Crack growth test data of Ni-base alloys under cyclic loading in simulated boiling water reactor (BWR) environment including the effects of load rising time (tr) were evaluated in the view points of both fatigue and stress corrosion cracking (SCC). When the test data were plotted in the relationship between da/dt and Kmax, da/dt monotonically decreased with increasing tr and the stress ratio (R). For alloy 182 weld metal under short tr and/or low R, the crack growth rate assuming SCC is much lower than those of the test data. For alloy 182 under tr = 30 and 1000 s at R = 0.8, the crack growth rate assuming SCC almost coincided with test data. For heat affected zone (HAZ) of alloy 600 base metal (600HAZ), the crack growth rate assuming SCC had much different slope of da/dNK relationship compared with the test data in the tested range of tr up to 3000 s. From these observations, the contribution of SCC is relatively small and the main mechanism of crack growth is thought to be fatigue for the tested range (tr=1 to 1000 s for weld metal, tr=1 to 3000 s for base metal and R = 0.1 to 0.8). It was assured that the fatigue crack growth formula proposed by the authors accounts the effect of SCC adequately at long tr. Additionally, the applicability of the fatigue crack growth rate formula for austenitic stainless steels to the long term cyclic load was investigated and it was found that the formula can be applied to tr=30000 s.

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