DHC Initiation at a Blunt Flaw: The Effect of Flaw Profile

Delayed Hydride Cracking (DHC) in Zr-2.5Nb alloy material is of interest to the CANDU (Canada Deuterium Uranium) industry in the context of the potential to initiate DHC at a blunt flaw in a CANDU reactor pressure tube. Modelling and experimental programmes have led to the development of a flaw evaluation procedure that is based on the process-zone methodology, and which has been incorporated into a Canadian Standards Association (CSA) Standard N285.8 for evaluation of fitness-for-service of Zr-2.5Nb pressure tubes. A key feature of the procedure is the importance that has been given to the effect of flaw profile, and in particular the effect of flaw root radius, which implicitly assumes that the flaw root profile is locally parabolic. It is against this background that the paper highlights the characteristics of smooth bottomed flaws that do not have a locally parabolic root profile. Such flaws, even though they are smooth bottomed can have a zero root radius of curvature, according to the accepted mathematical formula for root radius. The paper is a preliminary attempt at discussing the behaviour of non-parabolic root profile flaws, which clearly more closely resemble service flaws than parabolic root profile flaws, in view of their generality. It emphasises the desirability of linking the non-parabolic and parabolic flaw behaviours, so that, when evaluating a flaw found during service, advantage can be taken of the results from the parabolic flaw modelling and experimental programmes.