties at high temperatures.
TVI-06 C. Gao* | Y. Li Mechanics of Bio-Inspired Composites with 3D-Printed Sutural Tessellation
Abstract: In nature, sutural tessellations are observed in armor systems of various fauna and flora species across all length scales. These natural materials are typically composites with relatively stiff building blocks articulated via more compliant thin layer of interfacial tissues with zigzag morphology, showing a remarkable jigsaw-puzzle-shaped sutural tessellation. In this investigation, the microstructure of the seedcoat of Portulaca oleracea were first quantified via Scanning Electron Microscope (SEM), and then a biomimetic design of composite plates with sutural tessellation were developed and key geometry and material parameters were defined. To explore the mechanics of this biomimetic composite, mechanical specimens with different suture complexity were fabricated via a multi-material 3D printer (Objet, Connex 260) which enables the simultaneous printing of two different materials. The stiffer building block was printed as VeroWhitePlus (an acrylic-based photopolymer) and the wavy suture layer was printed as TangoBlackPlus (a rubber-like flexible material). Then, uni-axial tension experiments were performed to quantify the stress-strain behaviors of the 3D printed composite plates in two orthogonal directions. Digital image correlation (DIC) was used to track the deformation of the composites. Finally, finite element models were developed to simulate the experiments and systematically quantify the influence of suture complexity and material combination on the stiffness, strength, fracture toughness, and failure mechanisms of the composite plates. In the finite element models, damage mechanics model was used to capture the damage initiation and evolution of the 3D printed suture layer.
TVI-07 K. Miroshnichenko* | L. Liu | I. Tsukrov | Y. Li Mechanical Modeling of Biological Sutures with Fibrous Joints
Abstract: Biological sutures are mechanical joints connecting skeletal components with a thin interfacial layer of soft connective tissue, showing a complicated zigzag morphology. The interfacial layer is composed of aligned collagen fibers embedded in mesenchyme matrix. Biological sutures have multiple mechanical functions to provide protection by effectively transmitting load and dissipating energy, while also to supply flexibility by accommodating growth, respiration and locomotion.

Recently, theoretical models were developed to predict the stiffness and strength of sutures with various wavy morphologies. In these theoretical models, the soft interfacial layer was assumed to be isotropic. However, due to the aligned collagen fibers in the interfacial layer, the effective mechanical properties of the layer are anisotropic. Therefore, it is not clear how the anisotropic properties of the fibrous interfacial layer and the wavy suture morphology jointly influence the mechanical properties of biological sutures. In this investigation, a composite hierarchical suture model is developed in order to provide better understanding of the synergistic effects of suture morphology and the fiber orientation in the soft connective tissue. The model includes a soft thin interfacial layer connecting two bone pieces with saw tooth geometry. The layer is modeled as aligned fibers embedded in soft matrix. Therefore, the interfacial layer in the model is anisotropic, and its properties are determined by the fiber orientation, fiber volume fraction, and the mechanical properties of the fibers and the matrix. This theoretical mechanical model was used to systematically quantify the overall orthotropic in-plane stiffness of suture joints as a function of wavy morphology of the skeletal components, the fiber orientation, and matrix properties in the interfacial layer.

TVI-08 G. Lin* | J. Yin Dynamically Tunable Hierarchical Wrinkles as Multifunctional Smart Windows
Abstract: Residential and commercial buildings account for nearly 40% of the total energy consumed in the US, including 72% of the nation’s electricity use and 39% of carbon dioxide emission each year through their direct influence on heating, cooling, and lighting. Therefore, the design of new energy efficient materials and technologies is crucial for energy saving and CO2 emission reduction. Optical materials with reversible switching properties of light transmittance have immense potential as smart windows for architectural and vehicular applications for energy saving. The development of such dynamic glazing materials has so far focused primarily on chromogenic materials and devices but suffering from complicated fabrication processes. Recently, harnessing surface wrinkling instabilities for design of smart mechano-optical materials has attracted increasingly interest due to its simple and cost-effective fabrication process and control. However, most studies in smart window focus on the single properties of tunable optical transparency. An idealized smart window would perform multifunctionality not only in blocking or letting light into the building to save energy, but in efficiently harvesting and collecting freshwater from fog or dew to address the issue of the economic water shortage in arid regions. Therefore, simultaneous control of multiple dynamically tunable properties is highly desired for realizing multifunctional smart windows, which remains largely unexplored. In this work, we address open questions surrounding the design of multifunctional smart windows through harnessing surface wrinkling. Our multifunctional smart window is based on the hierarchically wrinkled silicone elastomer through sequential wrinkling with exhibited both nanoscale and microscale features. We demonstrated in this work, for the first time, on the simultaneous manipulation of dynamically tunable optical properties and controllable water droplet transport with hierarchical wrinkles through mechanical strain.
