Granular Metamaterial for Impact Mitigation

Classical granular assemblies exhibit a mechanical response that involves significant energy dissipation upon impact. However, subsequent dissipation-induced scattering and irreversible global structural changes limit their functional utility in effective impact mitigation. We introduce a new class of materials composed of discrete grains, specifically spherical shells, that are interlocked in 3D by loose links with conical frustum-shaped heads. Despite its granular construction, the resulting network is mechanically compliant and capable of dissipating energy while remaining cohesive and structurally reversible, under repeated impact. The internal degrees of freedom within the network structure allow it to rattle under impact, accessing various local configurations that help to dissipate impact energy rapidly by utilizing it to overcome the friction and other inelastic collisions between granular components. We also perform a series of impact tests under different structural configurations to demonstrate their effectiveness in impact mitigation. The drop test on a rigid platform reveals an almost perfectly inelastic collision response, with a low coefficient of restitution and little or no structural damage, an essential feature for reusable shock isolators to handle sensitive objects. Furthermore, projectile impact tests show that the peak of the impact force decays rapidly with an increasing number of particle layers in the structure, demonstrating the strong effectiveness of layering in energy attenuation.