This study uses the digital image correlation technique to measure the crack tip displacement field at various crack lengths in U71MnG rail steel, and the interpolated continuous displacement field was obtained by fitting with a back propagation (BP) neural network. The slip and stacking of dislocations affect crack initiation and growth, leading to changes in the crack tip field and the fatigue characteristics of crack growth. The Christopher-James-Patterson (CJP) model describes the elastic stress field around a growing fatigue crack that experiences plasticity-induced shielding. In the present work, this model is modified by including the effect of the dislocation field on the plastic zone of the crack tip and hence on the elastic field by introducing a plastic flow factor ρ, which represents the amount of blunting of the crack tip. The Levenberg-Marquardt (L-M) nonlinear least squares method was used to solve for the stress intensity factors. To verify the accuracy of this modified CJP model, the theoretical and experimental plastic zone errors before and after modification were compared, and the variation trends of the stress intensity factors and the plastic flow factor ρ were analysed. The results show that the CJP model, with the introduction of ρ, exhibits a good blunting trend. In the low plasticity state, the modified model can accurately describe the experimental plastic zone, and the modified stress intensity factors are more accurate, which proves the effectiveness of dislocation correction. This plastic flow correction provides a more accurate crack tip field model and improves the CJP crack growth relationship.

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Affected by the service environment, the actual service conditions of rail steel are complex, and the safety evaluation methods are limited. In this study, the fatigue crack propagation in the U71MnG rail steel crack tip was analysed by means of the DIC method, focusing on the shielding effect of the plastic zone at the crack tip. The crack propagation in the steel was analysed based on a microstructural approach. The results show that the maximum value of stress of the wheel–rail static contact and rolling contact is in the subsurface of the rail. The test grain size of the material selected along the L–T direction is smaller than that in the L–S one. Within a unit distance, if the grain size is smaller, the number of grains or grain boundaries will be greater so that the driving force required for a crack to pass through the grain boundary barriers will be larger. The Christopher–James–Patterson (CJP) model can well describe the contour of the plastic zone and can well characterize the influence of crack tip compatible stress and the crack closure effect on crack propagation under different stress ratios. The crack growth rate curve at the high-stress ratio is shifted to the left relative to the low-stress ratio, and the crack growth rate curves obtained under different sampling methods have good normalization.