PSI - Issue 13

H.E. Coules / Procedia Structural Integrity 13 (2018) 361–366

362

Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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not interact. In structural integrity assessment it is important to be able to account for the possibility of flaw interaction conservatively. As a result, structural integrity assessment procedures such as BS 7910 (BSi 2013) and R6 (EDF 2015) commonly include simple criteria for judging the significance of flaw interaction at the start of an assessment. If interaction is judged not to be significant then each flaw can be considered in isolation for throughout the rest of the analysis. On the other hand, if interaction is significant then the flaws may need to be re-characterised as a single large enclosing flaw and treated as such for the rest of the procedure. Alternatively, in some cases the system of interacting cracks could be modelled explicitly (using FEA) and a decision on the structure’s fitness -for-service could be made based on the results. The interaction criteria used in assessment procedures are designed to be simple, easy to use, and conservative, i.e. never to indicate non-interaction when the adjacent flaws have a significant effect on one another. In practice, they may be applied to a wide range of situations: flaws arising via different mechanisms, components which may be subject to tension, bending or thermal shock, and different materials of varying ductility. However, they are typically formulated on the basis of FEA results for uniform tension only. This may be done by comparing the increase in Stress Intensity Factor (SIF) which occurs the tip of a crack in an interacting pair to the SIF which occurs for a crack of the same geometry on its own, and using a maximum allowable increase in SIF to help determine an interaction criterion. This increase in SIF is characterised by an interaction factor : ( ) = ( ) =∞ ( ) (1) where is the SIF for a crack in an interacting pair and =∞ is the SIF for a crack of the same geometry but located far from any other crack. , and are all functions of position on the crack tip line, which is defined using the ellipse parametric angle (see Figure 1a). Increasingly, the possibility of ductile tearing is also considered and so the results of experimental analysis and/or elastic-plastic FEA may also be used in the formulation of interaction criteria (Bezensek & Coules 2018; Bezensek & Hancock 2004; Coules 2018). The possibility of non-uniform distributions of stress may also be considered (Chapuliot & Blouin 2018). In this article, factors which affect the interaction of cracks will be discussed. The aim is to highlight factors which should be considered while formulating flaw interaction criteria for use in fitness-for-service assessment. Two issues in particular are emphasized: the interaction of flaws subject to non-uniform distributions of stress, and the interaction of flaws under elastic-plastic conditions. 1.2. Flaw geometry Crack-like flaws in structures can have various shapes, and systems of interacting (or potentially interacting) flaws can have different forms. However, a particularly important class of flaw systems is that of co-planar surface-breaking defects in plates and pressure vessel walls. Surface-breaking defects can be formed by several mechanisms either during manufacture or in-service. For example, weld hot cracking, fatigue and environmentally-assisted cracking all typically result in relatively sharp surface-breaking flaws. Furthermore, crack-like flaws which lie out-of-plane from each other can often be conservatively idealised by considering a similar set of in-plane flaws. Consequently, sets of co-planar surface-breaking flaws are important from the point of view of assessment. Here, as in R6 (EDF 2015), the flaws themselves will be treated as being semi-elliptical which allows a good shape approximation to flaws from different growth processes. This differs from the practice used in ASME Boiler and Pressure Vessel Code Section XI, where defects are normally approximated as rectangular (ASME 2013).

2. Analysis 2.1. Identical flaws (LEFM)

Components can be subjected to non-uniform through-wall distributions of stress as a result of bending, welding residual stresses and thermal shock. To illustrate the effect that the through-thickness distribution of stress has on interacting flaws, pairs of identical semi-elliptical and co-planar flaws in a plate (as in Figure 1a-b) were analysed under two different loading conditions using FEA. In each case, the stress transverse to the crack plane was uniform