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Published September 1990 | metadata_only
Journal Article

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.

Additional Information

© 1990 Academic Press, Inc. Received December 8, 1986; revised February l, 1990. We have benefited from technical discussions with Jay Melosh and technical reviews from A. Vickery and A. Fujiwara. We thank Peter Schultz for his assistance in utilizing the Ames facility. This research was supported by NASA Grant NGL-05-002-105. Contribution 4405, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.

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August 19, 2023
August 19, 2023