An Improved Axial Surface Crack Model to Predict Crack Growth and Burst Pressures Based on FE Analyses For Jplastic And J-R Curve Methodology

Abstract

During an in-depth review of the MAT-8 method, it was noted that there were assumptions for determining the FAD-curve shape by matching three points along FAD curves from FE analyses to get the plastic contribution of the crack-driving force (Jpl). Because the FAD-curve shapes from the FE analyses for different flaw sizes (depths and lengths) of interest for liquid line operation were quite different from both the MAT-8 assumed FAD-curve shapes and the FAD-curve fitting function for the FE-based FAD curves, an alternative procedure was pursued. For the cases of deeper and longer flaws, FAD-curve shapes cannot be found easily due to the highly nonlinear plastic behavior. To develop a new FE-based crack-driving force equation, a modification of the GE/EPRI basic formulation with contributions for elastic and plastic crack-driving force relationships was implemented. Several hundred FE analyses for axial surface cracks in pipes were performed. The new procedure was developed first by separating the elastic contribution from the total Japplied, then using the plastic part of Japplied versus pressure (Jpl-P) curve to establish an analytical solution. A nonlinear-regression analysis of the FE results from the entire Jpl-P curve was used, not just a few selected points. The new Jpl-P analytical solution showed good agreement with the carefully conducted FE-based solution from short, shallow flaws to deep, long flaws. The MAT-8 based solutions were good for short flaws, but deviated from the FE-based solution with both larger flaw depths and lengths. In addition to the new procedure, we included the use of the J-R curve for surface cracks which has been found to be a function of the surface crack a/t in pipe and SEN(T) tests, i.e., toughness is not a constant for all surface crack geometries which is discussed in other papers. Predicted burst pressures from the various methods including the new procedure were compared to pipe tests are presented.