High-strength, fully dense granular crystals*

Typical granular materials are far from optimal in terms of mechanical performance: Random packing leads to poor load transfer in the form of thin and dispersed force lines within the material, inhomogeneous jamming, and strain localization. In addition, localized contacts between individual grains result in low stiffness, low strength and brittleness. Here we take a new approach where we consider granular materials as materials by design. We vibrated millimeter-scale 3D printed grains with rhombic dodecahedral or truncated octahedral shapes to assemble them into fully dense FCC and BCC granular crystals. These granular crystals are up to 10 times stronger than traditional randomly packed spheres, and they can display a rich set of mechanisms: Nonlinear deformations, crystal plasticity reminiscent of atomistic mechanisms, cross-slip, shear-induced dilatancy, geometric hardening, micro-buckling. To capture some of these mechanisms we developed a multiscale model that incorporate local friction between grains, resolved shear and normal stresses on available slip planes, and prediction of compressive strength as function of loading orientation. The predicted strength is highly anisotropic and agrees well with triaxial compression experiments.