High-selectivity ultrasonic imaging of closed crack is an important subject for avoiding not only underestimation but also misidentification of closed cracks. Thus far, the combination of load difference phased array (LDPA) and the crack opening method (GPLC) that combines global preheating (GP) and local cooling (LC) has been verified. Here the LDPA utilized the subtraction of the phased array (PA) images before and after the application of thermal stress induced by GPLC. However, as the cooling time increased, the change in wave velocity depending on the temperature within specimen is not negligible. This results in ghosts that degrade the selectivity of closed crack in subtracted images. In this study, on the basis of the finding that the change in crack response is faster than that in the wave velocity during GPLC, we propose a short time interval subtraction (STIS) method as an option in LDPA. This is based on the subtraction of a PA image from that obtained before a short time interval. It was experimentally verified in a closed fatigue crack specimen made of aluminum alloy.
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