Lorenzo Valdevit, University of California, Irvine
Katia Bertoldi, Harvard University
Tobias Schaedler, HRL Laboratories, LLC
Martin Wegener, Karlsruhe Institute of Technology
PM01.01: Shape Morphing and Origami/Kirigami Materials I
Monday AM, November 26, 2018
Hynes, Level 1, Room 102
8:00 AM - PM01.01.01
Shape Morphing Metamaterials
McGill University1Show Abstract
Thermal expansion can be challenging to control in applications that require thermal stability, yet it can be exploited to generate shape transformations that are highly predictable. In this talk, I will first present compliant building blocks that can deform under temperature, and then illustrate how their sequence can be encoded to attain macroscopic reconfigurations that are reversible. Investigated through theory and computations, the metamaterial behaviour is tested through proof-of-concept prototypes that show promise for deployable satellites, morphing components, and actuation devices.
8:30 AM - PM01.01.02
Mechanical Instability Tuning in Architected Magnetoelastomers
Phil Buskohl1,Vincent Chen1,2,Artemii Goshkoderia3,Matthew Robinson1,2,Carson Willey1,2,Stephan Rudykh4,Abigail Juhl1
Air Force Research Laboratory1,UES, Inc.2,Technion–Israel Institute of Technology3,University of Wisconsin–Madison4Show Abstract
Magnetoelastomers (MAEs) are an important class of soft, strain tolerant materials that generate a stiffness increase in response to a magnetic field. Stiffness tuning under magnetic field is advantageous due to the fast, reversible and non-contact control of the material properties, which is relevant for applications such as soft actuators, adaptive vibration dampers and acoustic filters. Architected MAE composites, such as laminates and periodic MAE inclusions in a non-active matrix, have been predicted to possess novel mechanical instabilities, due the spatial distribution of stiffness mismatch and the ability to tune the mismatch with magnetic field. To demonstrate these concepts experimentally, we fabricated MAE composites using a commercial silicone as the non-responsive soft matrix and a silicone loaded with carbonyl iron particles for the stiff, magnetoactive regions. The silicone matrix underwent several modifications to increase the stiffness ratio between the soft encapsulating matrix and the stiff MAE regions, including tuning of the crosslinker to polymer ratio, incorporation of hydride-terminated silicone to promote a linear network, and addition of silicone oil to further reduce crosslinking. Laminates and 2D periodic MAE architectures were constructed through a series of drop casting and molding steps using 3D printed molds. A custom compression test jig was developed to systematically load the laminate specimen in the presence of a magnetic field. The study provides feedback on the sensitivity of the buckling strain to experimental specimen sizing/edge effects and provides broader insight on the practical integration of MAE instabilities into functional devices.
8:45 AM - PM01.01.03
Field Responsive Architected Materials
Julie Mancini1,2,Mark Messner3,William Smith1,Logan Bekker1,Bryan Moran1,A. Golobic1,Andrew Pascall1,Eric Duoss1,Kenneth Loh2,4,Christopher Spadaccini1
LLNL1,University of California, Davis2,Argonne National Laboratory3,University of California, San Diego4Show Abstract
We present a method of creating field responsive architected materials by combining additive manufacturing with magnetorheological (MR) fluid. MR fluid, a fluid that commonly consists of oil and magnetic particles, goes through a change in its rheological properties under a magnetic field. This rheological change observed in the MR fluid is on the order or milliseconds and is highly reversible. We demonstrate these field responsive architected materials by infilling tubular cuboctahedron unit cells and lattices with MR fluid. The result is a dynamically tunable structure that can rapidly and reversibly change its effective stiffness through the simple application of a magnetic field without changing its overall form. We will also discuss a predictive model that we have created to aid in future design of these field responsive architected materials. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Supported by LDRD Strategic Initiative 14-SI-004. LLNL-ABS-753060.
9:00 AM - PM01.01.04
3D Uniaxial Chiral Metamaterials with Ultra-long Characteristic Length Scale
Patrick Ziemke1,Peter Gumbsch1,2
Karlsruhe Institute of Technology1,Fraunhofer Institute for Mechanics of Materials2Show Abstract
Ordinary linear elastic materials do not exhibit a chiral mechanical response. This means that they do not convert a stretch or a compression into a twisting deformation mode. Recent research into the fabrication and design of metamaterials has made this degree of freedom accessible. By stacking 3D laser printed unit cells in each spatial direction, Frenzel et al. created samples of a chiral mechanical metamaterial and successfully mapped its behavior onto a micropolar continuum. Micropolar continuum theory predicts that the magnitude of the twist decreases inversely proportional to the ratio of the sample size to the size of one unit cell. To maintain the global chiral response for larger samples, the scaling behavior must be manipulated to minimize the decay of the twist.
This can be approached by either increasing the intrinsic chiral response of the material or by decreasing the coupling strength for the chiral component of the distortion. We attempt the latter and approach it with three-dimensional lattices based on a geometrically simple uniaxial chiral unit cell. Such simple structures can be modeled with reduced-order finite element formulations (cf. ) yielding an efficient numerical technique which allows the execution of extensive parameter studies with large three-dimensional samples.
We combine chiral unit cells with non-chiral coupling elements to form different lattices. Chiral and non-chiral elements can then be arranged just like in binary crystals forming CsCl or NaCl structures and the like. By testing different structures and tuning the properties of the different unit cells, we have designed lattice structures exhibiting an ultra-long decay-length compared to previously presented chiral mechanical metamaterials. In fact, some of our structures show an increasing chiral response at small numbers of unit cells and do not show a significant decay in the chiral response up to an edge length of 20 unit cells.
 T. Frenzel, M. Kadic, and M. Wegener, Science 358, 1072 (2017)
 A. C. Eringen, Microcontinuum Field Theories: I. Foundations and Solids (1999)
 J. N. Reddy, Computer methods in applied mechanics and engineering 149, 113 (1997)
9:15 AM - PM01.01.05
Deployable Multi-State Structures via Pre-Strained, Multistable Elements
Jochen Mueller1,Jennifer Lewis1,Katia Bertoldi1
Harvard University1Show Abstract
Inspired by the shape memory of crumpled sheets and prior art on multistable structures, we present a new type of architectural metamaterial that can be fabricated flat and buckle out-of-plane to form complex shapes with defined load bearing capacity. The material is composed of discrete elements, of which the sides are exposed to different levels of pre-strain, locally induced during fabrication. Through experiment and simulation, we show that the elements buckle out of plane into different, stable states when exposed to small, orthogonal loads. The resulting, global shape can be pre-programmed via local pre-stress levels or changed on the fly by the activation sequence. By adding additional hierarchy levels, large displacements are achieved. All deformations are elastic and, hence, reversible. This reprogrammable and versatile material is expected to find application in a range of small and large-scale structures, such as deployable space frames and robotic manipulators.
10:00 AM - PM01.01.06
Morphing Sheets into Freeform Objects, at the Micro- and Macro-Scales
California Institute of Technology1Show Abstract
Morphing two-dimensional sheets into three-dimensional objects is a classical problem in mechanics, mathematics and art. Today, the ability to manufacture two-dimensional materials with an almost arbitrary microstructure, architecture and pre-stress distribution opens the door to new approaches for bending sheets into complex forms. In this talk, I will review recent progress in the design of micro- and macro-scale, nonuniform sheets that can bend into freeform objects. Engineering the distribution of residual stresses, stiffness gradients and/or cut patterns, we control the buckling of sheets at both local and global scales. The designed distribution of responsive materials within the sheets provides a time dependent control of the developing shapes. I will discuss examples of sheets that change shape in response to environmental stimuli or the application of point loads. Programming 2D sheets into rigid, 3D geometries expands the potential of existing manufacturing tools for efficient and versatile production of 3D objects.
10:30 AM - PM01.01.07
Atomic Origami—A Technology Platform for Nanoscale Machines, Sensors and Robots
Cornell University1Show Abstract
What would we be able to do if we could build cell-scale machines that sense, interact, and control their micro environment?
In Richard Feynman’s classic talk ``There's Plenty of Room at the Bottom" he foretold of the coming revolution in the miniaturization of electronics components. This vision is largely being achieved and pushed to its ultimate limit as Moore's Law comes to an end. In this same lecture, Feynman also points to the possibilities that would be opened by the miniaturization of machines. This vision, while far from being realized, is equally as tantalizing. For example, even achieving miniaturization to micron length scales would open the door to machines that can interface with biological organisms through biochemical interactions, as well as machines that self-organize into superstructures with mechanical, optical, and wetting properties that can be altered in real time. If these machines can be interfaced with electronics, then at the 10's of micron scale alone, semiconductor devices are small enough that we could put the computational power of the spaceship Voyager onto a machine that could be injected into the body. Such robots could have on board detectors, power sources, and processors that enable them to make decisions based on their local environment allowing them to be completely untethered from the outside world.
In this talk I will describe the work our collaboration is doing to develop 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 technologies behind our machines are graphene-glass and graphene-platinum bimorphs. These ultra-thin bimorphs bend to micron radii of curvature in response to small strain differentials. 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. With the incorporation of electronic, photonic, and chemical payloads, these basic elements will become a powerful platform for robotics at the micron scale. As such, I will close by offering a few forward looking proposals to use these machines as basic programmable elements for the assembly of multifunctional materials and surfaces with tunable mechanical, optical, hydrophilic properties.
11:00 AM - PM01.01.08
Reconfigurable Rotationally Symmetric Kirigami Structures and Their Applications
Erin Evke1,Dilara Meli1,Tristan Blanzy1,Max Shtein1
University of Michigan1Show Abstract
Kirigami design principles increasingly are being used to engineer auxetic materials and structures to achieve 2D to 3D transformations and precisely tune their physical properties. Here, we design, fabricate, and characterize rotationally symmetric kirigami (RSK) structures with complex elasticity, allowing for deformations of planar sheets to approximate curved surfaces and reversibly undergo large deflections. A combination of theory, finite element modeling, and experiments are used to depict the relationship between the cut pattern and the structure’s cross-plane deformation. We classify the RSK structures based on the order of rotational symmetry, defined by the number of sides and cuts along the perimeter. While in the planar, relaxed state, RSK structures exhibit inversion symmetry; this is broken upon cross-plane deformation, a property that may be of interest in controlling wave propagation. The deformed state can be interpreted as a superposition of deformed sub-units, in which the total deflection is a summation of the individual rings’ displacements. The rings can further be partitioned into beams that correspond to the cut geometry, which strongly influences the mechanical properties. We find the effective spring constant decreases with radial cut density but increases with azimuthal cut density, resulting in a theoretical 200-fold reduction in stiffness over the noncut membrane. To demonstrate the electrical characteristics and feasibility of RSK structures for flexible electronic applications, we fabricate conductive nets that maintain electrical properties even under large strains, well beyond 100,000% of the original profile. We also show the reconfigurability of the structures using thermally activated shape memory polymers and analyze their fatigue mechanism.
11:15 AM - PM01.01.09
Inflatable Origami-Inspired Structures
David Melancon1,Chuck Hoberman1,2,Jason Ku3,Erik Demaine3,Katia Bertoldi1,2
Harvard University1,Wyss Institute2,Massachusetts Institute of Technology3Show Abstract
Origami has long been used as a source of inspiration to design creative and esthetic constructions, from the iconic paper swan to facades of multi-story buildings. More recently, the rules of folding have been applied to fabricate architected materials with functional properties such as compactness, self-foldability, and multi-stability. These properties highlight the potential of origami to become a new design paradigm for rapidly deployable structures. Whereas multiple origami-inspired deployable surfaces have been reported in the literature, there is a lack of research on enclosed deployable geometries. In this work, we introduce a novel type of inflatable origami-inspired structure comprised of a polyhedron with triangular faces and elastic hinges. From simple geometry principles, this star-shaped structure possesses two compatible configurations – flat-folded and deployed – giving rise to a bi-stable behavior. Based on experiments of prototypes and simulations of an energy-based model, we characterize the mechanics of the deployment and explore the design space of this origami-inspired structure. The insights gained from the study of this simple geometry enable the understanding of the folding principles of a novel class of enclosed origami-inspired structures that can be deployed to different stable configurations through inflation.
