The stages of fatigue crack initiation and small crack propagation usually occupy most part of fatigue life. The multi‐scale fatigue crack propagation mechanism of 304 stainless steel using in situ optical microscopy was investigated in this paper. An analytical model was also proposed to predict the multi‐scale crack growth rate. Results indicated that grain boundaries inhibit the small crack propagation and lead to the fluctuation of crack growth rate when the applied stress is relatively low. The prediction results using the proposed model agree well with the experimental result. The effect of grain size distribution on crack growth rate can be identified by the present model. Moreover, the prediction capacity of the proposed model is further verified by using another two materials, for example 7075‐T6 and U720Li.

Fatigue crack growth behaviours of the titanium alloy Ti‐6Al‐4V, with two different microstructures, at different maximum stresses were identified by digital image correlation technique. Full‐field strains were monitored around fatigue cracks after consecutive cycles in fatigue crack growth experiments. Results indicated that the Ti‐6Al‐4V alloy with a bi‐modal microstructure had a better fatigue resistance than that with a primary‐α microstructure. Typical behaviours of small cracks and the evolution of multi‐scale fatigue cracks were clarified. The strain accumulations around the micro‐notch and fatigue crack increased with increasing number of load cycles. On the basis of von Mises strain mapping, it was found that crack growth rate could be characterized by crack‐tip plastic zone size.