Modelling of cyclic and viscous behaviour of pearlitic steels. Application to tread braked railway wheels.

Licentiate thesis, 2016

Railway wheel and rail materials are subjected to very high stresses and, in some cases, also elevated temperatures. The rolling contact loading results in a multiaxial stress state with a combination of compression and shear. The elevated temperature is caused by frictional heat generated between wheel and brake blocks at tread braking, or between wheel and rail. The main goal of this thesis is to improve modeling of the cyclic and viscous mechanical behaviour of wheel materials subjected to mechanical and thermal loadings. Finite element analyses of generic heavy haul wheel designs, subjected to high power drag braking loads, are carried out giving global wheel behaviour e.g. in terms of axial rim displacements and residual stresses. A plasticity model and a viscoplasticity model are calibrated against results from cyclic strain controlled experiments of a railway wheel steel of ER7 grade at different temperatures. The finite element analyses show a strong influence of the material model in particular for braking with high power. Next, a methodology to simulate full scale brake rig tests is developed. It includes three types of finite element analyses: an axisymmetric thermal analysis, a 3D mechanical wheelrail contact analysis and a 3D thermomechanical analysis. The wheel material behaviour is modelled by a plasticity model calibrated against results from cyclic strain controlled experiments. The results from the simulation methodology in terms of ratchetting failure show rather good agreement with full scale test rig results for three cases of initial velocity and brake block material. To further improve the material modelling, a viscoplastic model is calibrated against experimental data for cyclic loading and relaxation (low strain rates) as well as high strain rate tests. Preliminary results from the simulation methodology show that the viscoplastic model predicts larger plastic strains as compared to the plastic model.