The Nondestructive Testing Of Thin Metal Sheet By Ultrasonic Guided Wave

It is very difficult to test thin plates nondestructively by conventional ultrasonic methods, but Lamb wave techniques are rapid and efficient for nondestructive testing of large plate-shaped structure. The interaction between Lamb waves and defects were complex, due to absence of the knowledge, there are still a lot of inconsistent viewpoints and unresolved problems for Lamb wave techniques. In this research, the Lamb wave signals obtained in the defect-free plate and defected ones, respectively, are analyzed in time-frequency domain, on the basis of analyzing the Lamb modes and the Lamb mode conversion, characteristic parameters are extracted to characterize and evaluate the defects; besides, the relationship between the vibration displacement and the sensitivity of Lamb mode is researched.Slots with different depth and length are made in metal sheet of ST12. In these metal sheets, Lamb waves are excited and sampled. The digital Lamb wave signals are analyzed and the result shows: there is a good corresponding relationship between the defect size(the depth or the length of the slots)and the change of the average instantaneous frequencies of the signals; the main Lamb wave mode, S1, was isolated in time domain by the FFT band-pass filtering, and then the centroid of its envelope was extracted, good correlation of the centroid position with the slot depth was obtained; the change of Lamb wave's shape factor are correlative linearly with the defect size. On the basis of the EMD processing, the shape factor would be of great promise for detecting and evaluating the slight defect. Also, there is a good corresponding relationship between the defect sizes and the frequency centers of signals.The relationship between the vibration displacement and the sensitivity of Lamb wave testing is researched. The result shows: there is a good corresponding relationship between the vibration displacement and the sensitivity of testing, which provides guidance for the choice of the sensitive Lamb modes, and a convicting experimental support for the current viewpoint which is still disputed on.