Ultrasonic detection of spall damage induced by low-velocity repeated impact

This paper address the examination of spall damage in medium carbon steel subjected to a repeated impact testing. The experiments were performed well below the threshold spall-stress of 2.6 GPa and the damage introduced in the subsurface volume was investigated using the low frequency scanning acoustic microscopy. Based on B- and C-scan images (the images taken along and perpendicular to the impact surface) we made a qualitative and semi-quantitative evaluation of the damage type (voids in a ductile material or cracks in a brittle one) and its distribution. We found the spall damage development dependent on the amplitude and the duration of the stress pulses. In particular, we proved that the high, long stress pulses induce damage that resembles tensile failure of material, in which voids or cracks nucleate along the spall plane to form macro-cracks. This explains why spall-damage is not seen when the first impact is below the characteristic threshold spall-stress. However, when the tests consist of more than four impacts the spall damage is produced already under stress below the threshold-value.