Velocity of Delayed Hydride Cracking with Stress Intensity Factor in Zr-2.5Nb Pressure Tube

Delayed Hydride Cracking (DHC) has led many Zr- 2.5Nb pressure tubes to be replaced or fail during their operation in heavy water reactors. DHC was known to occur by diffusion of hydrogen to a crack tip, nucleation and growth of hydrides followed by fracturing of hydrides. Since some incidents of tube failures by DHC in 1970s, lots of researches have been conducted to understand its mechanism. Dutton and Puls suggested that a driving force for DHC is the tensile stress gradient formed at the crack tip. However, this model can't explain the unique phenomenon of DHC or the constant crack growth rate independent of the applied stress intensity factor or K. The DHC crack starts to grow only if the applied K exceeds the threshold stress intensity factor (or KIH) and its velocity increases quickly, which is termed Stage I, At Stage II, the DHC velocity becomes constant with an increase in the applied K. Besides, the spacing of striation lines on the fractured surfaces after DHC tests is wider close to the notch, becomes smaller gradually with increasing K and levels off to a constant at Stage II. The aim of this paper is to explain the cause for a change of the DHC velocity with increasing K based on the analysis of the stress state at the crack tip using a finite element method (FEM)