13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- mixed mode of fatigue, the da/dN are tended to increase along with the crack length at both α of 30° and 60°. In this case, the important issue is that whether the local microstructures still have a large influence or are there other factors that should be considered. The roles of ΔKI and ΔKII in the mixed mode crack growth also need to be elucidated. 100 1000 0 40 80 120 160 200 Crack extension Δa (μm) Cyclic numbers N (cycles) α=0, ΔK I/ΔK eff=1 α=30°, ΔK I/ΔK eff=0.92 α=60°, ΔK I/ΔK eff=0.65 Figure 2. The crack extension Δa versus cyclic numbers N at different loading angles 8.75 8.80 8.85 8.90 10-5 10-4 10-3 da/dN (mm/cyc) Crack length a (mm) α=0, ΔK I/ΔK eff=1 α=30°, ΔK I/ΔK eff=0.92 α=60°, ΔK I/ΔK eff=0.65 Figure 3. Fatigue crack growth rates at different loading angles 3.3. Crack growth path deflection Fig. 4 presents the macro-crack profile at various loading angles. For mode I loading, as shown in Fig. 4a, crack deflected for about 21° from the initial pre-crack and then fluctuated, and the resultant crack path was quite tortuous. When load mode was superimposed with mode II loading at α of 30° (Fig. 4b), the initial angles of branch cracks was increased to 27°. Under this angle, crack extended for about 150 μm, after which the crack was further deviated to an angle of about 45° in
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