Statistical Properties of a 2D Granular Material Subjected to Cyclic Shear

This work focuses on the evolution of structure and stress for an experimental system of 2D photoelastic particles that is subjected to multiple cycles of pure shear. Throughout this process, we determine the contact network and the contact forces using particle tracking and photoelastic techniques. These data yield the fabric and stress tensors and the distributions of contact forces in the normal and tangential directions. We then find that there is, to a reasonable approximation, a functional relation between the system pressure, $P$, and the mean contact number, $Z$. This relationship applies to the shear stress $\tau$, except for the strains in the immediate vicinity of the contact network reversal. By contrast, quantities such as $P$, $\tau$ and $Z$ are strongly hysteretic functions of the strain, $\epsilon$. We find that the distributions of normal and tangential forces, when expressed in terms of the appropriate means, are essentially independent of strain. We close by analyzing a subset of shear data in terms of strong and weak force networks.