Structural Metamaterials: Session III
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Additive Manufacturing Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Amy Wat, Lawrence Livermore National Laboratory; Brad Boyce, Sandia National Laboratories; Xiaoyu Zheng, University of California, Los Angeles; Fabrizio Scarpa, University of Bristol; Robert Ritchie, University of California, Berkeley

Tuesday 8:00 AM
March 1, 2022
Room: 304A
Location: Anaheim Convention Center

8:00 AM  
Single Test Evaluation and Design of Directional Elastic Properties in Anisotropic Materials: Jagannadh Boddapati1; Moritz Flaschel2; Siddhant Kumar3; Laura De Lorenzis2; Chiara Daraio1; 1California Institute of Technology; 2ETH Zürich; 3Delft University of Technology
     When the elastic properties of structured materials become direction-dependent, the number of descriptors of their elastic properties increases. In two-dimensions (2D), for example, an anisotropic material can be described by up to 6 independent constants, as opposed to 2, when the properties are direction-independent. Such high number of independent elastic constants expands the design space of structured materials, and leads to unusual phenomena, such as materials that can twist under uniaxial compression. However, this increased number of independent parameters also makes the experimental evaluation of anisotropic material properties more challenging.In this talk, we will present an experimental technique to evaluate the 6 independent elastic constants in 2D structured materials, from a single experiment. The technique combines digital image correlation, unsupervised machine learning, and a multi-axis load cell in a mechanical testing machine, to extract all 6 independent elastic constants, using displacements and global force data.

8:20 AM  Invited
Topological States and Bandgaps in Dimerized Minimal Surfaces: Massimo Ruzzene1; Matheus Rosa1; Yuning Guo1; 1University of Colorado
    We investigate periodic minimal surfaces as a platform for topological mechanical metamaterials. We specifically consider 1D and 2D lattices that are dimerized through parametrizations that respectively break C2 and C3v symmetries, and that form the bases for opening non-trivial band gaps, and for introducing interfaces that support topological valley modes. The existence of band gaps and of non-trivial interface modes is predicted through numerical simulations, and through vibration and wave propagation experiments conducted on additively printed samples. The results illustrate the confinement of topologically protected edge states along engineered interfaces and demonstrate the lack of significant backscattering at sharp corners. This study supports the vision of minimal surfaces as a general framework where geometrical modulations can be conveniently introduced in 1D, 2D and 3D assemblies to achieve novel and unusual mechanical and acoustic functionalities.

9:00 AM  
Machine Learning of Symbolic Expressions to Model Dispersion Curves in Metamaterials: Hongsup Oh1; Sharat Paul1; Alberdi Ryan2; Robbins Joshua2; Pai Wang1; Jacob Hochhalter1; 1University of Utah; 2Sandia National Laboratories
    Metamaterials can be tuned to prevent the propagation of a bandwidth of frequencies, referred to as a bandgap. This characteristic presents a unique opportunity to engineer materials and structures for wave control and vibration isolation. Research to date has largely consisted of the design of custom 3D metamaterial structures to tune the band gap. Here, we simulate a parameterization of metamaterial structures to form a relative band gap data training data set. Subsequently, genetic programming for symbolic regression (GPSR) is employed for the development of an interpretable machine learning approach to predict the relative band gap as a function the parameterized metamaterial geometry. This interpretable GPSR model essentially homogenizes the lower length-scale metamaterial behavior as an analytic expression that is natural for inclusion into existing engineering workflows. As a demonstration, an expression parser is implemented to automate the inclusion of GPSR-produced models into the PLATO topology optimization code.

9:20 AM Break

9:40 AM  
Large-strain Compressive Response and Failure Mechanisms of Additively Manufactured Cubic Chiral Lattices: Caterina Iantaffi1; Eral Bele1; Chu Lun Alex Leung1; Martina Meisnar2; Thomas Rohr2; Peter D. Lee1; 1University College London; 2ESA-ESTEC
     Developments in metal additive manufacturing (AM) technologies, such as Electron Beam powder bed fusion (EPBF), have facilitated the manufacturing of lattice structures with tailorable mechanical properties. EPBF provides an excellent opportunity for optimising the mechanical response of structural lattices; however, the mechanical response under loading conditions is not well understood. In here, we combined experimental and finite element modelling results to investigate large-strain compressive mechanical response of Ti-6Al-4V chiral quadratic lattice made by EPBF. The responses of the unit cell, and finite-size specimens are analysed numerically, including appropriate ductile failure initiation and propagation criteria. The elastic modulus and yield strength follow classic scaling laws of bending-dominated lattices; however, the sequence of deformation mechanisms, ultimate fracture strain, and energy absorption characteristics are dependent on the slenderness ratio of the constitutive struts.

10:00 AM  Invited
Exceptional Mechanical Properties of Additively Manufactured Nano-architected Materials with Complex Topologies: Lorenzo Valdevit1; 1University of California, Irvine
    Truss-based architected materials have been investigated for decades for their ability to provide tunable combinations of mechanical properties at low density. Recent progress in additive manufacturing technologies has dramatically expanded the design space, enabling materials designers to think more creatively about optimal topologies. In this presentation, we will discuss three novel topologies with intriguing mechanical properties: cube-octet plate lattices, spinodal shell lattices, and tensegrity truss lattices. All topologies are fabricated by two-photon polymerization Direct Laser Writing (2pp-DLW); subsequently, plate and shell lattices are pyrolyzed, resulting in ceramic nano-architected materials with feature sizes of the order of 100nm. At this scale, existing cracks are too small to induce brittle failure and the theoretical strength of the base material can be achieved. We show that the combination of optimally designed unconventional topologies and unique nanoscale size effects on the constituent material result in complex nano-architected materials with unique combinations of properties.

10:40 AM  
3D-printable Cactus and Spider-silk Hydrogel Composites for Next Generation Multifunctional and Sustainable Energy Absorptive Metamaterials: Graham Day1; Qicheng Zhang1; Gianni Comandini1; Adam Perriman1; Fabrizio Scarpa1; 1University of Bristol
    Energy absorptive materials are generally fossil-derived or metal-based in design. However, there are emerging opportunities to generate multifunctional composite materials derived from bio-based feedstocks for 3D-printable manufacture. For instance, natural fibre composites exhibit large loss factors in high-amplitude low frequency applications owing to fractal surface features and multiscale architecture and porosity. Here, we have utilized a 3D-printable alginate–pluronic hydrogel, which exhibits significant sound absorption properties as a filler in acoustic metamaterials; it can also be used as a scaffold for reinforcement with cactus-based fibres and mechanically impressive spider dragline silk. We have characterised the low frequency vibration damping properties of this hydrogel and measured loss factors comparable to auxetic foams. Notably, the dynamic modulus of this material is an order of magnitude higher that it’s static modulus, indicating behaviour like that of non-Newtonian fluids under dynamic conditions.