SUMMARY

Gas Metal Arc Welding (GMAW) is one of the primary techniques used to join thin structures together. Weld dimensions such as penetration depth, leg length, and throat thickness are key to the quality of welds. Therefore, it is crucial to accurately measure them. Previous research has shown that non-destructive evaluation using laser generated bulk waves and electromagnetic acoustic transducer (EMAT) reception is an efficient and effective way to monitor weld quality in thick structures. Laser generated Lamb waves have the potential to be used to monitor weld quality in thin structure. However, due to the fact that laser generated Lamb waves in thin structures are broadband and dispersive, the complexity of ultrasonic signals is greatly increased. A method named superimposed laser sources technique is proposed to reduce the complexity of signals. By using superimposed laser sources, one would have the flexibility to generate a desired wavelength of Lamb waves. The advantage of generating narrowband Lamb waves with a fixed wavelength is that the dominant frequency contents and traveling speeds of different wave modes can be determined from the dispersion curves. A signal processing procedure that combines wavenumber-frequency domain filtering and continuous wavelet transform is also proposed to further simplify received signals. It is also proposed to use reflection coefficients of different Lamb wave modes and wavelengths to measure weld dimensions. The practicability of the proposed methods will be validated by finite element simulations and experiments. In addition, the effects of welding parameters such as electrode extension, welding speed, arc voltage, and wire feed rate on weld dimensions will be studied. The correlation between these reflection coefficients and weld dimensions will also be studied, and empirical regression models will be developed. Finally, optimization of experimental setup and parameters of superimposed laser sources will be conducted.