Nonlinear surface-acoustic-wave phased array with fixed-voltage fundamental wave amplitude difference for imaging closed cracks

To achieve higher signal-to-noise ratios (SNRs) in imaging closed cracks in the vicinity of a surface, we propose a nonlinear surface-acoustic-wave phased array imaging method based on the fixed-voltage fundamental wave amplitude difference, which enables the inference of all the nonlinear components generated at cracks based on the energy movement between the fundamental and nonlinear components. After confirming its fundamental performance with a slit specimen, we applied it to two kinds of crack specimens: a thermal fatigue crack and a stress corrosion cracking extending from a mechanical fatigue crack. As a result, we succeeded in selectively visualizing the open and closed cracks with high SNRs. Furthermore, the analyses of delay-and-sum waveforms suggest that the proposed method can achieve a higher sensitivity than other nonlinear ultrasonic methods, which measure only a specific nonlinear component.