TVII-01 J. Li* | H. Godaba | Z. Zhang | K. Foo | J. Zhu A Soft Origami Robot Driven by Electrostatic Forces
Abstract: Soft robots are attracting more and more attention recently due to their interesting attributes including deformable structure, inherent compliance and safety. Origami robots are one kind of soft robots whose structures can be fabricated by origami technique. In general, one can fold an origami robot by simply transforming a 2D structure into a 3D one. The current origami robots are usually driven by shape memory alloys (SMA) or DC motors. The robots driven by SMA usually have low velocity, while those driven by DC motor have complex structures (due to accessory units for actuation).

In this paper, a soft origami robot is developed, which is driven by electrostatic forces. The robot consists of two pieces of paper, each smeared with compliant electrodes (carbon grease). Experimental results show that the robot can achieve 2D motion on land. It has weight of 7g and can achieve a speed of 0.45 body-lengths per sec. Finite element method and an analytical model have been employed to interpret the movement mechanism of the robot and then to simulate its motion. The simulations are qualitatively consistent with the experimental observations.

TVII-02 C. Barney* | A. Crosby Residual Stress Effects on Needle-Induced Cavitation
Abstract: Needle-induced cavitation measures stiffness of soft solids through pressurized fluid injection. Deep penetration and puncture of the material occurs prior to injection which can lead to significant residual stress at the tip of the embedded needle; however current models of cavitation neglect to account for this deformation history. The effect of this history was experimentally measured by monitoring the force on the needle during the cavitation process. Residual stress was found to increase the observed critical pressure. 
TVII-03 A. Kumar* Two-Potential Constitutive Model for Rubber Viscoelastic Materials
Abstract: The specialization of the two-potential constitutive framework – also known as the “generalized standard materials” framework – to rubber viscoelasticity is demonstrated. The framework is laid out for a general anisotropic, compressible material. Furthermore, many popular rubber viscoelasticity formulations, introduced over the years following different approaches, are shown to be obtainable from this framework. As a first application of practical relevance, the framework is utilized to put forth a new objective and thermodynamically consistent rubber viscoelastic model for incompressible isotropic elastomers. The model accounts for the non-Gaussian elasticity of elastomers, as well as for the deformation-enhanced shear thinning of their viscous dissipation governed by reptation dynamics. The descriptive and predictive capabilities of the model are illustrated via comparisons with experimental data available from literature for two commercially significant elastomers.
TVII-04 Y. Tang* Programmable Kiri-Kirigami Metamaterials
Abstract: We study the programmability of monotonic and non-monotonic structural generation in a parallel cut-based kirigami metamaterial through both global and local actuation, as well as its potential application as building envelopes for energy-saving adaptive architecture. After introducing patterned notches into the kirigami structure, stretching breaks the buckling degeneracy of the infinitesimal, linear modes, and the resulting structure has one, well-defined, global mechanism. Through simulation, we demonstrate that the kirigami structure can largely reduce the energy consumption in electricity for light and cooling. We break the degeneracy explicitly by engraving the flat-cut structure resulting in programmable control, generating a variety of inhomogenous structural configurations on demand. The engraving has a negligible effect on both the stress-strain curve and the extreme stretchability of the original kirigami structures. With these introduced notches, we demonstrate feasability of the kirigami approach to thick panels for expandability. We achieve similar programmable control of the titling directions of the cut units through thermal actuation by attaching thermal-shrinkable tapes to our kirigami structures. The reprogrammability is further demonstrated through the switch in the titling direction of kirigami structures controlled by temperature.
