Spallation fracture dependence on shock intensity and loading duration in single-crystal aluminum

Non-equilibrium molecular dynamics simulations were carried out to explore the effects of the shock intensity and loading duration on the spallation fracture mechanisms for single-crystal aluminum. First, the dynamic applicability of the potential function was validated by the shock Hugoniot relation. It was then found that compression waveforms ranging from triangular to rectangular closely depended on the loading duration. Coupled with atomic structural evolution, the spatio-temporal planes characterized by the density and stress well reproduced the spallation process dominated by void nucleation, growth, and coalescence. A larger shock velocity could accelerate the increase in the void volume fraction, while a larger loading duration could postpone the void nucleation but did not affect the variation trend of the void volume fraction. Based on the void morphology evolution at shock velocities of 1-3 km/s, the void volume fraction in the nucleation and growth (NAG) stages were effectively demarcated and then obtained, and the numerical values agreed closely with theoretical values calculated via the NAG model. The NAG model parameters were subsequently acquired, wherein the nucleation rate threshold and pressure sensitivity parameter were basically independent of the loading duration and shock velocity. However, they largely affected the nucleation threshold, growth threshold, and viscosity coefficient. In addition, the variation trend of the spallation strength depended on the predominance of the strain hardening and temperature softening effects. Finally, the correlation between the slope of the velocity pullback and damage rate was preliminarily discussed based on the NAG model.