Shock responses of nanoporous aluminum by molecular dynamics simulations

TL;DR

This study uses molecular dynamics simulations to analyze shock responses in nanoporous aluminum, revealing dislocation nucleation, void collapse mechanisms, an unusual thermodynamic phenomenon, and effects on spall fracture resistance.

Contribution

It introduces a continuum wave reflection theory and a resolved shear stress model to explain dislocation nucleation and void collapse mechanisms in nanoporous aluminum under shock.

Findings

Dislocation nucleation concentrates near void surfaces.

Two void collapse mechanisms identified: plasticity and internal jetting.

Nanoporous aluminum has lower spall strength but higher spall resistance than single-crystal aluminum.

Abstract

We present systematic investigations on the shock responses of nanoporous aluminum (np-Al) by nonequilibrium molecular dynamics simulations. The dislocation nucleation sites are found to concentrate in low latitude region near the equator of the spherical void surfaces. We propose a continuum wave reflection theory and a resolved shear stress model to explain the distribution of dislocation nucleation sites. The simulations reveals two mechanisms of void collapse: the plasticity mechanism and the internal jetting mechanism. The plasticity mechanism, which leads to transverse collapse of voids, prevails under relatively weaker shocks; while the internal jetting mechanism, which leads to longitudinal filling of the void vacuum, plays more significant role as the shock intensity increases. In addition, an abnormal thermodynamic phenomenon (i.e., arising of temperature with pressure dropping) in shocked np-Al is discovered. This phenomenon is incompatible with the conventional Rankine-Hugoniot theory, and is explained by the nonequilibrium processes involved in void collapse. The influences of void collapse on spall fracture of np-Al is studied. Under the same loading velocity, the spall strength of np-Al is found to be lower than that of single-crystal Al; but the spall resistance is higher in np-Al than in single-crystal Al. This is explained by the combined influences of thermal dissipation and stress attenuation during shock wave propagation in np-Al.