13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- the increment of strain amplitude. It can be observed that cyclic hardening rate of two X80 linepipe material is higher than that of static tension. It means that although the material apparently presents cyclic softening, since uniform distribution ability of deformation elevated by hardening, cyclic level is declined with the increment of strain amplitude [5]. Cyclic stress strain curve of two X80 linepipe material has been compared as figure 5 shown, it displays that cyclic softening level of X80 high-strain linepipe material is much lower than X80 conventional linepipe material under the same strain amplitude and the difference of stress amplitude between them gradually enhances as strain amplitude increases, which implies that X80 high-strain linepipe material has a much better cyclic strain resistance. Figure 4. Cyclic strain stress curve and static tension curve of X80 linepipe material: (a) conventional, (b) high-stain 3.3. Fatigue lifetime According to Manson-coffion equation, the relationship between strain and lifetime of X80 conventional and high strain linepipe material can be extrapolated as equation (1) and (2) after test data linear fitted in the logarithmic coordinates by least square method. f 0.0677 0.5572 t e f 0.0044(2 ) 0.3171(2 ) 2 2 2 p N N ε ε ε − − Δ Δ Δ = + = + (conventional) (1) f p 0.1158 0.5032 t e f 0.0073(2 ) 0.2159(2 ) 2 2 2 N N ε ε ε − − Δ Δ Δ = + = + (High-stain) (2) Figure 6 shows the strain-lifetime curve of two X80 linepipe materials. It indicates that lifetime of X80 high-strain linepipe material is much higher than X80 conventional linepipe material when stain amplitude is much lower as 0.4%~1.0%. When strain amplitude exceeds 1.0%, the lifetime of two X80 linepipe materials almost has no difference. Generally, the lifetime of X80 high-strain linepipe material is much higher than X80 conventional linepipe material. (a) (b)
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