Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E17: GSOFT Prize Session: Mechanics, Topology and GeometryFocus Prize/Award
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Sponsoring Units: GSOFT Chair: Christina Marchetti, Syracuse University Room: 276 |
Tuesday, March 14, 2017 8:00AM - 8:36AM |
E17.00001: Shaping through buckling in elastic gridshells: from camping tents to architectural roofs Invited Speaker: Pedro Reis Elastic gridshells comprise an initially planar network of elastic rods that is actuated into a 3D shell-like structure by loading its extremities. This shaping results from elastic buckling and the subsequent geometrically nonlinear deformation of the grid structure. Architectural elastic gridshells first appeared in the 1970’s. However, to date, only a limited number of examples have been constructed around the world, primarily due to the challenges involved in their structural design. Yet, elastic gridshells are highly appealing: they can cover wide spans with low self-weight, they allow for aesthetically pleasing shapes and their construction is typically simple and rapid. We study the mechanics of elastic gridshells by combining precision model experiments that explore their scale invariance, together with computer simulations that employ the Discrete Elastic Rods method. Excellent agreement is found between the two. Upon validation, the numerics are then used to systematically explore parameter space and identify general design principles for specific target final shapes. Our findings are rationalized using the theory of discrete Chebyshev nets, together with the group theory for crystals. Higher buckling modes occur for some configurations due to geometric incompatibility at the boundary and result in symmetry breaking. Along with the systematic classification of the various possible modes of deformation, we provide a reduced model that rationalizes form-finding in elastic gridshells. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 9:12AM |
E17.00002: The bifurcations of nearly flat origami Invited Speaker: Christian Santangelo Self-folding origami structures provide one means of fabricating complex, three-dimensional structures from a flat, two-dimensional sheet. Self-folding origami structures have been fabricated on scales ranging from macroscopic to microscopic and can have quite complicated structures with hundreds of folds arranged in complex patterns. I will describe our efforts to understand the mechanics and energetics of self-folding origami structures. Though the dimension of the configuration space of an origami structure scales with the size of the boundary and not with the number of vertices in the interior of the structure, a typical origami structure is also floppy in the sense that there are many possible ways to assign fold angles consistently. I will discuss our theoretical progress in understanding the geometry of the configuration space of origami. For random origami, the number of possible bifurcations grows surprisingly quickly even when the dimension of the configuration space is small. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E17.00003: Engineering out-of-plane actuation in thin-film polymer networks: an auto-origami box Fangfu Ye, Vianney Gimenez-Pinto, Robin Selinger Liquid crystal polymer networks can be programmed to display complex stimuli-responsive shape transformations given a particular non-uniform director microstructure. Inspired by the variety of morphing shapes that these materials exhibit, we implement finite element elastodynamics simulations to design and test two nematic director fields that will form an auto-origami box under external stimulus. This thin-film actuator is flat at a reference state and spontaneously folds with changes in nematic order. The first proposed director microstructure is based on arraying four twist-nematic domains, while the second implements a hybrid radial-azimuthal director with +1 topological charge to induce four-fold symmetry bending. These studies provide an insight on experimental observations of elastomers with similar nematic microstructures and show the value of finite element elastodynamics simulations for engineering liquid crystal polymeric devices. [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E17.00004: Environmentally Triggered Instabilities in Origami Structures Phil Buskohl, Ben Treml, Andrew Gillman, Richard Vaia Mechanically adaptive films and 3D structures that respond to environmental stimuli (light, heat, humidity) show promise as next-generation materials in devices, sensors, and regulators. The interplay between a material's response to an external environmental field and the deformation presents an opportunity for local material design to achieve targeted actuation. While response rates can be fast, the associated strains are often small and limit motion. To amplify the mechanical response, origami fold patterns are designed and embedded into environmentally responsive thin films. Design of these origami structures is accomplished through development of a topology optimization framework that discovers optimal fold patterns that achieve targeted metastable equilibrium positions through environmentally induced instabilities. These computationally designed patterns are experimentally realized through additive manufacturing of composite structures that exhibit active and inactive zones. These concepts are demonstrated for hygroscopic polymers subject to humidity gradient fields to illustrate how environmentally responsive materials with local patterning can amplify the mechanical response and enable enhanced functionalization for remote, autonomous locomotion. