Target Configurations for Plate-Impact Recovery Experiments

Normal plate impact recovery experiments have been perfomed on thin plates of ceramics, with and without a back momentum trap, in a one-stage gas gun. The free-surface velocity of the momentum trap was measured, using a normal velocity (or displacement) interferometer. In all recovered samples, cross-shaped cracks were seen to have been formed during the impact, at impact velocities as low as 27 m/s, even though star-shaped flyer plates were used. These cracks appear to be due to in-plane tensile stresses which develop in the sample as a result of the size mismatch between the flyer plate and the specimen (the impacting area of the flyer being smaller than the impacted area of the target) and because of the free-edge effects. Finite element computations, using PRONTO-2D and DYNA-3D, based on linear elasticity, confirm this observation. Based on numerical computations, a simple configuration for plate impact experiments is proposed, which minimizes the inplane tensile stresses allowing recovery experiments at much higher velocities than possible by the star-shaped flyer plate configuration. This is confirmed by normal plate impact recovery experiments which produced no tensile cracks at velocities in a range where the star-shaped flyer invariably introduces cross-shaped cracks in the sample. The new configuration includes lateral as well as longitudinal momentum traps.