The Investigations On Lamb Wave Testing Technique And HHT Time-frequency Analysis Method

Platelike material has been widely used in real of aerospace, automotive and other industries. Comparing with the longitudinal and transverse wave testing, character of lamb wave testing is more effective and fast. As the complexity of lamb wave theory and testing mechanism, there also exist many problems in the testing process, such as the selection and determination of mode, and explanation of signals, which greatly limit the application in industrial production.Lamb wave dispersion curve is an important basis for testing. Plotting dispersion curve easily and explaining the signal accurately are the key problems to solve for promote the application of lamb wave technology. Based on the ultrasonic guide wave theory, the plotting of disperse curve and time-frequency analysis of testing signal were investigated in this work. According to the character that disperse curve is the distribution figure of real solution of disperse equation, the process of solving equation and plotting curve has been simplified. Curve of Lamb wave propagation characteristics in aluminium plate was plotted by direct iteration method used Matlab software., including the curves of Lamb wave dispersion, excitation angle and wave structure. Experimental investigation was carried out on a aluminum plate with the thickness of 1.29 mm by a variable angle 2.5 MHz probe, and it is found that there is a good agreement between the theory calculation and experimental values of group velocity, phase velocity and exciting angle. A single mode (So) of the lamb wave was excited according to the disperse curve, against the non-stationary of lamb wave. Artificial defect hole with diameter of 2, 2.5, 3, 4 and 5mm was tested, and signals were analyzed by HHT time-frequency method. Signal reconstruction of the IMF components from EMD decomposition was carried out and envelope spectrum of the signal was obtained so as to determine the arrival time of defect wave packet. Lame wave signals of different hole diameter were characterized by the parameters of instantaneous frequency and magnitude from Hilbert transform. The results show that the value of instantaneous peak linearly decrease with increasing hole diameter, and the linear correlation coefficient are R~2 = 0.97 between them. Value of instantaneous peak well reflected the change of hole diameter, time-frequency analysis of HHT method provided a good reference for the application of the Lamb wave detecting defects targeted and quantitative analysis. Finally, plotting system of lamb wave testing curve was made by Matlab, and all the curves were collected in one interface. Calculation and plotting process were completed through simple human-computer interaction (such as inputting velocity of P-wave and S-wave in the plate). Our work provide a convenient method of plotting lamb wave testing curve and make the real-time obtained the curve possibly for testing personnel.