Ultra-stable shear jammed granular material

TL;DR

This study reveals that shear-jammed granular materials can evolve into ultra-stable states with unchanged particle configurations after many shear cycles, exhibiting nearly elastic responses.

Contribution

The paper demonstrates the existence of ultra-stable shear-jammed states in granular materials and characterizes their mechanical properties through experimental cyclic shear tests.

Findings

Ultra-stable states persist for thousands of shear cycles.

Particle configurations and contact forces remain unchanged in these states.

Stress response becomes nearly elastic in ultra-stable states.

Abstract

Dry granular materials such as sand, gravel, pills, or agricultural grains, can become rigid when compressed or sheared. At low density, one can distort the shape of a container of granular material without encountering any resistance. Under isotropic compression, the material will reach a certain {\it jamming} density and then resist further compression. {\em Shear jamming} occurs when resistance to shear emerges in a system at a density lower than the jamming density, and the elastic properties of such states have important implications for industrial and geophysical processes. We report on experimental observations of changes in the mechanical properties of a shear-jammed granular material subjected to small-amplitude, quasi-static cyclic shear. We study a layer of plastic discs confined to a shear cell, using photoelasticimetry to measure all inter-particle vector forces. For sufficiently small cyclic shear amplitudes and large enough initial shear, the material evolves to an unexpected "ultra-stable" state in which all the particle positions and inter-particle contact forces remain unchanged after each complete shear cycle for thousands of cycles. The stress response of these states to small imposed shear is nearly elastic, in contrast to the original shear jammed state.