Tests for crack diffraction enhancement based on phased array laser ultrasound
GAO Feng1, ZHOU Hong1, HUANG Chao2
1. School of Air Transport, Shanghai University of Engineering and Technology, Shanghai 201620, China;
2. School of Urban Rail Transit, Shanghai University of Engineering and Technology, Shanghai 201620, China
Abstract：Aiming at the problems of poor detection accuracy, low reliability of area detection and low signal-to-noise ratio of ultrasonic diffraction time difference method, a phased array laser ultrasonic crack detection method based on fiber picosecond laser and high-speed rotating mirror is proposed. The finite element method is used to simulate the thermoelastic mechanism, and a two-dimensional transient laser ultrasonic thermo-solid coupling model is established to generate S-waves and P-waves to diffract at the defect. The influence of different singularities of crack tip, different center frequency of acoustic wave and different position of phased array excitation source on acoustic diffraction is analyzed and verified by experiment. The results show that the numerical simulation results are in good agreement with the experimental results. Compared with the traditional single beam laser source, the phased array laser source has an obvious enhancement effect on the diffraction signal amplitude and signal-to-noise ratio, and the P-wave diffraction signal is ideal. The amplitude of the diffraction signal increases with the increase of the defect singularities and the decrease of the acoustic wave frequency. The signal-to-noise ratio increases with the increase of the tip singularity, and has no obvious change with the increase of the center frequency in a certain range. With the increase of the laser source distance, the signal-to-noise ratio first increases and then decreases, and the error between quantitative analysis of crack length and actual crack is less than 6.8%.
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