Creep damage, ductility and expected life for materials with defects

Defects can pre-exist and grow by creep in structures subjected to loading at high temperatures. As structural integrity is not necessarily conveniently predicted and managed by applying design and life assessment techniques intended for nominally defect-free material, it is important that methods are available for quantified and safe assessment of defects. In addition to the assessment methods, also materials behaviour will affect the likely outcome. In particular, ductility of the materials is important, and unfortunately ductility tends to decrease when shifting from short-term testing to long term creep conditions. In this paper, two examples are shown of materials with such ductility effects when combined with defects. The first example involves 316H stainless steel subjected to creep loading with an extensive crack-like defect, resulting in a transformation from microscopically ductile to brittle intergranular cracking within a relatively modest time span. The second example will demonstrate a corresponding shift in OFP copper that shows a radical ductility and life reduction in creep when including so small weld defects that they would be undetectable in conventional NDT.