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

Sammanfattning: One of the main sources of damage caused by Rolling Contact Fatigue (RCF) inrailway components is the large plastic deformations that accumulate in the surface layerof these components. Large plastic deformations in components made of pearlitic steelinduce anisotropy in the mechanical properties of the material. The objective of thisthesis is to investigate the effect of this anisotropy on the RCF properties of pearliticsteel components by utilizing material models and computational analysis.The first paper aims at formulating a material model for predicting large irreversibledeformations 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 ferritephase. Large plastic deformations in pearlitic steel lead to a re-orientation and alignmentof cementite lamellas in the microstructure. This is believed to be the main reason forevolution of anisotropy in the material. Therefore, a macroscopic model formulatedfor large strains is proposed that captures this re-orientation and its influence on themacroscopic yielding of the material. Thereby, the re-orientations lead to distortionalhardening of the yield surface. The proposed material model is calibrated againstexperimental 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 propagationof cracks in pearlitic rail steel is investigated. Experimental results in the literaturehave reported significant degrees of anisotropy in fracture toughness and fatigue crackpropagation rate in heavily deformed pearlitic structures. Indeed, such an anisotropyshould be taken into account when trying to predict the fatigue life of componentssubjected to large deformations. This anisotropy can also be attributed to the alignmentof cementite lamellas in the pearlitic microstructure which results in changes in theresistance against crack propagation in different directions. Micrographs of the surfacelayer of pearlitic steel rails, tested in a full scale test rig, show a transition from afully aligned microstructure (a high degree of anisotropy) at the surface, to a randomlyoriented lamellar structure (isotropy) at some millimeters from the surface. Based on theseobservations, an anisotropic fracture surface model is proposed to capture the anisotropicresistance against crack propagation and its dependence on the depth from the surface.The fracture surface model is employed in a computational framework for simulation ofpropagation of planar cracks. The framework is based on the concept of material forceswhere the propagation rate is linked to a crack-driving force. The results of simulationsshow that the characteristics of the surface layer have a substantial influence on the crackpath.