Ultra-stable shear jammed granular material

We report experiments that demostrate the existence of unexpected, nearly elastic states in a shear-jammed granular material.  These ``ultra-stable'' states are formed by applying low-amplitude, quasistatic cyclic shear to an initially shear-jammed system.  After a transient relaxation, all the particle positions and inter-particle contact forces remain unchanged after each complete shear cycle for thousands of cycles.  We perform experiments on a layer of plastic discs, using photoelasticimetry to measure all inter-particle vector forces.  For a given strain amplitude, the ultra-stable states are formed from shear-jammed states prepared by a sufficiently large initial shear strain.  The ultra-stable states display different mechanical behavior than the conventional shear jammed states, being nearly elastic and resisting shear reversal up to a finite yield strain.