PSI - Issue 13

H.E. Coules / Procedia Structural Integrity 13 (2018) 361–366 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

365

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Figure 4: Comparison of interaction factors for pairs of twin cracks calculated via finite element analysis (FEA) and via the method of weight functions (WF) for the uniform tension and MLOCA loading conditions. a. b.

Figure 5: Elastic-plastic crack interaction in a steel pipe subject to increasing internal pressurisation. a.) Elastic-plastic equivalent stress intensity factor. b.) Interaction factor. The effect that interaction between the cracks has on the elastic-plastic crack tip parameters is shown in Figure 5. With increasing levels of pressurisation, the interaction factor (now defined in terms of ) increases. For tougher materials, which are more prone to plastic collapse than fracture initiation, this may be less significant in for brittle materials. The degree of interaction between adjacent flaws in a structure is affected by the through-wall distribution of applied stress and the level of plasticity which occurs. Under certain non-uniform stress conditions and some conditions of elastic-plastic fracture, the effective level of interaction between flaws can be greater than is for the same geometry under uniform tension in LEFM. Therefore, when formulating flaw interaction criteria for use in structural integrity assessment procedures, non-uniform loading cases and ductile fracture should be considered carefully to ensure that the interaction criterion is conservative under all anticipated use cases. The use of weight functions for determining the elastic interaction between pairs of cracks has also been demonstrated. This method makes calculation of the interaction factor which occurs under an arbitrary through-thickness stress distribution straightforward. Weight 3. Conclusions