Fatigue crack growth rates in an irradiated 316 stainless steel were obtained at a load cycling frequency of 0.8 Hz with hold times of 1, 10, and 50 s and at 60 Hz at test temperatures ranging from 298 to 1023 K. Specimens were prepared from wrapper tube material, which was irradiated to a fluence level of 4 × 1026 n/m2 (E ⩾ 0.1 MeV) at 683 K in a fast breeder reactor (JOYO).
Results indicated that crack growth rate with hold time can be approximated by the form of dah/dN (with hold time) = da/dN (1 + Bth0.7), where th is the hold time and B is a proportionality constant which is a function of test temperature and helium content.
For the lower frequency of 0.8 Hz, crack propagation occurred with the crack tip branching at and below 673 K. Crack tip branching resulted from microcrack formation in the dislocation channel, where deformation by fatigue loading occurred preferentially. At high cyclic frequencies large strain rates near the crack tip suppressed crack branching to increase the growth rate. Above this temperature range, no crack tip branching occurred, and the frequency effect was not large.