Fatigue crack propagation behaviour in ultra-fine grained low carbon steel

Fatigue growth behaviour of long cracks in an ultra-fine grained steel (grain size 1 μm) was investigated. The influence of tensile mean stress on propagation threshold and propagation behaviour was analysed. The threshold for fatigue crack propagation is relatively low and results are in good agreement with previous evidence that showed that the fatigue propagation threshold for long cracks decreases with grain size. However, when compared at medium and high applied stress intensity range level and the same applied mean stress, the ultra-fine grained steel showed a lower crack propagation rate than a SM490 steel or a HT80 steel. Influence of the mean stress was observed until a given stress ratio for the total analysed ΔK range, including the near threshold crack growth. Good correlation with fatigue crack propagation rate was obtained when the (ΔK Kmax)1/2 driving force parameter was applied.