11:30 AM - PM01.01.10
Kirigami Inspired-Metamaterials—From Morphable Structures to Soft Robots
Harvard University1Show Abstract
In recent years kirigami has become an emergent tool to design programmable and reconfigurable mechanical metamaterials. Kirigami-inspired metamaterials allow the practitioner to exploit cuts in addition to folds to achieve large deformations and create 3D objects from a flat sheet. Therefore, kirigami principles have been exploited to design highly stretchable devices and morphable structures.
In this talk I will show that precreased folds are not necessary to form complex 3D patterns in kirigami, as mechanical instabilities in flat sheets with an embedded array of cuts can result in out-of plane deformation. Furthermore, by largely stretching the buckled perforated sheets, plastic strains develop in the ligaments. This gives rise to the formation of kirigami sheets comprising periodic distribution of cuts and permanent folds. As such, the proposed buckling-induced pop-up strategy points to a simple route for manufacturing complex morphable structures out of flat perforated sheets. Finally, I will also show that kirigami principles enable the design of morphable and transformable skins that facilitate the design of soft robots capable of locomotion.
11:45 AM - PM01.01.11
Kirigami Inspired Self-Folding
Arif Abdullah1,K. Jimmy Hsia2
University of Illinois at Urbana–Champaign1,Carnegie Mellon University2Show Abstract
Realization of complex programmable metamaterials where the structural characteristics and functionalities could be tuned beyond their original, as fabricated design remains as one of the challenging problems within the research field of architected materials. To that end, Kirigami - the art of cutting and folding flat sheets of paper, provides scalable routes to generate intricate three-dimensional shapes from thin, planar (two-dimensional) sheets of materials. Researchers have utilized mechanical force actuated Kirigami structures in functional disciplines as diverse as energy harvesting, actuation, optics, stretchable electronics, and soft robotics. Achieving shape reconfiguration of freestanding, Kirigami-cut sheets in a stimuli-responsive, autonomous manner would not only enable new functionalities but also contribute to self-assembly.
This work aims to understand the self-folding behavior of Kirigami-cut bilayers where one layer isotropically expands with respect to the other in response to an external stimulus. We investigate two distinct types of cut geometries namely squares with radial cuts and rectangles with side cuts through a combination of nonlinear finite element modeling and experiments with soft polymeric systems. The Kirigami cuts decompose the pristine squares and rectangles into interconnected beams (length >> width) and plates (length ~ width) of varying aspect ratios. Our finite element calculations reveal that it is possible to tune both the bending direction and curvature values of each individual geometric units within the Kirigami-cut structures and thus transform the bilayers into complex three-dimensional architectures with spatially varying bi-directional curvatures in an on-demand manner. To experimentally demonstrate the potential of our approach, we design planar bilayered samples with side cuts and swell them in organic solvents to generate letters from the English alphabet to make up "UIUC" (University of Illinois at Urbana–Champaign) and "MRS" (Materials Research Society). We also design bilayers with radial cuts, and as they transform shapes with varying mismatch strains (solvent concentrations), we show that it is possible to use them as freestanding tunable optical systems where the transmission and reflection windows for incident light could be controlled through the deformation behavior of individual geometric units between the cuts. We also use a combination of the cuts to realize polyhedral shapes (such as tetrahedron and cube) through the self-folding of planar bilayers. The design principles proposed in this work would be applicable to a variety of material systems across length scales and contribute toward the development of smart programmable metamaterials.
PM01.02: Shape Morphing and Origami/Kirigami Materials II
Monday PM, November 26, 2018
Hynes, Level 1, Room 102
1:30 PM - PM01.02.01
Sequential Mechanical Metamaterials
Martin Van Hecke1
Leiden University1Show Abstract
Ordered sequences of motions govern the morphological transitions of a wide variety of natural and man-made systems, while the ability to interpret time-ordered signals underlies future smart materials that can be (re)programmed and process information. Here we introduce two novel classes of mechanical metamaterials, that can (1) exhibit sequential output and (2) are sensitive to sequential input. To obtain metamaterials that translate a global uniform compression into a precise multistep pathway of reconfigurations, we combine strongly nonlinear mechanical elements with a multimodal hierarchical structure, and demonstrate multi-step reconfigurations of digitally manufactured metamaterials. To obtain metamaterials that are sensitive to a sequence of mechanical inputs, we introduce the notion of non-commuting metamaterials, and show their capability for storing and processing information. Our work establishes generic principles for infusing metamaterials with sequential input and output.
2:00 PM - PM01.02.02
In Situ Tunable Stiffness Using Multistable Kirigami Metamaterials
Yi Yang1,Marcelo Dias2,Douglas Holmes1
Boston University1,Aarhus University2Show Abstract
Materials with in situ tunable stiffness are needed for engineering programmable materials, shape-shifting structures, artificial muscles, and soft robotic actuators. While most examples of materials with in situ tunable stiffness focus on stimulus-responsive material properties, recent investigations on reconfigurable mechanical metamaterials opened another door to attain tunable material properties by systematically programming the microstructure of a constituent material. The mechanical properties of these architected materials depend on the topology of the substructure regardless of the constituent materials used. By introducing morphological structures into the unit cell, reprogrammable and reconfigurable metamaterials can be attained. Among various types of mechanical metamaterial, kirigami and origami metamaterials attracted tremendous attention due to its robust and straightforward ability to transform 2D sheets into 3D structures. Compared with its sister, origami metamaterials which have been extensively studied, understanding the mechanical behavior of kirigami metamaterials is limited. In this study, through a combination of experiments, simulation, and theoretical analysis, we demonstrate how a multistable microstructure inspired by kirigami provides a new design approach. By changing the spacing between the adjacent slits in the conventional linear parallel cutting patterns, we obtain multistable kirigami lattice structures composed of repeating unit cells whose structure is endowed with a bistable snap-through mechanism. Each stable state of the mechanical metamaterial exhibits a corresponding stiffness. By switching unit cells between the two stable states, we can tune the stiffness of this kirigami metamaterial in situ by a factor of five. Since this multistable kirigami approach is material independent, it could be integrated with stimulus-responsive materials, 3D printing technique, or combined with origami structures to be applied in multifunctional materials, deployable space structures, soft robotics, and biomedicine.
2:15 PM - PM01.02.03
Observation of Mechanical Activity in a 3D Chiral Metamaterial
Tobias Frenzel1,Muamer Kadic2,1,Martin Wegener1
Karlsruhe Institute of Technology1,Université de Bourgone Franche-Comté2Show Abstract
In a medium exhibiting optical activity, an incident transverse linear polarization of a light wave rotates because the eigenpolarizations of the chiral medium are not linear but rather circular, with a lifted degeneracy between left- and right-handed circular modes. Therefore, a linear incident polarization must be decomposed into the two circular eigenpolarizations, which propagate with different phase velocities. The resulting phase difference accumulated during propagation leads to a periodic rotation of the linear polarization axis versus the propagation coordinate. A rotation by 90 degrees corresponds to mode conversion.
Regarding elasticity, a different kind of mode conversion has recently been observed in two-dimensional (2D) centrosymmetric metamaterials . There, the conversion between in-plane longitudinal (compression) and in-plane transverse (shear) modes did not require chirality. Chirality is crucial in quasi 2D monolayers of non-centrosymmetric tungsten diselenide crystals, for which the presence of chiral phonons  was deduced via selection rules of optical transitions.
In this contribution, we report the first experimental observation of mechanical activity, the elastic counterpart of optical activity, in 3D chiral polymer microlattices . We have previously investigated related 3D chiral micropolar metamaterial samples made by 3D laser nanoprinting in the static regime . Our present experimental approach in the dynamic regime is based on cross correlations of microscopy images taken under phase-delayed synchronized stroboscopic illumination with a light-emitting diode and sinusoidal excitation of the sample with a piezoelectric actuator. This approach is distinct from the established mapping of out-of-plane surface excitations by interferometric laser detection and laser excitation (see, e.g., ).
For an incident linearly polarized transverse elastic wave, we demonstrate the polarization rotation by 90 degrees at 225 kHz frequency over as few as 10 unit cells in the axial direction, with 3-by-3 unit cells in the cross section. The experimental results are in good agreement with numerical finite-element calculations for finite beams as well as with band structure calculations for fictitious infinitely extended beams. These experiments pave the road for molding the polarization and the propagation direction of elastic waves in three dimensions by micropolar mechanical metamaterials.
 J. M. Kweun, H. J. Lee, J. H. Oh, H. M. Seung, Y. Y. Kim, Phys. Rev. Lett. 118, 205901 (2017)
 H. Zhu, J. Yi, M.-Y. Li, J. Xiao, L. Zhang, C.-W. Yang, R. A. Kaindl, L.-J. Li, Y. Wang, and X. Zhang, Science 359, 579 (2018)
 T. Frenzel et al., unpublished
 T. Frenzel, M. Kadic, and M. Wegener, Science 358, 1072 (2017)
 Y. Sugawara, O. B. Wright, O. Matsuda, M. Takigahira, Y. Tanaka, S. Tamura, and V. E. Gusev, Phys. Rev. Lett. 88, 185504 (2002)
2:30 PM - PM01.02.04
Multi-Stimuli Responsive Actuation of 3D Printed Bistable Beam-Based Structures
Yijie Jiang1,Lucia Korpas1,Jordan Raney1
University of Pennsylvania1Show Abstract
Nature provides many examples of systems that autonomously undergo morphological and functional changes in response to environmental stimuli or sequences of stimuli. For example, the Venus flytrap (Dionaea muscipula) is well-known for its ability to rapidly snap its leaves together to trap prey. However, in addition to this, it embodies complex logic, requiring, for example, repeated mechanical stimulation within a defined period of time in order for the leaves to be fully closed. Unlike analogous behavior in synthetic systems (e.g., robotics), which traditionally relies on an integrated mechatronic system of batteries, sensors, microprocessors, and actuators, nature achieve responsiveness via control logic that is embodied in the compositional (material) and structural (geometric) features. Inspired by nature, we combine anisotropic architected materials with geometric nonlinearity to design structures that precisely and sequentially self-actuate in response to multiple stimuli. We achieve this by 3D printing structures with geometric parameters that lie near nonlinear bifurcation points associated with a transition between bistable and monostable mechanical behaviors. We fabricate these structures using direct ink writing (DIW) with microfibrous architected materials, which are highly anisotropic due to shear-induced alignment of the fibers during printing. As a result, the materials swell anisotropically when exposed to suitable stimuli (non-polar solvents or water), which alters a key geometric control parameter defining the nonlinear behavior. If the control parameter passes through specific bifurcation points, rapid, large-amplitude self-actuation events can be triggered to occur at specific times, which can be harnessed to impart autonomous functional changes to structures. We demonstrate the utility of this bioinspired autonomous control strategy with several examples of structures that respond to their environment according to their embodied logic, without electronics, external control systems, or tethering.
2:45 PM - PM01.02.05
Origami and 4D Printing of Elastomer-Derived Ceramic Structures
Guo Liu1,Yan Zhao1,Ge Wu1,Jian Lu1
City University of Hong Kong1Show Abstract
Four-dimensional (4D) printing involves conventional three-dimensional (3D) printing followed by a self-shaping assembly step. It enables more complex shapes to be created than is possible with conventional 3D printing, and shape-morphing capabilities can improve the adaptability of structural materials to versatile application environments. However, 3D-printed ceramic precursors are usually difficult to be deformed, hindering the development of 4D printing for ceramics. To overcome this limitation, we developed elastomers which can be printed, deformed, and then transformed into ceramics, making the growth of complex ceramic origami and 4D-printed ceramic structures possible. In addition, strength-scalability synergy is achieved in the resultant ceramic architectures. This work starts a new chapter of printing geometrically complex and mechanically robust ceramics, and this concept is cost-efficient in term of time when a series of complex-shaped ceramics with similar geometries were required. With shape-morphing capabilities of elastomers, this work on origami and 4D printing of elastomer-derived ceramics (EDCs) could lead to structural applications of autonomous morphing structures, aerospace propulsion components, space exploration, electronic devices, and high-temperature microelectromechanical systems.
