Free ebook Shock-induced damage in rocks: application to impact cratering
Two representative terrestrial rocks, San Marcos granite and Bedford limestone, are chosen as test target. Impacts into the rock targets with different combinations of projectile material, size, impact angle, and impact velocity are carried out at cm scale in the laboratory.
Shock-induced damage and fracturing would cause large-scale compressional wave velocity reduction in the recovered target beneath the impact crater. The shock-induced damage is measured by mapping the compressional wave velocity reduction in the recovered target.
Both compressional velocity and attenuation are measured in three orthogonal directions on cubes prepared from one granite target impacted by a lead bullet at 1200 m/s. Anisotropy is observed from both results, but the attenuation seems to be a more useful parameter than acoustic velocity in studying orientation of cracks.
Our experiments indicate that the shock-induced damage is a function of impact conditions including projectile type and size, impact velocity, and target properties. Combined with other crater phenomena such as crater diameter, depth, ejecta, etc., shock-induced damage would be used as an important yet not well recognized constraint for impact history.
The Johnson-Holmquist strength and failure model, initially developed for ceramics, is applied to geological materials. Strength is a complicated function of pressure, strain, strain rate, and damage.
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