Wheel-rail interaction at short-wave irregularities

Abstract

Short-wave irregularities in the wheel-rail interface are at the basis of track and vehicle damage and deterioration. On the short term, they result into high dynamic train-track interaction forces and a high energy input into the system that must be dissipated in the different system components or "levels", leading on its turn to progressive deterioration on the long term. Furthermore, the short-wave defects grow into longer defects in the track geometry, due to the fact that the train is a travelling multi-body mass-spring system. The lifetime of the track and its components can be extended by adjusting the "path" of the dissipated power spectrum through the system and adjusting component and system properties with respect to their hysteretic behaviour. Instead of life-time extension, such measures may also aim at an extension of maintenance intervals, which is important to optimise the availability of e.g. high-speed lines. The present study investigates two particular types of short defects in detail: rail welds and wheel flats. In longitudinal direction and on a global scale, the contact between a rolling wheel and a rail can be distinguished into continuous single-point contact and transient double-point contact. The contact type that occurs depends on the actual geometry of the wheel-rail interface in the running direction. The first contact type leads to a dynamic amplification of the static axle load, whereas the second leads to wheel-rail impact. Especially the latter contact type is detrimental to the rail system and should be prevented as much as possible or detected at an early stage. The introduction of rail welds instead of the traditionally bolted connections reduced the dynamic forces at rail joints globally with a factor three. However, welds remain potential damage initiators due to the local geometrical and metallurgical discontinuity. Investigations show an approximately linear relationship between the extreme value of the dynamic wheel-rail contact force at a weld, the maximum absolute rail inclination and the train speed. The geometry of rail welds is traditionally assessed with the principle of vertical tolerances. A new assessment method for rail welds is proposed, with norm values for the allowable inclination depending on the line section train speed. This method is based on a relatively strong correlation between discretised maximum rail geometry inclinations (first derivatives) and extreme dynamic wheel-rail contact forces, relative to the poor correlation between tolerances and extreme forces. The method aims at a reduction and uniformisation of dynamic contact forces at rail welds, in order to avoid deterioration. Wheel flats are commonly assessed on the basis of their length and/or depth, or automatically detected by wheel impact load detectors in the track. This study has shown that the minimum circumferential wheel tread curvature is the critical parameter that governs the dynamic wheel-rail interaction in the presence of wheel flats. It determines which contact type occurs for a given flat geometry: continuous single-point contact, in the subcritical speed regime, or transient double-point contact, in the transcritical speed regime. It furthermore determines the magnitude of the contact force in the subcritical regime. Both speed regimes are shown to exhibit essentially different features with respect to the dynamic wheel-rail interaction: the track stiffness governs the interaction for low train speeds and long flats, whereas for high speeds and/or short flats the inertial properties of the wheel and the rail govern the interaction. The force-speed relationship is non-linear in the first regime, whereas linearity is a good approximation in the second regime.