PM01.03: Micro/Nano-Architected Materials I
Martin Van Hecke
Monday PM, November 26, 2018
Hynes, Level 1, Room 102
3:30 PM - PM01.03.01
Additive Manufacturing and Architected Materials—Design, Fabrication, Materials and Performance
Lawrence Livermore National Laboratory1Show Abstract
Material properties are governed by the chemical composition and spatial arrangement of constituent elements. Over the past decade, the field of architected materials has sought to design, fabricate, and demonstrate materials with performance that is fundamentally controlled by geometry at multiple length-scales rather than chemical composition alone. There have been many advancements ranging from the maturation of additive manufacturing technologies which can be used to realize these materials, inverse design methods such as topology optimization, and unique new material feedstocks which make up the structures. This presentation will touch on all aspects of the architected materials realization process as well as evaluate performance of some of those materials. Specifically, we have demonstrated designer properties of these architected materials in polymers, metals, ceramics and combinations thereof. In addition to novel properties such as ultra-stiff lightweight materials, negative stiffness, and negative thermal expansion, I will present multifunctional architected materials with energy storage capability and architectures that respond to external fields. Many of these architected materials derived from advanced design and optimization methods which we have been developing and were fabricated with custom additive manufacturing techniques. These include projection microstereolithography (PμSL), direct ink writing (DIW), electrophoretic deposition (EPD), volumetric additive manufacturing, computed axial lithography (CAL), and diode-based additive manufacturing (DiAM) to name a few. New materials including graphene aerogel, carbon fiber composite, and printed glass will also be touched upon.
Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. LLNL-ABS-753106
4:00 PM - PM01.03.02
Architected Nanocomposites as Model Materials for Armor Systems
Anna Guell Izard1,Jens Bauer1,Lorenzo Valdevit1
University of California, Irvine1Show Abstract
Armor systems must be carefully designed to mitigate two threats: blast and penetration. A successful armor dissipates the kinetic energy of the blast via plastic deformation while being able to resist the penetration of a projectile. These functional requirements require a material with high toughness and hardness, two properties that are generally mutually exclusive in monolithic materials. Current armor solutions consist of multi-layer systems encompassing one or more metallic or polymeric phases for energy absorption and a ceramic phase for penetration resistance. Here we propose a novel strategy based on the optimal design of functionally graded micro/nano-architected ceramic/polymer or ceramic/metal composites. The optimal intertwining of the two phases at the nanoscale has the potential to vastly outperform current solutions, while reducing weight. In this talk, we will present the performance of ceramic/metal and ceramic/polymer architected nanocomposites based on ordered lattice topologies and spinodal architectures.
4:15 PM - PM01.03.03
Mechanical Behaviour of Architectured Auxetic Hybrid Lattice Structures
Frédéric Albertini1,Justin Dirrenberger1,Andrey Molotnikov2,Cyrille Sollogoub1
CNRS1,Monash University2Show Abstract
Architectured Materials are an emerging class of advanced materials that bring new possibilities in terms of functional properties, filling gaps in Ashby’s material performance maps. The term architectured materials describes any heterogeneous material that exhibits improved specific properties due to a thoughtful and predetermined morphology and/or topology design. This usually induces characteristic length-scales comparable to the size of the final component being produced, i.e. the millimetre scale in the case of lattice structures. Different strategies have been studied in the literature for mitigating the surface defects of additively manufactured metallic lattices: chemical etching, electro-erosion, mechanical polishing.
A new proposition is presented in this work: polymer coating or embedding of metal struts, by analogy to the soft-hard turtle-like strategy for mitigating crack propagation. Besides processing of such architectured lattice structures, the present work brings experimental and numerical results concerning the mechanical behaviour in compression for negative Poisson’s ratio lattices, also known as auxetics. As a matter of fact, one engineering challenge is to predict the effective mechanical properties of architectured materials; computational homogenization using finite element analysis is a powerful tool to do so when considering quasi-static behaviour; difficulties arise when analysing the effective damping behaviour. A straightforward solution is to rely on full-field finite element dynamic simulation, accounting for both the intrinsic viscoelastic damping of the constitutive material, as well as the structural damping due to the geometrical definition of the lattice structure considered in the present work. Homogenized behaviour of architectured materials can thus be used in large structural computations, hence enabling the dissemination of such materials in the industry. Comparison is made between the metal and hybrid lattice structures.
4:30 PM - PM01.03.04
Architected Viscoelastic Impact Attenuators for Sports Padding Applications
Eric Clough1,2,Zak Eckel1,Alvin Escobar1,Jacob Hundley1,Tobias Schaedler1
HRL Laboratories1,University of California Santa Barbara2Show Abstract
Mitigating injury as a result of an impact between multiple players or between a player and the ground presents a significant challenge to the sports protective equipment designer. These designs must consider multiple injury criteria (e.g. concentrated loads, peak acceleration, peak torque) for multiple successive impacts, while balancing human factors such as comfort and player mobility. For padding components, maximal volumetric energy absorption is key to achieving impact attenuation performance without sacrificing player mobility. Typical padding materials include urethane and vinyl nitrile closed-cell foams which attenuate impacts through the collapse of internal pores when compressed. The stochastic internal geometry of foams limits optimal design of foam pads to essentially two design variables: composition and apparent density. Conversely, lattices provide the designer with a host of architectural degrees of freedom enabling the optimal design of lattice pads that can significantly outperform foams through tuning of buckling and post-buckling response. The open-celled architecture of lattices additionally affords improvements in airflow for comfort or enhanced energy absorption. In this talk, we will present our recent work on the design, fabrication, and optimization of lattice padding materials fabricated via HRL’s rapid and scalable self-propagating photopolymer waveguide process. We demonstrate that by optimal design of the architecture, in conjunction with tuning the viscoelastic properties of the parent solid, lattice pads with impact attenuation performance exceeding 1.3X that of state-of-the-art foam padding can be achieved.
4:45 PM - PM01.03.05
Plate Mechanical Metamaterials and Their Applications
University of Pennsylvania1Show Abstract
Recently, we introduced the concept of plate mechanical metamaterials—cellular plates with carefully controlled periodic geometry and unique mechanical properties—as well as its initial realization in the form of freestanding corrugated plates made out of an ultrathin film. We used atomic layer deposition (ALD) and microfabrication techniques to make robust plates out of a single continuous ALD layer with cm-scale lateral dimensions and thicknesses between 25 and 100 nm, creating the thinnest freestanding plates that can be picked up by hand.
More recently, we also fabricated and characterized plate metamaterials made from multiple layers of nanoscale thickness, whose geometry and properties are reminiscent of honeycomb sandwich plates or corrugated paper cardboard. The two layers are offset from each other but at the same time are connected using vertical-wall webbing, which prevents shear of the two layers with respect to one another. As a result, these “nanocardboard” plates orders-of-magnitude higher bending stiffness than the single-layer structures we reported earlier, while still possessing extremely low weight (as low as 0.5 g/m2) and mechanical robustness. The increase in the bending stiffness is expected, and its mechanism is similar to that used in conventional honeycomb sandwich plates, which offer the best possible combination of high bending stiffness and low mass. However, in contrast to sandwich composite plates, our nanoscale two-layer mechanical metamaterials can sustain extremely large deformations without fracture, fully recovering their original shape and not displaying any signs of internal damage.
Like the nanotruss-based mechanical metamaterials reported by other groups, plate mechanical metamaterials are extremely lightweight and resilient due to their nanoscale thickness and microscale cellular structure. However, in contrast to the cube-shaped metamaterials that typically form a lattice easily penetrated by the ambient air, our plates form continuous flat plates. Ultralow weight, mechanical robustness, thermal insulation, as well as chemical and thermal stability of alumina make plate metamaterials attractive for numerous applications, including structural elements in flying microrobots and interstellar light sails, high-temperature thermal insulation in energy converters, photophoretic levitation, as well as ultrathin MEMS/NEMS sensors and resonators. I will briefly discuss our experimental progress on all these applications, including demonstration of extremely robust thermal insulators that can sustain a temperature difference of ~1000 C across a micron-scale gap, macroscopic plates that levitate when illuminated by light, and hollow AFM cantilevers that offer greatly enhanced sensitivity and data acquisition rates.
Lorenzo Valdevit, University of California, Irvine
Katia Bertoldi, Harvard University
Tobias Schaedler, HRL Laboratories, LLC
Martin Wegener, Karlsruhe Institute of Technology
PM01.04: Micro/Nano-Architected Materials II
Tuesday AM, November 27, 2018
Hynes, Level 1, Room 102
8:00 AM - PM01.04.01
Materials by Design—Three-Dimensional (3D) Nano-Architected Meta-Materials
Julia Greer1,Andrey Vyatskikh1,Carlos Portela1,Xiaoxing Xia1,Arturo Mateos1
California Institute of Technology1Show Abstract
Creation of extremely strong and simultaneously ultra lightweight materials can be achieved by incorporating architecture into material design. We fabricate three-dimensional (3D) nano-architectures, i.e. nanolattices, whose constituents vary in size from several nanometers to tens of microns to centimeters. These nanolattices can exhibit superior thermal, photonic, electrochemical, and mechanical properties at extremely low mass densities (lighter than aerogels), which renders them ideal for many scientific pursuits and technological applications. The dominant properties of such meta-materials, where individual constituent size at each relevant scale (atoms to nanometers to microns) is comparable to the characteristic microstructural length scale of the constituent solid, are largely unknown because of their multi-scale nature. To harness the beneficial properties of 3D nano-architected meta-materials, it is critical to assess properties at each relevant scale while capturing the overall structural complexity.
We describe the fabrication and synthesis using Additive Manufacturing (AM) techniques, as well as the mechanical, biochemical, electrochemical, and thermal properties of nanolattices made of different materials with varying microstructural detail. Attention is focused on uncovering the synergy between the internal atomic-level microstructure and the nano-sized external dimensionality, where competing material- and structure-induced size effects drive overall response and govern these properties. Specific discussion topics include the nanofabrication and characterization of (often hierarchical) three-dimensional nano-architected meta-materials and their applications in chemical and biological devices, ultra lightweight energy storage systems, damage-tolerant fabrics, and photonic crystals.
8:30 AM - PM01.04.02
Nanocrystalline Aluminum Truss Cores for Lightweight Sandwich Structures
Tobias Schaedler1,Lisa Chan2,Eric Clough1,Morgan A. Stilke1,Lawrence Masur2,Jacob Hundley1
HRL Laboratories, LLC1,Xtalic Corporation2Show Abstract
Substitution of conventional honeycomb – composite sandwich structures with lighter alternatives has the potential to reduce the mass of future vehicles. Here we demonstrate nanocrystalline aluminum - manganese truss cores that achieve 2 - 4 times higher strength than aluminum alloy 5056 honeycombs of the same density. The scalable fabrication approach starts with additive manufacturing of polymer templates, followed by electrodeposition of nanocrystalline Al-Mn alloy, removal of the polymer, and facesheet integration. This facilitates curved and net-shaped sandwich structures, as well as cocuring of the facesheets, which eliminates the need for extra adhesive. The nanocrystalline Al-Mn alloy thin -film material exhibits high strength and ductility and can be converted into a three-dimensional hollow truss structure with this approach. Ultra-lightweight sandwich structures are of interest for a range of applications in aerospace, such as fairings, wings, and flaps, as well as for the automotive and sports industries.
