Elastic-plastic transition induced by small-amplitude cyclic shearing of shear-jammed granular solids*

We show experimentally that small amplitude cyclic shear has a dramatic effect on shear jammed granular packings. A shear-jammed state generated by forward shear can relax to an unjammed state or an elastically stable state, depending on the initial shear strain γ_i and the amplitude of the cyclic shear δγ. For γ_i less than a threshold value γ_c, cyclic shear on the order of 5% of γ_c causes the stress to relax to zero, and strobed particle dynamics is diffusive. The number of cycles needed to reach the unjammed state grows exponentially with increasing γ_i. For γ_i larger than γ_c, we see reversible elastic states lasting for thousands of cycles. The value of γ_c depends on both packing fraction and δγ. The long-lived elastic states have much smaller pressures than the initial shear-jammed packings but exhibit much stiffer responses to incremental shear strain.