The Effect of Testing Direction on DHC Growth Rate Using Zr-2.5Nb Plate

Delayed Hydride Cracking (DHC) is a fracture mechanism responsible for several failures of Zr-2.5Nb CANDU pressure tubes. The crack velocity is directionally dependant. Disk-shaped hydrides preferentially form on certain crystallographic planes that are hypothesized to be responsible for the difference in DHC growth rates. If bulk hydrides slow crack growth, no directional difference in crack velocity will be observed when no bulk hydrides are present. When bulk hydrides are present the crack velocities should begin to diverge as temperature is decreased. In this study, DHC crack velocities are determined using cantilever beam specimens with cracks propagating in both the longitudinal and through-thickness directions with and without bulk hydrides present, i.e., below and above TSSP. The DHC crack velocities in the longitudinal direction were observed to be higher than the corresponding through-thickness crack velocities with or without bulk hydrides present. Thus, the bulk hydride hypothesis is disproved. Alternative hypotheses are presented.