13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- loading. Figure 7. Fatigue crack growth path at a loading angle of 30° Figure 8. Fatigue crack growth path at a loading angle of 60° The FCG behavior at a loading angle of 60° is illustrated in Fig. 8. It is observed in Fig. 8a that the crack extends transversely among the lathy martensites and propagates directly through the grain boundaries. In this process, crack bifurcated along the lath interfaces and grain boundaries. The passing through of the crack into the grain boundary was also observed in Fig. 8b, and then the crack extended along the interface of the lathy martensites. Therefore, it can be concluded that the interfaces of lathy martensites and the grain boundary are potential crack initiation sites. With the
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