Sub-Yield Dynamics in Yield-Stress Materials

TL;DR

This study investigates the sub-yield behavior of yield-stress materials, demonstrating that they exhibit nonlinear viscoelasticity rather than plastic flow below the yield point, challenging existing models.

Contribution

The paper provides experimental evidence that yield-stress materials do not flow below yield and highlights the importance of nonlinear viscoelasticity in their sub-yield response.

Findings

Materials show bounded, periodic strain responses below yield.

Residual slip effects can be properly accounted for in rheometry.

Sub-yield behavior is governed by nonlinear viscoelasticity.

Abstract

The mechanical response of yield-stress materials below the yield point remains a subject of debate. Two of the most widely used constitutive models for these materials offer fundamentally conflicting views: one permits plastic flow at all stress levels, the other assumes entirely recoverable viscoelasticity below yield. Using parallel superposition rheometry, we test the sub-yield behaviour of a microgel and an emulsion. When residual slip effects are properly accounted for, both fluids exhibit bounded, periodic strain responses, offering compelling evidence that they do not flow in the studied regime. Our results indicate that the sub-yield regime is underpinned by nonlinear viscoelasticity and underscore the need for improved constitutive relations that capture such effects without treating yielding as a precursor for nonlinearity.