Short fatigue crack behaviour of LZ50 railway axle steel under multi-axial loading in low-cycle fatigue

An experimental study was carried out on the low-cycle fatigue of LZ50 railway axle steel under multi-axial loading. The initiation and propagation behaviour of a short fatigue crack was observed and analysed as per the principles of effective short fatigue crack. Our results showed that in the microstructurally short crack (MSC) stage, the crack propagation was strongly influenced by the microstructural features of the material and did not satisfy the linear elastic fracture mechanics. At the physically short crack (PSC) stage, the crack was long enough to not be directly influenced by the microstructures; nevertheless, linear elastic fracture mechanics still could not explain the law of crack propagation properly owing to the large-scale yield range of the crack tip. Based on these observations, a crack growth rate model under multi-axial loading was presented to explain the short fatigue crack behaviour. The non-linear elastic-plastic fracture mechanics parameter, J-integral (cyclic), was adopted as the driving parameter in this model and the crack closure effect was also taken into account. In the MSC stage, the crack growth rate fluctuated significantly when the cyclic J-integral was used owing to the influence of the microstructure. However, at the PSC stage, the crack growth rate fluctuations were small. A linear correlation for the PSC was obtained with R = 0.934 and this model could be applied to describe the short fatigue crack behaviour in the PSC stage.