Magnetostriction-Based Omni-Directional Guided Wave Transducer for High-Accuracy Tomography of Steel Plate Defects

An omni-directional (OD) magnetostriction-based electromagnetic acoustic transducer (EMAT) is suitable for use in the guided wave (GW) tomography of steel plate defects. Because the GW is characterized by multiple modes and frequency dispersion, the waveforms detected by the traditional magnetostriction-based OD GW EMATs are complex, which substantially reduces the accuracy of projection data extraction and then decreases the quality of GW tomography and the accuracy of defect quantification. This paper proposes a new magnetostriction-based OD EMAT to generate shear-horizontal (SH) GWs of a special mode to reduce the interference of multiple modes and the frequency dispersion of GWs. The new EMAT consists of a contra-flexure coil and a pre-magnetized thin circular nickel foil. The theoretical background and working principle of this EMAT are presented. The results of pitch-catch experiments on a healthy steel plate demonstrate that the new EMATs can transmit and receive purer single SH1-mode GWs and have improved the accuracy of projection data extraction, compared with the traditional EMATs. The performance of SH1 wave tomography of the traditional and new EMAT arrays are compared via experiments implemented on a 3-mm-thick steel plate with an artificial corrosion defect. The experimental results indicate that higher tomographic reconstruction quality, clearer normalized slowness curves with less noise and higher accuracy of tomographic defect quantification can be achieved by the new EMAT array, which has improved the projection data extraction accuracy, compared with the traditional EMAT array.