Blind zone defect imaging using multipath edge-reflected Lamb waves

This paper proposed a Lamb wave-based defect imaging method with multipath edge reflections, which can detect the crack-like defect in blind zones that is invisible for the conventional delay-and-sum algorithm. In the implementation process, mirror points of transducers with respect to all the four plate edges are firstly introduced as extra virtual transmitters and receivers. By assuming the defect position, all of the potential traveling paths of edge-reflected wave packets can be next traced. Considering it is always possible to find a matching path for a certain wave packet from these traced ones if there is really a defect at the assumed place, a damage index is thus established to estimate whether the assumption holds true. Based on that, the detection area can be imaged by altering the assumed defect position, calculating its index, and taking the index as pixel value. Subsequently, wave packets of different orders from various signals are also used to generate the corresponding images. A multiplication strategy is finally adopted to fuse all the results and eliminate the artifacts. In this manner, the final image of the detection area can be obtained. Both numerical and experimental cases have been carried out to prove the effectiveness and feasibility of the proposed method. Results show that it can locate through-thickness cracks in different blind zones accurately, and the minimum relative error of these cases is only 1.12%.