Imaging and Sizing of Surface Defects Using Synthetic Aperture Focusing of Laser-generated Surface Waves

The synthetic aperture focusing technique (SAFT) can be easily adapted to laser ultrasonic testing and can improve the signal-to-noise ratio (SNR) of laser ultrasonic images, to enhance defect detectability. In this study, the imaging and sizing of surface defects were carried out when SAFT was adopted for laser-generated surface waves. Six different surface breaking notches were made on an aluminum plate and were visualized using a laser ultrasonic scanning system consisting of a pulsed laser for ultrasonic generation and a laser interferometer for ultrasonic reception. B-scan images and SAFT-processed images for the six different notches were compared, which highlighted the advantages of the SAFT post-processing compared to B-scan imaging. A methodology to size the defects in SAFT-processed images was also proposed. The methodology utilizes the linear relationship between the suitable threshold decibel value, which determines the exact defect length with the use of an amplitude-drop method, along with the SNR of the defect echo in the SAFT-processed images. The experimental results demonstrate that the estimated defect lengths are similar to actual defect lengths, which showing that the proposed methodology can be effectively used for sizing of surface defects in SAFT-processed images.