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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. 

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, to inhomogeneous jamming, and to strain localization. In addition, localized contacts between individual grains result in low stiffness, strength and brittleness. Here we propose a granular material that simultaneously embodies three approaches to increase strength: geometrical design of individual grains, crystallization, and infiltration by an adhesive. Using mechanical vibrations, we assembled millimeter-scale 3D printed grains with rhombic dodecahedral shapes into fully dense FCC granular crystals. We then infiltrated the granular structure with a tacky, polyacrylic adhesive that is orders of magnitude weaker than the grains, but which provides sustained adhesion over large interfacial displacements. The resulting material is a fully dense, free-standing space filling granular crystal. Compressive tests show that these granular crystals are up to 60 times stronger than randomly packed cohesive spheres and they display a rich set of mechanisms: Nonlinear deformations, crystal plasticity reminiscent of atomistic mechanisms, cross-slip, shear-induced dilatancy, micro-buckling, and tensile strength. To capture some of these mechanisms we developed a multiscale model that incorporates local cohesion 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 the compression experiments. Once fully understood and harnessed, we envision that these mechanisms will lead to granular engineering materials with unusual combinations of mechanical performances attractive for many applications.