Interlocking of Sheared Non-convex Hexapods*

Packings of non-convex or elongated particles can form free-standing structures like walls or arches, due to particle interlocking effects leading to geometric cohesion. Interlocking effects have been studied for various particle shapes and aspect ratios, but the microscopic origins of the stabilization of rigid structures remains unclear. We report on experiments on hexapods that consist of three orthogonal sphero-cylinders, whose centers are bonded at one origin. The diameter and lengths of the sphero-cylinders are 3, 10, 20 and 30 mm, respectively. We subjected aggregates of spheres or hexapods to quasistatic direct shear and developed novel techniques to measure structural information such as particle positions, orientations, and bending. For spheres and 10 mm hexapods, we observed plasticity phenomena consistent with the Mohr-Coulomb model. For 20 and 30 mm hexapods, however, we observed strain-stiffening during shear. By analyzing X-ray micro-computed tomography data collected during the shearing process, we found that the onset of strain-stiffening is associated with particle bending and that particles that bend significantly tend to be aligned with the compression direction of the shear.