Multi-mode nonlinear ultrasonic phased array for imaging closed cracks

Nondestructive evaluation of closed cracks is one of the most challenging issues in ultrasonic testing. Thus far, we have developed a closed crack imaging method, subharmonic phased array for crack evaluation (SPACE) of using only longitudinal (LL) subharmonic waves. In this study, we investigated the imaging capabilities of SPACE with LL and mode-converted shear (LS) subharmonic waves. After formulating the imaging algorithm, this method was applied to a complexly branched stress corrosion crack (SCC). As a result, a few responses of the branched SCC were observed in the LL subharmonic image, whereas more multiple responses of the branched SCC with higher resolutions were observed in the LS subharmonic image. Importantly, the LS subharmonic image successfully visualized a lot of responses that were not visualized in the LL subharmonic image. This suggests that the use of both LL and LS subharmonic images will make SPACE more robust for practical application.