Models of rolling contact are used to predict the wear of the railhead due to passing wheels. It is commonly assumed in such models that material is worn away in proportion to the frictional work done in the slip zone of each contact patch. Moreover, a constant coefficient of friction is usually assumed. However, it is known that the friction coefficient between sliding surfaces decreases as the velocity of sliding increases. This paper describes a technique for including more sophisticated friction laws in a model of wheel-rail rolling contact. A non-Hertzian model is used based on a variational method. The resulting tangential stress distribution and slip velocity in rolling contact are examined and compared with results calculated using a constant coefficient of friction. For conditions of rolling contact with partial slip a stick-slip oscillation is found to occur when a velocity-dependent friction law is used. The pattern of material removed along the rail in rolling contact is also investigated. For low creep values and for existing broadband roughness, the inclusion of velocity-dependent friction does not result in significantly different predictions of wear. For higher creep values, velocity-dependent friction results in an unstable force-friction relationship which could lead to high variable wear. Creep-force relationships have been investigated using a velocity-dependent friction model and results have been compared with those for a constant friction coefficient. For low-creep values, there is little difference in the results obtained from the two models. For high-creep, this velocity-dependent friction model produces a creep-force curve with an adhesion maximum followed by reduced tangential loads at higher creep values. This model shows that rather than a smooth creep-force relationship at high creep values (as is often assumed in vehicle motion simulations), the relationship is in fact highly unstable with partial stick-slip oscillations and periods of full slip predicted in the contact. With the velocity-dependent friction, the creep-force relationship covers a range of values. The minimum converges to the friction law that is input to the model, whereas the average result, which corresponds to what may be measured in a test rig, is greater than the underlying friction law. This implies that the actual friction law used locally within the contact cannot be derived directly from measurements of the overall rolling friction. It is clear that the friction law used in a variational contact model can affect the prediction of the development of roughness. As an example, it is shown that if a regular, sinusoidal corrugation is assumed to be present on the rail, assuming constant friction to predict the wear pattern leads to a regular, sinusoidal wear pattern. When a velocity-dependent friction law is included in the model, the wear pattern is more variable. However, the difference between the wear predicted by the two models is found to be less noticeable if a broadband initial rail roughness is assumed. The pattern of material removed is then dominated by the initial roughness profile. For this reason, it is concluded that if creep is low and steady, there is little benefit in improving the representation of friction in the wear model. However, for higher creep, or for varying creep, there is a chance that uneven wear can occur due to the variability of friction with slip velocity. This may have a significance for the calculation of wear damage to rails in situations with high creep.
Velocity-dependent friction in a model of wheel-rail rolling contact and wear
Rail Corrugation and Roughness Growth, 2010 ; 1791-1802
Vehicle System Dynamics ; 49 , 8-12
2011
12 Seiten, 11 Bilder, 24 Quellen
Conference paper
English
Velocity-dependent friction in a model of wheel–rail rolling contact and wear
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