Spatial and shape distributions of ejecta from hypervelocity impact between rock projectile and metal target

• Ejecta shape was analyzed based on penetration holes and an empirical model. • Simulated solid fragment shape distribution was compared with experimental results. • Melt droplet size is comparable to that predicted by a physical model. Hypervelocity impact experiments of rock projectiles and steel targets were conducted at velocities ranging from approximately 3 km/s to 7 km/s. Ejecta with various ejection angles were analyzed using aluminum foil targets. The number density of ejecta was highest at 50° within the range of 25° to 50° relative to the projectile trajectory examined in this study. The major and minor axes of the ejecta were estimated from the corresponding axes of the foil holes, using an empirical relationship newly formulated in this study based on a previous study. No clear dependence of the ejecta axial ratio distributions on ejection angle was observed for impacts at 3 km/s and 5 km/s. For impacts at 7 km/s, the axial ratio of the ejecta tended to be higher than that observed at lower impact velocities. The axial ratio distribution of the ejecta exhibited a dependence on size, with the fraction of ejecta smaller than 10 µm having small axial ratios being suppressed at an impact velocity of 7 km/s compared to 3 km/s or 5 km/s, likely due to the inclusion of melt droplets that would have high axial ratios. On the other hand, ejecta in the larger size range (>20 µm) showed no change in axial ratio distribution with respect to impact velocity, suggesting that ejecta of this size were probably solid fragments. Observations of the ejecta captured in aerogel blocks revealed spherical structures ranging in size from a few to 10 µm that may have been melt droplets. The sizes were of the same order of magnitude as predicted by a previous physical model, which considers the balance between kinetic energy and surface energy of melt.