8:45 AM - PM01.04.03
Additive Manufacturing and Design of Multi-Functional 3D Architected Metamaterials
Virginia Tech1Show Abstract
Rapid progress in additive manufacturing methods has led to the creation of a new class of ultralight, stiff and strong 3D architected metamaterials comprised of a network of hierarchical, stretch-dominated, micro-scale unit cells. The performance of these micro-architected metamaterials, spanning across multiple length scales, will ultimately be limited by their tolerance to damage and defects, yet an investigation of their fracture toughness has remained elusive. Here, we provide the first experimental observations of different crack initiation modes activated by mode-I loading of notched low density, hierarchical metamaterials. We find that, through hierarchical micro-architected features, the low density, stretch-dominated metamaterials can achieve simultaneously higher fracture toughness and strength: properties that are usually mutually exclusive. Numerical and scaling relationships are also reported that accurately capture the measured fracture toughness and strength values. These are then used to develop design maps for the optimal hierarchical architectures as functions of the density of the metamaterial and failure strain of the parent material.
9:00 AM - PM01.04.04
Subtractive Post-Processing to Create Delicate Nano-Scale Structures
Andrew Gross1,Katia Bertoldi1
Harvard University1Show Abstract
Two photon polymerization (TPP) has become a popular technique for testing new devices and designer materials at the nano and micro scales. This technique provides much of the flexibility of a desktop 3D printer with access to physics that are uniquely scale dependent. Like every additive manufacturing technique, TPP is limited to a minimum achievable size for the voxel that can be rastered to create a structure. The constraint on reducing the size of the voxel is a practical limit arising from both the spatial photon density distribution within the diffraction limited focal volume of the laser illumination and the contrast curve of the photoresist that describes the degree to which it polymerizes in response to exposure at a certain dose. Here, we present an approach for achieving smaller feature sizes that is accessible to researchers without specialized equipment. This approach leverages the high resolution of a piezo stage to precisely raster the voxel into shapes that upon isotropic etching allow access to a smaller length scale. Free-standing fully three-dimensional structures with feature sizes under 100 nm are demonstrated. Additionally, this technique allows for the fabrication of structures which are otherwise too delicate to make. This is achieved both by simply etching a structure of interest to impart more slender features, and also by including secondary support material to create complex structures that cannot be fabricated by any other method at this length scale.
9:15 AM - PM01.04.05
Carbon-Based Nanolattice Materials
Lorenzo Valdevit1,Jens Bauer1,Anna Guell1,Cameron Crook1
University of California, Irvine1Show Abstract
From a traditional perspective, there is little room for further expansion of the accessible material property space by classical material fabrication methods. Single one- and two-dimensional nanoscale objects, such as nanowires and thin films, are known to exhibit exceptional physical properties. Yet, their properties are intrinsically coupled to their small size and their solitary nature, and therefore can hardly be accessed in actual materials of practical volume. If nanowires and thin films are simply scaled up, many of their exceptional properties, which relate to surface to volume effects, drop dramatically; similarly, if nanoscale objects are clustered in a composite material, interfaces dominate the overall behavior, again substantially reducing performance. Nanolattice materials, regular three-dimensional networks constructed from nanowires or thin films, can potentially resolve this dilemma. Nanolattice materials can be thought of as metamaterials which can scale up beneficial size effects by topological design of their architecture.
In our work, we have fabricated carbon-based nano-architected lattice materials by direct laser writing (DLW) and pyrolysis. The combination of DLW and pyrolysis facilitates complex three-dimensional carbon-based ceramic structures of uniquely high resolution, with feature dimensions below 100 nm. This enables to take advantage of pronounced size-dependent effects on material strength. We systematically investigate the effects of processing parameters, dimensional scale, topology and topology-dependent strain during pyrolysis on the mechanical and functional properties of nanolattices. Here, we offer an overview of manufacturing routes, with emphasis on the key processing parameters. The interplay of size-dependent effects with different topological and material design approaches is emphasized.
10:00 AM - PM01.04.06
Designing and Manufacturing Cellular Mechanical Metamaterials
Maximilian Wormser1,Carolin Körner1
FAU Erlangen-Nuremberg1Show Abstract
The geometrical structure of a material can drastically change its mechanical behavior. Mechanical metamaterials are architected structures with extreme properties that surpass the capabilities of conventional bulk material. Periodic cellular structures lend themselves to be used as metamaterials because of their high variability. In the past, manufacturing methods limited the possibility to create intricate three-dimensional cellular structures. This changed with the advent of additive manufacturing technologies. Nowadays, cellular structures can be printed as polymers, ceramics or metals.
Finding new shapes for cellular materials can be difficult. We developed a new method to find interesting unit cell designs by using eigenmode analysis on a basic unit cell. The eigenmode shapes of the basic cell then serve as new unit cell for further simulations.
Cellular materials can have an excellent ratio of stiffness to weight, making them highly suitable for lightweight construction. By adding additional metamaterial functions to these structures, we get a so-called intelligent or smart material. An example for such a feature is auxetic behavior, i.e. a negative Poisson ratio. Therefore, if the structure is compressed, it will also compress laterally to the applied force and reduce its volume as opposed to regular materials that expand laterally.
Other metamaterial effects influence the way structures interact with mechanical waves. Given the right geometry, a structure can be unable to resonate in a certain frequency band. This effect is called phononic band gap and it can be used for wave guiding and filtering.
Selective Electron Beam Melting (SEBM) is a powder-based additive manufacturing technology that enables us to produce complicated structures on a millimeter scale with very few limitations. We used this technology to create auxetic structures as well as phononic band gap structures using the above mentioned eigenmode analysis method and tested their properties. A new measurement method for mechanical wave transmission in lattice materials was developed. Piezoelectric transducers were used to create and measure the incoming and outgoing signal. The experimental results were compared to numerical simulations and were found to be in agreement with each other. By further improving the design based on gradient based optimization the phononic band gap of a macro-level unit cell could be pushed into the audible range below 20 kHz.
10:30 AM - PM01.04.07
Single Atom Scale Fabrication by Scanning Transmission Electron Microscopy
Ondrej Dyck1,Sergei Kalinin1,Stephen Jesse1
Oak Ridge National Laboratory1Show Abstract
Fabrication of atomic scale structures remains the ultimate goal of nanotechnology. The reigning paradigms are scanning probe microscopy (SPM) and synthesis. SPM assembly dates to seminal experiments by Don Eigler, who demonstrated single atom manipulation. However, stability and throughput remain issues. The molecular machines approach harnesses the power synthetic chemistry to build individual functional blocks, yet strategies for structural assembly remain uncertain.
In this presentation, I discuss research activity towards a third paradigm — the use of the atomically focused beam of a scanning transmission electron microscope (STEM) to control and direct matter on atomic scales. Traditionally, STEM’s are perceived only as imaging tools and beam induced modifications as undesirable beam damage. Our team and several groups worldwide have demonstrated that beam induced modifications can be more precise. We have demonstrated ordering of oxygen vacancies, single defect formation in 2D materials, and beam induced migration of single interstitials in diamond like lattices. What is remarkable is that these changes often involve one atom or small group of atoms and can be monitored real-time with atomic resolution. This fulfills two out of three requirements for atomic fabrication. I will introduce several examples of beam-induced fabrication on the atomic level, and demonstrate how beam control, rapid image analytics, better insight through modelling, and image- and ptychography based feedback allows for controlling matter on atomic level.
This research is supported by and performed at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, BES DOE.
10:45 AM - PM01.04.08
Facile Electrochemical Fabrication Route for Multicomponent and Multifunctional Inverse Opaline Films for Highly Sensitive VOC Detection
Pei-Sung Hung1,Yu-Szu Chou1,Shih-Cheng Chou1,Guang-Ren Wang1,Chuan-Jyun Wang1,Yu-Ting Cheng1,Pochun Chen2,Tsung-Eong Hsieh1,Kea Tiong Tang3,Pu Wei Wu1
National Chiao Tung University1,National Taipei University of Technology2,National Tsing Hua University3Show Abstract
Non-invasive diagnosis for early detection of diseases have drawn considerable attention during past few decades. Earlier, several biomarkers have been identified in exhaled breath specimens using GC-MS technique. However, the GC-MS analysis is a tedious process requiring high-end instruments, spacious facility, and well-trained personnel. With rapid development of artificial intelligent e-nose system, there is a promising potential serving as a portable alternative to the aforementioned analytical technique. With ultra-high specific surface area and activities, numerous nanomaterials have been explored for sensing applications. However, a dilemma often exists as researchers trying to increase the amount of materials used for enhanced sensitivity and meanwhile maintaining accessible surface area. Unlike 2D thin film sensors, 3D nanostructured sensing materials provide a larger surface-to-volume ratio with enhanced mass transfer due to their open cell configuration. Several architectures have been proposed and designed for these purposes. Among them, inverse opal (IO) is considered as a promising candidate as it possesses ordered and monodispersed porous scaffolds with numerous interconnected channels for superior permeability.
Herein, we report a facile fabrication route through electrochemical methods to construct inverse opaline sensing materials. Their pore sizes can be tailored from meso to macro scale, providing a large specific surface area and rendering a low tortuosity pathway for gas stream. In addition to the structural design, we further introduce heterojunctions into the sensing units by using composite metal oxides as the molding materials for enhanced sensitivity. Lastly, a top-bottom electrode configuration is adopted with a metal IO upper layer as a gas permeable electrode. Structural, morphological, and compositional characterizations are conducted by SEM, EDX, XRD, and XPS techniques. The responses of different composite IO films toward VOC vapors are then recorded and analyzed.
11:00 AM - PM01.04.09
Mechanics and Fabrication of Architected Graphene-Metal Lattice Materials
Richard Li1,Shruti Rastogi1,Jeffrey Kysar1
Columbia University1Show Abstract
Much of graphene research to date has focused on graphene in a 2D, planar geometry, but the idea of extending a 2D material to span three dimensions opens many interesting possibilities. A variety of techniques have recently emerged for assembling graphene into 3D configurations such as foams, aerogels, and lattice networks. However, the arrangement of individual graphene sheets is stochastic in most cases, and none have produced periodic graphene materials with precisely controlled and tunable 3D features at small scales. The creation of such a material would enable the systematic study of graphene in 3D space and could lead to many new insights.
We have taken a step towards this goal by developing a method for depositing graphene on micrometer-scale 3D periodic lattice structures. Polymer lattice templates are fabricated by two-photon lithography and then metallized by electroless plating of either copper or nickel-boron. After the polymer is removed, the hollow metallic lattice is used as a substrate for graphene growth by chemical vapor deposition (CVD). The result is a 3D periodic lattice with graphene-metal composite struts. We can then test the mechanical behavior of these composite lattices under uniaxial compression and observe the reinforcing effect of the graphene layers.
Raman spectroscopy and scanning electron microscopy reveal conformal sheaths of single-layer to few-layer graphene enveloping the metallic lattices. Compression tests indicate that the graphene coatings significantly enhance the mechanical properties of the lattice. These preliminary results demonstrate a potential pathway for the research and application of 2D materials in 3D space.
11:15 AM - PM01.04.10
Fabrication of Ideally Ordered Anodic Porous TiO2 and Its Application to Photonic Crystal
Toshiaki Kondo1,Shota Hirano1,Sanami Nagao1,Toko Tamura1,Takashi Yanagishita1,Hideki Masuda1
Tokyo Metropolitan University1Show Abstract
Ideally ordered nanohole array of TiO2 has attracted attention due to its wide applicability based on the periodicity of the nanoholes, high refractive index and semiconductor properties of TiO2. Various types of optical devices based on an ideally ordered nanohole array of TiO2 have been proposed, such as photonic crystal, photocatalyst, solar cells and so on. Until now, several kinds of fabrication process of nanohole array structure of TiO2 have been proposed. By applying semiconductor microfabrication process using electron beam lithography and focused ion beam etching apparatuses, an ideally ordered nanohole array could be obtained. However, this process is time consuming. By anodization process, porous TiO2 could be obtained in large sample area. However, it is difficult to obtain ideally ordered nanohole array structure because the nanoholes in anodic porous TiO2 usually form multi domain structure. In this presentation, fabrication process of anodic porous TiO2 having ideally ordered nanohole array and its application to photonic crystal will be presented [1~4]. Prior to the anodization of Ti, concaves were fabricated onto the surface of Ti through a texturing process. In subsequent anodization, each concaves acted as a starting point of the formation of nanoholes, resulting in the formation of an anodic porous TiO2 having ideally ordered nanohole array. Photonic crystal properties of the anodic porous TiO2 were evaluated by measuring reflection spectra. Then, formation of photonic band gap at visible wavelength range was confirmed. The present process is expected to be applied to fabricate various types of functional optical devices requiring an ideally ordered nanohole array structures, such as photonic crystals.
