Plate impact experiments to investigate shock-induced inelasticity in Westerly granite

In the present study, plate-impact experiments are employed to better understand the stress threshold for inelasticity in Westerly granite. The experiments are designed to obtain the Hugoniot Elastic Limit (HEL) as well as spall strength following shock-induced compression in the Westerly granite samples. The HEL for Westerly granite is estimated to be in the range 3.2 to 3.5. GPa, while the spall strength is ~50. MPa for a pulse duration of 3. μs, and is nearly independent of the compression stress level in the stress range employed in the experiments (i.e. 0.7-5.0. GPa). In addition, soft-recovery plate-impact experiments are employed to better understand the threshold for initiation of micro-cracking in the granite samples. These recovery experiments are conducted at the same stress levels as the normal plate impact experiments (~0.7. GPa), and the duration of the tensile pulse is varied from 20 to 500. ns by varying the pre-set gap between the specimen and the momentum trap. The results indicate that the stress pulse duration threshold for initiation of spall is about 120. ns at an applied tensile stress level of ~0.7. GPa. In addition, tensile fracture in Westerly granite is observed to be time dependent with the spall strength dependent on the duration of the tensile pulse. For the soft recovery experiments that do not show a clear spall signal, the recovered granite specimens show relatively large micro-cracks (30-40. μm in width) near the plane where the unloading waves from the flyer and target free surfaces intersect. The spall plane reveals a relatively rough and a tortuous fracture surface. At higher magnifications, several cracks and pits/holes are detected on the spall plane. The fracture/damage mode is seen to be predominantly brittle, with damage initiating at the weak grain boundaries