Projectile impact on soft, porous rock

Summary

Projectile Impact on Soft, Porous Rock

An experimental investigation was conducted to study the penetration phenomena associated with the normal impact of 60° conical and hemispherically-tipped cylindrical projectiles with a 6.35 mm diameter on green shale in the velocity range from 18 to 40 m/s. In addition to initial velocity, a special device measured the velocity history of the striker during penetration, and the deformation pattern of the target was obtained from grid lines inscribed on a vertical diametral section of the specimen which was reassembled by emplacement in a steel ring before testing. The results showed that the tip geometry controlled the penetration depth which was quadratically related to the initial kinetic energy. Comparison with other data using spherical strikers indicated that only the contact geometry, and not the configuration nor mass of the rest of the striker determined the indentation depth for the same initial bullet energy within the present range of impact parameters.

The tests also indicated that target deformation was comprised of material compaction varying in degree from a maximum at the initial contact point towards the target interior and equivolumnial material distortion. A computational model was constructed incorporating both of these features based on an experimentalempirical relation between hydrostatic pressure and volumetric strain and two representations for the effective stress-strain relation of an elastic-perfectly plastic comportment of the material. It was found that good agreement was obtained between the observed deformation pattern and the predictions of this model when flow stress was considered to be independent of volumetric strain, but less so when this quantity was taken as a function of this strain. The model also adequately predicted the velocity history of the projectile both for the present tests and those involving spherical strikers conducted previously.