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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.

Creep damage gradually proceeds in high temperature components in thermal power plants featuring creep void nucleation and growth. Accurate prediction of the creep damage is necessary for reliable operation and life extension of these components. Microstructural changes of a 1%CrMoV steel and a longitudinal welded tube of 9%Cr steel during creep were observed with creep test interrupted specimens by means of SEM. It was suggested that void growth is controlled mainly by a diffusion mechanism affected by matrix creep deformation and constraint of surrounding grains. Based on microstructural observation and theoretical considerations, a void initiation and growth model was proposed, and a void simulation program incorporating the void initiation and growth model was developed. It was demonstrated that void initiation and growth behaviour in homogenous materials as well as in heat affected zone in a welded tube was quantitatively predicted by the void simulation program. This suggests that local damage evolution in components may be predicted by the proposed method. [ABSTRACT FROM AUTHOR]