Strain hardening by sediment transport

TL;DR

This paper investigates how sediment beds respond to fluid shear stress cycles, revealing strain hardening, weakening, and memory effects that influence river-bed armor formation and breakup.

Contribution

It explores the role of shear jamming and stress history in sediment transport, providing new insights into bed compaction, dilation, and anisotropic effects.

Findings

Direction-dependent strain hardening occurs for low stresses.

Dilation weakens the bed at higher stresses.

Memory loss happens after large stress cycles.

Abstract

The critical fluid-shear stress for the onset of sediment transport, $\theta_c$, varies with the history of applied shear. This effect has been attributed to compaction, but the role of shear jamming is unexplored. We examine the response of a granular bed to fluid-shear stress cycles of varying magnitude and direction, and determine isotropic and anisotropic contributions. Creep and bed-load transport result in direction-dependent strain hardening for $\theta/\theta_c < 4$. Dilation-induced weakening, and memory loss, occurs for larger stresses that fluidize the bed. Our findings provide a granular explanation for the formation and breakup of hard packed river-bed 'armor'.