F-SAFT based on laser-induced directional multimode ultrasound for defect imaging

In conventional laser ultrasonic (LU) single-mode frequency domain synthetic aperture focusing technique (F-SAFT) imaging, the precision of the imaged defect profile is limited due to the influence of the directionality of laser-induced ultrasound and the directionality of detection. To solve this problem, LU-based F-SAFT imaging with the shear wave, longitudinal wave, and mode-converted wave is proposed in this article. In the experiment, multimode bulk waves are excited by a pulsed laser, and a laser Doppler vibrometer is employed to detect ultrasound waves. The F-SAFT imaging is performed with single-mode and multimode ultrasound wave signals, respectively. The results show that F-SAFT imaging fusing three mode signals (shear, longitudinal, and mode-converted waves) improves the range of the upper surface of the imaged defect by nearly two times. In addition, the profile of the imaged defect is more complete than that of using a combination of both longitudinal and shear waves. By analyzing the influence of the directionality of laser-induced ultrasound and the detection directionality caused by the out-of-plane ultrasound component on the imaging results, a theoretical approach is used to select a reasonable excitation–detection distance for the use of multimode-fused F-SAFT algorithm. Furthermore, the feasibility of the theoretical approach is verified by the experimental results.