Impact spallation experiments: Fracture patterns and spall velocities

Spall velocities were measured for nine experimental impacts into San Marcos gabbro targets. Impact velocities ranged from 1 to 6.5 km/sec. Projectiles were iron, aluminum, lead, and basalt of varying sizes. The projectile masses ranged from a 4-g lead bullet to a 0.04-g aluminum sphere. The velocities of fragments were measured from high-speed films taken of the events. The maximum spall velocity observed was 30 m/sec, or 0.56 percent of the 5.4 km/sec impact velocity. The measured velocities were compared to the spall velocities predicted by the spallation model of Melosh (1984). The compatibility between the spallation model for large planetary impacts and the results of these small-scale experiments is considered in detail. The targets were also bisected to observe the pattern of internal fractures. The series of fractures was observed, whose location coincided with the boundary between rock subjected to the peak shock compression and a theoretical “near-surface zone” predicted by the spallation model. According to the model, between this boundary and the free surface, the target material is expected to have received reduced levels of compressive stress as compared to the more highly shocked region below.