TVII-05 P. L. Floch* | X. Yao | Q. Liu | Z. Suo | Y. Sun | L. Jia Bio-Inspired Wearable and Washable Conductors for Active Textiles
Abstract: Stretchable conductors need elastomeric coatings in order to ensure good insulation against humidity, contaminants, but also good dielectric properties, even upon large deformations. Nature offers examples of such a system: a myelinated axon is a hybrid of electrolyte and dielectric. The electrolyte is the saline solution inside and outside the axon. The dielectric is the myelin sheath of the axon. The electrolyte-dielectric hybrid functions as fast conduit for electrical signal, much like a transmission line, or an earphone. Recent works have mimicked the myelinated axon using a hydrogel as the electrolyte and an elastomer as a dielectric. Such an artificial axon is highly stretchable and transparent, and has enabled many devices of unusual characteristics. Examples include stretchable loudspeaker, sensory skin, electroluminescence, touchpad, and liquid-crystal display. Artificial axons, however, may lose water in the open air, or exchange water and solutes in contact with living tissues. Water-permeability of the coating is the key design parameter, as dehydration is the main source of failure of the device. Envisioning textile-scaled devices requires excellent barrier properties from the elastomeric coating. Current hydrogel-elastomer systems are mainly focused on the use of PDMS as a coating. According to literature and our experiments, Butyl Rubber – 100 times less permeable than PDMS – is the most water-impermeable rubber. However this elastomer is not good enough to protect hydrogel fibers from drying over years. We show that a combination of a coating and a hygroscopic salt can tremendously increase the lifetime of fibers, thus making them wearable over years in typical conditions of Relative Humidity. Butyl Rubber can be dip coated around ionic conductors, and in-situ bonding between the hydrophilic gel and the hydrophobic rubber can be achieved (interfacial toughness of 79 J/m). We provide proofs of unprecedented wearability (increased lifetime in air, slow diffusion of salt in water) and washability (5 cycles or more in a real washing machine) of fibers coated with a thin layer of Butyl Rubber (less than 100 μm), opening new opportunities for active textiles.
TVII-06 Y. Jiang* | Y. Li Bio-Inspired Auxetic Chiral Cellular Solids for Color Change and Particle Release
Abstract: Inspired by the camouflaging mechanism of Cephalopods skin, new auxetic cellular solids were designed, in which chirality-induced rotation was used to generate unique sequential cell opening mechanism. The biomimetic designs were fabricated via a multi-material 3D printer (Objet Connex 260). Mechanical experiments were performed to quantify the cell-opening mechanism and the rotation of cells. Finite element simulations of the periodic cellular solids were also performed. Excellent quantitative agreement was found among numerical and experimental results. Then, systematic FE simulations were performed to explore the effects of geometry and material combination on the mechanical behavior of the biomimetic design. Also, potential applications of this material on color change and drug delivery are illustrated and quantified. It is found that by utilizing this unique cell-opening mechanism, the cellular solids can be used for changing colors based on different level of strain. Also, medicine particles with different sizes can be released sequentially at different level of overall strain.
TVII-07 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.
TVIII-01 M. Monsef* | Y. Li A Scaling Law for 3D-Printed Fractal Interlocking
Abstract: Topological interlocking is an effective joining approach in both natural and engineering systems. Hierarchical/fractal interlocking were also found in many biological systems. Fascinating examples of the fractal interlocking in nature include the cranial sutures of mammals, the ammonite septal sutures, inter-cellular joints between the epidermis cells of some seed-coats and the linking girdles of the single cell algae diatom. However, few fractal interlocking was found in existing engineering systems. This is mainly because of less understanding in the mechanics of fractal interlocking. To shorten this gap, in this investigation, the influence of fractal geometry on the mechanical properties of interlocking will be explored via analytical modeling, numerical simulation, 3D-printing and mechanical experiments.  The Koch fractal interlocking with different number of iterations N were designed. Contact mechanics model was used to analytically capture the mechanical behavior of the fractal interlocking under infinite small deformation. Then finite element (FE) simulations were performed to study the deformation mechanism of fractal interlocking under finite deformation. In the FE models, 3D elements were used, hard contact with tangential behavior were defined between the contacted pieces. Damage mechanics was used to capture the damage initiation and evolution of the materials. It was found that by increasing the number of iterations, the contact area increases and the interlocking stiffness and strength are also increases significantly. The gap and friction coefficient of the contact play an important role in the mechanical properties of the interlocking.
