Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study

TL;DR

This study uses molecular dynamics simulations to investigate how stress-triaxiality influences void growth and fracture behavior in copper under high strain rates, revealing detailed mechanisms and thresholds relevant to dynamic fracture.

Contribution

It provides new insights into the atomistic mechanisms of void growth under different stress conditions and validates macroscopic assumptions with detailed simulation data.

Findings

Void growth thresholds align with experimental spall strength values.

Void evolution exhibits four distinct regimes: elastic, yielding, saturation, and failure.

Stress-triaxiality significantly accelerates void growth and influences fracture modes.

Abstract

The effect of stress-triaxiality on growth of a void in a three dimensional single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular dynamics (MD) simulations using an embedded-atom (EAM) potential for copper have been performed at room temperature and using strain controlling with high strain rates ranging from 10^7/sec to 10^10/sec. Strain-rates of these magnitudes can be studied experimentally, e.g. using shock waves induced by laser ablation. Void growth has been simulated in three different conditions, namely uniaxial, biaxial, and triaxial expansion. The response of the system in the three cases have been compared in terms of the void growth rate, the detailed void shape evolution, and the stress-strain behavior including the development of plastic strain. Also macroscopic observables as plastic work and porosity have been computed from the atomistic level. The stress thresholds for void growth are found to be comparable with spall strength values determined by dynamic fracture experiments. The conventional macroscopic assumption that the mean plastic strain results from the growth of the void is validated. The evolution of the system in the uniaxial case is found to exhibit four different regimes: elastic expansion; plastic yielding, when the mean stress is nearly constant, but the stress-triaxiality increases rapidly together with exponential growth of the void; saturation of the stress-triaxiality; and finally the failure.