Visualization of wave modes generated by electromagnetic acoustic transducers with the photoelastic imager

Typical electromagnetic acoustic transducers (EMATs) applied in steel industry are designed for the generation of transversal waves. The probe type investigated here is designed for perpendicular incidence but exhibits several unwanted signals in a typical A-scan that are difficult to identify and to assign to possible wave modes. In this study a photoelastic imager setup is used to allow ultrasonic waves to be generated and observed in a glass sample. Image enhancement techniques were applied to improve the signal-to-noise ratio so that even weak wave modes become visible. By generating waves in conductive but non-magnetic, non-conductive but magnetic and conductive and magnetic materials the contributions of Lorenz-, magnetic and magnetostrictive forces could be separated. It is found that transversal waves have their origin in Lorenz- and magnetostrictive forces, while the largest contribution to longitudinal waves comes from magnetic forces leading to an unfavourable propagation direction. Several unexpected wave modes were detected that most probably origin from deviations of the realization from ideal configurations. Understanding these effects will help to improve the EMAT probe design for future applications.