The paper discusses the problem of crack extension in a weld in an engineering structure, and how this relates to crack extension in a small specimen of weld material. For the case where the parent material is softer than the weld material, it is argued that the J integral crack growth resistance curve for the small specimen can be above that for the structure, for which the curve depends on the flow properties of both the weld and parent materials, and also the crack size and weld thickness. The onset of crack extension can be conservatively predicted using the weld material’s toughness properties and the parent material’s flow properties, but nonconservative results may arise if the same approach is used to make instability predictions. The focus of attention is the case where the weld crack is parallel to the plane of the weld, a situation for which the J integral is path-independent with regard to any contour surrounding a crack tip.

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