13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- To clarify the reason for the difference in high humidity susceptibility of fatigue limit between the SA-P and DA-U steels, successive observation of surface failure due to stress repetitions was conducted. Figs. 4 and 5 show crack growth curves and relation between crack growth rate and stress intensity factor range in 25% and 85%, respectively. It can be seen that the initiation and early propagation of cracks short than a few grain sizes are definitely accelerated by high humidity and this trend is more remarked at lower stress levels, because the lower the stress level, the longer the crack initiation life and the stronger the influence of humidity. In other words, it is the acceleration of crack initiation and its early growth that caused the fatigue strength decrease in high humidity. Figure. 3 S-N curves Figure 4. Crack growth curves 0 1x10 5 2x10 5 3x10 5 1E-3 0.01 0.1 1 10 σ a = 750MPa Crack length, a mm Number of cycles, N cycle RH 25% 85% SA-P DA-U 10 5 10 6 10 7 10 8 0 100 200 300 400 500 600 700 800 RH 25% 85% SA-P DA-U Stress amplitude, σ a MPa Number of cycles to failure, N f cycle
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