Elastic-plastic calculations have been performed for the geometry containing twin non-aligned through-wall flaws in tension. Such geometries develop localised shear flow fields in which neighbouring flaws interact over much larger separations than currently allowed for in the codes [1–3] under elastic-plastic conditions. J values have been obtained in the annular zone spanning directions directly ahead of the crack, to the geometric angle which connects the two crack tips. It was found that elastic-plastic conditions give rise to a local mixed mode problem, where maximum J values rotate off the plane of the flaw, towards the neighbouring flaws. Interaction factors have been defined by comparing elastic-plastic stress intensity factors for twin flaw configuration with the values for the single flaw. A significant interaction has been observed for geometries with vertical separation less than the length of the flaw (H<L). Interaction based on the ratio of the local to global limit loads has also been examined and shows interaction up to the vertical separations of H=1.5L, largely independent of the horizontal separation.

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