13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- increased. Cyclic fatigue lifetime of two X80 linepipe materials is gradually shortened as strain amplitude increases. In the earlier phase of cyclic deformation with strain amplitude exceeding 0.8%, X80 conventional linepipe material shows a cyclic saturation feature, while X80 high-strain linepipe material presents a cyclic hardening behavior. The softening rate of X80 high-strain linepipe material is much higher than X80 conventional linepipe during the whole process of cyclic deformation. 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. 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. The fatigue lifetime of X80 high-strain linepipe material is much higher than X80 conventional linepipe material. In initiation zone of X80 high strain linepipe material has a much smoother feature than X80 conventional linepipe material and its step is also much lower, which implies that high-strain linepipe material has a relative lower crack initiation rate. the amount of second crack and the spacing of fatigue striation of X80 conventional linepipe material is much higher than X80 high-strain linepipe material, which means that fatigue crack propagation rate of X80 conventional linepipe material is much higher. X80 high-strain linepipe material presents a much obvious tire-shaped patter in crack propagation zone which presents a typical brittle fatigue feature. Substructure evolution investigation reveals that the primary deformation mode of two X80 linepipe material is islocation slipping. As strain amplitude increased, dislocation cell developed gradually from dislocation line and tangled dislocation in two X80 linepipe materials. Dislocation density in X80 high strain linepipe material is much higher than X80 conventional linepipe material under various strain amplitude. The formation of dislocation cell or dislocation realignment of X80 high-strain linepipe material is always later than X80 conventional linepipe material. Ferrite deformation has been restrained in X80 high-strain linepipe material due to the higher amount of M/A and ferrite which postponed the formation and development of dislocation cell and caused a much lower softening rate. Acknowledgements Thanks for the financial support of Major Science and Technology Project of the 2nd WEGP of China National Petroleum Corporation. References [1] Levin SI. Causes and requency of failures on gas mains in the USSR [J]. Pipes and pipelines international, 91 (30): 149-176. [2] Hagiwara N, Meziere Y, Oguchi N, et al. Fatigue Behavior of Steel Pipes Containing Idealized Flaws under Fluctuating Pressure [J]. JSME International Journal, 42(4): 610-617. [3] Hagiwara N, Oguchi N. Fatigue Behavior of Line Pipes Subjected to Severe Mechanical Damage [ J ]. Journal of Pressure Vessel Technology, 121(4): 369-374. [4] Fowler JR, Alexander CR, Kovach PJ, et al. Fatigue Life of Pipelines with Dents and Gouges Subjected to Cyclic Internal Pressure[C]. Proceeding of the Energy-Sources Technology Conference and Exhibition. Houston: ASME, 69: 17-35.
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