Anisotropy in pearlitic steel subjected to rolling contact fatigue - modelling and experiments

Doktorsavhandling, 2014

In rails and wheels subjected to severe rolling/sliding contact, large plastic deformations accumulate in the surface layer. This decreases the fatigue resistance of components and makes this layer prone to formation of common rolling contact related defects. In pearlitic steel railway components, accumulated plastic deformations result in microstructural changes which, in turn, lead to anisotropic characteristic of properties like fracture toughness. The aim of the thesis is to investigate the influence of material anisotropy on damage mechanisms of pearlitic rail steels subjected to rolling/sliding contact. The interaction between the pearlitic microstructure and cracks in the surface layer of rail samples is studied. Based on microstructural investigations, an anisotropic fracture surface model is proposed to account for the directional dependence of resistance against crack propagation. The fracture surface model is employed in a computational framework where propagation of planar cracks is simulated. The simulation results show that the degree of anisotropy in the surface layer has a significant influence on the crack propagation path. In particular isotropic material characteristics will result in crack propagation towards the surface. This is a fairly benign type of fracture as compared to the transversal rail breaks that may result if the propagation deviates into the bulk material. To include large plastic deformations and the resulting anisotropy in simulations, a hybrid micro-macromechanical material model for pearlitic steels is proposed. Results from High Pressure Torsion (HPT) tests were used to calibrate the model. In HPT tests, samples are deformed under similar loading conditions to that of the rail-wheel contact i.e. a high compressive force and simultaneous large torsional straining. The HPT deformation procedure is simulated in the commercial finite element package ABAQUS. Numerical results agree well with experimental data demonstrating the high potential of the proposed material model in analyses including large deformations of pearlitic steel. In addition, the influence of different homogenization techniques in the material model is investigated. Two models proposed for a pearlitic colony are calibrated against micro-compression test data. The macroscopic response of a 3D model of pearlitic steel during simple shear deformation is compared with the response predicted by the developed hybrid material model. The hybrid model was found to give stress-strain responses that are qualitatively similar, but around 12% lower in stress magnitudes compared to the other two models. This should be contrasted towards the superior computational performance of the hybrid model.