To assess the criticality of a crack-like defect in a component, it is necessary to consider the value of crack driving force under combined primary and secondary loads, which can then be compared to a representative material's fracture toughness. The value of the crack driving force used can be determined from elastic-plastic finite element analysis or, more conveniently, by making use of simplified methods such as those contained in the R6 assessment procedure. A range of methods exist to describe how primary and secondary stresses combine. However, each of these methods has different levels of associated conservatism. The main reason for these different levels of conservatism is the underlying theory, or fit to finite element analyses, that have been used to define each respective approach. This is because the effect of elastic follow-up may be significantly different over a range of cases will lead to different levels of plastic enhancement to the contribution of the secondary stress to the total crack driving force. The work presented here provides finite element analyses to optimise the level of elastic follow-up for comparison to these assessment methods. The work has shown that the recently published approach by Ainsworth was not able to conservatively predict V/V_0 for case considered here, even if the elastic follow-up factor, Z, was allowed to become unrealistically high. A re-derivation of the Ainsworth approach, with the initial assumption that the secondary self-induced plasticity has little effect on the level of secondary reference stress, has been presented and compared to the finite element results where it has been shown to provide a much improved fit with more realistic values of Z.
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