Ultrasonic imaging of stress corrosion crack formed in high temperature pressurized water using subharmonic phased array

Closed stress corrosion cracks (SCCs) have been generated in high temperature pressurized water in weld parts of important structure of atomic power plants. The depth of closed SCCs is often underestimated or overlooked, since the ultrasound is transparent through the closed cracks. This leads to catastrophic accidents. To solve this problem, we have developed a novel imaging method, namely, the subharmonic phased array for crack evaluation (SPACE), on the basis of subharmonic waves and a phased array algorithm. In this study, we formed an SCC in heat affected zone of weld part of austenitic stainless steel in high temperature pressurized water (280 deg C) to simulate those formed in actual atomic power plants. Then we imaged the SCC using SPACE with LN array transmitter. As a result, the subharmonic images visualized the crack tip which was deeper than in the fundamental images corresponding to linear phased-array image. Thus, we demonstrated that SPACE with LN array transmitter is useful in reducing the underestimation of closed-crack depths.