Ultrasonic assessment of wheel-rail contact evolution exposed to artificially induced wear

One of the most important effects that wear originates on the wheel-rail system is represented by the significant changes introduced in contact area features and contact pressure distribution. In fact, in most cases, even regular operation gradually modifies the profile of the contacting elements so as to transform the original Hertzian counterformal conditions into a conformal contact characterized by lower average pressures and relevant alterations in the size and shape of the contact patch. On the basis of the aforementioned considerations, this study presents the results of a series of experimental tests carried out with an ultrasonic non-invasive technique, with the final purpose of assessing to what extent wear induces modifications in contact parameters. To this end, in a regular flanged wheel, increasing degrees of wear were artificially produced at the centre of the tread. The interface of the wheel-rail systems thus obtained was investigated with high-frequency ultrasonic waves and the observed coefficient of reflection (which is known to be related to contact conditions) was graphically processed to build contact maps from which to extract information about the size and shape of the contact area and pressure distribution. The results allowed the monitoring of the transition from counterformal to conformal contact, and helped to establish an empirical relationship between the amount of wear and both the contact area value and average pressure. Moreover, contact pressure distribution (obtained with a dedicate calibration procedure) showed that conformal contacts lose the original 'one-peak' pressure trend and exhibit a multi-peak configuration. Thus, the ultrasonic method confirmed its capabilities to faithfully monitor contact conditions even in the case of randomly altered geometries of the contacting elements and, in the specific case, may represent an effective tool in validating the wear simulation models.