Improved Failure Assessment Diagrams to Describe Delayed Hydride Cracking Initiation at a Blunt Flaw

Abstract

Delayed Hydride Cracking (DHC) in Zr-2.5 Nb 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 nuclear reactor pressure tube. The material is susceptible to DHC when a hybrided region forms at the flaw tip. The hydrided region could then fracture to the extent that a crack forms, and is able to grow by the DHC crack growth mechanism. A process-zone based methodology for evaluation of DHC initiation at a blunt flaw that takes into account flaw geometry has been developed. In a paper presented at the 2000 ASME PVP Conference, the process-zone methodology was used to develop failure assessment diagrams in such a way that the geometry dependence of the failure assessment curves was minimized. This was achieved by defining the ordinate of the failure assessment diagrams in terms of the ratio of the applied elastic peak stress divided by the threshold peak stress for DHC initiation at the tip of a deep flaw. However, the resultant failure assessment curves for Mode I loading did not have the simple form as the curves for Mode III loading, where the Mode III case was modelled in order to clearly see the interplay between material and geometry parameters. The present paper demonstrates that the irregular shapes of the Mode I curves were due to the relation for the threshold peak stress for the deep flaw that was used in the Mode I failure assessment curves. In the 2000 ASME PVP Conference paper an exact relation for the threshold peak stress was used for Mode III loading, while an approximate relation was used for Mode I. In the present paper a more accurate relation for the threshold peak stress for a deep flaw was used for Mode I loading, and the resultant Mode I failure assessment curves have a simpler form, which leads to more practical applications of the approach. Agreement between the improved Mode I failure assessment diagram predictions and experimental results is reasonable.