Predicting the life of steel rails

Abstract

A model of plastic strain accumulation, wear, and rolling contact fatigue (RCF) crack initiation in rail steel has been developed. Local to the contact zone, material is subject to severe cyclic stresses taking it beyond yield and leading to incremental accumulation of plastic deformation (ratcheting). This model is based on a ratcheting law derived from twin-disc, rolling-sliding contact experiments and can simulate thousands of ratcheting cycles with corresponding strain hardening. The model is being further refined to account for detailed microstructural changes. Sections of worn and fatigued rail, removed from service, have been metallurgically analysed. To obtain further data on rail-steel deformation and RCF crack initiation, twin-disc tests have been performed using discs cut from across a railhead and wheel rim. Two heat treatments were applied to some rail discs to investigate the effect of pro-eutectoid ferrite phase distributions and volume fractions. Tests were run to failure (defined by an eddy current crack-detection system) and to percentages of fatigue lives. Micro- and nano-hardness tests, and microstructural observations, have been used to suggest a micromechanism of fatigue crack initiation for the model. Application of this model will contribute to reduced maintenance costs and an improved understanding of RCF development.