Molecular dynamics studies on energy dissipation and void collapse in graded nanoporous nickel under shock compression

We present systematic investigations on energy dissipation and void collapse in graded nanoporous nickel by non-equilibrium molecular dynamics simulations. It is found that void size gradient influences the time history path of the energy dissipation. Under strong shock loading conditions, when the voids are completely collapsed, the total energy dissipation is dependent only on the porosity, and is independent of the void size gradient in the shock direction. The total energy dissipation increases with the increase of porosity. However, when the porosity increases to a critical value of 6%, the total energy dissipation reaches an upper limit. Increasing the porosity beyond this critical value would not result in further increase in energy dissipation. The simulations show that voids collapse is attributed to the combined effect of transverse and longitudinal plasticity flow of void wall. Two mechanisms of voids collapse are revealed: the plasticity mechanism and internal jetting mechanism. Under relatively weaker shocks, the plasticity mechanism, which leads to transverse collapse of voids, prevails; while at the stronger shock strengths, the internal jetting mechanism, which leads to longitudinal flow, plays a more significant role. The earliest appearing dislocations in a void may either nucleate at the front half surface or on the back half surface, depending on the position of the void in the sample and the void size gradient. Moreover, the simulations provide quantitative descriptions about the effects of loading intensity on energy dissipation rate and void collapse rate. We show that the energy dissipation rate can be well represented by a quadratic polynomial function of the shock loading velocity, and the void collapse rate is a linear function of the shock loading