A feasibility study on in-service non-destructive inspection of railway rails by an ultrasonic technique based on travelling electromagnetic acoustic transducers

This study explores the feasibility of electromagnetic acoustic transducers (EMATs) for ultrasonic rail inspection, focusing on bulk wave generation from the rail head and on defect detection at the central part of the rail foot. As a contactless method, EMATs could overcome some known limitations of conventional ultrasonic techniques, but require further validation. Different campaigns of experimental tests were performed, evaluating, by means of a probability of detection approach, the response of the technique to several artificial semi-elliptical flaws of increasing size and by considering two sensors characterized by different working frequencies. In contact, static tests allowed to assess the basic feasibility of the inspection technique and showed a linear response to defect size, saturating when defect width exceeded the rail web thickness. Dynamic tests allowed to introduce the effects of lift-off on signal responses. During all tests, the higher-frequency sensor outperformed the lower-frequency one. Finally, full-scale bogie tests on an indoor permanent track installation, comprehensive of defective rails, confirmed the higher flaw detection rates of the higher-frequency sensor, with minimal detection failures despite occasional false alarms. EMATs showed encouraging results for in-motion rail inspection: with further technical development and optimization, this technique could enhance ultrasonic rail inspection by diagnostic trains.