  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 remains unclear. We report on
experiments and numerical simulations of sheared columns of "hexapods,"
particles consisting of three mutually orthogonal sphero-cylinders whose
centers coincide. We vary the length-to-diameter aspect ratio, $\alpha$, of the
sphero-cylinders and subject the packings to quasistatic direct shear. For
small $\alpha$, we observe a finite yield stress. For large $\alpha$, however,
the column becomes rigid when sheared, supporting stresses that increase
sharply with increasing strain. Analysis of X-ray micro-computed tomography
(Micro-CT) 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 the particles are collectively under tension along
one direction even though they do not interlock pairwise. These tensions comes
from contact forces carrying large torques, and they are perpendicular to the
compressive stresses in the packing. They counteract the tendency to dilate,
thus stabilize the particle cluster.
