Numerical Simulation Study of Laser Ultrasound Synthetic Aperture Focusing Imaging Technology in Subsurface Defects

Accurate detection of subsurface defects in materials is critical to ensuring the structural integrity and service reliability of workpieces. In order to address the challenge of visual detection of subsurface defects in workpieces, in this paper, a laser ultrasonic subsurface defect imaging method employing a dynamic two-dimensional (2D) rectangular window based on the time-domain synthetic aperture focusing technique (T-SAFT) is proposed. Firstly, the directional amplitude distribution of ultrasound generated by a pulse laser inside the workpiece and the propagation time delay characteristics of mode-converted acoustic waves are analyzed, and the longitudinal wave converted to shear wave (LS) signal to be used for the imaging of subsurface defects. Then, a numerical simulation model of laser ultrasonic subsurface defects is established, and the laser ultrasonic imaging method based on T-SAFT is adopted for the detection of subsurface defects. In order to achieve quantitative imaging detection of subsurface defects, a 2D dynamic rectangular window of appropriate size is introduced to extract the characteristic signals of subsurface defects. The visualization detection of subsurface defects with diameters ranging from 0.1 to 1.2 mm and burial depths ranging from 0.3 to 1.0 mm is successfully realized with the average detection error controlled within 7%. The proposed technique provides an effective detection method for the imaging detection of subsurface defects in laser ultrasonic technique filed.