The fatigue crack growth region is the region of stable development after fatigue crack nucleation and initiation. Fatigue bands generally appear in the fatigue crack growth zone. Fatigue bands show the initial development of fatigue cracks and are the most obvious characteristics of the fatigue crack growth zone. Figure 1 shows the low magnification morphology of fatigue crack growth zone on fatigue fracture surface of low alloy HT250, reflecting the characteristics of fatigue crack growth zone of low alloy HT250.

Many curved tear edges can be found in Fig. 1 (a) and (b). In Fig. 1 (a), there is a river pattern (shown in solid frame) with point crack source radiating around. In Fig. 1 (b), tear edges and smooth cleavage surface can be found at the junction of two fracture surfaces, indicating that the fatigue fracture of gray cast iron has the characteristics of quasi cleavage fracture.

Fig. 2 shows the morphology of casting hole crack initiation and fatigue strip in the fatigue crack propagation zone of low alloy HT250 fatigue fracture.

It can be seen from Fig. 2 (a) that after the crack nucleates and initiates at the casting hole, it expands around to form a fatigue strip (shown by the solid line arrow). The fatigue strip is usually considered as some alternating grooves and uplift lines, or some step marks surrounded by these grooves and uplift lines. The fatigue strip represents the direction of fatigue crack growth, The appearance of fatigue bands shows that the grains have weak toughness, and each fatigue stripe corresponds to a cycle of fatigue crack propagation under cyclic load. It is found from Fig. 2 (b) that the extension direction of the fatigue strip is not on the same plane, indicating that the crack propagation is not carried out in a single direction, but in multiple directions at the same time (shown by the solid line arrow).