Research On Signal Processing Of Laser Ultrasonic Testing Of Metal Surface Defects

The surface defects is easy to cause the damage and aging of metal materials and it is necessary to detect the metal surface defects.Laser ultrasonic detection has many advantages,such as non-contact excitation and detection,wide frequency band,wide range of applicable materials,accurate measurement and so on.In this paper,the finite element method is used to analyze the laser ultrasonic in metal materials.Combined with simulation and experiment,the noise reduction method of laser ultrasonic signal and the change of defect characteristic signal in time and frequency domain are studied.The main contents of this paper are as follows.(1)The finite element software COMSOL is used to simulate the ultrasonic field and surface displacement waveforms excited by laser on the surface of a aluminum block without and with defects under the thermoelastic mechanism.The reflection and transmission effects of surface defects on the ultrasonic wave excited by laser are analyzed.An experimental system to detect surface defects with linear light source is built.(2)Aiming at the difficulty of feature extraction caused by low signal-to-noise ratio of laser ultrasonic signal,the methods of wavelet threshold and empirical mode decomposition(EMD)are used to de noise the signal.This paper mainly discusses the selection of wavelet parameters for noise reduction,EMD signal-to-noise modal sorting and reconstruction criteria.Combined with wavelet threshold and EMD,the mode aliasing in EMD is improved.The wavelet threshold and EMD(wavelet threshold de-noising in critical layer and after IMF)are compared through the denoising effects of the simulation signal and the experimental signal,and the time-frequency analysis of the experimental signal after denoising.The results show that the wavelet threshold denoising effect is better.(3)The effects of metal surface defects and micro surface defects on far-field and near-field surface acoustic waves are studied.The time-domain and frequency-domain characteristics of direct wave,reflected wave and transmitted surface acoustic wave at different excitation positions are analysed.The results show that when the laser is excited at the defect,the direct wave and reflected wave will overlap in time domain,which corresponds to the amplitude increasing at the central frequency of direct wave spectrum.The sound velocity of surface acoustic wave excited by laser in different test blocks is measured.The contact and non-contact detection systems are built to collect laser ultrasonic signals at the same time,and the surface defects are located by pulse echo method.The non-contact laser ultrasonic detection system based on dual wave hybrid interferometer is determined to have higher accuracy for defect location.