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E17.00005: Graphene-based bimorphs for the fabrication of micron-sized, autonomous origami machines. Marc Miskin, Kyle Dorsey, Baris Bircan, Michael Reynolds, Peter Rose, Itai Cohen, Paul McEuen We present a new platform for the construction of micron sized origami machines that change shape in fractions of a second in response to environmental stimuli. The enabling technology behind our machines is the graphene-glass bimorph. We show that graphene sheets bound to nanometer thick layers of glass are ultrathin actuators that bend in response to small strain differentials. These bimorphs can bend to micron radii of curvature using strains that are two orders of magnitude lower than the fracture strain of graphene. By patterning thick rigid panels on top of bimorphs, we localize bending to the unpatterned regions to produce folds. Using panels and bimorphs, we can scale down existing origami patterns to produce a wide range of machines. These machines can sense their environments, respond, and perform useful functions on time and length scales comparable to microscale biological organisms. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E17.00006: Patterns in highly indented elastic shells Matteo Taffetani, Dominic Vella Depending on its geometry, its mechanical properties and the loading conditions, an elastic shell shows a large variety of behaviors when indented. Although the classical picture is that, under indentation, the preferred low energy configuration is an axisymmetric `mirror buckled' shape, this ideal shape is only observed rarely. More often indentation gives rise to wrinkling or polygonal buckling, depending on the presence or absence of an internal pressure. We consider the `Near Threshold' behavior of such systems (to determine the buckling transition) but then focus on the evolution of instability `Far from Threshold'. In particular, we use finite element simulations, together with analysis of the shallow shell equations to study the spatial variation of the instability's wavenumber, as well as the evolution of this pattern with increasing indentation. In so doing we offer some new insights into why these wrinkled and crumpled structures are `better' than mirror buckling. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E17.00007: Origami Metamaterial based on Pattern Rigidity Yan Chen, Zhong You Origami inspired mechanical metamaterials are made from a tessellation of origami units. Their mechanical behaviour is closely related to the behaviour of the origami units used. In this article, we focus on a family of metamaterials that are created by the tessellation of the square twist origami units. Generally a square twist origami unit can have four distinct hill-valley crease arrangements, two of which are rigidly foldable whereas the others are not. The rigidly foldable unit has, in general, lower stiffness than that of the non-rigidly foldable one if the facets can easily rotate about the creases. We shall show that it is possible to put rigidly foldable and non-rigidly foldable units together to form a geometrically compatible tessellation, and the stiffness of the overall structure based on such a tessellation is primarily decided by the number of non-rigid units. By astutely placing such units in a tessellation, we are able to create a metamaterial with a tunable stiffness. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E17.00008: Tensile Instability in a Thick Elastic Body Johannes Overvelde, David Dykstra, Rijk de Rooij, Katia Bertoldi A range of instabilities can occur in soft bodies that undergo large deformation. While most of them arise under compressive forces, it has previously been shown analytically that a tensile instability can occur in an elastic block subjected to equitriaxial tension. Guided by this result, we conducted centimeter-scale experiments on thick elastomeric samples under generalized plane strain conditions and observed for the first time this elastic tensile instability. We found that equibiaxial stretching leads to the formation of a wavy pattern, as regions of the sample alternatively flatten and extend in the out-of-plane direction. Our work uncovers a new type of instability that can be triggered in elastic bodies, enlarging the design space for smart structures that harness instabilities to enhance their functionality. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E17.00009: Geometry and Mechanics of Kirigami Suraj Shankar, Michael Moshe, David R. Nelson, Mark J. Bowick \emph{Kirigami}, the art of cutting and folding paper, often has dramatic effects on the 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 softening in response to external forces, by trading stretching for bending energy. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E17.00010: Design of Multistable Origami Structures Andrew Gillman, Kazuko Fuchi, Giorgio Bazzan, Gregory Reich, Edward Alyanak, Philip Buskohl Origami is being transformed from an art to a mathematically robust method for device design in a variety of scientific applications. These structures often require multiple stable configurations, e.g. efficient well-controlled deployment. However, the discovery of origami structures with mechanical instabilities is challenging given the complex geometric nonlinearities and the large design space to investigate. To address this challenge, we have developed a topology optimization framework for discovering origami fold patterns that realize stable and metastable positions. The objective function targets both the desired stable positions and nonlinear loading profiles of specific vertices in the origami structure. Multistable compliant structures have been shown to offer advantages in their stability and efficiency, and certain origami fold patterns exhibit multistable behavior. Building on this previous work of single vertex multistability analysis, e.g. “waterbomb” origami pattern, we are expanding the solution set of multistable mechanisms to include multiple vertices and a broader set of reference configurations. Collectively, these results enable an initial classification of geometry-induced mechanical instabilities that can be programmed into active material systems. [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E17.00011: Snapping to reshape origami sheets Anne S. Meeussen, Martin van Hecke The hunt for pluripotent materials is on. We identify origami-like sheets, shapeable via stretching or bending, as promising game. Local, explosive yet elastic snap-through events let these sheets switch between multiple stable shapes. We study the mechanics of such snapping sheets, focusing on the interplay between macrogeometry, energy and curvature. [Preview Abstract] |
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