This paper shows the group-velocity change of shear horizontal (SH) plate modes in aluminum plates and steel pipes on which slot-shape defects were fabricated, with the purpose of developing a new non-destructive inspection method of the remaining thickness. The SH plate modes were generated and detected by a periodic-permanent-magnet electromagnetic acoustic transducer (PPM-EMAT) in a non-contacting manner. It was found that the group velocity changed depending on the depth at the defects, and drastically increased at a specific value of thickness, called the cut-off thickness. The velocity change was only observed in the higher modes, and this was explained by considering the mode conversion of the SH modes from a higher mode to the lower ones at the thinned wall.

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