154 / 1971-01-01 00:00:00

Comparison And Analysis On Low-cycle Fatigue Behavior Of Two X80 Linepipe Materials

Strain amplitude, Low-circle Fatigue, Lifetime, Dislocation, M/A

Low circle fatigue tests were carried out with conventional and high-strain X80 linepipe material respectively. Results showed that high-strain X80 linepipe material exhibited a lower cyclic softening rate and a longer low-circle fatigue life than those of conventional X80 linepipe material. Cyclic response curves showed that the cyclic softening occurred at all strains amplitude (0.4%~1.4%) on two X80 line pipe materials. For conventional X80 linepipe material, cyclic softening was found after slight cyclic stress saturation at high strain amplitude of 1.0% ~1.4%. However, for high-strain X80 linepipe material, cyclic softening was observed after slight cyclic hardening at high strain amplitude of 1.0% ~1.4%. Fractography analysis suggested that transgranular fracture with well-developed fatigue striations and obvious second crack is the predominant failure mode. The amount of second crack was fewer detected in high-strain X80 linepipe material than that in conventional X80 linepipe material. TEM examination reveals that the primary deformation mode of two kinds of material was dislocation slipping. Abundant dislocation glide bands and dislocation cells were formed at grain boundary as strain amplitude increased. The number of dislocation blocked and wall thickness of dislocation cells in conventional X80 linepipe material are much greater than that in high-strain X80 linepipe material. The better cyclic deformation resistance of high-strain X80 linepipe material was ascribed to relative more M/A islands distributed inside the grains or grain boundaries.