The reference stress for axial (longitudinal) surface cracks in cylinders is compared using equations from the 2016 API 579-1/ASME FFS-1 and BS 7910:2013 engineering standards, and by using J-integral values from elastic-plastic Finite Element Analysis of three-dimensional crack meshes to compute crack front reference stress. The cylinder axial surface crack reference stress solutions from the two standards differ, and further examination and comparison is desired.
To evaluate if a crack is unstable and may cause catastrophic structural failure, the Failure Assessment Diagram method provides an evaluation using two ratios: brittle fracture and plastic collapse. The FAD vertical axis gives the Kr stress intensity to toughness ratio, and the FAD horizontal axis gives the Lr reference stress to yield strength ratio. The details of the FAD method are described in both standards, along with stress intensity and reference stress solutions for various geometries and crack shapes. Since the cylinder axial surface crack reference stress solutions from API 579 and BS 7910 differ, J-integral values are used to compute reference stress trends that provide additional insight and reveal if there is agreement with one or the other or neither standard.
Computing reference stress from crack front J-integral results is described in API 579 Annex 9G Section 9G.4. A 3D crack mesh is created for each crack and cylinder size. Along the crack front the focused mesh pattern uses initially coincident groups of nodes at each crack front position. The group of nodes at each location on the crack front are initially coincident and can separate to help model the blunting at the crack front as the loading increases and local plasticity occurs. Post processing calculations use the J-integral versus load trend and the material specific Kr at Lr = 1 ratio to determine the reference stress geometry factor. The reference stress is computed at each crack front node to find the maximum crack front reference stress value for comparison to the engineering standards’ reference stress solutions.
A range of surface crack sizes in thin to thick wall cylinders with internal pressure are used to examine reference stress trends. Standard pipe sizes and typical pipeline steel material is used in the analysis.
The difference in reference stress solutions was found during an engineering critical assessment, so the J-integral approach was used to improve the solution to reduce conservatism and allow the component to remain in service.