Enhancement of Shear Wave Purity in Spiral Coil EMATs for Defects Testing in Non-Ferromagnetic Metal Material

In some mainly shear wave electromagnetic acoustic transducers (EMATs) such as spiral coil EMATs, an existing longitudinal wave distorts the shear wave response and complicates the echo pattern. In order to enhance the shear wave purity, a finite element model has been developed for predicting the eddy current, the static and dynamic magnetic fields in an aluminum specimen. And a method of predicting the shear wave purity is introduced based on the predicted fields. The effects of an impedance matching network and a band-pass filter on the bulk wave amplitudes in the ultrasonic receiving system were investigated by experiments. Subsequently, the influences of magnet dimensions and the thickness of Si -steel backplate on shear wave purity were also studied. Results indicate that a band-pass filter and an impedance matching network designed at a center frequency that is the same as the excitation current, a larger diameter and smaller thickness magnet, and a thicker Si-steel back-plate can help to improve the shear wave purity, but it is at some sacrifice of the shear wave sensitivity.