Cheng-Ying Ho

Subcritical crack growth behavior for hydrided Zircaloy-4 plate

Abstract To study the subcritical crack growth (SCG) behavior for hydrided Zircaloy-4 plate, sustained load tests were performed on fatigue precracked specimens in Ar atmosphere at 200, 250, and 300 °C with hydrogen contents up to 290 ppm. Log crack velocity versus stress intensity curves showed typical three-stage crack growth behavior. The values of threshold stress intensity (Kth) were not sensitive to both temperature and hydrogen content and were about a constant value, 21 MPa m . The crack growth velocity at stage II, (da/dt)II, increased with increasing temperature. In general, the SCG behavior is not consistent with the classical Dutton-Puls delayed hydride cracking model, except for the specimen containing 110 ppm H tested at 200°C. On the basis of metallographic and fractographic results, the crack propagation process is proposed. The critical stress intensity, Kc, decreased with increasing hydrogen content, which indicates that the effect of increasing hydrogen concentration is to shorten the K range in stage II.

Effect of hydrogen gas on the mechanical properties of a zirconium alloy

Abstract To study the mechanical properties of Zircaloy-4 in hydrogen gas, notched Zircaloy-4 plate specimens were tested in a hydrogen gas environment of various pressures up to 2020 kPa at 25, 100 and 200°C. The results showed that, for all test temperatures, the notch tensile strength of Zircaloy-4 was slightly increased with hydrogen pressure until 101 kPa and then decreased with increasing hydrogen pressure. A ductile-brittle transition of reduction of area at 25°C was found when the alloy was tested at hydrogen pressure between 101 and 1010 kPa. From the fractographic finding, the ductile-brittle transition was closely related to the precipitation of brittle hydrides. The ductile-brittle transition became less distinct at 100°C and above. This was attributed to the improved ductility of zirconium matrix with increasing temperature.