Using active ultrasonics to measure wheel-rail contact during a running-in period

Flange climb derailment is most likely to occur during the wheel-rail running-in phase, such as just after the railway vehicle wheels have been re-profiled or during morning rush hour. It is thought that the underlying cause is an increase in traction due to changes in the contact between wheel and rail. However, the mechanism of this increase in traction remains a subject of continuing controversy. Active ultrasonic measurements have been effectively used to measure wheel-rail contact conditions, particularly the contact stiffness. In this study, the ultrasonic method is used to characterise the wheel-rail contact during running-in whilst the surface roughness of the wheel reduces due to cyclic loading against the rail. An array of sensors was mounted behind the wheel flange so as to reflect ultrasound directly from the contact. An initial investigation was carried out to understand the measurement resolution of this ultrasonic array transducer. A static measurement was then performed by traversing the ultrasonic array transducer across the wheel when statically loaded against the rail. Following this, the same array transducer was fixed to the wheel and dynamic measurements were carried out. These measurements were cyclic to investigate the effect on changing surface topography. From this, the relationship between the slip and the contact stiffness was investigated. It was found that an increase of contact stiffness was measurement after slip motion was recognized. Both the static and dynamic measurements showed a difference in the wheel-rail contact conditions. In the case of the rolling condition, the peak position of the contact stiffness in the contact area shifted from the centre of the contact to the exit side. From the cyclic rolling tests, it was observed that the change in surface topography resulted in an increase in interfacial contact stiffness. This phenomenon indicates that there is an increase in real area of contact between the wheel flange and the rail gauge corner during running-in.