Creep-fatigue life evaluation of circular notch specimen on a CrMoV forging steel under multiaxial stress gradient condition

Abstract

Creep-fatigue damage preferentially exceeds at stress concentration portions in high temperature components such as steam turbine rotors during cyclic start up- shut down operation. Therefore development of an accurate crack initiation life assessment method under creep-fatigue conditions at the stress concentration portions under multiaxial stress states is necessary to maintain reliable operation. In this study, creep-fatigue tests using circular notch bar specimens on a CrMoV forging steel have been conducted to clarify effect of stress conditions on failure life. Three dimensional finite element elastic-plastic creep analyses have been performed. Fatigue lives of the notch specimens are about 10 times shorter than those of plain specimens, and the lives of the notch specimen under the creep-fatigue condition are about 1/5 times of the plain specimen. From observation of notch root of the creep-damage specimens, it suggested that most of the creep-fatigue life was occupied by crack propagation up to 1mm from the notch surface. It was also indicated that triaxial tension stress state occurs around the notch root surface and stresses decrease with distance from the notch root surface by the inelastic analysis. A new creep-fatigue life evaluation procedure of notch specimens, in which concept of a damage evaluation area was introduced, was proposed by considering damage extension and the stress component distribution around the notch root. As a result, creep-fatigue lives of the notch specimens were accurately predicted by coupling of the new procedure and the nonlinear damage accumulation rule.