Feasibility study for real time measurement of wheel-rail contact using an ultrasonic array

Failure of a wheel-rail contact is usually by wear or fatigue of either component. Both mechanisms depend on the state of stress, which in turn depends on size and location of the contact patch. In this work, the feasibility of an ultrasonic approach for measuring the contact, real time on a rail, has been evaluated. The approach is based on the physical phenomenon of ultrasonic reflection at an interface. If the wheel and rail surfaces make contact, and are under high stress, they will transmit an ultrasonic pulse. However, if there is no contact, or the contact is under low stress, then the wave is completely or partially reflected. By measuring the proportion of the wave reflected, it is possible to deduce the extent of the contact area and also estimate the pressure distribution. In a previous work (Marshall, Lewis, Dwyer-Joyce, Olofsson, and Bjorklund, 2006, “Experimental Characterisation of Wheel-Rail Contact Patch Evolution,” ASME J. Tribol., 128(3), pp. 493–504), static wheel-rail contacts were scanned using a transducer to build up a two-dimensional (2D) map of the contact. The procedure was time consuming and could in no way be used for measurements online. In this work, a method is presented that could be used at line speeds, and so provide wheel-rail contact measurements in field trials. The scan is achieved by using an array transducer that performs a one dimensional electronic line scan. This, coupled with the speed of travel of the contact patch past the sensor location, enables a 2D map of the contact to be produced. Specimens were cut from wheel and rail sections and loaded together hydraulically in a biaxial frame. An array transducer was mounted beneath the rail specimen. The array transducer consisted of 64 ultrasonic elements that could be pulsed independently, simultaneously, or with controlled phase difference. The signals were reflected back from the contact to effectively produce a line scan. The transducer was physically moved to simulate the translation of the contact patch and so generate a series of 2D reflection profiles. Contacts under a range of normal and lateral loads have been measured and compared with some simple results using a pressure sensitive film. While the map produced by ultrasonic reflection is relatively coarse, the results agree well with measurements from the pressure sensitive film. The work concludes with a discussion of how this array measurement procedure might be implemented at full line speed, and what resolution could potentially be achieved.