Creep damage assessment of circular notch specimen on a CrMoV forging steel based on the void growth simulation

Creep damage preferentially extends at a stress concentration portion in high temperature components such as steam turbine rotors. Therefore development of an accurate damage assessment method for creep damage process at the stress concentration portion under multiaxial stress states is necessary to maintain reliable operation. In this study, creep tests using a plain specimen and two kinds of round bar notch specimens with different notch radius on a CrMoV forging steel have been conducted to clarify effect of stress conditions on creep damage extension process. Three dimensional finite element creep analyses have been performed to discuss relation between the creep damage and stress conditions. Creep rupture time of the plain specimen is shorter than those of the notch specimens. Creep rupture time of the notch specimen with lower stress concentrate factor is longer than that with higher stress concentration factor. In the notch specimens, the maximum stresses occur at notch root at initial loading and portions of the maximum stresses gradually change to inside of the specimens with time due to stress redistribution. Triaxial tensile stress yields at the notch root sections with different distributions of the triaxiality factor depending on the notch radius. Void number densities at the maximum stress portion in the notch specimens are ten times larger than that in the plain specimen. The void growth simulation method developed previously was applied to predict the void number density under multiaxial stress states in the notch specimens. Distributions of the void number density from notch root to center of the specimen both in the notch specimens were quantitatively predicted by the void growth simulation method. An equation, which predicts change of void number density with time under a certain maximum stress and a triaxiality factor, was derived based on the void growth simulation under different multiaxial stress states.