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Delayed hydride crack velocities in Zr-2.5 wt% Nb alloys with different ther-momechanical treatments were measured. Materials with higher strength have higher crack velocity, and the stepwise crack propagation occurred by smaller increments associated with a smaller zone of crack tip hydrides. A series of load reduction experiments were performed on specimens with an active delayed hydride crack. An incubation period was required for the specimen to resume cracking after reducing the applied K to a level still significantly above the threshold stress intensity factor K 1H. The length of the incubation period depended on the amount of K reduction, material strength, temperature, and the final K in which cracking occurred. Crack velocity increases with the amount of hydrogen in solution in the matrix. Crack velocity increased as a function of the peak temperature reached in the initial cooldown thermal cycle. There is hysteresis in hydride solubility which results in different levels of hydrogen in solution depending upon the thermal history. The implication of this in terms of crack velocity is discussed.

The effects of hydride morphology on the axial fracture toughness of cold-worked Zr-2.5Nb pressure tube material have been determined between room temperature and 240°C. Tests were performed on small compact tension specimens machined from samples of material prepared with different morphologies and hydrogen concentrations. The morphologies were characterized by a parameter referrred to as the hydride continuity coefficient (HCC), which provides a measure of the extent to which hydrides are oriented in the axial-radial plane of the pressure tube. Hydrides in this orientation are known to be detrimental to the fracture properties of the tube. Fracture toughness was characterized by a J-R curve technique, from which it is possible to estimate the maximum stable size of a through-wall axial crack for typical reactor operating conditions. Material with HCC values greater than 0.5 exhibited low toughness from room temperature to 240°C, at which temperature there was an abrupt transition to an upper shelf toughness value. As HCC decreases, the transition to upper shelf toughness occurs more gradually and is complete at a lower temperature.

Delayed hydride cracking (DHC) is an important crack initiation and growth mechanism in Zr-2.5Nb alloy pressure tubes of CANDU nuclear reactors. DHC is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth, and fracture of a hydrided region at a flaw tip. In-service flaw evaluation requires analyses to demonstrate that DHC will not initiate from the flaw. The work presented in this paper examines DHC initiation behavior from V-notches with root radii of 15 μm, 30 μm, and 100 μm, which simulate service-induced debris fretting flaws. Groups of notched cantilever beam specimens were prepared from two unirradiated pressure tubes hydrided to a nominal hydrogen concentration of 57 wt. ppm. The specimens were loaded to different stress levels that straddled the threshold value predicted by an engineering process-zone (EPZ) model, and subjected to multiple thermal cycles representative of reactor operating conditions to form hydrides at the notch tip. Threshold conditions for DHC initiation were established for the notch geometries and thermal cycling conditions used in this program. Test results indicate that the resistance to DHC initiation is dependent on notch root radius, which is shown by optical metallography and scanning electron microscopy to have a significant effect on the distribution and morphology of the notch-tip reoriented hydrides. In addition, it is observed that one tube is less resistant to DHC initiation than the other tube, which may be attributed to the differences in their microstructure and texture. There is a reasonable agreement between the test results and the predictions from the EPZ model.