Mode conversion of torsional waves generated by electromagnetic acoustic transducer

Change in group velocity of torsional guided waves induced by mode conversion is investigated for wall-thinning inspection of pipe. The fundamental and second torsional modes are generated and detected by the Lorentz-force-type electromagnetic acoustic transducers (EMATs) in aluminum pipes with fabricated-thinning region. Traveling time of the higher mode in the axial direction varies depending on the thickness of the thinning regions and drops suddenly when the thickness is smaller than the cut-off thickness. From comparison with calculated dispersion relation, the drop is attributed to mode conversion from a higher mode to a lower mode with higher group velocity.