Stress Wave Induced Damage in Rock
Abstract
Blocks of San Marcos Gabbro and Bedford Limestone were impacted with high-velocity projectiles and longitudinal elastic velocity measurements were carried out on oriented cubes throughout a cross-section of each block. For both rock types the velocity increases rather uniformly with distance from the impact site, reaching the unshocked velocity at a distance of approximately 1 crater radius. The maximum observed velocity reduction is to slightly < half the unshocked seismic velocity for both gabbro and limestone(ls); however, the average velocity reduction within the damaged zone is significantly greater for ls. Observed anisotropy in velocity reduction in the gabbro sample clearly can be related to the preferred orientation of macroscopic cracks, suggesting that velocity measurements can be a powerful tool for characterizing crack density and orientation in shocked rock. Microscopic observations of the gabbro indicate that both the number of cracks/unit area and crack size increase as seismic velocity decreases. Observational estimates of crack density are generally slightly lower than estimates made from theoretical consideration of observed velocity reduction. Microscopic observation of the porous ls indicates that both the percentage of fractured grains and their degree of comminution increase as seismic velocity decreases. Anisotropy of crack orientation and seismic velocity is less pronounced in the ls than in the gabbro, possibly because fracture growth is controlled by grain-grain contacts which tend to be distributed homogeneously. The peak dynamic pressures induced by the quasi-spherical shock waves reached maxima of about 0.97 GPa and 0.84 GPa in the gabbro and ls, respectively, immediately below the crater floor.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 01, 1989
- Accession Number
- ADA211599
Entities
People
- Allan M. Rubin
- Thomas J. Arhens
Organizations
- California Institute of Technology