 T. Kondo, S. Nagao, T. Yanagishita, N. T. Nguyen, K. Lee, P. Schmuki, H. Masuda, Electrochem. Commun., 50, 73 (2015).
 T. Kondo, S. Nagao, T. Yanagishita, H. Masuda, J. Electrochem. Soc., 163, E206 (2016).
 T. Kondo, S. Nagao, S. Hirano, T. Yanagishita, N. T. Nguyen, P. Schmuki, H. Masuda, Electrochem. Commun., 72, 100 (2016).
 T. Kondo, S. Hirano, T. Yanagishita, N. T. Nguyen, P. Schmuki, H. Masuda, Appl. Phys. Express, 9, 102001 (2016).
11:30 AM - PM01.04.11
Strength and Fracture Toughness of Hierarchical Nanoengineered Composites Reinforced with Aligned Nanoscale Fibers
Xinchen Ni1,Reed Kopp1,Nathan Fritz1,Estelle Kalfon-Cohen1,Carolina Furtado2,Albertino Arteiro2,Gregor Borstnar3,Mark Mavrogordato3,Lukas Helfen4,5,Ian Sinclair3,Mark Spearing3,Pedro Camanho2,Brian Wardle1
Massachusetts Institute of Technology1,University of Porto2,University of Southampton3,European Synchrotron Radiation Facility4,Karlsruhe Institute of Technology5Show Abstract
Aerospace-grade unidirectional carbon fiber reinforced plastic (CFRP) composite laminates were reinforced in the resin-rich weak interfaces using high densities of aligned nanoscale fibers, in particular, aligned carbon nanotubes (A-CNTs), in a hybrid architecture termed “nanostitching.” Here, we investigate the effect of A-CNT interlaminar reinforcement on laminate strength and toughness via ex situ and in situ mechanical testing, leveraging both synchrotron radiation computed tomography (SRCT) and lab-based high resolution micro-computed tomography (μCT). Mode I and II interlaminar fracture toughness were assessed by conducting ex situ double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. We find a ~6% lower steady-state mode I fracture toughness and no improvement of precracked mode II fracture toughness over baseline laminates. However, unique crack propagating behaviors in A-CNT reinforced laminates were revealed by scanning electron microscopy and μCT of fractured specimens. A-CNTs were found to force the crack into the intralaminar region in both DCB and ENF loadings, suggesting a tougher interlaminar region with the addition of A-CNTs. The measured toughness values of the A-CNT reinforced laminates were found to be associated with the toughness inside the ply. In situ SRCT tensile testing of double edge-notched tension (DENT) specimens shows a ~9% increase in ultimate tensile strength (UTS) over the baseline specimens. No significant differences in progressive damage features (up to 90% of UTS) near the notch were observed regardless of the presence of A-CNTs. 3D visualization and damage segmentation software identified matrix cracking and fiber/matrix interfacial debonding as dominant damage mechanisms for loads up to 90%. However, large interlaminar delaminations, which are known to be a primary damage mechanism suppressed by A-CNT interlaminar reinforcement, were revealed by post-mortem CT of DENT specimens. These findings reveal for the first time the multiscale strengthening and toughening mechanisms induced by A-CNTs, which influence the macroscopic behavior of composite laminates. Future work will focus on acquiring 3D data beyond 90% UTS of DENT configurations and performing in-situ mode I testing to non-destructively elucidate the crack propagation behavior in real-time.
11:45 AM - PM01.04.12
Self-Assembly of Polyhedral Metal-Organic Framework Nanoparticles
Cefe Lopez2,Civan Avci1,Inhar Imaz1,Arnau Carné1,Jose Aangel Pariente2,Javier Perez Carvajal1,Maria Isabel Alonso2,Alvaro Blanco2,Marjolein Dijkstra3,Daniel Maspoch1
Catalan Institute of Nanoscience and Nanotechnology1,Consejo Superior de Investigaciones Científicas2,University of Utrecht3Show Abstract
Self-assembly of particles into long-range, three-dimensional, ordered superstructures is crucial for the design of a variety of materials, including plasmonic sensing materials, energy or gas storage systems, catalysts and photonic crystals. This kinds of structures have been demonstrated mostly with spheres that, owing to their entropically favoured fcc packing, form the densest structures possible. Polyhedra have been tested but so far only with metals and oxides.
Here, we have combined experimental and simulation data to show that truncated rhombic dodecahedral particles of the metalorganic framework (MOF) ZIF-8 can self-assemble into millimetre-sized superstructures with an underlying three dimensional rhombohedral lattice with photonic crystal properties.
These superstructures feature a photonic bandgap that can be tuned by controlling the size of the ZIF-8 particles and is also responsive to the adsorption of guest substances in the micropores of the ZIF-8 particles.
Superstructures with different lattices can also be assembled by tuning the truncation of ZIF-8 particles, or by using octahedral UiO-66 MOF particles instead. These well-ordered, submicrometre-sized superstructures might ultimately facilitate the design of three-dimensional photonic materials for applications in sensing.
 J. F. Galisteo-López, et al., Adv. Mater. 2011, 23, 30.
 Z. Quan, et al.. Nano Today 2010, 5, 390.
 C. Avci, et al., Nat. Chem. 2017, 10, 78.
PM01.05: Micro/Nano-Architected Materials III
Tuesday PM, November 27, 2018
Hynes, Level 1, Room 102
1:30 PM - PM01.05.01
Robust Architected Materials—Deformation and Failure of Microlattices Under Cyclic Loading
Karlsruhe Institute of Technology1Show Abstract
Cellular materials with designed nanoarchitectures have redefined the limits of the accessible material-property space throughout different disciplines. Having characteristic features in the micro to nanometer regime, such architected materials exhibit exceptional mechanical properties at low density, including ultrahigh strength, damage tolerance, and stiffness. However, the design of lightweight structures that are both strong and damage tolerant is still a challenge and requires a better understanding of cyclic deformation and failure of the strength-optimized structures. Not only the absorbed energy during one loading cycle is important, but also the ability of the lattices to recover, maintain the strength, and be able to absorb energy during additional cycles.
The highest resolution in manufacturing is currently achieved by 3D direct laser writing, which enables the fabrication of highly customized polymeric microlattices. We then apply additional processing steps, such as additional heat treatments and ceramic or metal coatings, to further improve or tailor the effective mechanical properties. While a ceramic coating in general results in enhanced strength, the failure mechanisms depend on the coating thickness. Brittle fracture of the struts rather than buckling is observed, when comparing the effect of 10 nm and 100 nm thick alumina coatings. However, in addition to high strength our solid-beam microlattices exhibit significant energy dissipation during cyclic loading experiments. Overall, the energy dissipation is a function of progressively failing ligaments, while upon multiple loading cycles to the same strain stable cyclic behavior is approached. In situ experiments showed that post-yield softening was induced by plastic buckling and crushing of individual ligaments. The contributions of the different deformation and failure mechanisms to the specific dissipated energy were quantified through progressive cyclic loading of the microlattices.
2:00 PM - PM01.05.02
Architected Interfaces with Enhanced Fracture Toughness
Kevin Turner1,Simon Heide-Jørgensen2,Sumukh Pande1,Michal Budzik2
University of Pennsylvania1,Aarhus University2Show Abstract
Additive manufacturing has opened new opportunities for engineering the mechanical properties of materials via geometry. While there have been significant efforts in the architected materials field to exploit geometry to realize materials with unique properties such as high specific stiffness and the ability to recover from large deformations, the engineering of fracture properties and crack growth resistance via geometry is less studied. Here, we use a combination of experiments and computational modeling to investigate the role of geometric structuring on the toughness of interfaces containing arrays of pillars. Specifically, we establish quantitative relationships between the effective interface toughness and the pillar geometry, pillar arrangement, and mechanical properties of the constituent materials. Analytical and finite element fracture mechanics models are used to establish quantitative relationships between toughness and geometry and to demonstrate how compliance introduced by structuring can reduce the strain energy release rate at crack tip, resulting in higher effective toughness. Toughness of architected interfaces made via laser patterning as well as additive manufacturing were measured using double cantilever beam experiments. Measurements on a relatively brittle acrylic (PMMA) and higher toughness ABS structured specimens were performed. The experimental results are in agreement with model predictions. A key and significant finding of this work is that less material can lead to higher toughness when the geometry of the interface is designed based on an understanding of the mechanics. The experiments, mechanics modeling, and an overall design strategy for using geometry to enhance interface toughness will be presented.
2:15 PM - PM01.05.03
Control of Interfacial Crack Behavior via Architected Cellular Materials in 3D Printed Structures
Chengyang Mo1,Jordan Raney1
University of Pennsylvania1Show Abstract
Natural materials, with their remarkable fracture toughness and low density, have long been a source of inspiration to engineers. In recent years, researchers have made great strides in cataloging the specific mechanisms by which these properties are attained. These mechanisms arise from the very complex microstructures that are observed in natural materials, including mechanisms that arise due to internal geometric parameters (e.g., structural hierarchy, interface density, interface angle, etc.) and due to material parameters (e.g., interfacial adhesion between two distinct material types). It is often difficult to isolate the respective contributions, particularly in an experimental context, where it is difficult to precisely and uniformly control the interfacial properties between two materials along an internal interface. As one example, the Conch Shell has three different levels of hierarchy to prevent shell fracturing under puncture loading allowing dissipation of a large amount of energy by crack diversion, with characteristic hard calcium carbonate sheets separated by soft protein layers where initial cracks form during loading. The goal of this research is to separate the effects of geometry from that of the material interface, and to thereby determine how toughness can be maximized purely through a structure’s internal geometry. To do this, we fabricate structures (from a single material) that contain internal lattices, which act as weak interfaces due to their decreased relative density. Rather than relying on a material interface, internal cellular structures act as crack arresting regions with variable effectiveness based on such properties as relative density and lattice shape. We experimentally characterize fracture behavior, measuring properties such as fracture toughness, energy dissipated, and crack path in 3D printed single edged notched bend samples (SENB). We study geometric parameters including defect region orientation, lattice density, and lattice shape for crack diversion to occur and maximize energy dissipation. The effect of lattice parameters to critical defect region orientation is studied analytically and compared with experiments. Numerical simulation is also implemented to better predict the critical angle taking account of differences in fracture toughness of cellular materials under different modes.
2:30 PM - PM01.05.04
Selective Laser Melting of Ti6Al4V Lattice Structures with Hollow Struts—Processability, Mechanical Behaviour and In Vitro Response
Ezgi Onal1,Wencheng Liu1,Bernard Chen1,Jessica Frith1,Xinhua Wu1,Andrey Molotnikov1
Monash University1Show Abstract
Advances in additive manufacturing are enabling fabrication of multitude of complex cellular structures for numerous applications in aerospace and biomedical fields. Among periodic lattice structures hollow lattice structures are gaining popularity due to their improved mechanical properties, such as higher flexural and inelastic buckling strength  and higher compressive strength due to greater resistance to plastic buckling when compared to the equivalent solid lattice structures . Furthermore, additional channels provided by hollow struts provide ample space for tissue in-growth and cell colonization. The latter aspect is of a particular interest since our recent study demonstrated , that pore size and distribution significantly affect the cell migration in the lattice scaffolds.
