Extension of the Simplified Method for the Interaction of Primary and Secondary Stresses

The R6 defect assessment procedure, used commonly in the UK nuclear industry to assess the significance of defects in structures, uses the Failure Assessment Diagram to evaluate limiting parameters whilst accounting for the effects of plasticity. The interaction of primary and secondary stress is accounted for within R6 through the use of the ρ, or an equivalent V, term. Previous work has developed an alternate method to that in R6 and has been termed the “Simplified Method”. This Simplified Method has the benefit of being less conservative than the current R6 method and of not requiring ρ (or V factors, and hence not requiring the use of the ‘Look-up’ tables. This paper builds upon the work presented previously and considers the reduction in displacement controlled secondary stress through primary stress induced plasticity by proposing a new function, g(), which is based upon an Option 2 failure assessment curve. Comparisons are also made upon a range of cracked geometries using both the current R6 Method and the derived Simplified Method including g(). The comparisons are made upon both a thermally induced bending stress and a weld residual stresses obtained from a detailed weld simulation. The range of cases provided is to further validate the method and provide strong evidence not only for different geometries but also for realistic residual stress fields. The application of the Simplified Method to the thermally induced bending stress field show improved estimated of K_J over the R6 Method and provide added value to both the Simplified Method and g(). Application to the weld residual stress field shows potential non-conservatism (of both the R6 and Simplified Method) and potential reasons are discussed.