Evolution and prediction of rolling contact fatigue for rails with surface indentations

The support, guidance and high-speed operation of trains were realised by rails, which can be regarded as a key and critically important component of the track structure. Rail surface defects caused by external factors will directly affect the safe and reliable operation of trains. Therefore, in order to ensure safe service performances of defective rail, the formation and prediction of RCF (Rolling Contact Fatigue) for defective rails is deeply and systematically studied. The research objective of this thesis is to estimate critical sizes of surface defects and reveal the evolution and mechanism of materials with defects, which have important theoretical significance for the maintenance of rail with defect in the field. The research methodologies used for the realisation of the defined objective are based on scaled laboratory experimental techniques, finite element numerical modelling and a novel designed prediction procedure. 

In this paper, surface indentations, representing surface defects, were artificially prepared on the rail sample during the experimental study in this thesis. Then, a wheel-rail rolling testing machine was used to carry out tests on the RCF behaviour of rails with surface indentations considering effects of indentation geometric characteristics, entry of a water medium and operating parameters. Rail material RCF behaviour was systematically analysed by means of optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), etc. The evolution process of material around rail surface indentations and critical sizes of surface indentations are first experimentally conducted, including effects of indentation geometric characteristics on rail material RCF behaviour. And the mechanism of material around surface indentations under wet conditions was investigated to further study the influence of the existence of the indentations on rail materials. Then, as most railway lines are in an open and complex service environment, RCF behaviour of rail with surface indentations under various wheel-rail operation conditions was systematically studied. However, due to the impossibility of tracing contact stresses and strains around surface indentations and predicting RCF crack initiation in the experimental studies, based on the proposed procedure of RCF prediction, a finite element model for wheel-rail roller including surface indentations and a full-scale wheel-rail model were established to investigate contact stresses around surface indentations and to predict RCF crack initiation life of rails with surface indentations. Also, the geometrical relationship between wheel-rail rollers and full-scale wheel-rail was investigated. 

This research work contributes knowledge to the evolution mechanism of microstructure changes and RCF behaviour of rail material with surface indentations during the wheel-rail rolling contact process under various operating conditions. The critical size determination of surface indentations and the RCF crack initiation prediction of rail material with surface indentations that are currently unavailable and has therefore not been implemented in the existing railway engineering domain. This study confirms the capability for enhancement of the productivity, efficiencies, and safety of railways that will ultimately be a contribution toward sustainable transportation.