Homogenisation of a Row of Dislocation Dipoles from Discrete Dislocation Dynamics

TL;DR

This paper develops a matched asymptotic approach to model the collective behavior of dislocation dipoles in crystalline materials, capturing their dynamics at a coarse-grained level and linking microscopic structures to macroscopic properties.

Contribution

It introduces a novel multiscale method that incorporates dislocation dipoles into continuum models, addressing limitations of traditional homogenisation techniques.

Findings

Dislocation pair width evolves faster than pair density.

Hierarchy in time scales enables coupled evolution equations.

Transition between equilibrium patterns explains slip band formation.

Abstract

Conventional discrete-to-continuum approaches have seen their limitation in describing the collective behaviour of the multi-polar configurations of dislocations, which are widely observed in crystalline materials. The reason is that dislocation dipoles, which play an important role in determining the mechanical properties of crystals, often get smeared out when traditional homogenisation methods are applied. To address such difficulties, the collective behaviour of a row of dislocation dipoles is studied by using matched asymptotic techniques. The discrete-to-continuum transition is facilitated by introducing two field variables respectively describing the dislocation pair density potential and the dislocation pair width. It is found that the dislocation pair width evolves much faster than the pair density. Such hierarchy in evolution time scales enables us to describe the dislocation dynamics at the coarse-grained level by an evolution equation for the slowly varying variable (the pair density) coupled with an equilibrium equation for the fast varying variable (the pair width). The time-scale separation method adopted here paves a way for properly incorporating dipole-like (zero net Burgers vector but non-vanishing) dislocation structures, known as the statistically stored dislocations (SSDs) into macroscopic models of crystal plasticity in three dimensions. Moreover, the natural transition between different equilibrium patterns found here may also shed light on understanding the emergence of the persistent slip bands (PSBs) in fatigue metals induced by cyclic loads.