This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada
The objective of this paper is to evaluate a cracked piping bend, i.e. elbow, to determine the limiting critical crack sizes due to internal pressure and to compare these results to a straight pipe, i.e. cylinder. A measured material JR resistance curve provides the pipeline material’s toughness. If the material is ductile and operating on the toughness upper shelf, a ductile tearing instability fracture assessment can be used to evaluate the cracked piping bend and cylinder. The benefit of a ductile tearing instability analysis is a larger critical crack size or higher critical load by using the full JR curve instead of a single toughness value, which is often the case when using the Failure Assessment Diagram (FAD) method. A rising JR resistance curve gives additional toughness as stable ductile tearing occurs. Both the ductile tearing resistance and FAD assessment methods are described in the engineering best practice standard API 579-1/ASME FFS-1 and fracture mechanics text books.
A piping bend geometry was chosen as an example to examine the modeling details including: creating the 3D crack models, obtaining convergence in the elastic-plastic finite element analysis (FEA) solver, computing the crack front J-integral values, and determining the ductile tearing instability. 3D crack meshes are used to model a range of crack sizes and aspect ratios in the piping bend to accurately model the geometry, crack location, and crack orientation. The needed crack front mesh refinement is shown and described. The crack mesh region is connected to the larger model using tied contact. The elastic-plastic FEA uses the pipe material’s stress-strain curve and captures crack front blunting at the evaluation pressure. The piping bend model includes the straight pipe sections at each end of the bend, and the model uses the non-linear geometry option in the FEA solver to capture any ovalization of the bend. Axial cracks are examined. The JR resistance curve and applied J curve (from the FEA) are converted into their corresponding non-dimensional tearing modulus curves, providing an easier way to determine the instability point, which is at the intersection of the two curves. The same modeling approach is used for the cylinder with axial surface cracks to obtain limiting flaw sizes for comparing to the piping bend to address the question if a cylinder model provides a useful approximation to the cracked bend geometry, or if the results differ enough to require the more detailed analysis.
A benefit of using a ductile tearing instability analysis is obtaining a critical flaw size or limiting load larger than when using a single toughness value, which could justify reducing or delaying inspections, and could allow for a longer service life in cyclic fatigue before repair or replacement is needed.
|Date||18th June 2019|
|Organisation||Quest Integrity Group|