Dynamic characterisation of the wheel/rail contact using ultrasonic reflectometry

The contact condition between the wheel and the rail is paramount to the lifespan, safety and smooth operation of any rail network. The wheel/rail contact condition has been estimated, calculated and simulated for many years, but accurate dynamic measurement has still not been achieved. Methods using pressure sensitive films and controlled air flow have been employed, but are both limited. Ultrasonic reflectometry has been successfully used to characterise various static contacts. This is achieved by pulsing an ultrasonic wave packet through a material that is either fully or partially reflected at an interface. The reflected waveform is then measured and information about the contact can be extracted from the A-Scan. The interface is modelled using a quasi-static spring approach. The asperities behave like springs, as the contact load is increased, the asperities deform accordingly allowing a greater amount of ultrasonic transmission. A reflection coefficient can be determined by calculating the ratio of the reflected ultrasound to the ultrasound incident on the interface. This can be used to determine the interfacial stiffness. Stiffness can be related to contact pressure by performing a calibration procedure. Previous work has relied on a focusing transducer and a 2-dimensional scanning arrangement which results in a high resolution image of the wheel/rail contact, but is limited to static loading of a specimen cut from a wheel and rail. The work described in this paper has enabled the measurement of a dynamic wheel/rail contact patch using an array of 64 ultrasonic elements mounted in the rail. Each element is individually pulsed in sequence to build up a linear measurement of the interface. These cross-sectional, line measurements are then processed and collated resulting in a 2-dimensional contact patch. Measurements have been taken at different speeds and loads. Work is now underway to increase the speed of the measurement.