Ultrasonic evaluation of spall damage accumulation in aluminum and steel subjected to repeated impact

This paper shows that the spall damage accumulated in target plates during repeated impact tests can be effectively evaluated by invoking ultrasonic techniques. The experimental data obtained for commercially available, pure aluminum and medium carbon steel subjected to three times repeated plate impact test, serve as a proof of efficiency and accuracy of our method. The spall damage was evaluated using a low frequency scanning acoustic microscope (C- and B-scan images) as well as measurement of ultrasonic velocity, attenuation (amplitude change of B2–B1 echo) and backscattering intensity. We provide here a detailed account of the damage history in essentially ductile aluminum. This proves that the voids nucleated during the first impact form the defect array undergoing coalescence when the stress of second impact is lower than the level applied in the first one. The case of aluminum contrasts the behavior of carbon steel, governed by the development of microcracks generated already during the initial impact. Our conclusions are based on the output of B- and C-scan images consistent with the results of ultrasonic measurements. While providing insights into the evolution of the spall damage, the non-destructive ultrasonic technique promoted by the present authors offers advantages of efficiency, direct applicability to the components of commercial structures, and for early damage prediction.