Shock responses of nanoporous aluminum by molecular dynamics simulations

摘要

We present systematic investigations on the shock responses of nanoporousaluminum (np-Al) by nonequilibrium molecular dynamics simulations. Thedislocation nucleation sites are found to concentrate in low latitude regionnear the equator of the spherical void surfaces. We propose a continuum wavereflection theory and a resolved shear stress model to explain the distributionof dislocation nucleation sites. The simulations reveals two mechanisms of voidcollapse: the plasticity mechanism and the internal jetting mechanism. Theplasticity mechanism, which leads to transverse collapse of voids, prevailsunder relatively weaker shocks; while the internal jetting mechanism, whichleads to longitudinal filling of the void vacuum, plays more significant roleas the shock intensity increases. In addition, an abnormal thermodynamicphenomenon (i.e., arising of temperature with pressure dropping) in shockednp-Al is discovered. This phenomenon is incompatible with the conventionalRankine-Hugoniot theory, and is explained by the nonequilibrium processesinvolved in void collapse. The influences of void collapse on spall fracture ofnp-Al is studied. Under the same loading velocity, the spall strength of np-Alis found to be lower than that of single-crystal Al; but the spall resistanceis higher in np-Al than in single-crystal Al. This is explained by the combinedinfluences of thermal dissipation and stress attenuation during shock wavepropagation in np-Al.