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

Fracture properties of an 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 the 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 the X80. It is shown that the predicted $J$-$R$ curves developed in this paper agree 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.