Stress relaxation for granular materials near Jamming under cyclic compression

We have explored isotropically jammed states of semi-2D granular materials through cyclic compression. In each compression cycle, systems of either identical ellipses or bi-disperse disks, transition between jammed and unjammed states. We determine the evolution of the average pressure, P, and structure through consecutive jammed states. We observe a transition point, {\phi}m, above which P persists over many cycles; below {\phi}m, P relaxes slowly. The relaxation time scale associated with P increases with packing fraction, while the relaxation time scale for collective particle motion remains constant. The collective motion of the ellipses is hindered compared to disks, due to the rotational constraints on elliptical particles.