Impact fragmentation of nanoscale projectiles at ultrahigh striking velocities

Molecular-dynamics simulations of the classic Taylor experiment are performed to investigate some general trends of impact fragmentation at ultra-high striking velocities. The striking velocities of flat-ended, monocrystalline, nanoscale pillars (nanoprojectiles) range from 0.34 km/s (Mach 1) to 30 km/s to explore qualitative effects on the fragment mass distribution. These atomistic simulations offer insight into evolution of the fragment distribution and its dependence upon the striking velocity. According to the simulation results, distribution of the fragment masses following hypervelocity impacts of energy sufficient to ensure that the fragmentation problem is statistically well posed, is well represented by the bilinear (bimodal) exponential distribution commonly observed during high-energy homogeneous fragmentation events. At more moderate striking velocities, a mixing of fragments from different fragmentation intensity events—that is, the more pronounced statistical heterogeneity—results in the distribution of fragment masses that appears to follow the trilinear (trimodal) exponential distribution due to the occurrence of a large-fragment tail in addition to the bilinear exponential part. The maximum fragment mass is studied from the standpoint of the striking velocity as well as a set of state parameters: the instantaneous kinetic temperature and the selected stress and strain invariants; corresponding phenomenological relationships are suggested for the investigated hypervelocity impact range.