Abstract

When a crack is found in essential equipment two common questions are (1) will it break, and (2) when will it break? The API 579-1/ASME FFS-1 fitness-for-service (FFS) standard describes methods to answer those questions. When the available API 579 crack solutions are not sufficient for complex equipment geometry, Abaqus is used with 3-D crack meshes to calculate the values needed for the FFS assessment.

Important Abaqus capabilities include the J-integral calculation for the focused mesh at the crack front, non-linear material given by stress-strain curves, tied contact to easily connect the crack mesh with the larger mesh, and non-linear geometry for cases with large strains.

Analysis examples presented include an assessment using a custom failure assessment diagram (FAD), a ductile tearing instability analysis, and a crack growth analysis. The custom FAD for the crack assessment is determined using elastic-plastic J-integral results and can reduce conservatism compared to the default API 579 FAD. The ductile tearing instability analysis uses multiple crack sizes to compute the elastic-plastic J-integral values for comparison to the material resistance curve. A ductile tearing analysis can further reduce the conservatism in a crack assessment permitting larger cracks or larger applied loads. To answer the second question, a crack growth analysis using the Paris equation calculates the crack growth rates. An incremental remeshing method uses Abaqus results to calculate the elastic stress intensity values to update the crack length and crack depth growth equations. The growing crack is evaluated to determine the critical failure size.

Presenter Bio - Greg Thorwald

Greg Thorwald is a principal engineer at Quest Integrity in Boulder, Colorado, which is now part of Baker Hughes. He uses Abaqus finite element analysis for structural, fracture, and fatigue crack growth assessments in a variety of fitness-for-service consulting projects. He is the lead developer of the commercial FEACrack software, an automated parametric 3D crack mesh generation program creating Abaqus input files to compute crack front stress intensity and Jintegral values needed for Level 3 assessments. He also contributes to the Signal Fitness-forService software used for Level 2 fitness-for-service assessments. He is a contributing member of the API/ASME fitness-for-service committee that provides the API 579/ASME FFS engineering standard.