2017 Volume 66 Issue 1 Pages 43-50
Thick high temperature components in thermal power plants such as turbine rotors are subjected to creep-fatigue loading under cycles of start - steady state - stop operation. The creep-fatigue damage gradually proceeds in these components. Therefore it is important to clarify damage extension mechanism under the creep-fatigue loading to develop accurate life assessment methods for reliable operation. In this study, a stress control creep-fatigue test with 10 minutes stress hold time at the maximum stress was conducted in a scanning electron microscope to clarify void growth behavior during the test. A strain control creep-fatigue test with 5 hours strain hold was also performed to observe void initiation and growth condition by interrupting the tests. From the observation, round shape voids initiated around 15% of the life and grew by changing the shape to a crack-like, and rate of void growth is considered to be accelerated by cyclic loading based on the fact that size of the maximum void under the creep-fatigue loading is larger than that under the static creep loading. According to the observation results, the previous proposed void growth rate equation was multiplied by acceleration factor. The acceleration factor is assumed to be dependent on applied strain range. The maximum void growth behaviors under both the stress and strain creep-fatigue loadings were accurately predicted by the extended void growth simulation procedure to the creep-fatigue loading considering influence of hold time on the acceleration factor.
