Rolling Contact Fatigue of Railway Rails with Emphasis on Crack Initiation
Sammanfattning: Increased traffic density and the heavier axle loads of modern trains have led to severe problems with rolling contact fatigue (RCF) which cause initiation of surface cracks in railheads. In this thesis, a specific type of surface crack, the head check, is studied numerically with emphasis on fatigue crack initiation. Head checks form on railhead surfaces in the strongly plastically shear deformed surface layer without any influence from material imperfections or macroscopic faults.A strategy for fatigue life prediction of RCF crack initiation in rails is developed. It combines finite element (FE) analyses, multiaxial fatigue crack initiation models used together with the critical plane concept, and fatigue damage accumulation; the results from the numerical analyses are compared with experimental results and observations made at a commercially used track. A newly designed numerical tool for the analysis of RCF in railheads is presented. It consists of two FE models that are coupled using a sub-modelling technique. In simulations, the FE tool accounts for both the global track response and the three-dimensional local elastic-plastic material response caused by the wheel-rail rolling contact. It is utilised for a real train traffic situation at a Swedish test site in a curved section of a track, with rails made of pearlitic steel, where head checks initiate on the gauge corner of the railhead. The predicted results show good agreement with tests and test site observations of the position and orientation of the crack plane and the number of cycles to crack initiation. Moreover, two-dimensional rolling contact simulations are carried out to: (i) evaluate fatigue crack initiation models, (ii) compare three elastic-plastic material models with cyclic plasticity that has a decaying ratchetting rate, and (iii) compare variable amplitude loads and constant loads for accumulation of damage and predicted time to fatigue crack initiation. Finally, the influence of variable amplitude loads on the stabilised plasticity-induced crack closure level is investigated in experiments and numerical analyses of fatigue crack propagation in Mode I loading. The investigation is made for specimens that are designed to fulfil plane strain conditions. The crack closure level is found to depend on the crack length: not on the stress range of the fluctuations in the variable amplitude load history. Statistical uncertainty analyses made of the fatigue life show that the uncertainties in initial crack length and load levels have a greater influence on the uncertainty in fatigue life than the fluctuation level of the load.
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