Flaw interaction under bending, residual stress and thermal shock loading

Crack-like surface flaws in pipes, pressure vessels and other structural components sometimes occur near to one another. When this happens, it is often necessary to take the mutual interaction of the flaws into account when performing fracture-mechanics-based fitness-for-service assessment. Integrity assessment procedures including ASME BPVC Section XI, BS 7910 and R6 include interaction criteria which are used during flaw characterisation to ensure that interaction is accounted for conservatively. This paper considers the interaction which occurs when flaws are loaded by a non-uniform through-wall distribution of stress, as may arise due to bending, thermal shock or welding/cladding residual stresses. Using parametric finite element analysis of a large number of crack pairs subjected to different distributions of stress, it is shown that the degree of flaw interaction can be enhanced under non-uniform loading. Therefore, care should be taken when performing integrity assessment using interaction criteria based on uniform tension loading only.