An Experimental Tomography Study of Impact-induced Damage Beneath

Introduction: Large scale reductions in compressional velocity caused by impact-induced damage is an important but largely unstudied feature of impact craters [1]. Damage depth beneath impact craters can be used to extract information about the impact cratering process. A new non-destructive method is developed to study the damage structure beneath impact craters in the laboratory. Experimental setup: (1) Rifle shot: To produce the crater in the target, a 0.220 inch caliber rifle is used to launch the commercial lead bullet into the San Marcos granite block, with dimension of 20x20x15 cm, and density of 2.657 g/cm. Two breakwires are placed on the path of the bullet for velocity measurement. The impact velocity of this shot is ~1200 m/s. The initial pressure of this impact is calculated using impedance match method [2]. (2) Tomographic set up: a 2-D tomography method is used to study the shock-induced velocity reduction in the recovered target [3]. Fig. 1 is the tomographic ray diagram. A tungsten carbide ball (diameter 0.08 cm) is launched by a laboratory constructed air gun to produce the source wave. The new apparatus allows 1cm spatial resolution and velocityr resolution of ±0.2 km/s. Two P-wave transducers are used; one is placed next to the mechanical source, the other is placed on the surfaces of the target at other 34 stations, with 1cm increment. The frequency of the transducers is 5 MHz. A 0.5 mm thick anvil of WC is placed between the impact source and the target surface to prevent mi-