In assessing the integrity of structures, complex multiple flaws located in close proximity to each other are generally characterised as one, larger, single flaw. Guidance for the characterisation of multiple flaws is provided in codes such as R6 and BS 7910, which are routinely used in the UK and elsewhere in the structural integrity assessment of structures and components. Recent studies have shown that the current characterisation rules may be non-conservative under some circumstances. A combined experimental and analytical programme of work is underway within the UK in order to further investigate this potential non-conservatism for situations where the possibility of cleavage failure may have to be taken into account when assessing structures or components containing multiple flaws. Details of early stages of the analytical programme were reported at the 2006 ASME PVP Conference and comprised a number of finite element analyses to evaluate cleavage failure probability, via a master-curve based approach, for interacting twin flaws and the corresponding characterised single flaw under an applied tensile load. These analyses considered surface-breaking semi-elliptical flaws of the same depth but three values of aspect ratio and a range of separations. It was found that non-conservatism was indicated for flaws of high aspect ratio in contact. This paper describes further analytical work that has been undertaken to extend the results previously reported. The further work described has been centred on: • extending the three aspect ratios (a/c = 0.2, 0.44 and 0.8) for the twin surface flaws previously considered to a/c = 1. • extending the study to include an applied bending load. • extending the study on surface breaking flaws to include embedded flaws (for a/c = 0.8 and tensile loading only). The paper also includes results of the experimental work completed to-date.

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