2019 年 85 巻 878 号 p. 18-00428
Creep damage preferentially extends at a stress concentration portion in components such as steam turbines and boilers. Therefore, in order to maintain reliable operation of these components, it is necessary to clarify the relationship between the creep damage extension process and the stress states in the stress concentration portion. In this study, creep tests using four kinds of round notch bar specimens with different notch radius (notch tip radius 0.1mm(R0.1), 0.5mm(R0.5), 2.0mm(R2.0) and 4.0mm(R4.0)) on a Mod. 9Cr-1Mo steel have been conducted to clarify influence of stress conditions on creep damage extension process and rupture time. Creep rupture time increases with increasing elastic stress concentration factor under the same nominal stress. Distribution pattern of measured void number density from notch root surface to specimen center was different depending on the notch radius. The void number density around the notch root surface is higher than that around the specimen center in R0.1 and R0.5, while the void number density increases toward the specimen center in R2.0 and R4.0. Finite element creep analyses of the round notch bar specimens indicate that triaxial tensile stress yields on the notch root section with different distribution of the triaxiality factor depending on the notch radius. The distribution of the triaxiality factor of each round notch bar specimen corresponded to the distribution of the void number density. Rupture time of the round notch bar specimens can not be predicted by applying the representative stress such as the maximum stress and Mises equivalent stress. A new rupture time prediction procedure using an area average creep damage evaluation method, in which total creep damage accumulated on the notch root section of the specimen was divided by area of the section, was proposed. Rupture times of the notch specimens were accurately predicted by the proposed procedure without showing any dependency of the elastic stress concentration factor.
