Application of Constraint Corrected J-R Curves to Fracture Analysis of Pipelines

Fracture properties of API X80 pipeline steel have been developed using a set of single edge notched bend (SENB) and single edge notched tension (SENT) specimens with shallow and deep cracks to generate different crack-tip constraint levels. The test data show that the J-R curves for X80 pipeline steel are strongly constraint dependent. To facilitate transfer of the experimental J-R curves to those for actual cracked components, like flawed pipeline, constraint corrected J-R curves are developed. The two-parameter J-A₂ formulation is adopted to quantify constraint effect on the crack-tip fields and the J-R curves. The constraint parameter A₂ is extracted by matching the J-A₂ solution with finite element results for a specific crack configuration. A constraint corrected J-R curve is then formulated as a function of the constraint parameter A₂ and crack extension Δa. A general method and procedure to transfer the experimental J-R curves from laboratory to actual cracked components are proposed. Using the test data of J-R curves for the SENB specimens, a mathematical expression representing a family of the J-R curves is constructed for X80. It is shown that the predicted J-R curves developed in this paper match well with experimental data for both SENB and SENT specimens. To demonstrate its application in assessing flaw instability, a pipeline with an axial surface crack is considered. For a crack depth of 50% of the wall thickness, the predicted J-R curve is found to be higher than that for the SENB specimen with the same crack length to width ratio. From this predicted J-R curve and crack driving force obtained by finite element analysis, the failure pressures of the pipeline at the crack initiation and instability are determined and discussed.