In this work, we explore the potential use of hollow lattices for bone implant scaffolds. We have designed and fabricated Ti6Al4V hollow body centered cubic (BCC) lattice structures with different hollow thicknesses and unit cell pore sizes using selective laser melting process. We also included additional hollow z-struts at the nodes and centres of each BCC unit cell to study the effect of vertical channels. The mechanical properties and failure behaviour of the obtained scaffolds are investigated using compression testing. Furthermore, in-vitro studies with pre-osteoblast cells are conducted to study cell migration and colonization behaviour in channels and hollow struts. This study explores the use of novel lattice structure designs for orthopaedic implants and contributes to understanding of mechanical and biological response of hollow architectured materials.
 Wang Y., Jing S., Liu Y., Song G., Qie L., Xing H., Advances in Mechanical Engineering 2018, 10(3), 1–12
 Queheillalt D.T., Wadley H.N.G., Materials Science and Engineering A 2005, 397(1), 132-137
 Onal E., Frith J.E., Jurg M., Wu X., Molotnikov A., Metals 2018, 8(4), 200
2:45 PM - PM01.05.05
Collective Magnetic Behavior and Domain Formation in Magnetite Nanoparticle Assemblies
Matthias Elm1,Nils Neugebauer1,Alexander Fabian1,Michael Czerner1,Christian Heiliger1,Peter Klar1
Justus-Liebig-Universität Gießen1Show Abstract
Ferromagnetic nanostructures, such as arrays of magnetic nanodots, are of high interest for applications on the field of high density storage media, non-volatile logic or spintronic devices, as the magnetic properties of the arrays can be tuned by their shape and their arrangement. An alternative approach to patterning magnetic thin films is the fabrication of arrangements of ferromagnetic nanoparticles. Such low dimensional assemblies show collective magnetic behavior due to dipolar coupling between the nanoparticles, which offers an additional degree of freedom in manipulating the magnetic interactions inside the magnetic elements. However, in order to tune the magnetic interactions of the assemblies a detailed understanding of the magnetic coupling phenomena is necessary.
Here we present the investigation of the magnetic properties of magnetite nanoparticle assemblies using angle-dependent ferromagnetic resonance measurements . Nanoparticle assemblies of different shape were prepared using the meniscus force deposition method. For this, electron beam lithography was used to prepare PMMA openings. In a horizontal dip-coating process the nanoparticles with an average diameter of 20 nm are arranged in these openings by self-assembly. To investigate the influence of the shape on the magnetic interactions between the particles, rectangular assemblies with different aspect ratio varying from 1:1 to 1:1000 were prepared. The angle-dependence of the resonance was described using the Smit-Suhl formalism. For assemblies with aspect ratios below 1:10 the demagnetization factors obtained are in good agreement with the values expected from the aspect ratio revealing a single magnetic domain behavior due to dipolar coupling. For larger aspect ratios the ratio of the demagnetization factors is much smaller than the expectation indicating the formation of a magnetic domain structure inside the assemblies. This assumption is further supported by magnetic simulations of the magnetic structure and probably arises due to inhomogeneities in the filling of the openings. Furthermore, magnetic coupling between the arrangements was investigated by varying the distance between circular shaped arrangements. Due to their shape, isolated arrangements reveal an isotropic behavior of the angle-dependence of the resonance field in in-plane geometry. However, below a critical distance coupling between the arrangements occurs which is manifested by an angular dependence of the resonance field.
 Alexander Fabian, Matthias T. Elm, Detlev M. Hofmann and Peter J. Klar, J. Appl. Phys. 121, 224303 (2017).
3:30 PM - PM01.05.06
Nano to Macro Architectured Materials and Structures—Processing Challenges and Opportunities
University of Virginia1Show Abstract
The development of light metals and ceramics with internal topologies that offer novel combinations of properties has been greatly accelerated by the recent development of additive methods for their fabrication. These enable the fabrication of complex shaped structures containing internal features with length scales that range from a few tens of nanometers to the centimeter scale. However, each of the processing methods suffer from various deficiencies such as a limited materials palate, difficult to control internal surface roughness, the presence of residual stresses and distortions of structure, and properties of the solid material that are sometimes inferior to those of conventionally processed counterparts. As a result, innovative applications of traditional fabrication/processing methods can still compete with additive approaches in many areas. Here three approaches for making microarchitectured materials are described and the challenges confronting their further development discussed. At the centimeter scale, we describe the use of electron beam melting approaches for making Ti-6Al-4V octet lattice structures and show how surface roughness of the trusses impact mechanical performance, especially as the strut diameter decreases towards 1 mm. This approach is then compared with a perforated sheet folding/braze bonding approach for making stainless steel octet lattices with sub-millimeter diameter trusses. In this case, surface roughness is not the challenge, instead the precision of the perforation and sheet folding processes together with the knock down in braze region properties limit the smallest cell size that can be achieved. To reach cell sizes in the micron to tens of nanometer range, we have begun to investigate the use of template electroplating and related micro - particle space holding based methods. While this approach holds great promise for making nano-architectured materials, the challenges of scaling the approach to meter scale structures still remains.
4:00 PM - PM01.05.07
Additive Manufacturing of 3D Nano-Architected Metals
Andrey Vyatskikh1,Stephane Delalande2,Akira Kudo1,Xuan Zhang3,Carlos Portela1,Julia Greer1
California Institute of Technology1,PSA Group2,Tsinghua University3Show Abstract
Most existing methods for additive manufacturing (AM) of metals are inherently limited to feature sizes of 20-50 μm, which renders them inapplicable for generating complex 3D metallic structures with smaller dimensions. We demonstrate a lithography-based process to create complex 3D nano-architected metals with ~100 nm resolution. We synthesize hybrid organic-inorganic materials that contain Ni clusters and use them to produce a metal-rich photoresist. We use two-photon lithography to sculpt the computer-designed architectures out of the resist and pyrolyze them to volatilize the organic constituents, which results in a > 90wt% metal architecture. Using this approach, we demonstrate the fabrication of Ni octet nanolattices with a unit cell size of 2 μm, and beam diameters of ~300 nm. TEM analysis reveals that the microstructure of Ni beams in the lattice is nanocrystalline and nanoporous, with a 20 nm mean grain size and 10-30% porosity within each beam. In-situ nanocompression experiments show the specific strength of as-fabricated octet nanolattices to be 2.1-7.2 MPa g-1 cm3, which is comparable to that of metal lattices with 0.1-1.0 mm beam diameters fabricated using alternative metal AM technologies. These findings suggest an efficient pathway to create complex three-dimensional metallic structures with nano-scale resolution.
4:15 PM - PM01.05.08
Alignment of Barium Titanate Platelets for Textured 3D Piezoelectric Architectures Using Direct Writing
Rebecca Walton1,Richard Meyer1,Gary Messing1
The Pennsylvania State University1Show Abstract
Direct writing textured piezoelectric ceramics can allow for increased flexibility of 3D designs for a variety of applications through greater geometric possibilities, as well as novel orientations of crystallographic alignment. Generally, direct writing of ceramics and ceramic composites involves the extrusion and deposition of a ceramic containing paste which is then gelled to maintain its shape. Some studies on the alignment of platelets using a circular nozzle have explored the densification of textured ceramic as a function of position in the filament cross-section and length of the deposition nozzle. This work will highlight the fabrication of textured Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramic designs with crystallographic orientations that are difficult to fabricate with traditional forming techniques. The effect of shear field shape and duration and ceramic paste rheology on the alignment of barium titanate templates and the fidelity of the printed geometries will be explored. Increasing the magnitude of the shear field in the nozzle during direct writing and controlling the rheology of the ceramic paste via pH control of the aqueous phase allows for the fabrication of dense, textured Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics with varied geometries and alignment directions.
4:30 PM - PM01.05.09
Small Scale Materials with Tunable Impact Resistance Based on Electrospun Fibers via Integrated Additive Manufacturing Methodologies
Arief Budiman1,Avinash Baji2
Singapore University of Technology & Design1,La Trobe University2Show Abstract
We used additive manufacturing methodologies to fabricate impact resistant composites. Similar to natural materials, such as mantis shrimp dactyl club, these lightweight synthetic analogues display superior structural properties and functionalities – such as fracture toughness and impact resistance. The present study is based on the hypothesis that synthetic impact resistant materials with extraordinary fracture toughness can be obtained and tuned by the 3D hierarchical micro-architectured molecular structures – often enabled with nanoscale sub-structures – and that such 3D structures and mechanical properties could be enabled by additive manufacturing approach, especially with an integrated electrospinning technique that combine multi-length scale fabrication capabilities. Further, more specifically, this study investigated the fracture toughness and impact resistance properties of the 3D helicoidal architecture inspired by the mantis shrimp dactyl club and elucidate the associated deformation mechanisms. Having layers of multiscale fibers with rotated alignments in each layer, the 3D helicoidal architecture could potentially lead to very efficient (high rate) as well as highly tunable damage dissipation mechanism. We further explore the effect of 3D hierarchy micro-architectured structures and the possible size effects of the substructures on the fracture toughness and impact resistance properties of the resulting composite materials as well as the associated damage mechanisms. In this study, we fabricated structures by controlling the orientation of the polymer fibers within the composites. This is enabled by recent advances in additive manufacturing technologies. We plan to use electrospinning further as well as a novel integrated approach involving melt near-field electrospinning (NFES) technique and another deposition method called the fused deposition modeling (FDM). The 3D architecture would enable the composites to absorb mechanical energy before they fracture. Ultimately this high performance, impact-resistant composite technology will have other widespread technologically important applications, such as in aerospace, space, military/defense to automotive and biomedical/prosthetics. We also aim to enable design of such advanced composite materials through creation of new manufacturing technologies for micrometer and nanoscale materials and for their assembly and integration at larger scale.
4:45 PM - PM01.05.10
Complete Space Filling Cellular Network of Graphene Bubbles for Ultralight, Strong and Superelastic Materials
Seon Ju Yeo1,Min Jun Oh2,Hyun Min Jun2,Minhwan Lee3,Jung Gun Bae3,Yeseul Kim2,Kyung Jin Park2,Seungwoo Lee2,Daeyeon Lee4,Byung Mook Weon2,Won Bo Lee3,S. Joon Kwon1,Pil J. Yoo2
Korea Institute of Science and Technology1,Sungkyunkwan University2,Seoul National University3,University of Pennsylvania4Show Abstract
Advanced materials with low density and high strength will have transformative impacts in construction, aerospace and automobile industries. These materials can be realized with assembling well-designed modular building units into interconnected structures. This study uses a hierarchical design strategy to demonstrate a new class of carbon-based, ultralight, strong, and superelastic closed-cellular network structures. In contrast to conventional top-down approaches such as three-dimensional (3D) printing, the building units are prepared by a multi-scale design approach starting from the controlled synthesis of functionalized graphene oxide nanosheets at the molecular- and nano-scale, leading to the microfluidic fabrication of solid-shelled bubbles with shape diversity at the micro-scale. Then, bubbles are strategically assembled into meso-scale 3D structures. Subsequently, these structures are transformed into self-interconnected and structurally-reinforced closed-cellular network structures through post-treatment, leading to the generation of a 3D graphene lattices with rhombic dodecahedral honeycomb structure at the centimeter-scale. The 3D graphene suprastructure simultaneously exhibits the Young’s modulus above 300 kPa while retaining a light density of 7.7 mg/cm3 and sustaining the elasticity against up to 87% of the compressive strain benefited from efficient stress dissipation through the complete space-filling closed-cellular network. The fabricated 3D graphene closed-cellular structure opens a new pathway for designing lightweight, strong, and superelastic materials.
Lorenzo Valdevit, University of California, Irvine
Katia Bertoldi, Harvard University
Tobias Schaedler, HRL Laboratories, LLC
Martin Wegener, Karlsruhe Institute of Technology
PM01.06: Modeling/Design Strategies
Wednesday AM, November 28, 2018
Hynes, Level 1, Room 102
8:00 AM - PM01.06.01
Exploring Vast Design Spaces—Computational Optimization Methods for Additive Manufactured Lattice Structures
ETH Zurich1Show Abstract
Recent advances in Additive Manufacturing (AM) enable the fabrication of complex 3D lattice structures at many scales and with many materials. Through computational design optimization methods for AM lattice structures, it is possible to control the distribution of materials and structure within objects with a high degree of precision. This has the potential to dramatically improve structural performance and even enable new functionalities that can not be realized with conventional manufacturing. However, this is highly challenging for designers due to the vast design space of new possible shapes, materials and structures and often unknown AM fabrication constraints.
