A local degraded adhesion model for creep forces evaluation: An approximate approach to the tangential contact problem

The calculation of the creep forces has always been one of the fundamental goals for a wheel-rail contact model. At the same time, a wheel-rail contact model has necessarily to consider the phenomena related to degraded adhesion because they deeply affect both creepages and creep forces. Degraded adhesion is extensively observed at the wheel-rail interface. However, the development of an accurate and efficient adhesion model is quite a challenge due to the complexity of the system and the difficulty in collecting detailed experimental data. In this framework, the paper presents a local degraded adhesion model (based on local slidings/creepages and local pressures inside the contact area) able to provide a more accurate description of the contact phenomena characterizing the wheel-rail degraded adhesion with respect to analogous global models (involving global sliding/creepages and forces). The new model is able to consider important phenomena like the energy dissipated at the contact and the consequent surface cleaning effect leading to an adhesion recovery between wheel and rail. To reach this goal, the Kalker's FASTSIM algorithm has been properly extended and improved to include the previous phenomena. Furthermore, the new model allows the achievement of a good trade-off between accuracy and numerical efficiency. Thank to its efficiency, the model can be implemented inside general railway vehicle multibody models to perform large scale dynamical simulations. The model has been finally validated through experimental data coming from on-field tests and consisting of specific braking maneuvers under degraded adhesion conditions.