TVIII-02 J. Sun* | K. T. Wan | A. Gu | S. Muftu Adhesion of a Solid Sphere onto a Rigid Planar Substrate in the Presence of Moisture
Abstract: Adhesion of an elastic sphere onto another sphere or a rigid substrate is ubiquitous in many branches of science and technology such as colloidal particles, storage of transportation of glass powder, and micro-electromechanical systems (MEMS) especially the typical dimension shrinks to submicron scale. Exposure to moist air leads to meniscus formation at the contact interface and could post serious problems of operation hindrance and reliability. In the present study, a new adhesion model on an elastic sphere adhering onto a rigid substrate in the presence of moisture is established. The adhesion-detachment mechanics is constructed based on Hertz contact theory and Laplace-Kelvin equation.  The inter-surface attraction is the solely provided by the Laplace pressure within the meniscus.  Interrelation between the applied load, contact radius and approach distance is derived based on a force balance.  “Pull-off” is predicted with the critical tensile force larger than the Derjaguin-Muller-Toporov (DMT) limit that serves as only in saturated environment. The new model shows a monotonically decreasing function of relative humidity though “Pull-off” force approaches the DMT limit. When relative humidity drops to roughly 30%, continuity of water molecules breaks down and effect of meniscus become relatively unimportant. The new model accounts for the finite size of water molecules which is missing in the classical Johnson-Kendall-Roberts (JKR) limit.
TVIII-03 D. Hu* | J. Papadopoulos | G. G. Adams Prying Action in a Bolted Connection – A Practical Problem of Receding Contact
Abstract: We investigate the common example of a bolted tensile joint as a receding contact problem. In this configuration there are two flat parts which are fastened together by the tension of bolts and are subjected to repeated tension loading. Due to the flexibility of the parts, large “prying” forces arise which can cause the bolt forces to be much greater than the applied force. Hence such joints are prone to failure especially due to fatigue under repeated loading. In the American Institute of Steel Construction Manual, the design procedure is based on the idea of a fixed lift-off region, which can significantly underestimate the prying force. In contrast, we treat this configuration as a compliant receding contact problem where the flexural deformation of the components induce variablelocation prying forces. The results of our analysis for the bolt force are incorporated into the Goodman diagram in order to predict fatigue life of a material under cyclic loading. This analysis can be used for guidance on joint design including specification of the preload to prevent failure caused by repeated prying actions. We have obtained results using an analytical approach with beam theories (Euler-Bernoulli and Timoshenko) and also using a more comprehensive three-dimensional finite element analysis. By comparing the results of these three approaches, we are able to provide guidance on the most appropriate model to use under a specified set of conditions.
TVIII-04 N. K. Mohammadi* | G. G. Adams Self-Excited Vibration of a Finite-Height Elastic Layer Sliding Against a Rigid Surface
Abstract: This study considers the dynamic stability of the steady frictional sliding of a finite-height elastic layer pressed against a rigid surface. The finite-height layer is fixed on its bottom; on its entire top surface the rigid surface slides with constant speed and with a constant friction coefficient. The plane-strain equations of motion for a linear isotropic elastic solid are solved analytically for small dynamic disturbances. The analysis shows that the steady solution is dynamically unstable. Eigenvalues with positive real parts lead to self-excited vibration which occurs for any sliding speed and for a wide range of the Poisson ratio and coefficient of friction. This is in contrast to the behavior of an elastic half-space sliding against a rigid surface in which the instability occurs only if the coefficient of friction is greater than unity. This work, and its extensions, are expected to be relevant in the theoretical aspects of sliding friction as well as in brake dynamics.
TIX-01 C.M. Portela* | D. M. Kochmann | J. R. Greer Nodal Effects on the Stiffness Scaling of Lattice Architectures
Abstract: Advanced techniques such as two-photon lithography and stereolithography allow for the manufacturing of complex three-dimensional structures that span a large range of relative densities, stiffnesses, and strengths. In structural applications, lattice architectures that are manufactured through these processes require proper mechanical characterization as well as predictability of the mechanical properties. However, typical lattice architectures in load-bearing applications possess high relative densities ρ∗ and strut slenderness ratios (r/l), where the nodes (i.e., strut junctions) become a large fraction of the architecture and dominate the mechanical behavior. As a consequence, beam theory is no longer applicable but instead computationally intensive continuum element models are necessary to predict the mechanical behavior of these architectures. In the current study, we present a definition for lattice nodes and perform a systematic study to isolate their effect on the stiffness scaling of lattice architectures. Namely, we explore how modifying the nodal geometry—while keeping other parameters such as relative density and strut slenderness constant—has an effect on the stiffness of lattice architectures. We perform uniaxial compression experiments on both rigid (octahedron) and non-rigid (tetrakaidecahedron) polymer lattice architectures, manufactured through two-photon lithography, and identify the effect of rigidity on the nodal contribution to stiffness. In some cases, for a given relative density, the nodal modifications on an architecture result in up to a 30% increase in stiffness. In addition, we formulate finite element models of the 5 × 5 × 5 polymer lattices and confirm the effects of nodes as
observed in experiments. These computational models utilize a combination of 3D continuum elements (at nodes) and beam elements that accurately capture the stiffness scaling of the compression experiments, with a significant reduction in computational cost compared to typical fully-resolved continuum element models. We discuss the possibility of applying our findings on nodal effects in a global stiffness scaling for cellular solids that can be valid beyond the limits of classical beam theory.
