Preserving elastic anisotropy with tessellations of granular packings.

Multiscale periodic metamaterials have been designed for numerous applications, such as impact absorption, acoustic cloaking, photonic band gaps, and mechanical logic gates. This prior work has focused on optimizing mesoscale structure for desired bulk isotropic properties. In contrast, we seek to develop materials with highly anisotropic elastic properties. To quantify elastic anisotropy, we introduce two rotationally invariant, normalized quantities that characterize the anisotropic response to shear and compression, respectively, A_{G} and A_{C}. A_{G} (A_{C}) is the normalized variance of the shear (uniaxial compressive) modulus obtained by changing the coordinate axes. We find that typical crystalline solids possess average elastic anisotropy A[over ¯]_{G}≈0.15 and A[over ¯]_{C}≈0.09. Compared to atomic crystals, jammed granular materials can attain elastic anisotropies that are several orders of magnitude larger. Since grain rearrangements reduce anisotropy in granular materials, to preserve strong elastic anisotropy, we design tessellated granular materials that consist of multiple connected grain-filled voxels, which limit rearrangements and enable highly anisotropic elastic properties. Bulk granular packings with N grains prepared at pressure p have maximal anisotropy for pN^{2}∼1 and become isotropic in the large-pN^{2} limit. We show that homogeneously tessellated granular systems can inherit the elastic response of the constituent voxel configurations with elastic anisotropy up to 100 times that of crystalline compounds over a range of pN^{2}. We show further methods to enhance the elastic anisotropy of tessellations by designing heterogeneously patterned voxel configurations and tessellations that allow large boundary deformations.