Abstract
We present systematic investigations examining the shock responses of nanoporous aluminum (np-Al) by nonequilibrium molecular dynamics simulations. The dislocation nucleation sites are observed to be concentrated in the 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; the internal jetting mechanism, which leads to longitudinal filling of the void vacuum, plays a more significant role as the shock intensity increases. In addition, we observed that the temperature rises while the pressure drops for tens of picoseconds in the shocked np-Al. This thermodynamic phenomenon is different from that observed in single-crystal Al, where the temperature and pressure immediately reach the Hugoniot constants. The influences of void collapse on spall fracture of np-Al are studied. Under the same loading velocity, the spall strength of np-Al is observed to be lower than that of single-crystal Al; however, the spall resistance is higher in np-Al than in single-crystal Al. This resistance is explained by the combined influences of thermal dissipation and stress attenuation during shock wave propagation in np-Al.
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