13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- In case of SA-P steel subjected to fatigue in long life region in RH 85%, corrosion debris (Fig. 6a) and multi cracks initiated from a corroded pit (Fig. 6b) are confirmed on specimen surface, implying that crack initiation was promoted by anodic dissolution. Figure 7 shows SEM morphology of SA-P steel fractured in both low and high humidity. In case of low humidity, slip steps appear on a facet in the vicinity of crack initiation site, as shown in Fig. 7a-1)), meaning that slip deformation or stage I cracking dominate crack nucleation [8]. On the other hand, in high humidity, crack initiation is characterized by many facets, especially river pattern-like facets (Fig. 7b-1), suggesting that hydrogen embrittlement assisted fracture occurred. Figure 5. Crack growth rate vs. stress intensity factor range (a) Corroded area (b) Cracks initiated at corrosion pit Figure 6. Surface state of SA-P specimens subjected to fatigue in long life region in RH 85% (Arrows indicate crack tips) 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% SA-P DA-U
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