Picosecond laser ultrasonic imaging detection of near-surface micro defects using PCS and SAFT algorithm

The effective detection and imaging of near-surface micro defects has been the key problem in the ultrasonic inspection field for a long time. Laser ultrasound has high resolution and sensitivity which is the effective technology to detect micro defects. In this paper, the time and frequency domain characteristics of the picosecond laser ultrasound and the interaction mechanism between micro defects and ultrasonic waves are studied by simulation and experiment. A picosecond pulsed laser is used to excite ultrasonic waves in samples with near-surface micro defects of different depths, and the propagating ultrasonic waves are detected by an optical interferometer. Through the analysis of the experimental results, the near-surface micro defects with diameters of 0.13 mm and depths of 0.3 mm, 0.5 mm, 1 mm and 2 mm can be detected. In addition, a defect imaging algorithm combining principal component subtraction (PCS) and synthetic aperture focusing technique (SAFT) is proposed, which can effectively suppress the detection blind areas and artifacts caused by Rayleigh (R)-wave interference. Compared with the conventional total focusing method (TFM) of water-immersion ultrasound, the average SNR is increased by 25.57 dB. This research is poised to contribute to the resolution of quality assessment challenges in product manufacturing and service processes.