Characterisation of small defects using miniaturised EMAT system

Many surface breaking defects, such as those caused by thermal fatigue or stress corrosion, have finite lateral dimensions. However, much of the research considers significantly larger simulated defects. This paper considers defects with mm-dimensions, and presents a method for characterisation of their length and depth. This is done using non-contact ultrasonic techniques, including a pair of electromagnetic acoustic transducers (EMATs) with significantly reduced size compared to standard industrial EMATs. Defects with dimensions of 1-11 mm length and 0.5-2 mm depth are measured. All information is obtained from a single raster scan of a sample, considering transmission and enhancement of Rayleigh waves, and introducing the defect cross-section. The lateral size resolution for the scan steps chosen is +/- 1 mm, and depth resolution is 0.5 mm. The method, being non contact, is also demonstrated on a sample with a 110 m thick coating.