Defect identification in variable thickness waveguides using SH₀ guided wave

Guided waves, especially Lamb waves, propagating in plate-like structures with constant thicknesses have been investigated intensively in the past decades in the context of structural health monitoring (SHM) and non-destructive testing (NDT). However, structures with variable thicknesses are also very common in practice, such as ship hulls whose thickness varies for satisfying various load conditions, and defect identification for such structures is also of importance. In this paper, we compare the propagation characteristics of both Lamb modes and SH₀ mode in variable thickness plates and propose to use the SH₀ mode for defect identification in such waveguides because of its non-dispersive property. First, the non-dispersive property of the SH₀ mode in waveguides with variable thickness is demonstrated in theory. Then, a wave prediction method is adopted to illustrate the differences between the Lamb modes and the SH₀ mode when propagating in waveguides with constant or variable thickness. Finally, experiments are conducted on a variable thickness plate to verify the non-dispersive property of the SH₀ mode and further compare the results with those of the Lamb modes. Recently developed thickness-poled omnidirectional shear horizontal piezoelectric transducers (OSH-PTs) are used to produce pure SH₀ waves. It is observed that the velocity of the Lamb modes may change significantly and the group velocity of the S₀ mode even decreases by 55.1% during propagation, which hampers Lamb modes from defect identification in variable thickness plates. By contrast, the SH₀ mode is non-dispersive thus promising for damage detection, and its effectiveness for defect localization in such structures is validated experimentally.