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.
Rolling Contact Fatigue
crack propagation
Anisotropy
plasticity
pearlitic steel
material forces