Procedures for structural integrity assessment such as R6 and BS7910 often need to consider the possibility of closely-spaced structural defects interacting with one another. Typically, this comes in the form of a simplified interaction rule: if a criterion based on the size, shape and separation of the defects is satisfied, then they are judged to interact and cannot be analysed independently. When interaction is judged to occur, the defects may be re-characterised as a larger, enclosing defect. Rules for predicting defect interaction and then specifying the size of the re-characterised defect need to be conservative across a wide range of situations, so they are often informed by studies using parametric finite element analysis and laboratory testing. This article discusses the interaction of defects in inelastic materials, focussing on the determination of limit loads and the elastic-plastic crack driving force for co-planar crack-like defects. New methods for parametric modelling and data analysis are introduced, with examples demonstrating the use of inelastic modelling in formulation of interaction criteria.

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