The structure-property relations of mechanical metamaterials have allowed for the realization of novel tough and lightweight material morphologies that exhibit unique mechanical properties including high stiffness-to-mass ratios and energy absorption. Unfortunately, to date, most experimental investigations into this property space have been limited to quasi-static mechanical testing. As such, the high-strain-rate responses of these materials remain poorly understood and largely unexplored. Investigations into these extreme regimes are of paramount interest for the realization of mechanical metamaterials in impact absorption and energy dissipation applications.
Towards the fulfillment of this potential, we present efforts using a custom miniaturized direct-impact Kolsky bar, with bar diameters less than 1 mm, capable of characterizing metamaterial responses across a broad range of strain rates, spanning from 1e3 to ~1e5 1/s. Stresses exerted on the sample are measured using normal-displacement interferometry, while high-speed imaging is used to determine the kinematic responses of the materials at the unit-cell level. To enable rapid prototyping and characterization, we use two-photon lithography capabilities to produce high-resolution microscale samples. Through exploration on a series of stretching- and bending-dominated architectures, with varying degrees of symmetry and periodicity, we discuss the rate-dependent properties of these materials as a function of their morphology. Specifically, we investigated the mechanics of truss-based morphologies, triply periodic minimal surfaces, and aperiodic shell-based architectures. By studying these materials across decades of strain rates, we discuss differences in their rate-dependent responses to decouple the constituent material’s rate dependence from that of the metamaterial. The results of these experiments have implications for the improved application of mechanical metamaterials in impact absorption.
