Yielding and Rigidity of Sheared Columns of Hexapod Granules

Granular packings of non-convex or elongated particles can form free-standing structures like walls or arches. For some particle shapes, such as staples, the rigidity arises from interlocking of pairs of particles, but the origins of rigidity for non-interlocking particles remain unclear. We report on experiments and numerical simulations of sheared columns of “hexapods,” particles consisting of three mutually orthogonal rods whose centers coincide. We vary the length-to-diameter ratio, α, of the rods and subject the packings to quasistatic direct shear. For small α, we observe a finite yield stress. For large α, however, column is rigid against shear, and stresses within it increase with increasing strain. Analysis of X-ray micro-computed tomography data collected during the shear, reveals that the stiffening is associated with a tilted, oblate cluster of hexapods near the nominal shear plane in which particle deformation and average contact number both increase. Simulation results show that geometric cohesion effects are negligible for small α. For large α (α=10), we directly observe that particles are collectively under tension even though they do not interlock pairwise. The tensile stress counteracts the tendency to dilate in direction normal to the shear plane.