Crack sizing in welded T-joint using an enhanced ultrasonic TOFD approach by a sparse representation framework

Under complicated stress and loading conditions, cracks are prone to develop at the tip-toe area of a welded T-joint, yet the accurate sizing of these cracks has always been challenging, partly due to complex geometry. In this study, an enhanced ultrasonic time-of-flight diffraction (TOFD) technique in a sparse signal representation framework is proposed for quantitative sizing of weld-root crack at a single-weld T-joint. First, a scanning scheme using one transmitting transducer and two receivers on both sides of the weld seam that enables the measurement of two diffracted waves from different angles. Next, the two TOFD signals are sparsely represented by a nonconvex regularization algorithm based on a specially tailored overcomplete parametric dictionary, motivated by the knowledge that the measured signals contain only a few meaningful wave packages thus preserving a sparse nature. Sparse signal representation enables direct extraction of the physical information of each wave in the signals, most importantly their arrival times. Next, a scheme for crack-tip localization is presented utilizing the retrieved arrival times of the tip-diffracted waves, yielding the length and angle of the crack. The proposed method is first evaluated by a simulation study, demonstrating excellent accuracy for crack tip localization as well as crack characterization, even for noisy signals with wave overlapping. An experimental study is also presented where the method is employed for crack inspection in a real welded T-joint to further verify its effectiveness, confirming good performance and accuracy.