13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Both the initiation (a few grain sizes) and its early propagation (~0.16 mm) are accelerated by high humidity. Most of fatigue life (70-90%) is spent in growing a crack up to ~1 mm. On the other hand, the influence of humidity on crack growth rate is not remarkable when cracks extend to ~8 grain sizes of prior austenite, and neither is the effect of hardness, as shown in Fig. 6, indicating that the accelerations to crack initiation and early crack propagation are the main reasons for the decrease of fatigue strength in high humidity. Figure 7 shows the effect of humidity on the feature of typical small cracks observed in Steel C. The edges of a crack corroded in RH85% look relatively wide opened in comparison with those in RH25%, suggesting the promotion of crack initiation in high humidity through anodic dissolution. The effect of humidity on crack morphology is shown in Fig. 8. In case of Steel A, a crack that propagated in a zigzag way along grain boundaries in high humidity grew almost vertically to stress axis in low humidity. In case of Steel C, however, the influence of humidity is hardly recognized by merely optical surface observation. Figure 9 shows morphology of fracture surfaces in Steel C. It is found that even in Steel C, brittle facets are also observed in the vicinity of crack initiation site in high humidity, arrowed in Fig. 9b, though no brittle facet is found in low humidity (Fig. 9a), which means that hydrogen embrittlement caused the acceleration of crack propagation in high humidity. Figure 6. Crack growth rate vs. stress intensity factor range Figure 7. Cracks observed in Steel C in low (RH25%, left) and high (RH85%, right) humidity, respectively (Axial stresses applied in the horizontal direction) 10 100 10 -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 Crack growth rate, da/dN m/cycle Stress intensity factor range, ΔK MPa･m1/2 RH 25% 85% Steel A Steel C
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