Calculation of the delayed hydride cracking velocity vs. KI curve for Zr-2.5 Nb by critical hydride cluster length

In Zr-2.5 Nb alloy the delayed hydride cracking (DHC) velocity versus stress intensity factor KI relation exhibits a three stage curve. The first two stages of this curve have been numerically simulated by the authors in previous work, using the experimentally measured critical lengths of the hydride cluster for fracture and a numerical analysis of time-dependent hydride growth at the crack tip (D. Yan and R. L. Eadie, J. Mater. Sci.35 (2000) 5667; Idem., Scripta Mater. 43 (2000) 89). A modified experimental method was developed and this method allows hydride clusters formed under different KI to be separated so that the critical hydride length can be measured (Yan and Eadie, 2000). The numerical analysis was based on a simplified model, proposed by Shi and co-workers, that assumes a cylindrically symmetric stress distribution at the crack tip (S. Q. Shi, M. Liao and M. P. Puls, Modeling Simul. Mater. Sci. Eng.2 (1994) 1065; S. Q. Shi and M. P. Puls, J. Nucl. Mat.218 (1995) 30). In this work both the experimental work and the simulation of the DHC velocity versus KI curve are extended to include a different test temperature and the simulation results are compared with experimental measurements. This result is unique since both velocities and hydride cluster lengths have been determined in the same specimen over the range of KI right down to KIH. The two temperatures used 150 and 250°C cover the temperature range of most practical interest in CANDU pressure tubes. It was found that the simulated curves were in reasonably good agreement with the experimental measurements. The results also predicted the experimentally observed change in critical hydride length (striation size) with temperature.