Development of guidance for leak-before-break analysis of complex shaped cracks

Within the context of leak-before-break (LbB) methodology, this paper presents results from finite element (FE) analyses of a range of complex through-wall cracks in an infinite flat plate subject to combined primary membrane and secondary bending loads. The aim is to provide a simplified approach for the treatment of converging/diverging cracks in LbB assessments. Expressions have been derived from curve fits to FE results, which allow the calculation of the crack opening area (COA) for a complex crack. This expression is based on the crack mouth opening displacement for a straight fronted crack under membrane loading, with modification factors that take account of the crack shape, section thickness and the level of imposed bending stress. The variation of J through the plate thickness has also been studied. Some of the flat plate analyses have been repeated for a straight crack using elastic-plastic material properties. Elastic values of COA are always found to be conservative (i.e. an underestimate) compared to inelastic values. Elastic analyses of a cylinder containing a circumferential defect have also been carried out. These indicate that the crack opening on the inside wall is generally smaller than the crack opening from the corresponding flat plate analysis.