Structural Capability of Austenitic Piping and Shells With Parallel Offset Cracks

Boiling water reactor (BWR) components made of austenitic stainless steel are exposed to the high temperature water environment. Under the combination of susceptible material condition (sensitization due to welding or cold work), weld residual stresses and the oxygenated high temperature water environment, reactor internals such as the core shroud and internal core spray piping experience stress corrosion cracking. The cracking is typically in the weld heat affected zone on both sides of the weld and is parallel to the weld direction. The determination of the structural capability of the component with parallel offset cracks is the subject of this paper. The Section XI, ASME code position on parallel planar flaws states that there is no interaction (i.e. no change in load capability due to multiple parallel cracks) for cracks that are separated by 12.5 mm (1/2 inch) or more. While this is reasonable under linear elastic fracture mechanics conditions, it is not conservative for ductile failure under limit load conditions. Alternatively, assuming that the parallel cracks are in a single plane for the purpose of determining the load capability is over-conservative and underestimates the structural capability. This paper is an extension of earlier work on the interaction of parallel flaws and considers the combines load capability as a function the crack separation. The interaction rules developed here were based on analysis and validated by comparison with extensive test data from different sources. For cracks in two parallel planes, the proposed rules for ductile materials (where limit load is the governing failure mechanism) allow them to be considered as separate cracks if the distance between the two planes is greater than 3t (where t is the shell thickness) and the limit load for cracking in each plane is determined separately. For planes separated by less than t, they are combined and assumed to be in one plane and the limit load is calculated for the combined crack. Linear interpolation is used when the separation distance, d is such that t≤d ≤3t.