Experimental hypervelocity impact of sedimentary and metamorphic rocks: Reconstruction of crater shapes and volumes

The shapes and volumes of crater forms have been determined using 3-D laser scan data acquired following the hypervelocity impact of rock. Forty-four shots (23 sedimentary targets and 21 metamorphic targets) at impact velocities from 2.50 to 7.85 km/s have been assessed from the Multidisciplinary Experimental and Modeling Impact Network (MEMIN). Best fits have been determined for parabolic, hyperbolic and power law shapes for the central (penetration) crater, including the hemispheric form for the spall (lateral) crater. For the central craters, both the remnant and reconstructed forms are evaluated. In both cases, the best shape fits are hyperbolic, followed by parabolic then power law. The maximum angles at which the reconstructed central craters intersect the surface are 60-65° for hyperbolic, 80-90° for parabolic and 70-90° for power law. Maximum ejecta angles captured by high-speed video during the experiments are most closely matched by the predicted hyperbolic angles. Reconstructed central crater depth-diameter relations are between 0.20 and 0.40 for most target rocks (neglecting outliers in sedimentary targets), with the ratio being velocity and impact energy invariant. The depth-diameter ratio for the metamorphic targets is more constrained at an average of ∼0.20. For the spall craters, the best shape fits are power law, followed by hyperbolic then parabolic and hemispheric. Spall craters are close to conic in form. With increasing impact energy the contribution of spall to total crater volume increases relative to central crater volume. As the overall best fit for the central crater shape, the hyperbolic form has the highest volume (25% more than the parabolic) and the lowest (broadest spread) ejecta angles. This indicates that for brittle materials in the strength regime a greater ejecta volume would be generated and dispersed over a wider area relative to the other forms, with implications for protective system design and applications.