The effect of longitudinal modes waveguide conversion on the feature identification of defects in pipes

A comprehensive analysis of a defect's reflection in guided wave-based pipeline inspection usually provides sufficient information about the defect. The interaction of the longitudinal mode waveguide with the defect results in a double mode conversion: the L(0,2) mode and L(0,1) mode. The L(0,1) mode, which is converted from the L(0,2) mode after the interaction with defects, provides significant information about the defect. The reflection coefficients of L(0,1) and L(0,2) mode from defects with different depths and circumferential extent has been investigated using finite element analysis and laboratory experiments. The results show that the double mode conversion only occurs when the L(0,2) mode interacts with non-through-thickness defects; For a defect of a given circumferential length, increasing the depth of the defect at first increases the reflection coefficient of L(0,1) mode and then starts to decrease it; The reflection of the L(0,1) is as strong as that of the L(0,2) mode when the defect depth is less than half of the wall thickness. Results from this research provide a reference for the assessment of defect depth in pipeline inspection.