INSPECTION OF WELD PENETRATION DEPTH IN THIN STRUCTURES USING LASER-GENERATED LAMB WAVES AND EMAT RECEIVER
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Using laser/EMAT ultrasonic technique to inspect penetration depths of welds in thin structures is limited by lack of effective signal interpretation methods. The laser-generated ultrasounds in thin structures are very complicated. This research conducts a systematic study to investigate how to effectively apply laser/EMAT ultrasonic technique to measure penetration depths of welds in thin structures. First, the signals acquired during inspections are characterized using 2-D Fourier Transform and wavelet analysis to establish a good understanding of laser-generated Lamb waves in thin structures. Secondly, interactions of laser-generated Lamb waves with welds are investigated by measuring full wave field signals and 3-D Fourier Transform analysis. Thirdly, feasibility of using transmission coefficients of Lamb waves to predict weld penetration depths is studied both numerically and experimentally. A continuous wavelet transform-based algorithm proves effective to fast calculate transmission coefficients of Lamb waves from single acquired signals. Finally, welds of varying penetration depths are manufactured to test the transmission coefficient-based method. Lamb waves sensitive to weld penetration depths are selected and their transmission coefficients are used to develop a neural network to predict weld penetration depth in thin structures accurately. Some considerations regarding the implementation of the developed method are also discussed.