To address these challenges, a Design for Additive Manufacturing (DfAM) framework is developed that starts with AM material and process characterization from which quantitative models are created. These models are then used within design optimization methods for discrete lattice structures considering design variables of material, member size, shape and topology. Two methods are focused on including a Generalized Optimality Criteria (OC) method for multi-material optimization and optimization for anisotropy considering displacement, local yield stress and Euler buckling constraints. A Sequential Linear Programming (SLP) method is also discussed for the efficient optimization of member size and shape of truss lattice structures considering local yield stress and Euler buckling constraints. While the methods are process and material independent, results are shown for the characterized materials of the multi-material polyjet printing process. The results highlight the necessity of including AM fabrication constraints and tested material properties in the optimization process and the tight coupling needed among material understanding, the AM process and optimization methods to achieve optimized results.
8:30 AM - PM01.06.02
Three-Dimensional Multiscale Design Using Neural Net Surrogate Models of Lattice Material Response
Seth Watts1,Bill Arrighi1,Jun Kudo1,Dan Tortorelli1,Dan White1
Lawrence Livermore National Laboratory1Show Abstract
Topology optimization is a methodology for assigning material or void to each point in a fixed design domain in a way that extremizes some objective function, such as the stiffness of a structure under given loads, subject to various imposed constraints, such as an upper bound on the mass of the structure. This generative design approach has been used to design both macro-structures, such as bridges, as well as lattice micro-structures, comparable to the familiar octet truss architecture. In both settings, the structural response is evaluated with the continuum finite element method, and a design sensitivity analysis provides the first-order derivatives of the objective and constraint functions to a gradient-based optimization algorithm, which iteratively updates the design to improve its performance.
Even with today’s supercomputers, it is generally not feasible to design both the macrostructure and microstructure simultaneously on the same computational mesh; the necessary spatial resolution and domain size require too great a computational cost. Thus, multiscale topology optimization methods use some form of information mapping between the scales, enabling separate, smaller analyses at each scale. Homogenization is the most common mapping in the linear elastic regime we are interested in. Except in the case of certain specialized microstructures, e.g. ranked laminates, homogenization still requires solution of the elasticity PDE for several assumed test strains for each different microstructural design, and thus the overall design problem can still be quite costly, especially in three dimensions.
Our solution to reduce the overall computational cost of the multiscale topology optimization design problem is to generate neural net surrogate models of the homogenized microstructural response. Our particular interest is in open truss micro-architectures, which can be described in a very low-dimensional manner as a union of rods with given endpoint locations and cross-sectional diameters. Fixing the endpoint locations within a unit cell reduces the dimensionality further and guarantees a priori structural continuity from one unit cell to another. The off-line expense of creating the surrogate models is amortized over the solution of many design problems.
We generate a separate model for each of the 21 independent components of the homogenized elasticity tensor, as well as the volume fraction occupied by solid material (the balance is void space). These surrogate models are very fast to evaluate, enabling efficient design of the macrostructure while retaining accurate microstructural response, and additionally enable recovery of the full microstructure when the design is complete. We have generated surrogate models for a number of open truss micro-architectures, including the octet truss, ORC truss, and isotruss. We demonstrate our multiscale design capability via minimal-compliance designs on domains with millions of macro elements.
8:45 AM - PM01.06.03
Closed-Cell Architected Materials at the Hashin-Shtrikman Upper Bound
Cameron Crook1,Jens Bauer1,Lorenzo Valdevit1
University of California, Irvine1Show Abstract
Over the past decade, many beam-based micro- and nanolattice materials with exceptional mechanical properties have been reported. However, no beam-based lattice capable of achieving the Hashin-Shtrikman (HS) upper bounds for isotropic elastic strength or stiffness has been shown. Honeycombs, though mechanically efficient, are generally used in corrugate structures which make them highly anisotropic. This is unideal in large-scale materials and engineering design. Very recently, specific close-cell, shell-based designs were numerically predicted to achieve the HS bounds for stiffness, but no experimental verification has been produced to date.
Here, we experimentally demonstrate closed-cell cube-octet micro- and nano-shell-based architected materials achieving the Hashin-Shtrikman upper bounds for both isotropic elastic stiffness and strength. Cube-octet materials with relative densities of 20-60% were fabricated by two-photon polymerization Direct Laser Writing (DLW) using a Nanoscribe Photonic Professional GT. These structures were in-situ compression tested to failure. Furthermore, a second batch of structures was pyrolyzed, converting the polymeric material to glassy carbon. Approaching the “ultimate” HS strength upper bound, which corresponds to an optimal topology combined with a constituent material achieving the theoretical material strength, these glassy carbon architected materials are shown to outperform all known beam-based micro- and nanolattice materials in specific strength.
9:00 AM - PM01.06.04
The Representative Cellular Element (RCE) Method – Model, Implementation and Validation with Additive Manufacturing
Arizona State University1Show Abstract
Material modeling approaches for cellular materials broadly fall into three different categories, depending on the level of discretization at which the property of interest is modeled. In order of the scale at which material behavior is represented, these models can operate at either the level of the material point (bulk property models), the connecting member (member models) or finally, at the level of the cell (homogenization models). Bulk property models are often not representative of the microstructure and surface effects at the scale of the members that constitute cellular structures in Additive Manufacturing. Member models are challenging to characterize due to the small sizes of the structures involved and the specimen geometries do not always accurately capture behavior of the cellular material. Homogenization models, while efficient to implement computationally, are shape and size dependent and are unlikely to represent irregular and stochastic cell shapes and distributions accurately.
In this work, being presented for the first time, we propose a Representative Cellular Element (RCE) method for modeling cellular materials. Our approach follows from the concept of the Representative Volume Element (RVE) developed for heterogeneous materials. The RCE method essentially involves the identification of a structural element that represents the cellular material for the property of interest. For the model to be both valid and accurate, we show that it must represent the cellular material in three critical ways: geometry, processing history and the physics of its deformation and failure. We assess the performance of the RCE model against prismatic cellular structures (honeycombs) in a wide range of conditions: different shapes (regular, graded and stochastic), Additive Manufacturing processes and materials (ABS with Fused Deposition Modeling, Inconel 718 with Laser Powder Bed Fusion, and Ti6Al4V with Electron Beam Melting) and strain rates (ranging from 100 to 10-6 s-1). The model is also evaluated against a multi-material hexagonal honeycomb made with a nylon composite and Kevlar continuous fiber. In each case, we examine the model's predictability of the nonlinear stress-strain response in the elastic and plastic regimes. Three different design strategies for the RCE are compared under these conditions to identify the ones that yield the best results. Challenges in implementation and limitations of the RCE approach are also discussed.
9:15 AM - PM01.06.05
Multi-scale Geometric Design Principles Applied to 3D Printed Architected Materials
Seyed Mohammad Sajadi1,Peter Owuor1,Cristiano Woellner2,Varlei Rodrigues2,Robert Vajtai1,Jun Lou1,Douglas Galvao2,Chandra Tiwary1,P. M. Ajayan1
Rice University1,State University of Campinas2Show Abstract
The emergence of 3D printing has enabled scientists to innovate complex geometrical designs in materials which were unattainable using conventional synthesis methods. The topological material design is becoming a common occurrence aided by 3D printing. Here we use inverse methods (function-to-structure) to design multifunctioanl material. This work reports 3D porous structures with negative Gaussian curvatures, which forms a rigid foam-like structure with unusual mechanical and electronic properties. The mechanical behavior of these structures across different length scales is investigated after these geometries are 3D printed at centimeter length scales based on molecular models. Molecular dynamics and fiite elements simulations are used to gain further understanding on responses of these complex solids under compressive loads and kinetic impact experiments. The results show that these structures hold great promise as high load bearing and impact-resistant materials due to a unique layered deformation mechanism that emerges in these architectures during loading. Easily scalable techniques such as 3D printing can be used for exploring mechanical behavior of various predicted complex geometrical shapes to build innovative engineered materials with tunable properties.
10:00 AM - PM01.06.06
When Trusses Become Continua—Predicting and Optimizing the Mechanics of Truss-Based Metamaterials
Greg Phlipot2,Dennis Kochmann1,2,Raphael Glaesener1,Bastian Telgen1,Claire Lestringant1
ETH Zurich1,California Institute of Technology2Show Abstract
Truss networks have attracted significant attention as metamaterials whose mechanical, optical, thermal or acoustic properties can be controlled by the truss architecture (including the network topology, truss geometry and base materials). Enabled by advances in additive manufacturing, large arrays of periodic, hierarchical or functionally graded truss unit cells have been assembled at the micro- to nanoscales, producing new ultra-lightweight cellular solids. With enhanced experimental capabilities comes the need for new theoretical and computational tools to describe and predict the performance of truss networks containing millions and more of individual struts and junctions, where classical approaches such as direct numerical calculations incur prohibitive computational expenses. We show that such truss networks can be described efficiently by advanced constitutive models and numerical tools that replace the discrete truss network by an equivalent continuum. Our formulation captures nonlinear and inelastic material behavior as well as finite deformations. We discuss the theoretical and numerical techniques (involving nonlocal homogenization based on both translational and rotational degrees of freedom), and we present representative examples of truss lattices undergoing not only homogenous deformation but also instabilities such as buckling and shear banding. Finally, we combine the methodology with topology optimization to predict optimal cellular networks with spatially varying truss architecture. Unlike classical two-scale optimization, this approach results in compatible non-uniform truss architectures that can readily be fabricated by methods of additive manufacturing and offer opportunities for lightweight metamaterials with optimized mechanical properties.
10:30 AM -
10:45 AM - PM01.06.08
Fracture Toughness of Truss and Shell-Based Architected Materials
Meng-Ting Hsieh1,Lorenzo Valdevit1
University of California, Irvine1Show Abstract
The fracture toughness of brittle 2D lattices, such as triangular and kagome lattices, has long been investigated using elastic K-field approaches. However, much less is known about the fracture toughness of 3D cellular materials, particularly those built from ductile components. Here, we numerically investigate the linear elastic and elasto-plastic fracture toughness of octet lattices and spinodal shell-based architected materials. We address the issues of mesh dependence where stress singularity arises by incorporating a characteristic element size. To extract R-curves for cellular materials, local damage is modeled using the Johnson-Cook criterion and fracture toughness is obtained by the J-integral compliance with SENB (single-edge notched bend) specimens. We compare the performance of truss and shell-based architected materials in terms of scaling laws and R-curve behavior, and extract design principles. Numerical results are verified by fracture toughness experiments.
11:00 AM - PM01.06.09
On the Correlation Between Topology and Elastic Properties of Imperfect Truss-Lattice Materials
Panos Pantidis1,Andrew Gross2,Katia Bertoldi2,Simos Gerasimidis1
University of Massachusetts, Amherst1,Harvard University2Show Abstract
The pursuit of new materials with properties superior to the current state of the art, has led many investigators to examine the behavior of materials with a truss-lattice microstructure which accommodates member sizes in the range of micro- and nanometers. Aided by the immense progress of 3D additive manufacturing techniques, such as self-assembly (bottom-up techniques), material scientists have been enabled to fabricate novel materials with complex architectures which can attain unique, unprecedented and tunable properties. However, defects of various forms and concentrations are unavoidable in any fabrication process, and it is anticipated that the application of self-assembly techniques to larger three-dimensional volumes will increase the concentration of defects. Of particular interest for the self-assembly of truss-lattice materials is the influence of struts that are missing from the network, in various defect forms such as missing blocks (clusters) or randomly missing members. In this study, the dependence of the elastic properties on the concentration and distribution of missing struts is investigated for several three-dimensional lattice-truss materials of varying coordination number. This work constitutes a systematic experimental and numerical approach to examine and identify the mechanical elastic regime of defected architected metamaterials. The experimental part of this project is conducted with a two-photon lithography approach, an advanced additive manufacturing technique capable of printing struts with sub-micron cross-sectional dimensions, while the numerical part utilizes finite element simulations accounting for the randomness of the damage spatial distribution through exhaustive Monte Carlo simulations. Focusing on a variety of elastic mechanical properties (Young’s, bulk and shear modulus), their evolution is monitored as the total defect percentage increases in magnitude, providing a comprehensive picture of the defected architected metamaterials elastic property space. Finally, this work thoroughly explores the connection between defected truss-lattices and well-established homogenization techniques for composite mediums, elaborating on the applicability of the latter methods to accurately describe the response of defected lattice-based materials.
