Effects of the size and distribution of hydrides on delayed hydride cracking (DHC) in Zr - 2.5 wt%Nb pressure tube material was studied by static load test and fractographic examinations. Crack velocities were measured in hydrided specimens that were cooled by ice-water quenching and furnace cooling from solution heat treatment temperature. Fractographic features after delayed hydride cracking were examined with use of scanning electron microscope (SEM) and transmission electron microscope (TEM) as a function of stress intensity factor (K) and the size and distribution of hydrides. Crack velocities in specimens with finely dispersed hydrides increased but were essentially independent of applied stress intensity factor. The incubation period before cracking commences and activation energy for crack growth decreased in specimens with finely dispersed hydrides. The size and distribution of hydrides did not influence fracture morphology significantly. The inter-striation spacing was independent of stress intensity factor in specimen with fine hydride. The results can be explained on the basis of Simpson - Puls model that is applicable for specimens with the distribution of coarse hydrides. Effects of preexisting hydrides around crack - tip and hydride phase associated with different cooling rate from solution heat treatment temperature, however, must be considered to explain the reduction of incubation time and activation energy for crack growth.
