Modelling of cyclic and viscous behaviour of thermomechanically loaded pearlitic steels; Application to tread braked railway wheels

Abstract: In service, railway wheel and rail materials are subjected to high stresses and, in some cases, elevated temperatures. The high stresses are caused by the rolling contact between wheel and rail. Furthermore, heat generated from tread braking and/or sliding between wheel and rail gives additional stress due to constrained thermal expansion. The main goal of this thesis is to improve modelling of the temperature dependent cyclic and viscous behaviour of pearlitic wheel and rail steels subjected to thermomechanical loadings. Finite element (FE) analyses are carried out of generic heavy haul wheel designs subjected to thermal loading from high power drag braking. In these analyses, the results from using a plasticity and a viscoplasticity model are compared. Both models are calibrated against results from cyclic strain controlled (low strain rate) experiments with hold-time of ER7 wheel steel at different elevated temperatures. The comparison shows an increasing influence of the choice of material model with power of the drag braking. Also, a methodology to simulate full scale brake rig tests is developed. It includes an axisymmetric thermal analysis, a 3D structural wheel-rail contact analysis and a 3D structural analysis with a traversing contact load. The wheel material behaviour is modelled by a plasticity model calibrated against cyclic strain controlled (low strain rate) experiments of ER7 steel. In addition, the infuence of important operational parameters such as axle load, maximum vehicle speed and block material is investigated with respect to the ratchetting life of the wheel tread. To improve the modelling of the behaviour of ER7 steel for a wider range of loading rates and multiaxial loading, a viscoplasticity model is adopted and calibrated against test data of ER7 steel at different temperatures for slow cyclic strain controlled tests with hold-time, ratchetting tests with rapid cycles and cyclic biaxial tests. A simulation of a brake rig experiment is used to highlight the importance of using the viscoplasticity model in the prediction of the ratchetting fatigue life. Finally, a cyclic plasticity model incorporating phase transformations is developed to examine what phases and residual stresses that are obtained in a railway wheel after repeated short term local heating followed by rolling contact. This model can be used to study thermal damage mechanisms in rail and/or wheel steels that may lead to initiation of cracks (e.g. squats (studs) in rails and crack clusters in wheels).