Multiple Focused EMAT Designs for Improved Surface Breaking Defect Characterization

Abstract

<p>Ultrasonic Rayleigh waves can be employed for the detection of surface breaking defects such as rolling contact fatigue and stress corrosion cracking. Electromagnetic Acoustic Transducers (EMATs) are well suited to this technique as they can directly generate Rayleigh waves within the sample without the requirement for wedges, and they are robust and inexpensive compared to laser ultrasonics.</p> <p>Three different EMAT coil types have been developed, and these are compared to assess their ability to detect and characterize small (down to 0.5 mm depth, 1 mm diameter) surface breaking defects in aluminum. These designs are: a pair of linear meander coils used in a pseudopulse-echo mode, a pair of focused meander coils also used in pseudo-pulse-echo mode, and a pair of focused racetrack coils used in pitch-catch mode. The linear meander coils are able to detect most of the defects tested, but have a lower signal to noise ratio and give limited sizing information. The focused meander coils can also detect all defects tested, but have the advantage that they can also characterize the defect sizes on the sample surface, and have a stronger sensitivity at their focal point. The focused racetrack coils have been used to characterize smaller defects, including depth characterization for defects shallower than ~1.5 mm depth. To remove problems due to EMAT orientation relative to a defect a set of four confocal racetrack coils (one generating and three detecting) have been designed to find defects in both transmission and reflection. Measurements using all four EMAT designs will be presented and compared for high resolution imaging of surface-breaking defects.</p>