Surface defects in rails - Potential influence of operational parameters on squat initiation

Licentiatavhandling, 2015

Despite significant efforts throughout the last decades, the mechanisms behind the formation of squats — a form of rolling contact fatigue (RCF) damage in rails — are not fully understood. Proposed causes of initiation involve, but are not limited to, small initial rail surface irregularities which yield high contact stresses, rail corrugation and varying friction conditions. To complicate matters further, a very similar rail defect — the stud — has started to appear during the last ten years. This defect lacks common signs of RCF initiated damage, such as large scale plastic deformations, and is commonly found in connection with so-called white etching layers. The first paper of this thesis (Paper A) concerns a simplified two-dimensional model which is used to evaluate the dynamic interaction between a train and a flexible track. Wheel–rail contact stresses (and the resulting contact forces) are used to make assessments of the RCF impact due to rail surface irregularities under varying operational conditions. Excitation due to isolated rail defects and rail corrugation are considered. Differences in predicted RCF impact using a two-dimensional and a (computationally more expensive) three-dimensional contact model are investigated. A computational framework for more detailed RCF assessment is also established (Paper B). Wheel–rail contact stresses from the dynamic vehicle–track model are used as prescribed loads imposed onto a refined continuum finite element model of a rail section. This makes it possible to compute resulting stress and strain fields in the rail material. The propensity of RCF initiation is quantified using accumulated strain and the Jiang–Sehitoglu fatigue parameter. Finally (in Paper C), the aforementioned computational framework is utilised to perform detailed analyses of interesting operational scenarios that have been identified in Paper A. Further, the influence of interacting surface irregularities and varying friction conditions along the rail are investigated. The aim of this thesis is to increase the knowledge regarding squat initiation by means of numerical modelling. Such an improved understanding of squat initiation will also be beneficial in order to understand and mitigate the corresponding form of damage occuring on wheels — so called RCF clusters.