13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3. RESULTS 3.1. Cyclic stress response behavior The relationship between stress amplitude and lifetime of two X80 linepipe steel material has been investigated with different strain amplitude 0.4%~1.4% as shown in figure 3. Comparing figure 3 (a) and (b), some common cyclic fatigue characteristics of X80 linepipe material suffered with different strain amplitude can be concluded: i. the stress amplitude of two X80 linepipe materials declines with the increment of cyclic time Nf, which means that material shows a general cyclic softening during the process of cyclic deformation with different strain amplitude. ii. softening rate is relative lower when material suffers with low strain amplitude 0.4%~0.8%, softening rate greatly increases as strain amplitude exceeds 0.8%, total softening rate of material shows an incremental tendency as strain amplitude increased. iii. cyclic fatigue lifetime is gradually shortened as strain amplitude increases. According to figure3, some different cyclic deformation feature also can be found: i. in the earlier phase of cyclic deformation, e.g. Nf lower than 4, two X80 linepipe material shows an obvious cyclic softening feature when materials suffer with lower strain amplitude 0.4%~0.8. However, as strain amplitude exceeds 0.8%, X80 conventional linepipe material shows a cyclic saturation feature, while X80 high-strain linepipe material presents a cyclic hardening behavior in which hardening level is prompted as stress amplitude increased. ii. the softening rate of X80 high-strain linepipe material is much higher than X80 conventional linepipe during the whole process of cyclic deformation with different strain amplitude. iii. under the same strain amplitude, cyclic stress amplitude of X80 high-strain linepipe material is always higher than X80 conventional linepipe material, which maybe ascribes to the much higher strength of X80 high-strain linepipe material. Figure 3. Relationship between stress amplitude and lifetime of X80 linepipe: (a) conventional, (b) high-strain 3.2. Cyclic stress strain behavior Cyclic strain resistance in steady state of metallic material always can be indicated by cyclic stress strain curve. When comparing it with static tension curve, the feature of cyclic strain behavior can be found. In terms of relationship between /2 σΔ and p /2 εΔ , cyclic stress strain curve can be obtained after test data linear fitted in the logarithmic coordinates by least square method. Static tension curve and cyclic stress strain curve of two X80 linepipe material is shown in figure 4. It is found that two materials present cyclic softening characteristic in the range of strain amplitude. Softening level is much stronger under lower strain amplitude, while, softening level decreases with (a) (b)
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