Omnidirectional shear horizontal wave based tomography for damage detection in a metallic plate with the compensation for the transfer functions of transducer

Guided-wave based damage detection for plates has been widely studied for structural health monitoring. Because most of the earlier studies used dispersive Lamb waves, substantial efforts had to be made to alleviate the dispersive and multi-modal nature of Lamb waves and the effect of surface conditions. In contrast, shear-horizontal (SH) waves have better propagation characteristics suitable for the detection of damages in plates, but SH waves have not been widely used due to the lack of efficient methods to generate and sense omnidirectional SH waves. The objective of this study is to construct diagnostic images of damaged plates by using omnidirectional SH waves with a special emphasis on the compensation of the frequency-dependence of the SH wave transducers. The compensation is necessary to have reliable diagnostic images because its frequency-dependent characteristics considerably can affect imaging quality if they are not considered. Consequently, simplified, yet effective, models representing the transfer functions of the omnidirectional SH wave magnetostrictive patch transducer (OSH-MPT) are developed in this paper. To visualize the position and shape of the structural damages in a metallic plate, the virtual time-reversal imaging method is used and two alternative techniques are considered to compensate for the effects of the transfer functions of transducers in the imaging processes. The imaging results after the compensation appear quite promising, suggesting that the omnidirectional SH wave based enhanced time-reversal imaging can be an efficient inspection method for plate structures.