Wavelet and wavelet-packet analysis of lamb wave signatures in laser ultrasonics

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. LBU uses a pulsed laser to generate the ultrasound and a continuous wave (CW) laser interferometer to detect the ultrasound at the point of interrogation to perform ultrasonic inspection. LBU enables in-process measurements since there is no sensor contact or near-contact with the workpiece. The authors are using laser-generated plate (Lamb) waves to propagate from one plate into the weld nugget as a means of detecting defects. 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 discrete wavelet and wavelet packet analysis to untangle the Lamb wave signature. For signatures of Lamb waves captured in laser ultrasonic data in tailor-welded blanks, this has led to straightforward detection of weld defects. Furthermore, both techniques are realizable in the highly parallel cascaded-lattice architecture, and are well suited for on-line real-time monitoring of laser ultrasonic signals.