Differential Coil EMAT for Simultaneous Detection of In-Plane and Out-of-Plane Components of Surface Acoustic Waves

Surface acoustic waves, such as Rayleigh or Lamb waves, can be used to characterise the geometry of surface-breaking defects. For accurate measurement of defect length and depth, the in-plane (IP) and out-of-plane (OP) components of the displacement or velocity can be measured and compared. Electromagnetic acoustic transducers (EMATs) offer a simple, non-contact method of doing this, as it is possible to design detection EMATs such that they are sensitive predominantly to either the IP or the OP velocity component. Here we present a new design of differential coil EMAT which measures both components simultaneously, and through simple processing gives the IP and OP velocities. This is compared to previous designs, showing that these components are obtained accurately. The system is used to characterise three artificial surface-breaking defects in aluminium and one thermal fatigue crack in 10 mm thick steel plate under thin coating.