Penetration into low-density media: In situ observation of penetration process of various projectiles

In order to understand the penetration process of projectiles into lower-density targets, we carry out hypervelocity impact experiments using low-density (60 mg cm−3) aerogel targets and various types of projectiles, and observe the track formation process in the targets using a high-speed camera. A carrot shaped track, a bulbous, and a “hybrid” one consisting of bulbous and thin parts, are formed. The results of the high-speed camera observations reveal the similarity and differences on the temporal evolution of the penetration depth and maximum diameter of these tracks. At very early stages of an impact, independent of projectile type, the temporal penetration depth is described by hydrodynamic models for the original projectiles. Afterward, when the breakup of projectiles does not occur, intact projectiles continue to penetrate the aerogels. In the case of the breakup of projectiles, the track expands with a velocity of about a sound velocity of the aerogel at final stages. If there are large fragments, they penetrate deeper and the tracks become a hybrid type. The penetration of the large fragments is described by hydrodynamic models. Based on these results, we discuss the excavation near the impact point by shock waves.