DELAYED HYDRIDE CRACKING OF ZIRCONIUM-2.5 PERCENT NIOBIUM

The effects of load and temperature on the delayed hydride cracking of Zr-2.5%Nb have been investigated by use of tapered double-cantilever beam specimens. These samples were manufactured from Candu pressure tube material that was subsequently hydrided to the range of 92-298 ppm hydrogen. The progress of the hydride fracture was monitored by recording the acoustic emissions that were generated by the cracking samples. This was followed by fractographic and metallographic studies of the specimens.; The steady state cracking rate has been found to depend on prior conditions of both temperature and load. Generally faster cracking rates resulted when the load was increased to the test load, or when the sample was cooled to the test temperature.; In terms of temperature, hydride cracking in samples containing at least 92 ppm hydrogen was found to be bounded below by cracking rates too low to be considered significant, and bounded above by a crack arrest temperature, Tcat, which occurred in the range of 320{dollar}spcirc{dollar}C. Above this temperature the loaded samples went into a state of cracking remission. A long incubation period was then required to re-establish cracking.; Many features appeared on the fracture surfaces that could not be attributed to the progression of a purely brittle fracture. Comparison of the acoustic emission history with fractographic features of tests samples showed that the two were only related in a general way. This, and other observations, indicated that fracture processes other than the fracture of hydrides occur but do not produce significant acoustic emissions in the frequency range monitored.; Metallographic studies showed that the delayed hydride fracture produced multiple branching cracks that were often discontinuous in the plane of observation.; The delayed hydride fracture process is thought to consist of the propagation of two crack fronts: a brittle crack front, and a ductile crack front. A new qualitative five stage model has been developed for delayed hydride cracking which includes these two modes of fracture.