Nucleation properties of isolated shear bands

TL;DR

This study investigates the nucleation of isolated shear bands in materials, revealing a critical shear stress for their formation and linking it to dislocation mechanisms, with implications for understanding material failure.

Contribution

It provides experimental evidence of shear band nucleation properties and their relation to dislocation dynamics, expanding knowledge of plastic instability mechanisms.

Findings

Existence of a critical shear stress for shear band nucleation.

Nucleation stress is proportional to shear modulus and independent of normal stress.

Transition from shear banding to homogeneous flow occurs below nucleation stress.

Abstract

Shear banding, or localization of intense strains along narrow bands, is a plastic instability in solids with important implications for material failure in a wide range of materials and across length-scales. In this paper, we report on a series of experiments on the nucleation of single isolated shear bands in three model alloys. Nucleation kinetics of isolated bands and characteristic stresses are studied using high-speed in situ imaging and parallel force measurements. The results demonstrate the existence of a critical shear stress required for band nucleation. The nucleation stress bears little dependence on the normal stress and is proportional to the shear modulus. These properties are quite akin to those governing the onset of dislocation slip in crystalline solids. A change in the flow mode from shear banding to homogeneous plastic flow occurs at stress levels below the nucleation stress. Phase diagrams delineating the strain, strain rate and temperature domains where these two contrasting flow modes occur are presented. Our work enables interpretation of shear band nucleation as a crystal lattice instability due to (stress-assisted) breakdown of dislocation barriers, with quantitative experimental support in terms of stresses and the activation energy.