Air-Coupled Ultrasonic Testing of Rails

Under sponsorship by the Federal Railroad Administration (FRA), the University of California at San Diego is developing a system for non-contact rail integrity evaluation. The system uses ultrasonic air-coupled guided wave signal generation and air-coupled signal detection to detect transverse and longitudinal defects in the rail. The fully air-coupled ultrasound transduction presents an evolution over the previous-generation prototype that used pulsed laser ultrasound excitation and air-coupled detection. The system also features a real-time statistical data analysis that minimizes false positive rates and maximizes true detection rates, as well as a specialized filtering approach and impedance matching to overcome the inherently poor signal-to-noise ratio of air-coupled ultrasonic measurements in rail steel. Special targets of the inspection are Detail Fractures, Transverse Fissures and Vertical Split Head defects that were responsible for several accidents in recent years. The design of the prototype was guided by rigorous Finite Element Analysis simulations that revealed fundamental aspects of air-coupled wave propagation and interaction with internal defects in rails. The non-contact air-coupled prototype has been tested at the Transportation Technology Center in Pueblo, CO, in October 2014 (at walking speed) and in November 2015 (at speeds of 5 mph, 10 mph and 15 mph). Results from these tests are presented in terms of Receiver Operating Characteristic (ROC) curves that assess the performance of the system in terms of the unavoidable trade-off between true detection rates and false positive rates. The preliminary analysis of the ROC curves from the latest November 2015 tests is quite encouraging.