Detection of Shear Horizontal Guided Waves Propagating in Aluminum Plate with Thinning Region

Abstract

We propose a new method for wall-thinning inspection based on mode conversion of shear horizontal (SH) guided waves, which is represented as group velocity change. Group velocities of higher modes of SH guided wave propagating in a plate-like wave guide depend on the thickness and frequency due to dispersive characteristic. There exists a cut-off thickness for each higher mode, below which the associated mode cannot propagate and mode conversion to lower ones is expected, leading to discontinuous velocity change. Here, the fundamental SH0 and first asymmetric SH1 modes are generated by the periodic-permanent-magnet electromagnetic acoustic transducer (PPM-EMAT) and propagated in aluminum plates containing thinning regions. A broad-range piezoelectric pinducer is used to detect the propagating wave in order to explore the mode conversions phenomena. We directly observed that the generated SH1 mode converts to SH0 mode in thinning region and converts back to SH1 mode when the thickness returns to original.