Challenges for accurate and reliable determination of effective stress intensity factor range for structural assessments supporting incredibility of failure claims 1

Abstract

Operation of a civil nuclear pressurised water reactor (PWR) facility results in a complex variable loading sequence arising from combined mechanical and thermal loadings, which must be evaluated in conjunction with welding residual stresses. This presents a challenge for fatigue crack growth (FCG) assessments at the design stage, with no load history, in support of a safety case for components where gross failure must be shown to be incredible within the design life. Assessment methods for FCG are generally contained within in-service inspection codes, e.g. RSE-M, ASME XI. The codified methods were not developed for use at the design stage and can be overly conservative as a result. This study considers the treatment of crack closure in the RSE-M code in the case of a low alloy steel component. The aim is to understand the reasonableness of the approach and if this assessment parameter is a potential source of significant conservatism. The primary focus is the treatment of the start-up/shutdown transients, with a negative R-ratio modified via the inclusion of welding residual stress. The study has been carried out using elastic-plastic cracked-body finite element models with a constant defect size i.e., propagation is not explicitly modelled. Crack growth is inferred by examination of behaviour ahead of the crack-tip and crack-tip opening displacement (CTOD) to determine an effective load range on an un-cracked elastic model to subsequently derive an effective stress intensity factor range $\mathrm{\Delta }K$ .