On the modeling of anisotropy in pearlitic steel subjected to rolling contact fatigue

Licentiatavhandling, 2012

One of the main sources of damage caused by Rolling Contact Fatigue (RCF) in railway components is the large plastic deformations that accumulate in the surface layer of these components. Large plastic deformations in components made of pearlitic steel induce anisotropy in the mechanical properties of the material. The objective of this thesis is to investigate the effect of this anisotropy on the RCF properties of pearlitic steel components by utilizing material models and computational analysis. The first paper aims at formulating a material model for predicting large irreversible deformations in components made of pearlitic carbon steel. On the microscopic level, pearlitic steel is a two phase material consisting of cementite lamellas and a softer ferrite phase. Large plastic deformations in pearlitic steel lead to a re-orientation and alignment of cementite lamellas in the microstructure. This is believed to be the main reason for evolution of anisotropy in the material. Therefore, a macroscopic model formulated for large strains is proposed that captures this re-orientation and its influence on the macroscopic yielding of the material. Thereby, the re-orientations lead to distortional hardening of the yield surface. The proposed material model is calibrated against experimental results from cold drawing of pearlitic steel wires reported in the literature. In the second paper, the influence of the anisotropic surface layer on the propagation of cracks in pearlitic rail steel is investigated. Experimental results in the literature have reported significant degrees of anisotropy in fracture toughness and fatigue crack propagation rate in heavily deformed pearlitic structures. Indeed, such an anisotropy should be taken into account when trying to predict the fatigue life of components subjected to large deformations. This anisotropy can also be attributed to the alignment of cementite lamellas in the pearlitic microstructure which results in changes in the resistance against crack propagation in different directions. Micrographs of the surface layer of pearlitic steel rails, tested in a full scale test rig, show a transition from a fully aligned microstructure (a high degree of anisotropy) at the surface, to a randomly oriented lamellar structure (isotropy) at some millimeters from the surface. Based on these observations, an anisotropic fracture surface model is proposed to capture the anisotropic resistance against crack propagation and its dependence on the depth from the surface. The fracture surface model is employed in a computational framework for simulation of propagation of planar cracks. The framework is based on the concept of material forces where the propagation rate is linked to a crack-driving force. The results of simulations show that the characteristics of the surface layer have a substantial influence on the crack path.