Particle scale anisotropy controls bulk properties in sheared granular materials

TL;DR

This study experimentally links particle-scale anisotropies in contact and force networks to bulk properties like stress and friction in sheared granular materials, validating the Stress-Force-Fabric relation.

Contribution

It provides the first experimental validation of the SFF relation connecting particle-scale anisotropies to bulk properties in dense granular systems.

Findings

Particle-scale anisotropies predict bulk stress components.

The SFF relation accurately models transient behaviors.

Fabric anisotropy correlates with bulk friction coefficient.

Abstract

The bulk dynamics of dense granular materials arise through a combination of particle-scale and mesoscale effects. Theoretical and numerical studies have shown that collective effects are created by particle-scale anisotropic structures such as grain connectivity (fabric), force transmission, and frictional mobilization, all of which influence bulk properties like bulk friction and the stress tensor through the Stress-Force-Fabric (SFF) relationship. To date, establishing the relevance of these effects to laboratory systems has remained elusive due to the challenge of measuring both normal and frictional contact forces at the particle scale. In this study, we perform experiments on a sheared photoelastic granular system in an quasi-2D annular (Couette) cell. During these experiments, we measure particle locations, contacts, and normal and frictional forces vectors during loading. We reconstruct the angular distributions of the contact and force vectors, and extract the corresponding emergent anisotropies for each of these metrics. Finally, we show that the SFF relation quantitatively predicts the relationship between particle scale anisotropies, the stress tensor components, and the bulk friction coefficient, capturing even transient behaviors. As such, this method shows promise for application to other dense particulate systems where fabric anisotropy can provide a useful measure of bulk friction.