Three-dimensional molecular dynamics simulations of void coalescence during dynamic fracture of ductile metals

TL;DR

This study uses large-scale 3D molecular dynamics simulations to analyze void coalescence in ductile metals, revealing the critical conditions and dynamics leading to fracture, and critically evaluating common simulation practices.

Contribution

It provides new insights into the onset and dynamics of void coalescence during fracture, including the critical ligament distance and effects of boundary conditions.

Findings

Critical ligament distance is about one void radius.

Void interaction does not affect volumetric growth rate.

Periodic boundary conditions can produce misleading results.

Abstract

Void coalescence and interaction in dynamic fracture of ductile metals have been investigated using three-dimensional strain-controlled multi-million atom molecular dynamics simulations of copper. The correlated growth of two voids during the coalescence process leading to fracture is investigated, both in terms of its onset and the ensuing dynamical interactions. Void interactions are quantified through the rate of reduction of the distance between the voids, through the correlated directional growth of the voids, and through correlated shape evolution of the voids. The critical inter-void ligament distance marking the onset of coalescence is shown to be approximately one void radius based on the quantification measurements used, independent of the initial separation distance between the voids and the strain-rate of the expansion of the system. The interaction of the voids is not reflected in the volumetric asymptotic growth rate of the voids, as demonstrated here. Finally, the practice of using a single void and periodic boundary conditions to study coalescence is examined critically and shown to produce results markedly different than the coalescence of a pair of isolated voids.