SummaryIn the autumn, railroad traffic is often interrupted due to the occurrence of slippery tracks. The Dutch main operator (NS) and the infrastructure manager (ProRail) struggle with repeated delays. The layer between wheel and rail causing this phenomenon has not yet been identified, but can be quantified by rheological properties determined by correlating the measured friction between wheel and rail and the developed friction model. To this end, the research described in this thesis is twofold: theoretical; in which the wheel -rail contact is modelled in terms of contact and friction, and experimental; in which the friction between wheel and rail is measured to validate the developed model. The contact model between wheel and rail is approximated to be elliptical, which is shown to be accurate for the described purpose. The low friction situation is assumed to be caused by an interfacial layer, which is acting like a lubricant. Combining the previous two assumptions, a mixed lubrication friction model is developed for the Hertzian elliptical contact situation in which the interfacial layer is acting as a lubricant governed by the Eyring model. The mixed lubrication friction model results in the so-called Stribeck curve and/or the traction curve, which both take frictional heating and starved conditions into consideration. In addition, the changes in attack angle are also taken into account and a general viscoelastic model is proposed for both interfacial and boundary layers. In this way, most of the situations occurring in the wheel -rail contacts are covered, with the recommendation that future research should include the effect of spin. The validation of the friction model was performed by conducting laboratory experiments on general elliptical contacts in the presence of a lubricant. Field experiments are conducted for extracting the rheological properties of the interfacial layer causing the low friction by using the presented model. For the field experiments, two pioneer devices were developed and successfully used on the real track. One is a sliding sensor which measures the coefficient of friction between a curved shaped specimen pressed against and sliding along the rail, i.e. simple sliding conditions. A second device is developed and mounted on a train for measuring traction curves between a measuring wheel and the head (top) of the rail, i.e. rolling/sliding conditions, at velocities of up to 100 km/h. The measuring campaign was scheduled over 20 nights during the autumn of 2008 on three tracks of the Dutch rail network (Hoekselijn, Zeeuwselijn and UtrechtArnhem -Zwolle). Five characteristic traction curves were identified by the use of a statistical approach and rheological properties were extracted for the layer causing the low friction situation. Most of the approximately 6000 measured traction curves showed good agreement with the friction model, in which the viii Summary interfacial layer is represented by the Eyring behaviour. However, for some of them, viscoelasticity had to be taken into ...

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This paper deals with the prediction of the Stribeck and traction curves, by proposing a mixed lubrication model for highly loaded elliptical contacts. The model represents an extension on the mixed lubrication model of Gelinck and Schipper and comprehends both the asperity component or the so called boundary lubrication component (BL) and the elastohydrodynamic component (EHL). The asperity component is calculated from a fully deterministic contact perspective, where an equivalent rough surface is in contact with a smooth and rigid surface. In EHL regime, the film thickness is calculated according to Nijenbanning et al. and the separation for the asperity component is derived from Johnson, with a small adaptation, which was possible due to the deterministic contact model approach. In this way, the separation is calculated from the volume conservation theory of Johnson and even if this can be negative, the film thickness remains above zero permitting the calculation of highly loaded contacts. For the traction curve calculation, an elastic-plastic approximation for BL micro-contacts as proposed by Gelinck and Schipper for line contacts is used.