Distribution of contact pressure in wheel-rail contact area

The authors have recently assessed the feasibility of applying an ultrasonic technique for investigating contact problems between the wheel and rail. In particular, the size and shape of the contact area have been determined, obtaining good agreement between the experimental data and Hertz's theory. Real contact area (RCA) has also been evaluated and experimental results have demonstrated that under normal loads (i.e. for contact pressures of 600-800 MPa), this area is just a small fraction of the nominal one, its value depending strongly on initial roughness of the surfaces in contact. In this paper, the ultrasonic method has been used to estimate the distribution of contact pressure in a real wheel-rail system. To achieve this, a preliminary series of tests has been conducted on a simple sphere-plate contact in order to analyse ultrasonic reflection from the interface. A finite element model (FEM) has also been developed to calculate pressure distribution over the whole contact area. In this way, an empirical relationship between contact pressure and ultrasonic reflection has been established, useful for calibrating the experimental set-up. Ultrasonic wave reflection was then analysed for a wheel-rail system and the reflection coefficient matrices obtained from the scans for each load applied were converted into contact pressure matrices by means of the calibration curve. The results agree substantially with the Hertz's theory, though significant differences have been observed in terms of regularity of the pressure ellipsoid shape. Nonetheless, the ultrasonic method seems to confirm its features of simplicity and reliability that render it potentially useful for investigating the wheel-rail contact parameters