Crater ellipticity in hypervelocity impacts on metals

The change in crater shape of hypervelocity impacts as a function of impact angle has been studied for impacts at 2 and 5 km s(-1) of millimeter-sized spherical projectiles of cellulose acetate, aluminum and stainless steel impacting aluminum, and stainless steel, and lead targets. The variation in crater depth, length, and diameter with impact angle is presented and discussed. While the main dependence on impact angle is found to be of a cos theta nature, cases are found where the magnitude of the dependence changes at discrete angles of impact. This is interpreted as evidence of disruption of the projectile affecting the final size of the crater. Crater depth is found to depend on (cos theta)(n), where n is strongly dependent on the projectile density and not the traditionally assumed constant value of two-thirds. For aluminum impacting aluminum at 5 km s(-1), n is found to be near unity. Reproducing the criteria used in analysis of crater shape in published work for the Long Duration Exposure Facility satellite yields a mean density for small particles in Low Earth Orbit of 3400 kg m(-3). Normal incidence impacts of non-spherical projectiles have also been studied and found to yield irregularly shaped craters, which can appear as oblique angle impacts of spheres. No sensible cuts on crater measurements permit clean separation of these two types of impacts.