Fractures and Craters Produced in Sandstone by High-Velocity Projectiles

Abstract

Abstract The mechanics of impact crater formation in rock, particularly sandstone, has been studied, the velocity range being approximately that normally associated with oilwell gun perforators. The bullets were small steel spheres having diameters of 3/16, 9/32 and 7/16 in.; impact velocities ranged from 300 to 7.000 ft/sec. The craters have two distinct parts - a cylindrical hole (or burrow) with a diameter the same as that of the impacting sphere, and a wide-angle cup comprising most of the volume of the crater. The burrow is formed as material in front of the projectile is crushed and pushed aside, forming a cylindrical hole surrounded by a high-density zone. The cup forms as fractures are initiated in front of the projectile and propagate along logarithmic spirals, approximating maximum shear trajectories, to the free surface of the rock. A most significant observation (made for the first time) was that, below the base of the cup in one type of sandstone, there are a group of similar fractures, not extending to the surface, which are spaced uniformly a few millimeters apart. Each fracture follows roughly the contour of the base of the cup and appears to require a certain threshold impulse to initiate it. These fractures comprise a relatively high fraction of the total, newly exposed surface area. The volume of the material removed by crushing varies as the first power of the impact velocity and the volume removed by fracturing, as the second power of the impact velocity. Penetration varies linearly with the impact velocity and is inversely proportional to the specific acoustic resistance of the target material, the proportionality constant being dependent upon the shape of the projectile.