Ultrasonic Identification of Rail Tracks from Natural Wheel Excitations: Potential for High-Speed and High-Redundancy Rail Inspection

Rail inspection is commonly carried out by ultrasonic techniques that involve a pitch-catch or pulse-echo scheme to detect internal flaws. This method is usually implemented by Rolling Search Units (RSUs) whose maximum test speed is ~ 20-30 mph. These inspections must be carried out by specialized test vehicles requiring careful scheduling to minimize unavoidable disruptions to normal train traffic operations. This paper will present the concept and to-date results from a radically new ultrasonic testing method for rails. This technique exploits the natural wheel excitations of the rail and extracts a stable ultrasonic Transfer Function (Green’s function) of the rail by utilizing pairs of (air-coupled) ultrasonic transducers. Hence the sensing head is completely non-contact and it does not require an active or controlled ultrasonic excitation. The key to this concept is a special signal processing that is applied to the received signals to eliminate or minimize the effect of the (uncontrolled) wheel excitation. Importantly, this method only works if the wheels excite a significant amount of ultrasonic energy at the appropriate frequencies (~ between 20 kHz and 100 kHz). Current studies are aimed at understanding the operational limitations of the method. Results from two field tests conducted at the Transportation Technology Center (TTC) in Pueblo, CO, at testing speeds as high as 80 mph, will be shown to highlight the potential for this “high risk – high reward” approach. The “high reward” derives from (a) the possibility to inspect the rail at speeds of regular revenue traffic or higher, which would enable a new concept of “smart train” that performs inspections seamlessly during regular operations, and (b) the opportunity for great redundancy given the multiple passes of trains over the same track, naturally resulting in an improved probability of detection for any flaw that may be present. While this high-speed mode of testing is not likely to replace conventional ultrasonic inspection, at least at the current stage, it would constitute an important tool in the hands of railroads and regulatory agencies to increase the safety of rail transportation and the efficiency of rail maintenance operations.