Quantitative understanding of the onset of dense granular flows

TL;DR

This paper develops a simple friction-based model to understand when dense granular materials begin to flow, explaining the transition from solid-like to liquid-like behavior and the influence of history and preparation.

Contribution

It introduces a novel equation for the onset of granular flow based on frictional aging, capturing non-monotonic stress-strain behavior independent of shear rate.

Findings

The model explains below-threshold deformations in aging granular media.

It describes the solid-to-liquid transition in granular flows.

The results highlight the importance of history and initial conditions.

Abstract

The question when and how dense granular materials start to flow under stress, despite many industrial and geophysical applications, remains largely unresolved. We develop and test a simple equation for the onset of quasi-static flows of granular materials which is based on the frictional aging of the granular packing. The result is a non-monotonic stress-strain relation which - akin to classical friction - is independent of the shear rate. This relation suffices to understand the below-threshold deformations of aging granular media, and its solid-to-liquid transition. Our results also elucidate the (flow) history dependence of the mechanical properties, and the sensitivity to initial preparation of granular media.