Threshold stress criterion for delayed hydride crack initiation at a blunt notch in zirconium alloys

The flaw evaluation procedures, currently used for CANDU (Canada Deuterium Uranium) nuclear reactor zirconium alloy pressure tubes, are based on the-prediction of the initiation of delayed hydride cracking (DHC) at a blunt notch by assuming that failure of hydrided material occurs when the peak tensile stress in the vicinity of a notch attains a critical value p(c). However. since failure involves plastic straining of the hydrided material, the value of p(c) cannot be unique. The paper quantifies this non-uniqueness in terms of the ratio p(c)/p*, where p* is the threshold stress appropriate to the base-line case where a hydrided region forms at the planar surface of a semi-infinite solid, that is subjected to a uniform tensile stress, and where no allowance is made for the compressive stress induced by hydriding. The extent to which p(c) exceeds p* depends upon the ductility of the hydrided material and the stress gradient in the vicinity of a notch. In the latter contest, it is shown how the ratio p(c)/p* increases as the hydrided region length decreases (smaller number of thermal cycles), and as the notch root radius decreases. Copyright (C) 1996 Elsevier Science Ltd.