Nondestructive Evaluation of Change in Spall Damage under Repeated Plate Impact Tests by Flyer Plate of Different Thickness

Repeated plate impact tests on medium carbon steel were carried out on the identical target plate being impacted by flyer plate of the different thickness in each impact test. Evolution of spall damage under repeated impacts was evaluated nondestructively with the B- and C-scan images by low frequency scanning acoustic microscope as well as the ultrasonic variables (ultrasonic velocity, amplitude ratio of second to first backwall echo and backscattering intensity). The spall damage density and sizes after the second impact have increased in the impact surface side, when the second impact stress is lower than the first. On the other hand, when the second impact stress is higher than the first, the spall damage has increased in the back surface side. The change in B- and C-scan images of the spall damages under repeated impacts is well correlated with the change in ultrasonic variables. These methods give us an advanced means to evaluate nondestructively spall damage and to make clear the mechanisms of the spall damage growth under repeated impacts.