Any catastrophic rupture scenarios of a steel pipe should be taken into considerations in the design and during the maintenance stage as the loss-of-containment may be accompanied by either property damage or fatal accidents. Ductile fracture of wrinkled (buckled) steel pipes on the tensile side of the cross-section is studied in this research as the most plausible case of ultimate failure for pressurized buried pipelines being subjected to monotonically increasing curvature. The results from two full-scale bending tests on X80 line pipe specimens that are pressurized up to 60% of specified minimum yield strength (SMYS) are considered as an input for the current study. The specimens possess the same dimensions and are made of X80 steel grade with different yield strength to tensile strength ratios (Y/T) of 90% and 83%. The specimen with higher Y/T ratio ruptured on the tensile side of the cross-section while experiencing post-buckling deformations. However, the specimen with lower Y/T ratio was unloaded after the formation of the local buckling.
Finite element analysis (FEA) of the full-scale tests were conducted and verified using the experimental data. The power law is calibrated to model the post-necking plasticity of steel using material test data, and, cumulative fracture criterion in conjunction with general fracture strain locus for the pipelines’ high-strength steel is implemented to predict the ductile fracture initiation in the pipe's wall. It is shown that the FE model accurately reproduces the load-displacement response and final rupture of the specimen with the higher Y/T ratio. For the other specimen, numerical simulation shows no rupture until the inner surface of the buckle comes into contact with itself which reveals that the lower Y/T ratio reduces the chance of rupture. Further numerical studies postulate that both Y/T ratio and internal pressure have a coupled effect on the rupture of wrinkled pipes and play a key role in triggering that kind of failure. That is, higher values of Y/T ratio and internal pressure increases the probability of the rupture of wrinkled pipes.
Separation is often observed after Charpy V-notch tests or Drop Weight Tear Tests (DWTT). Separation is defined as the fracture morphology whereby many cracks were formed parallel to the rolling plane. On the other hand, an arrowhead fracture is often observed near the surface of DWTT. The morphology of the arrowhead fracture is similar to that of separation. In this study, the relationship between arrowhead fracture and microstructure was investigated as compared with that between separation and microstructure. From this study, it was thought that the mechanism of arrowhead fracture formation was the same as that of separation formation although stress constraint in arrowhead fracture was different from that in separation.
High strength line pipe steels, with API (American Petroleum Institute) X80 grade yield strength or higher, have been used for many pipeline projects because of the lower costs to transport natural gas. Crack arrestability of brittle and running ductile fractures is one of the required properties for high strength line pipe steels as cracks must be arrested, even if a brittle fracture occurs from welds such as girth welds. Cracks must also be arrested if the line pipe body is subject to ductile fractures.
The DWTT (Drop Weight Tear Test) (Eiber, (1979)) is a primary test method that evaluates the crack arrestability of brittle fractures. This test evaluates whether a ductile crack is transferred from a brittle fracture after a brittle crack is initiated just under the notch. Previous results (Amano, (1986)) indicated that the crack speed fell below 450 m/s and that the crack was subsequently arrested in the full crack burst test, for line pipes with a DWTT shear area of more than 40%. However, a DWTT shear area of 85% or higher is required for such specifications as those of the API because DWTT shear area scattering is taken into account in a circumferential direction.
Østby, Erling (SINTEF Materials and Chemistry) | Kolstad, Gaute T. (NTNU (Norwegian University of Science and Technology)) | Thaulow, Christian (NTNU (Norwegian University of Science and Technology)) | Akselsen, Odd M. (SINTEF Materials and Chemistry, and NTNU (Norwegian University of Science and Technology)) | Hauge, Mons (NTNU (Norwegian University of Science and Technology), and Statoil ASA)