Hypervelocity impact of anorthosite: Excavation, spallation and crater reconstruction

Abstract

We report the results of eleven unconfined hypervelocity experiments, each involving the impact of a 316 stainless steel projectile (3.175 mm diameter sphere) with an anorthosite rock target. Impact velocities ranged from 1.75 to ∼6.7 km/s. The experiments were performed using a two-stage light gas gun. Three successive impact stages are distinguished: (1) initial contact, with shock wave and, at impact velocities >4 km/s, impact flash generation (<0.1 ms duration); (2) crater excavation (<0.4 ms duration), which is correlated with an initial ejecta event, where the ejecta velocity is supersonic and proportional to impact energy; and (3) spallation (>0.4 ms duration), which is associated with a later, second ejecta event, where the ejecta velocity is subsonic and unrelated to impact energy. Anorthosite mass loss, via cratering and spalling, is proportional to impact energy. Resulting crater shapes in the anorthosite were assessed via 3-D laser scanning. They show an overall increase in crater depth with velocity. Three mathematical fits were evaluated to define crater shape: parabolic, hyperbolic and power law. Each of the fitted equations was then used to predict the shape of the transient crater. Out of nine samples that retained viable crater shapes, the hyperbolic fit performed best in seven cases, and power law best in two. Monte Carlo simulation was deployed to reconstruct pre-spall crater volume. Total material loss versus calculated crater volume shows increasing divergence above ∼5.5 km/s, which is attributed to the growing role of spall in removing target material at increasing velocity. Shock calculations based on known densities, zero-pressure bulk sound velocities and particle velocity coefficients for target and projectile allow the determination of shock wave velocity, particle velocity and pressure relations, as well as penetration velocity; the latter indicating underdriven excavation conditions. The calculated maximum contact shock pressures attained for the eleven shots range from 24 GPa (1.75 km/s) to 149 GPa (6.69 km/s). For the impact velocities experienced and following crater reconstruction (to pre-spall state) the results indicate that (1) crater shape and depth/diameter ratios are velocity independent, and (2) consideration be given to the hyperbolic form for craters generated by hypervelocity impact.