High resolution imaging of stress corrosion cracking by a confocal subharmonic ultrasonic phased array

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To ensure the safety and reliability of structures such as nuclear power plants, it is necessary to detect closed crack by nonlinear ultrasound. Though the novel imaging method, subharmonic phased array for crack evaluation (SPACE)is a practical solution, the lateral resolution was not high enough. In this research, we developed confocal SPACE with a single array transducer which can transmit a large amplitude focused wave and detect scattered wave using a single array sensor. We then succeeded to image an SCC in a heat affected zone (HAZ)of type 304 stainless steel specimen, formed in high temperature pressurized water, and succeeded in visualizing the closed crack tip in subharmonic image with high selectivity, irrespective of strong linear scattering at grain boundary. We also succeeded in 2D simulation of these waves. This result will significantly contribute to improve the safety and reliability of nuclear power plants.