Comparing inelastic deformations and strength in dense FCC and HCP granular crystals: Experiments and models

Randomly distributed granular materials offer a rich landscape of mechanisms but their tunability is limited. Taking inspiration from crystallography and granular mechanics, we fabricated and tested fully dense cohesive FCC and HCP granular crystals, and developed granular crystal plasticity models to investigate their relative strength and deformation mechanisms. Geometrically, switching from FCC to HCP is remarkably simple and only involves a 60° rotation about the midplane of individual dodecahedral grains. However, the effect of this transformation on crystallography, properties and mechanics are profound. This rotation breaks several symmetries, and while additional slip systems are made available (prismatic, pyramidal.) compared to the {111} family in FCC, each of the families in HCP contain a smaller number of total slip planes. As a result, slip in HCP is in general more difficult to activate resulting in an average strength 50% greater than in FCC. We also observed mechanisms that are unique to granular crystals: micro-buckling in FCC and HCP, and splaying in HCP crystals loaded along the c-axis. These granular crystals offer powerful and versatile platforms for new generation mechanical metamaterials with tunable inelastic deformation, energy absorption and strength. For example, the granular architecture amplifies the properties of the adhesive by about one order of magnitude, so that attractive rheologies maybe be translated into useful responses in compression.