Mechanisms for slow strengthening in granular materials

TL;DR

This paper investigates the mechanisms behind slow strengthening in granular materials, highlighting the roles of particle rearrangement, humidity, compaction, and shear stress cycling in increasing material strength over time.

Contribution

It identifies multiple mechanisms, including particle rearrangement and humidity effects, that contribute to slow strengthening in granular materials, providing detailed experimental insights.

Findings

Strength increases logarithmically with waiting time under shear stress.

Humidity influences strengthening but is not the sole factor.

Compaction and shear stress cycling can also enhance static friction.

Abstract

Several mechanisms cause a granular material to strengthen over time at low applied stress. The strength is determined from the maximum frictional force F_max experienced by a shearing plate in contact with wet or dry granular material after the layer has been at rest for a waiting time \tau. The layer strength increases roughly logarithmically with \tau -only- if a shear stress is applied during the waiting time. The mechanisms of strengthening are investigated by sensitive displacement measurements and by imaging of particle motion in the shear zone. Granular matter can strengthen due to a slow shift in the particle arrangement under shear stress. Humidity also leads to strengthening, but is found not to be its sole cause. In addition to these time dependent effects, the static friction coefficient can also be increased by compaction of the granular material under some circumstances, and by cycling of the applied shear stress.