The Nondestructive Testing Of Ultrasonic Lamb Wave In Thin Metal Sheet

Compared with the conventional ultrasonic nondestructive testing techniques, Lamb waves method is rapid and efficient and especially suitable for nondestructive testing of large plate-shaped structure. The nondestructive test in metal sheet is the most early application domain of Lamb wave technique, however, because of the complication of the theory and mechanism of Lamb waves, this technique has not yet achieved significant breakthrough.There are also a lot of inconsistent viewpoints and unresolved problems which have greatly limited its application in the industry.Short-time Fourier Transform (STFT) is used to recognise the modes of Lamb waves propagating in steel sheet. By analyzing the signals of Lamb waves in time domain, frequency domain and time-frequency domain, the artificial defects is detected nondestructively, the effects of different defects on propagation of Lamb waves, and positioning of defects are studied. The vibration displacement formula is deduced and the vibration displacement curves of Lamb waves are plotted. Also, The relationship between the vibration mode and sensitivity of Lamb waves is discussed according to the experimental data. Finally, a new non-stationary signal processing method, Hilbert-Huang Transform (HHT), is apply to analysis the Lamb wave signals. Instantaneous amplitude is extracted to characterize the defects in metal sheet, the rationality of this method for Lamb wave signals analysis is also discused from experimental viewpoint.Through experimental analysis and research, conclusions can be drawn as following: (1) Lamb wave modes can be effectively recognised by STFT. (2) hole-type defects and slot-type defects in the sheet can all be detected effectively by time domain analysis method, frequency domain analysis method or STFT method. (3)hole-type defects are not prone to cause mode conversion and Lamb wave energy changes simply with the increase of the diameter; slot-type defects are prone to cause mode conversion and Lamb wave energy changes complexly with the increase of slot depth. (4)Time-delay Estimation method and the method calculating by theoretical time-frequcncy distribution curves can both position the given defects, the former is simpler for calculation and the latter is preciser for positioning. (5) The direct proportion relationship between the vertical component of the vibration displacement and the energy of the reflected wave in this depth is determined primarily. (6) Multimode Lamb waves singels can be analysed effectively by improved HHT.