Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 150479

150479 - Multiscale Mechanics of Adaptive Hierarchical Granular Metamaterials for Tunable Impact Mitigation 

Granular materials are discrete, ordered assemblies of solid grains that interact through dissipative contact interactions. They possess unique wave propagation characteristics such as the formation of solitary waves and acoustic vacuums that, in combination with their nonlinear contact interactions, make them suitable for applications within impact engineering. The wave propagation characteristics of ordered granular materials are typically modified by introducing defect grains that are fabricated by varying the constitutive material or geometry of the grain. However, granular material designs that are realized through this process possess a fixed dynamic response that cannot be altered in real time. Smart materials with mechanical responses that are reactive to external stimuli are of great practical interest for various engineering applications as they enable the realization of a range of mechanical behaviors within a single material system. Recent studies into achieving granular metamaterials with real-time adaptivity have shown that external fields, such as magnetic fields, can tune the propagation of impact waves within these complex materials by creating tunable, rheological defects. Adaptive mechanical behaviors have also been experimentally demonstrated in various periodic, cellular metamaterials such as in truss based lattice structures. The mechanical behavior of cellular metamaterials is controlled by the geometry of the smallest periodic unit. As such, they possess low relative densities, low peak transmitted stresses and exotic mechanical properties such as auxeticity or negative Poisson's ratio that are not found in conventional bulk solids like metals/plastics. Due to this, cellular materials demonstrate great promise as energy absorbers. This work presents a novel granular metamaterial assembly containing two orders of structural hierarchy and when loaded, possesses an adaptive mechanical response under the action of external magnetic fields. Cellular grains containing hollow, re-entrant auxetic geometries infilled with magnetorheological fluids were fabricated to create grains with magnetoactive tunability. Structural hierarchy was realized by assembling individual magnetoactive grains to create granular metamaterials. The magneto-mechanical response of an individual, hollow, cellular grain and the hierarchical granular metamaterial has been experimentally demonstrated through quasi-static compression and drop-tower-based impact experiments at various magnetic-field strengths. The design space of the cellular grain was explored through experimentally validated finite element models to map the mechanical response of various field-driven structure-property combinations. Modeling the combined field-driven magnetoactive response of an individual grain as an effective material response, a contact law that describes the nonlinear interactions between two cellular, cylindrical grains has been calibrated through finite element analyses for various geometric configurations and magnetic field strengths. Furthermore, a discrete element numerical model employing the contact law has been developed to understand the dynamic response of a finite adaptive granular metamaterial. Beginning at the level of an individual grain, results have demonstrated the static and adaptive tunability of the hierarchical granular metamaterial as a function of the geometric properties of the grain and the strength of the external magnetic field. The influence of the cellular material geometry on the contact behavior between the constituent grains of the hierarchical granular metamaterial has been extensively studied through numerical investigations. Finally, for a given cellular grain geometry, the role of the spatial distribution of magnetoactive grains on the propagation of impact waves within the fabric of the metamaterial has been numerically quantified at various magnetic field strengths.

Presenting Author: Prajwal Bharadwaj Worcester Polytechnic Institute

Presenting Author Biography: I currently a doctoral candidate and research assistant in the experimental structures and materials laboratory that is a part of the aerospace engineering department at Worcester Polytechnic Institute. My research interests are in the development of novel, functional, materials and structures for advanced engineering applications through a combination of experiments and numerical simulations. I am also interested in exploring the utility of data-driven methods to create novel reusable and sustainable material technology through conventional and additive manufacturing techniques.

Authors:

Prajwal Bharadwaj Worcester Polytechnic Institute
Nikhil Karanjgaokar Worcester Polytechnic Institute