Plastic deformations and strain hardening in fully dense granular crystals

Granular crystals are intriguing constructs at the intersection between granular matter and architectured materials, offering new combinations of tunable mechanical properties, healing and recyclability. We have recently fabricated and tested fully dense granular FCC crystals based on millimeter size rhombic dodecahedral grains fabricated using 3D printing. Compressive experiments on these granular crystals show that they are more than ten times stronger than traditional granular materials, and that they display a rich set of mechanisms: Nonlinear deformations, crystal plasticity reminiscent of atomistic mechanisms, shear-induced dilatancy, micro-buckling. An intriguing feature of these granular crystals is strain hardening and delocalization, which contrast with typical granular materials which are governed by shear bands and strain softening. To explore these mechanisms, we used discrete elements simulations of FCC granular crystals to duplicate the triaxial compression experiments, and to capture strength and micromechanics of deformation. The models also show that compressive jamming of the grains at the intersection between slip planes is the source of strain hardening in the granular crystal. The implications in terms of size effects, confinement, free boundaries and "polycrystalline" granular crystals are also discussed.