Recent progress on nonlinear ultrasonic phased array for closed-crack imaging

Abstract

Closed cracks are challenging defects for ultrasonic testing since they are transparent to conventional ultrasound techniques. This can cause the underestimation or overlook of closed cracks, resulting in potential catastrophic accidents. To solve this problem, several types of nonlinear ultrasonic phased array has been developed by combining nonlinear ultrasonics with phased array (PA). In this study, we introduce the recent progress on nonlinear ultrasonic PA. The first one is called fundamental wave amplitude difference (FAD), which is based on the nonlinear incident-wave-amplitude dependence of fundamental responses. The key to the success of FAD is how high incident wave amplitude can be generated in samples to cause the contact vibration of crack faces. However, increasing the incident wave amplitude at MHz frequencies is not easy. To enhance the applicability of nonlinear ultrasonic PA, we have also developed ultrafast imaging (MHz range) with pump excitation (kHz range). We have successfully captured high-speed crack dynamics using plane wave imaging (PWI) during the pump excitation that can generate a large displacement of more than 1000 nm. Furthermore, we will talk about ongoing work toward 3D crack imaging. [Work partially supported by JSPS KAKENHI (19K21910, 21H04592, 22K18745) and JST FOREST program (JPMJFR2023).]