Experimental analysis of linear and nonlinear ultrasonic scatterings at closed fatigue crack using fixed-voltage fundamental wave amplitude difference with radarlike display

Although the understanding of the nonlinear ultrasonic scattering at closed cracks is essential for the practical application of nonlinear ultrasonic phased array, it has yet to be elucidated because of the lack of experimental techniques. This study proposes fixed-voltage fundamental wave amplitude difference (FAD) with radarlike display. We first describe the principle and imaging algorithm of the proposed method. In order to demonstrate the proposed imaging technique, we formed a closed fatigue crack in an aluminum-alloy specimen. After confirming the imaging capability of confocal fixed-voltage FAD, we examined the linear and nonlinear ultrasonic scatterings depending on incident angles with the radarlike display. As a result, we found that the nonlinear ultrasonic scattering was more sensitive to the incident angle than the linear one. We also interpreted the results with the absolute displacement of the incident wave amplitude. Thus, we demonstrated that fixed-voltage FAD with radarlike display is useful in obtaining physical insights and optimizing inspection conditions.