EMAT measurements of in- and out-of-plane ultrasonic signals

Ultrasonic testing using electro-magnetic acoustic transducers (EMATs) is becoming a more well known technique for defect detection in electrically conducting and/or magnetic samples. Recent improvements to EMAT design have been possible through modelling of the EMAT behaviour, with the combination of experiments and modelling explaining measurement results and suggesting new applications. Rayleigh waves generated on thick samples are of particular interest for detection and characterisation of surface-breaking defects, for example rolling contact fatigue on the railhead, and such cracks often propagate at an angle to the surface normal. Measurements of both the in- and out-of-plane components are useful for characterising such cracking. Typical EMAT systems detect predominantly in-plane signals, with out-of-plane signals measurable using a much more expensive interferometer system. We have developed EMATs to measure predominantly in- or out-of-plane signals, and these detection EMATs have been used to measure the behaviour of Rayleigh waves close to defects, in order to understand the signal enhancement close to the crack opening. Measurements and modelling of closely spaced slots and angled defects identify the benefits of using both in- and out-of-plane signals for defect characterisation.