Influence of Initial Defect Distribution on the Life of the Cold Leg Piping System

There are two basic design philosophies to avoid fatigue fracture presently contained in the ASME Boiler and Pressure Vessel Code. One, safe life, provides for fatigue resistance by avoiding the initiation of cracks, whereas the second, damage tolerance, presumes the existance of cracks and defects and guards against their unstable growth. This paper examines the utility of these approaches in the context of the sensitivity of piping system life to the character of the presumed initial defect distribution. The importance of defect detectability and growth monitoring is illustrated and the significance of parameters like defect size and shape to damage tolerant approaches discussed, in terms of piping system life. Results of the paper are interpreted in light of relevant NDI capabilities and requirements.