11:15 AM -
11:30 AM - PM01.06.11
Analysis of a Three-Dimensional Spider Web Architecture
Isabelle Su1,Zhao Qin1,Tomás Saraceno2,Markus Buehler1
Massachusetts Institute of Technology1,Studio Tomás Saraceno2Show Abstract
Spiders are abundant in most ecosystems in nature, making up more than 47,000 species. This ecological success is due to the web architectures and the exceptional mechanical properties of spider silk. Silk’s combination of strength, elasticity, toughness, and robustness originates from its hierarchical structure and has been a template for high-performance material design. In particular, spiders have optimized and adapted their web architecture to survive in their environment.
The most studied and familiar spider web is the 2D orb web which is composed of radial and spiral threads. However, 3D webs, such as sheet, funnel, or cob webs, are more common in nature. In contrast to 2D webs, where the spider is vulnerable to attacks, 3D webs surround the spider and offer a defensive advantage by warning the spider of intruders, blocking its predators and entangling prey.
Here, we investigate the architecture and mechanical properties of a Cyrtophora citricola 3D web. For the first time, we build a model of the 3D spider web generated through automatic laser scanning. The web is scanned by taking a high resolution picture of slices of the web illuminated by a sliding sheet laser. Using image processing algorithms, we construct a 3D fiber network. We study the response of a realistic web structure to mechanical loads using a coarse-grained bead-spring models based on the network model created through scanning and image processing.
Using this new method to trace the fiber network, we can study the connection between material and performance of numerous 3D spider webs. Understanding the roles of structure and material in the functionality and evolutionary fitness of spider webs could lead to innovative 3D spider web-inspired structures such as high performance light-weight long-span structures or fiber reinforced composite materials.
11:45 AM - PM01.06.12
Harnessing Design Principles from Glass Sponges for Structurally Robust Lattices
Matheus Fernandes1,James Weaver2,1,Katia Bertoldi1,2
Harvard University1,Wyss Institute2Show Abstract
Glass sponges are predominately deep sea sponges that live in ocean depths of 100-2000m. Beyond their fracture propagation inhibiting material composition, these sponges are perceived to exhibit large structural rigidity and strength against buckling. Since these sponges are primarily made of ’brittle silica’, buckling strength may be a crucial property in making them resistant to impact and environmentally applied stresses. Structurally, they exhibit a base square-grid architecture and regular ordering of vertical and horizontal struts that form the skeletal system. Furthermore, their base structure is overlaid with double diagonal reinforcement struts, which create a checkerboard-like pattern of open-closed cell structure. This diagonal reinforcement design is conjectured to give the sponge greater buckling resistance and strength to localized damage then it would experience having a single diagonal reinforcement strut (while allocating the same amount of mass to the diagonal reinforcement.) Analogous to the sponge, many engineering structures, such as buildings and bridges, exhibit diagonal reinforcement struts as a stability mechanism. Based on this similarity, we explore the following research question: Can we generate design principles for diagonal reinforcements of square beam lattices that are optimally designed to avoid global structural buckling? Here, we present a numerical analysis of the structure deformation under various loading conditions as well as survey different arrangements within similar design space of the sponge. Furthermore, we present experimental evidence that supports our numerical analysis.Through the various design iterations we look for the critical buckling strain and the elastic load caring capacity. Finally, we compare the results from the sponge design to what is typically used in engineering of structures such as buildings and bridges.
PM01.07: Acoustic Metamaterials
Wednesday PM, November 28, 2018
Hynes, Level 1, Room 102
1:30 PM - PM01.07.01
Advancing Acoustic Application with Architectured Metamaterials
Massachusetts Institute of Technology1Show Abstract
Today, sound is an indispensable component in numerous industrial and consumer products, such as musical instruments, cars, building technology, medical diagnostics, and many others. Acoustic characteristics are among their most important properties, greatly influencing their function and our society at large. Recent development of acoustic metamaterials opens a door to an unprecedented large design space for acoustic properties such as negative bulk modulus, negative density, and refractive index. These novel concept expands paves the way for the design of a new class of acoustic materials and devices with great promise for diverse applications, such as broadband noise insulation, sub-wavelength imaging and acoustic cloak from sonar detection.
In this invited talk, I will present our development of advanced design and micro/nanofabrication techniques, to enable exploration architectured meta structures for acoustic waves. These structures show promise on focusing and rerouting ultrasound through broadband metamaterials. As example, our study on the sound absorption of thin composite aerogel foams using a bimodal porous structure predicts a possible route to perfect thin film absorber by increasing the amount of epoxy resin. In a second case, stimuli responsive acoustic metamaterials are demonstrated to be able to extend the 2D phase space to 3D through rapidly and repeatedly switching signs of constitutive parameters with remote magnetic fields. Lastly I will report our study on a prototype hydraulic hydrogel transducers with excellent optical and sonic transparency.
2:00 PM - PM01.07.02
Design of a Resonant Laser Beam Scanner Based on a Topologically Protected Twist Edge State
Julian Köpfler1,Tobias Frenzel1,Muamer Kadic1,2,Jörg Schmalian1,Martin Wegener1
Karlsruhe Institute of Technology1,Université de Bourgogne Franche-Comté2Show Abstract
The concept of band gaps in periodic mechanical systems allows, for example, the design of waveguides and cavities. However, the corresponding modes are often fairly sensitive with respect to perturbations. Therefore, the more recent idea of topologically protected boundary modes has provided a new twist. Aiming at applications as one-way waveguide architectures, recent work [1,2] designed and realized mechanical structures exhibiting topological two-dimensional (2D) band gaps. 1D topological band gaps exhibiting longitudinal and bending interface modes were achieved using a beam composed of a 1D periodic arrangement of elastic unit cells .
In this paper, we present an application of topological mechanical band gaps that specifically takes advantage of chirality. We design a 1D chain of two different alternating 3D elastic chiral unit cells . The individual unit cells have been inspired by our previous work . The chain’s topological band gap, a result of the alternation of unit cells combined with their chirality, guarantees a protected edge state at one end of the beam, corresponding to a resonant localized twist mode. This mode can be excited by an axial motion at the other end of the beam, via evanescent modes in the gap. The topological robustness of the edge state allows us to add a micro-mirror, turning the arrangement into a resonant laser beam scanner with scalable operation frequency and adjustable quality factor.
Our work starts from calculations based on a simplified 1D mass-spring model, which we solve analytically. The system resembles two Su-Schrieffer-Heeger (SSH) models , one for the longitudinal displacement and one for the twist, which are chirally coupled via additional springs. We justify this model by the fact that the longitudinal (or pressure) and the twist mode on the one hand, and the two transverse (or shear) modes on the other hand, live in orthogonal subspaces according to micropolar continuum theory for chiral media. Within this model, the topological protection is due to a combination of time-reversal and a mirror symmetry, generalizing the classification of mechanical metamaterials . The results of the model are verified by numerical finite-element calculations for the complete 3D microstructures. These microstructures should be amenable to laser nanoprinting.
 R. Süsstrunk and S. D. Huber, Science 349, 47 (2015)
 R. Süsstrunk and S. D. Huber, Proc. Natl. Acad. Sci. U.S.A. 113, E4767 (2016)
 J. Yin, M. Ruzzene, J. Wen, D. Yu, L. Cai, and L. Yue, Sci. Rep. 8, 6806 (2018)
 J. Köpfler et al., unpublished
 T. Frenzel, M. Kadic, and M. Wegener, Science 358, 1072 (2017)
 W. P. Su, J. R. Schrieffer, and A. J. Heeger, Phys. Rev. Lett. 42, 1698 (1979)
2:15 PM - PM01.07.03
Dynamically Tunable Topological States in Soft Elastic Metamaterials
Jianfeng Zang1,Shuaifeng Li1
Huazhong University of Science & Technology1Show Abstract
Topology describes the properties of space under continuous deformation in mathematics. The concept has been used to explain band structures in condensed matter physics, resulting in the theoretical predication and experimental observation of topological insulator in electronic system, and recently also in photonic and phononic systems. Topological elastic metamaterials offer insight into classic motion law and open up opportunities in quantum and classic information processing. Theoretical modeling and numerical simulation of elastic topological states have been reported, whereas the experimental observation remains relatively unexplored. Here we present an experimental observation and numerical simulation of tunable topological states in soft elastic metamaterials. The on-demand reversible switch in topological phase has been achieved by changing filling ratio, tension, and/or compression of the elastic metamaterials. By combining two elastic meta-materials with distinct topological invariants, we further demonstrate the formation and dynamic tunability of topological interface states by mechanical deformation, and the manipulation of elastic wave propagation. Moreover, we provide a topological phase diagram of elastic metamaterials under deformation. Our approach to dynamically control interface states in soft materials paves the way to various phononic systems involving thermal management and soft robotics requiring better use of energy.
3:30 PM - PM01.07.04
Non-Reciprocal Wave Phenomena via Mechanical Modulation of Discrete and Continuous Elastic Lattice Systems
Michael Haberman1,Samuel Wallen1,Benjamin Goldsberry1
The University of Texas at Austin1Show Abstract
Non-reciprocal acoustic and elastic wave propagation is of significant interest due to its potential to enable direction-dependent devices that augment mechanical wave sensing and transmitting capabilities. They also open up the possibility for the construction of novel materials or structures for isolation from vibration and impacts. However, non-reciprocity can only occur under very specific circumstances, many of which are very difficult to achieve in practice. This work considers mechanical modulation as a potential means to obtain non-reciprocal elastic wave propagation in architected materials. The specific case studied is the application of a slowly-varying, large amplitude, mechanical pump wave whose motion is orthogonal to the direction of wave propagation. This pump wave acts as a spatio-temporal modulation of the mechanical structure, resulting in time and space varying effective material properties and non-reciprocal elastic wave phenomena. The mechanical system of interest is modeled using both discrete and continuous mechanical lattices. A detailed analysis that quantifies the degree of non-reciprocity for excitation with finite bandwidth, as well as robustness to geometric variability associated with known additive manufacturing techniques, is provided and demonstrated via numerical examples.
4:00 PM - PM01.07.05
Two-Step Manufacturing of Ultrathin Acoustic Metasurfaces and Non-Planar Acoustic Metasurfaces
Hanchuan Tang1,Youzhou Yang1,Xuefeng Zhu1,Jianfeng Zang1
Huazhong University of Science & Technology1Show Abstract
Acoustic metasurfaces that can manipulate and control sound waves at two-dimensional subwavelength scales open new avenues to unusual applications, such as sound barriar, super-resolution imaging, and particle manipulation. However, the long-standing goals of pushing frontier metamaterials research into real practice are still severely constrained by cumbersome configuration and rigid structure of the existing metamaterials. Here we fabricate an ultra-thin metasurface (10-300 μm in thickness, up to ~λ/650, λ the wavelength) that is capable of imparting sound wave with a non-trivial phase shift with high transmittance (>80%) in the range of 5 kHz and 30 kHz by electrospinning method. Besides, we incorporate the traditional paper-cutting art to carve the ultrathin metasurface into hollow-out patterns, resulting in a variety of remarkable functions, including acoustic vortex, focusing, and super-resolution. Our hollow-out patterning approach innovates