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.
Keywords: CJP model; crack closure; crack propagation rate; crack tip plastic zone; rail steel.