Effect of a back plate on SH-wave generation in EMATs

An electromagnetic acoustic transducer (EMAT) can generate or detect ultrasonic wave in electrically conductive or magnetic materials by means of Lorentz force and/or magnetostriction effects. EMATs are non-contact transducers that allow fast moving inspection under hot and a hostile environment. Especially, horizontal polarized Shear-waves (SH-waves) are useful for NDE of dissimilar or anisotropic austenitic inspection due to absence of mode conversion as compared to SV- and L-waves. The efficiency of the EMAT ultrasonic generation and detection is dependent on the whole system including a coil, a magnet, current pulse generator and samples. Also, it is well known that a lift-off between a metal sample and the exciting coil has an influence on eddy current generation between the coil and the metal sample, the equivalent coil inductance. If there is no electrically conducting back plate between permanent magnet and EMAT exciting coil, ultrasonic wave may occur within the electrically conducting magnet as well as a metal sample by means of Lorentz force from the interaction between an eddy current with the static magnetic field and/or magnetostriction due to the dynamic field by EMAT exciting coil. Then, the ultrasonic generation in the magnet can cause the unwanted signal from a metal sample. The purpose of this research is to investigate the influence of the metal back plate on SH-wave generation by EMATs. Also, we investigate the response of the EMAT system to some changes in a design parameter such as an air gap between EMAT exciting coil and magnet