Effect of Scale, Material Strength, and Loading on Ejecta Formation from Explosively Driven Aluminum

Abstract

When a shock wave reaches the free surface of a material with surface asperities, particles can be ejected from the surface. The mass and velocity of the ejecta depend on the strength and profile of the shock wave, the material in which the wave travels, and the finish of the free surface. In the present study, aluminum targets with machined triangular perturbations on the free surface were shock loaded by high explosives to 12.0 and 19.4 GPa and by plate impact to 14.5 GPa. In all experiments, the aluminum remained in the solid phase. Two scales of perturbations were tested: 30-μ m-deep and 500-μ m-deep V-shaped grooves with a 60° tip angle. The perturbation growth and ejecta formation were quantified using photonic doppler velocimetry and piezoelectric pins. It was found that the maximum observed velocity from the perturbed surface was nearly identical for both scales but that ejecta formed only when the larger scale perturbations were used. This result may be attributed to a scale effect caused by the smaller perturbation being on the scale of the grain size of the material. When the shock loading was removed by placing an air or vacuum gap between an explosive and the aluminum target, no ejecta was detected to within the instrumentation limits.