抄録
Growth or curtail of spall damage in aluminum plates under repeated impacts has been monitored with a C-scan acoustic microscope (C-SAM) and a PVDF focused transducer. The spall damage or ductile voids were formed by plate impact tests using a gas gun which has a special recovery unit for target plates. The target plate has been impacted up to a maximum of three times with velocities ranging from 100m/s to 200m/s. This experiment has revealed the following: (1) From the C-scan and B-scan images of the spall damage, we can observe the change in three dimensional distribution of voids in aluminum plates under repeated impact. (2) When the second impact stress is higher than the first, which is greater than the spall threshold stress, the voids grow in size and number. (3) On the contrary, a part of voids disappear when the first impact stress is higher than the second, only if the latter is greater than some critical value. (4) The B- and C-scan images overestimate the void size when the voids are less than the ultrasonic beam diameter in size at the focus and are aligned closely in a plane. (5) The changes in B- and C-scan images under repeated impact are well correlated with the change in attenuation (amplitude change of B2 to B1 echo).
