Shear banding as a cause of non-monotonic stress relaxation after flow cessation

TL;DR

This paper explains the counter-intuitive non-monotonic relaxation of shear stress after flow cessation in soft materials, attributing it to shear band dynamics and frustrated local stresses, supported by simulations of the soft glassy rheology model.

Contribution

It demonstrates that shear banding prior to flow cessation can cause non-monotonic stress relaxation, providing a mechanistic understanding through extended modeling.

Findings

Shear bands decay after flow stop, influencing stress relaxation.

Elastic recoil in unsheared bands can increase macroscopic stress.

Non-monotonic relaxation may be common in disordered soft solids.

Abstract

Recent flow cessation experiments on soft materials have shown a counter-intuitive non-monotonic relaxation of the shear stress: following the switch-off of a steady imposed shear flow, the stress initially decays before later increasing again. By simulating the soft glassy rheology model in a form extended to allow steady state shear banding, we show that the presence of shear bands prior to flow cessation can give rise to this phenomenon. We give a mechanistic understanding of the basic physics involved, in terms of (i) the decay of the shear bands after flow cessation, and (ii) the evolution of frustrated local stresses, governed by different time scales for plastic relaxation in each band. In particular, an elastic recoil in the unsheared band gives rise to negative local frustrated stresses, the slow release of which can cause an increase in macroscopic stress. Given that shear banding and frustrated local stresses arise widely across disordered soft solids, we argue that non-monotonic stress relaxation after flow cessation may occur in many different materials.