TIX-02 A. J. Mateos* | J. R. Greer Failure Response of Hollow Octet Nanolattices
Abstract: Understanding of a material’s resistance to fracture initiation is essential to classifying its use for fracture-safe applications. A full representation of fracture takes into account the complex stress states of naturally occurring flaws. In this study, the mechanics of failure for architected materials with prescribed flaws was investigated. Center-notched tension specimens were fabricated with a hollow-tube octet lattice architecture and their failure behavior under mixed-mode loading was studied by inclining a central crack with respect to the applied tensile load. Experiments revealed that unnotched and notched lattices are sensitive to the pre-defined flaw and failed catastrophically in an elastic-brittle fashion. Post-fracture surface morphology shows that the global preferred path for failure is confined at nodal planes perpendicular to the applied load, independent of notch orientation. Global failure occurred at the notch with cracks localized to nodal junctions. Tube wall fracture initiated failure at the crevices of the nodes and dominated crack path formation. The failure response of an equivalent ideally-brittle continuum is compared to the experiments and is predicted to follow a similar scaling of stresses at the onset of failure. These hollow-tube octet nanolattices exhibited a high specific tensile strength, outperforming current architected materials and bulk materials with low relative densities.
TXI-01 X. R. Zheng* Printing With Light: Scalable Additive Manufacturing Processes for Producing Large Area Ultralight Weight Nano-Architected Metamaterials (Poster Cancelled)
Abstract: It has been a long research and engineering pursuit to create lightweight and mechanically robust and energy efficient materials with interconnected porosity. These cellular materials are desirable for a broad range of applications including structural components, lightweight transportation, heat exchange, catalyst supports, battery electrodes and biomaterials. However, the required outstanding properties have remained elusive on lightweight materials (<10kg/m3), constrained by the inherent coupling of material properties and the lack of suitable processes to generate these artificial materials. For example, graphene aerogels have among the lowest record densities ~1kg/m3, but their strength have been degraded to tens to hundreds of Pascal (<10-8 of that of carbon nanotubes). The attainment of low density has come with a price — significant reduction of bulk scale properties. In this talk, I will present the design and manufacturing of multi-scale materials with controlled three-dimensional architectures from the macroscale to nanoscales. These 3D bulk metamaterials (polymer, metal, ceramic and combinations thereof) possess weight density comparable to that of carbon aerogel, but with over 104 higher stiffness and strength. By designing and studying their hierarchical architectures, material compositions and feature sizes spanning multiple length-scales, we create a wide range of decoupled material properties such as programmable stiffness, tunable strength and fracture toughness as well as programmable possion ratio. With the possibility of incorporating precise control of topological architectures across unprecedented length-scale sets, we enter into a paradigm where nanoscale material properties can be harnessed and made accessible in large scale objects, opening a wide range of applications of these materials in energy, health care and flexible electronics.
TXI-02 N. K. Mohammadi* | M. R. H. Yazdi Investigation of Thermoelastic Damping on Out-of-Plane Vibration of Curved Micro Beam
Abstract: A novel size-dependent curved micro beam model is developed based on the classic theories. The higher-order governing equations and related boundary conditions are derived using the variational principles. Effect of thermoelastic damping is considered for a simply supported electrostatic actuated curved beam with an isothermal boundary condition. We have applied thermoelastic models to the out of plane vibration equations of the curved beam with uniform rectangular cross-section. The presented analysis is based on the two models; first Zener’s classical model of thermoelastic loss for uniform beams, and second, the recent refinement of Zener’s analysis by Lifshitz and Roukes.

Using both approaches, numerical predictions of Q-factor are developed and compared. The relationships between geometry scale and Q-factor are explored. Then, the critical radius and critical thickness that cause maximum thermoelastic damping are illustrated. The size-effect induced by thermoelastic coupling would disappear when the thickness of the micro-beam is over the critical values. The critical values depend on the material properties and the boundary conditions. Some of the results are compared with the previously published results to establish the validity of the present formulation.

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