Wavelet and wavelet-packet analysis of Lamb wave signatures in real-time instrumentation

A persistent problem in the analysis of Lamb wave signatures in experimental data is the fact that several different modes appear simultaneously in the signal. The modes overlap in both frequency and time domains. Attempts to separate the overlapping Lamb wave signatures by conventional signal processing methods have been unsatisfactory, As might be expected, the transient nature of Lamb waves makes them readily tractable to wavelet analysis. The authors have used the discrete wavelet transform and the wavelet packet transform to untangle the Lamb wave signature. Furthermore, both techniques are realizable in the highly parallel cascaded-lattice architecture, and are well suited for on-line real-time instrumentation. For signatures of Lamb waves captured in laser ultrasonic data in tailor-welded blanks, this has led to straightforward detection of weld defects and demonstration of principle that weld defects can be classified according to the type of defect as revealed by features in wavelet space. This technique has considerable commercial value for online monitoring of manufacturing processes. For example, laser-based ultrasonic (LBU) measurement shows great promise for on-line monitoring of weld quality in tailor-welded blanks. Tailor-welded blanks are steel blanks made from plates of differing thickness and/or properties butt-welded together; they are used in automobile manufacturing to produce body, frame, and closure panels.