SUMMARY

Welding is one of the most important joining processes currently used in industry and weld penetration depth (WPD) is a key measure of weld quality. Thus, the development of techniques to assess the depth of penetration is crucial. Among the various non-destructive testing (NDT) techniques, the laser/EMAT ultrasonic (LEU) method has demonstrated great potential due to its non-contact characteristics. However, the interpretation of signals under this technique has proved challenging when compared with traditional ultrasonic testing (UT). Previous work has focused on the inspection of WPD in thick structures by measuring the time of flight (TOF) of laser-generated bulk waves. As the thickness of the structure decreases, however, Lamb waves become dominant in laser-generated ultrasound, and the TOF measurement technique becomes less useful.

The objective of this work is to develop new signal interpretation methods to extend the capability of the LEU technique to thin structures. First, LEU signals in thin structures will be characterized to gain insight into the properties of laser-generated Lamb waves within such structures. Next, the study will apply a new experimental method to investigate the capability of the LEU technique to detect the presence of weld defects. In addition, the effect of WPD on the propagation of Lamb waves will be studied both numerically and experimentally. And a new signal interpretation method based on transmission coefficients will be proposed. Finally, a more efficient denoising method will be proposed to improve the current LEU system.

Successful completion of this work will solve several of the most significant aspects of applying the LEU technique to inspect weld quality in thin structures. The fundamental physics underlying this technique will be established, and a new and efficient signal interpretation methodology will be developed.