Ultrasonic testing (UT) has often encountered the problem that the crack closure due to compressive residual stress or by oxide film generation between crack faces weakens crack response. Furthermore, other unknown linear scatterers (e.g., coarse grains, welds, geometric change) can lower signal-to-noise ratio (SNR). To realize high-selectivity crack imaging, various types of nonlinear ultrasonic phased arrays that combine nonlinear ultrasonics with PA have been studied. Most existing techniques rely on the contact vibration of crack faces induced by probe waves in the MHz range. However, their practical applicability is limited by the maximum achievable displacement amplitude of the incident waves. To overcome this difficulty, we have developed a promising method that combines pump wave (kHz) with ultrafast PA imaging (MHz) based on plane wave incidence. Note that the pump wave (kHz) is utilized to excite a very high displacement of 1,000 nm, which is much greater than the upper limitation for MHz frequency. The high-speed contact vibration of crack faces caused by the pump excitation is captured by ultrafast PA imaging at thousands of frames per second. In this study, we will show some imaging results of closed cracks obtained by the proposed method.
Yoshikazu Ohara (Wed,) studied this question.