13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 105 106 107 100 150 200 250 300 350 P1200 (In air) Pitted (In air) Corrosion fatigue P1200( In air, not failed) Maximum Stress / S (MPa) Number of cycles to failure / N (cycles) Fig. 2. S-N data for air and artificial seawater environments. Figs. 3 show the maximum stress within the pit. The maximum stress is not always at the bottom of corrosion pit and it moves to the mouth with the aspect ratio a/2c increasing. The maximum stress occurs at the bottom when a/2c is less than 1/7 under tension loading and a/2c less than 1/10 under bending loading. (a) a/2c =1/7 (b) a/2c =1/6 (c) a/2c =1/10 (d) a/2c =1/9 Fig. 3.The maximum stress distribution at various aspect ratios under tension (a), (b) and bending (c), (d). Cracks initiate from the point where the stress concentration is highest. Then the corrosion pit transfers to crack when the stress intensity factor for the equivalent surface crack growth for the pit reaches the threshold stress intensity factor for the fatigue crack growth, and the corrosion fatigue crack growth rate exceeds the pit growth rate. The SCF is largely influenced by the aspect ratio (a/2c) and the type of loading mode, as shown in Fig.4. The SCF increases greatly with increasing aspect ratio when a/2c is less than 1, and slow down when a/2c is between 1-2. It remains unchanged as a/2c is greater than 2. The tension loading produces bigger SCF than the bending. Also compared is the depth effect and width effect, illustrated in Fig. 5. The pit depth has much bigger effect than the pit width. 6. Conclusion The corrosion pit size and shape development and its effect on crack initiation and fatigue life were reviewed. It suggested the following: 1) Various relationships were developed for corrosion pit depth and pit width. But the literatures mostly suggest that the pit width follows a linear relationship with time and the pit depth is linearly proportional to the square root of time. 2) The higher the stress amplitude the more corrosion pits formed. Compared to artificially-induced pits, real corrosion pits have a smaller stress concentration factor, but lead to shorter fatigue lifetimes.
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