Homogenization of a row of dislocation dipoles from discrete dislocation dynamics

Conventional discrete-to-continuum approaches have seen their limitation in describing the collective behavior of the multipolar 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 homogenization methods are applied. To address such difficulties, the collective behavior 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 slow-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 the way for properly incorporating dipole-like (zero net Burgers vector but nonvanishing) dislocation structures, known as the statistically stored dislocations, 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 in fatigue metals induced by cyclic loads.