Gas Metal Arc Welding (GMAW) is one of the primary techniques used to join thin structures together. The quality of the weld plays an important role in structure integrity and product safety. Weld dimensions in butt joint welding of thin plates such as penetration depth, bead width, and reinforcement height are key to the quality of welds. Therefore, it is very crucial to accurately measure them. In this paper, a system that uses laser generated Lamb waves and electromagnetic acoustic transducer (EMAT) reception is used to inspect welds. Lamb waves are widely used in structural integrity inspection and defect detection in thin structures because of their potentials to inspect large areas and their abilities to detect various kinds of defects. The use of lasers to generate Lamb waves is beneficial due to its noncontact nature. 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 applied 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 applied to further simplify received signals. Reflection of ultrasounds occurs due to the presence of weld joint and reflection coefficients of different Lamb wave modes and wavelengths can be measured and used to quantify weld dimensions. In addition, the effects of welding parameters such as contact tip to work distance (CTWD), welding speed, arc voltage, and wire feed rate on weld dimensions is investigated. The correlation between these reflection coefficients and weld dimensions is studied, and empirical regression models are developed as well. This research results in a new non-destructive and non-contact sensing technique for measuring important weld dimensions of butt joint welding in thin structures. It will help to improve the quality and efficiency of the GMAW process, and reduce material waste and associated costs.

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