Improvement of Closed Crack Selectivity in Nonlinear Ultrasonic Imaging Using Fundamental Wave Amplitude Difference

To realize the high selectivity of closed cracks, we propose a fundamental wave amplitude difference (FAD) method based on the threshold behavior of fundamental waves caused by the contact vibration of closed cracks. This is realized by the subtraction of a fundamental array (FA) image at a small input amplitude multiplied by the amplification factor from that at a large input amplitude. The formulation clarified that FAD can selectively image closed cracks while eliminating other linear scatterers, which cannot be completely eliminated by the subharmonic wave amplitude difference (SAD) method. Furthermore, FAD was experimentally verified in a closed fatigue crack specimen using the closed-crack-imaging method of subharmonic phased array for crack evaluation (SPACE). Thus, we demonstrated that FAD is useful for achieving the higher selectivity of closed cracks against other linear scatterers than previous amplitude difference methods without filtering out the subharmonic or superharmonic waves.