Effect of temperature and hydrogen concentration on the threshold stress intensity factor of radial delayed hydride cracking in fuel cladding

Three-point bending tests provide the ability to measure the propagation of axially oriented cracks in thin-walled fuel cladding in a radial outside-in direction. Via three-point bending, unirradiated Zircaloy-2 cladding containing 155 and 305 wppm hydrogen was mechanically loaded at different temperatures, sufficient to initiate delayed hydride cracking (DHC) whereupon the unloading rate was fast enough to arrest further propagation. The crack front dimensions were measured through fractography and implemented with the final load into a finite element model (FEM). Given the average crack length and final load, the FEM was able to back-calculate the stress intensity factor. Over a temperature range of 210-330 degrees C, around 6 MPa root m was calculated as the minimum threshold stress intensity factor for DHC. The trend shows that the ideal temperature for DHC with lowest threshold stress intensity factors may depend on the hydrogen concentration, and that the threshold stress intensity factors quickly increase with higher test temperatures. It has also been shown that creep affects the threshold stress intensity factor for optimal DHC less for lower hydrogen concentrations than for higher hydrogen concentrations. At low hydrogen concentration, the temperature of terminal solid solubility for precipitation (TSSP) is low enough so that creep-induced crack tip rounding plays a less significant role compared to the hydrogen kinetics. (C) 2022 The Authors. Published by Elsevier B.V.