13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 240 260 280 300 320 340 360 380 50000 100000 150000 200000 250000 300000 Fatigue life/cycle Stress/MPa Retired sample 4 years 12 years 20 years Figure 3. Specimens’ stress-life curve The Fig.3 indicated that the S-N curve moved down along with the increase of parking time, and the fatigue life declined. 3. The life analysis model for the fatigue critical component 3.1. The life model based on the dynamic S-N curve Using the S-N curve： SN C α = （2） Where, C and αare the shape parameters of material. With the service time prolonging, if the loading was not changed, the effect of environment could result in the performance decline of component. That is to say, C and α could alter with the change of time, as the formula (3) and Fig. 4 shown [10]. (Parking Time, ) (Parking Time, ) C C T T α α ⎧ = ⎨ ⎩ = （3） Figure 4. Dynamic S-N curve Assuming the stress level for component was n at some loading spectrum, according to the linear accumulated damage theory, the fatigue life N could be shown as followed:
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