Auxetic Metamaterials under Direct Impact Loads in a Structural Health Monitoring Framework

Report No. ARL-TR-6306
Authors: James Ayers, Ben Chamish, Tzi-Kang Chen, Anindya Ghoshal, Chandan Kittur, Michael Coatney, Natasha Bradley, Tyrone Jones
Date/Pages: January 2013; 26 pages
Abstract: Under direct impact from ballistic loads, acoustic waves typically propagate throughout a structural component causing damage to sensitive areas. For structures with intelligent embedded monitoring systems, such as mounted piezoelectric actuators, a direct impact between 150200 m/s of small-caliber rounds has produced catastrophic results. This report focuses on redirecting acoustic waves using non-traditional structural configurations. The technical approach utilizes periodic and graded metamaterials (produced from auxetic cellular, lattice topology and material composition), which are lightweight and can be assembled for extreme anisotropy and phononic bandgaps, which can be exploited to alter the propagation path of high amplitude stress waves. This research focuses on the initial stage of understanding how to tailor periodic lattices for highly concentrated impact and blast loads, which generally produce a broadband frequency response and yield only partial bandgaps. Specific attention is given to square, hexagonal, re-entrant, and modified re-entrant topologies. As a baseline comparison, a solid aluminum plate is examined against uniform and graded re-entrant unit cell lattices. The plates are individually subjected to both in- and out-of-plane direct impact loading conditions of a 0.22-caliber fragment simulating projectile traveling at 300 m/s. A less than 10% marginal difference in peak stress amplitude exists between the loading conditions for a given single through-the-thickness unit cell.
Distribution: Approved for public release
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Last Update / Reviewed: January 1, 2013