  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.
