13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 10. Micro mophology of fitigue crack propagation zone of X80 linepipe material ( /2 σΔ =0.4%): (a) conventional，(b) high-stain 4. Sub-structure observation and Discussion Low-circle fatigue tests results of two X80 linepipe materials as presented above show that cyclic deformation behavior of both X80 linepipe materials is cyclic softening. In addition, the softening rate of X80 high-strain linepipe material is much lower than that of X80 conventional linepipe material, the fatigue lifetime of X80 high-strain line pipe material is much longer as well. Generally, cyclic softening behavior always decides by the evolution of micro sub-structure of material suffered for cyclic loads. Usually, possible reasons for cyclic softening are reduction of dislocation density resulted by opposite screw dislocation meetion and counteraction, dislocation realignment and formation of sub-grain or dislocation cell, decline of friction stress induced by dislocation off-pining which original pinned by solute atoms. Therefore, some investigations on both X80 linepipe materials after cyclic deformed have to be performed to discover the different cyclic strain behavior of both X80 linepipe materials. Since ferrite is relatively soft and has much fewer precipitated carbide, plastic deformation always preferentially happens inside it. Therefore, dislocation motion in ferrite can reflect the substructure evolution of material. Figure 11 presents substructure morphology of two ruptured X80 linepipe materials which suffered with lower cyclic strain amplitude 0.4%. X80 conventional linepipe material has a much heavier density of dislocation inside ferrite, and an incomplete dislocation cell is has formed by tangled dislocation as shown in figure 11 (a). However, it can be observed from figure 11 (b) that dislocation in ferrite and bainite is fairly higher and abundantly tangled on the grain boundary, while dislocation cell has not formed yet. The presence of dislocation of X80 high-strain linepipe material mainly is outcrop and dislocation line in ferrite due to much higher density of M/A constituent that can improve the deformation coordinate capability of material and postpone the occurrence of dislocation cell in ferrite. The difference of dislocation configuration between two materials results in the lower cyclic softening rate of X80 high-strain linepipe. (b) (a)
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