Evaluation for Environmental Fatigue of Notched Specimen of Ni-Base Alloy

Abstract

Fatigue tests were conducted on the notched specimens of Ni-base-alloy NCF600 in high temperature water. Notch root strain was analyzed by finite element method (FEM) to calculate the notch root strain rate and the environmental fatigue correction factor Fen. Relationship between notch root fictitious stress amplitude and corrected fatigue life in water FenNwater are compared with the fatigue data of smooth specimen in air (i.e. best-fit curve) and it was found that the corrected fatigue life data in water shows a little shorter but almost agree with best-fit curve in air for the specimen with Kt = 1.4A. For the specimen with Kt = 3, corrected fatigue life in water is longer than that in air and the difference between both lives becomes larger with decreasing stress amplitude. The longer fatigue life in sharp notched specimen than that in dull notched specimen at the same notch root stain amplitude is thought to be dominated by the difference in the crack propagation life since the stress distributions on the cross section are decisively different. It is concluded that the fatigue life of notched specimen in high temperature water is adequately predicted using the modified rate approach method when the notch root strain is appropriately estimated e.g. by FEM analysis even though it gives excessively conservative prediction for sharp notch at low stress amplitude. The aspect of environmental fatigue of notched specimens are summarized on carbon steel and stainless steels with Ni-base alloy studied in Japan Nuclear Energy Safety Organization (JNES) Environmental Fatigue Test (EFT) project.