Research On Ultrasonic Lamb Wave Detection Method Based On Time Reversal

Ultrasonic Lamb wave has many advantages, such as longer propagation distance and lower attenuation. It has been widely used in non-destructive testing of large-scale plate structure. The present research can realize the two-dimensional imaging of the crack in the thin plate and provide a method to detect the crack of the metal sheet. However, the depth of the cracks in the thin plate has not been effectively detected. This research will focus on the problem. In this paper, the experimental object is aluminum slab which is widely used in practical engineering.Combining with the time reversal method and the ultrasonic Lamb wave, this paper carries out some research on Lamb wave signal reconstruction, signal enhancement,signal denoising, and depth localization. The main contributions are as follows:The propagation law and dispersion characteristics of Lamb wave are analyzed.Based on the frequency equation of Lamb wave, the relationship between group velocity and phase velocity, the multimodal characteristics of group velocity and phase velocity of Lamb wave in aluminum thin plate were researched by using MATLAB. The modal identification of the Lamb wave detection signal in the aluminum plate is carried out by using the short time Fourier transform. The time reversal method and the time reversal characteristic of Lamb wave are researched,and the received multimode signal is reconstructed into a single mode signal by time reversal. The propagation law of Lamb wave in time reversal process is analyzed.The finite element modeling and simulation of the propagation of Lamb waves in the aluminum plate is carried out.In order to reduce the influence of noise on the Lamb wave detection signal and increase the precision of feature extraction, a denoising method based on fractional differential is proposed. Firstly, the amplitude spectrum of the noisy Lamb wave signal is differentiated by fractional differential to identify the frequency domain characteristic parameters. Then reconstructs the amplitude spectrum of the original signal according to the Tsallis model, finally obtaining the denoised Lamb wave signal combine with the phase information.The segmentation method of calculating the crack depth is researched. Firstly,the relationship between the crack depth and the frequency response function is obtained by segmented calculations. Then, the time reversal method is used to establish the correspondence relation between the waveform damage index (DI) of the reconstructed signal and the crack depth. The crack depth can be calculated from this relation. The combination of this method and the time reversal method not only reduces the computational complexity but also completely loads the relevant information of the crack location. The simulation results show that the segmentation method can detect the crack depth accurately.