Currently, structural integrity safety cases are based on avoidance of fracture initiation, providing a pessimistic estimate of structure life. However, crack behaviour at heterogeneous junctions is an area where understanding is particularly limited. Improved data regarding the behaviour of defects at such junctions after initiation may lead to improved assessment of real structures containing welded joints. Furthermore, justification of stable tearing at a material interface would result in increased demonstrable safety and higher plant availability. An experimental study has been carried out to investigate the ductile fracture behaviour of a crack approaching material interfaces. The aim has been to examine and refine current guidelines in the R6 defect assessment procedure. The experiments have been supported by 3D finite element modelling of the specimens and by R6 calculations. A further assessment of the standard (25mm) CT specimen results indicated that the R6 guidance on usage of the toughness in the crack tip medium may not always be conservative. However, the assessment points are conservative when the lowest yield strength or the weighted average yield strength values are used in the R6 fracture assessment.

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