Evaluating the Applicability of a Local Limit Load Solution for Assessing Defects in Complex Structures

Abstract The limit load of a cracked structure is one of the key parameters when understanding fracture assessments using the R6 fracture assessment procedure. The limit load is important as it not only specifies the load at which plastic collapse occurs but can also have a significant bearing on J-integral evaluations using the reference stress methodology in an elastic-plastic fracture assessment (i.e. inherent in the R6 methodology). R6 provides a compendium of limit load solutions for a range of cracked geometries for use in plant assessments. Where the assessed geometry or loading condition cannot be represented by the solutions in R6, alternative methods, such as the so-called Connors local limit load solutions, are typically employed. Work undertaken within the R6 Development Programme, assessed the applicability of the limit load solution being developed at EDF Energy for assessing defects in a pipe branch connection. The work indicated that, as a result of the complex nature of the branch geometry, elastic-plastic redistribution of stresses around the complex geometry can result in increased bending when high levels of plasticity are seen. Use of the elastic stresses for these regions has the potential to lead to non-conservative estimates of J and highlighted that further understanding of the stress redistribution in a branch connection was required. Modification factors were derived as part of a method for the inclusion of the stress enhancement when calculating the limit load. Modification factors were derived by considering the elastic and elastic-plastic through-wall stresses within an uncracked branch for a range of geometries and attempting to characterise the modification factors with generic equations. Further work was undertaken to investigate defects in different locations and pipe branch geometries so that the applicability of the limit load solution has been assessed for a wide range of scenarios.