13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- the final stage of growth of 40 μm. With the ΔKII further increased, i.e., α equals 60° in Fig. 4c, the initial branch cracks deflected about 52°. The crack extension values along the crack path in the X and Y-axis directions are depicted in Fig. 5. By linear fitting of the data, the deflection angles were consequently calculated to be 25° and 50° at α of 30° and 60°, respectively. This agreed well with the work reported by Kim [20] who obtained similar values in a rail steel. Figure 4. Macro-crack growth path at various loading angles: (a) α = 0, (b) α = 30°, and (c) α = 60° 0 40 80 120 160 200 0 40 80 120 160 α=30° α=60° Linear fitting Linear fitting Δa y(μm) Δa x(μm) θ =50° θ =25° Figure 5. Fatigue crack extension in mixed mode loading conditions 4. Discussion 4.1. Influence of loading angle on FCG Fig. 6 shows the FCG under pure mode I loading. At the end of the pre-crack, the crack propagated with two branches, as illustrated in Fig. 6b. One of the branch crack (lower side of the pre-crack) stopped growth after initiation along the grain boundary, whereas the other branch crack deflected at an angle of 21° and propagated to become the main crack. The competition between the two branch cracks to form the main crack was observed at N around 513 cycles in Fig. 6c. Though the original main crack continued growing forward, the lower side branch crack grew with a higher rate in a transgranular mode, and finally produced the main crack (Fig. 6d). In this process, it seemed that the crack tended to orient itself into a pure mode I condition at which the crack path was normal to the external loading direction. The alternation of the main crack was also observed when N was around 727 cycles, as shown in
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