Inspection And Signal Processing Of Lamb Wave To The Plate Defect

Metal plate has been widely used in modern engineering. Defects in the metal plate affect the quality of components. Comparing to conventional ultrasonic testing technology, Lamb wave is a new testing technology with characteristics of large-scale and long-distance. However, because of the complication of Lamb wave transmission mechanism, interaction between Lamb wave and defect in plate is still unclear. The response characteristics of Lamb wave to the defects in thin aluminum plate is investigated to afford experimental basis to practical testing in this paper.Respectively using 2.5MHz and 5MHz variable angle probes to detect the through-hole defects of different length in the aluminium plates with single-sensor method. The results showed that two kinds of frequency all had a linear relationship between the defect echo amplitude and the defect size, but 5MHz was more sensitive and had a higher linear regression coefficient of echo amplitude and defect size than 2.5MHz. With bi-sensor method, it was difficult to emit the Lamb wave that contained only one mode. But the number of modes by 5MHz probe emitted was less than 2.5MHz.To compare the two send-receive methods, the single-sensor method can locate the defect, but the bi-sensor method has a higher linear regression coefficient of echo amplitude and defect size.Short Time Fourier transform (STFT) and Hilbert-Huang Transform (HHT) were used to analyze the signals which were collected in the experiment. Time-frequency diagrams of echo signal were obtained by STFT process, showing the changes of frequency in the process of wave propagation. The echo signal was decomposed by the empirical mode decomposition(EMD) of HHT. Signal reconstruction of the intrinsic mode function(IMF) components from EMD decomposition was carried out. The correlation coefficient obtained between reconstructed signal and the original signal. Finally, the IMF components had been done a Hilbert transformation, and the Hilbert-Huang spectrum was obtained. Using HHT to process and analyze the testing signal ofΦ3mm pylome that were obtained by different frequencies and different send-receive methods. The results showed the echo signal that was obtained in single-sensor method had a more obvious frequency dispersion phenomenon. To compare two kinds of signal processing method, HHT is more suitable than the STFT to process the testing signal of Lamb wave.