Research On Detection Method Of Structural Internal Defects Based On Laser Ultrasound

Laser ultrasound combines the advantages of non-contact laser and internal sensitivity of ultrasound to information,and has a broad application prospect in the detection field.Laser-excited ultrasound is a comprehensive process of multiple physical fields,and its physical mechanism is complex.It can simultaneously excite ultrasonic waves of longitudinal wave,transverse wave,surface wave and other modes,and each mode has its own propagation law.This paper reveals the propagation law of longitudinal wave and transverse wave of laser-excited ultrasound through the methods,and based on this,carries out the detection research of internal defects of structures.In this paper,aiming at the multi-mode ultrasonic phenomenon of laser pulse excitation,the aluminum plate is taken as the research object,and the different-side transceiving and the same-side transceiving modes are adopted respectively.By adjusting the distance between laser excitation and receiving,the time-domain signals of each mode with different receiving distances are extracted,the complex propagation law of multiple reflections of transverse wave and longitudinal wave in the structure body and the reason of ultrasonic aliasing of each mode wave are revealed,in the process,the sound field and direction diagram of longitudinal wave and shear wave on upper and lower surface are reconstructed.Subsequently,the sound field on the lower surface of the metal-composite bonding structure with the same thickness is studied.According to the best reception distance of the longitudinal wave and transverse wave,the internal defects of the metal-composite bonding structure are scanned and detected point by point by using the transmission method.And that the signal-noise ratio of the longitudinal wave and the transverse wave excited by the lase are improved by using the variational mode decomposition method,the results show that the defect area can be completely reconstructed by the longitudinal wave and the transverse wave under the condition of the optimal pointing angle receiving.At last,the reflection method is used to detect the internal defects of the flat-bottomed hole in the aluminum material,and the defect depth information is obtained according to the transit time of the echo.The propagation law and directivity of longitudinal wave and transverse wave in laser ultrasound studied in this paper are of guiding significance for detecting the internal defects of structures.