The use of failure assessment diagrams to describe delayed hydride cracking initiation at a blunt flaw

Abstract

Delayed hydride cracking (DHC) in Zr–2.5Nb alloy material is of interest to the CANadian Deuterium Uranium (CANDU) (1) industry in the context of the potential to initiate DHC at a blunt flaw in a CANDU reactor pressure tube. The existing CANDU blunt flaw DHC evaluation procedure is based on a threshold peak flaw-tip stress for DHC initiation that is independent of flaw geometry. Work is underway to improve the existing blunt flaw DHC evaluation procedure by developing a methodology that takes into account the effect of flaw geometry parameters. The methodology is based on representing the stress relaxation due to hydride formation and crack initiation by a process zone. A process-zone model was used to develop failure assessment diagrams in the conventional format of R6 as described in this paper, although the coordinates of the diagram have a different physical meaning. The failure assessment diagrams were shown to be independent of the scale of the configuration as well as specific values of material properties. The failure assessment curves as derived in the conventional R6 format exhibit a strong dependence on the geometry of the flaw due to the stress concentration and associated stress gradient. By redefining the ordinate of the failure assessment diagram in terms of peak flaw-tip stress the geometry dependence of the failure assessment curves was reduced significantly. This minimal geometry dependence of the failure assessment curves is valuable with regards to practical engineering flaw evaluations. Agreement between the failure assessment diagram predictions and experimental results is reasonable.