13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Figure 12. Substructure morphology of X80 linepipe material by 0.8% strain amplitude: (a)~(b) conventional (Nf =1027), (c)~(d) high-strain (Nf =1145) Figure 13 presents the substructure morphology of two X80 linepipe material suffered 1.2% strain amplitude. For X80 conventional linepipe material, dislocation cell has been rapidly formed and cell wall-thickness obviously increases due to the dislocation pilling up nearby as indicted by figure 13(a). It is observed in figure 13 (b) that dislocation cell has been formed in ferrite surrounded by M/A, which further demonstrated the decrease of softening rate resulted by M/A that can obstruct deformation and postpone the development of dislocation cell in ferrite of X80 high-strain linepipe material. Figure 13. Substructure morphology of X80 linepipe material by 1.2% strain amplitude: (a) conventional (Nf =340), (b) high-strain (Nf =343) 5. Conclusion Generally, two X80 linepipe materials exhibit a cyclic softening behavior under the cyclic load. Softening rate of two X80 linepipe materials show an incremental tendency as strain amplitude (b) (a) (c) (d) (b) (a)
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