A Rail Mounted Ultrasonic Sensor for Contact Patch Measurement

Failure of a wheel/rail contact is usually by wear or fatigue and both of these depend on the size and location of the contact patch. One contact measuring approach that shows promise is by the use of ultrasonic reflection. If the wheel and rail surfaces make contact and are under high stress they are more likely to 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 previous work [1] static specimens of wheel and rail were measured by scanning a transducer to build up a 2D map of the contact. Whilst this produced good results and agreed well with contact modeling, it is a time consuming process (typically takes 30 minutes for a scan) and could in no way be used for the measurement on-line. In this paper we describe a method that potentially could be used at line speeds and so provide wheel rail contact measurements in field trials. The 2D scan is achieved by using an array transducer that performs a simultaneous line scan. This coupled with the speed of travel of the contact patch over the sensor location can achieve a map of the contact. 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 may be pulsed independently, simultaneously, or with controlled phase difference. In this work all transducers were pulsed simultaneously at repetition rates of 20 kHz. 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 reflection profiles. Contacts under a range of normal and lateral loads have been measured and compared with some simple results using pre-inked paper. The paper concludes with a discussion of how this array measurement procedure might be implemented at full line sped and what accuracy could potentially be achieved.Copyright © 2008 by ASME