Observation of strain-rate softening behavior in jammed granular media

TL;DR

This study uncovers a rate-softening behavior in jammed granular media, driven by decreasing surface friction with increasing strain rate, validated through experiments and FEM simulations, highlighting the importance of inter-particle tribology.

Contribution

It reveals a novel rate-softening behavior in granular materials and identifies surface friction reduction as the key mechanism, supported by experimental and simulation data.

Findings

Rate-softening observed in granular media at certain strain rates

Surface friction decreases with increasing strain rate

Confining pressure influences normal stress and frictional behavior

Abstract

The strain-rate sensitivity of confined granular materials has been widely explored, with most findings exhibiting rate-strengthening behaviors. This study, however, reveals a distinct rate-softening behavior across a certain strain rate range based on triaxial tests on particle clusters of various materials with different surface properties, particle sizes, shapes, and stiffness. This softening effect is especially pronounced in the case of common rice particles. By examining the behavior of rice particles under different confining pressure and surface conditions, and directly measuring the frictional coefficient across various loading rates, we find that the reduction in surface frictional coefficient with the increasing strain rate predominantly contributes to this rate-softening behavior. This conclusion is validated by results from Finite Element Method (FEM) simulations. Additionally, we identify confining pressure as a critical factor regulating the normal stress between particles, and thereby enhancing frictional behavior. Rheometer tests reveal that the shear modulus exhibits a similar rate-softening trend. This study of rate-softening behavior in granular materials enhances our understanding of the mechanisms during their deformation under confining pressure. It also suggests that local inter-particle tribology significantly impacts overall granular behavior.