In LCF tests, it has been reported that the fatigue lives under creep-fatigue interaction conditions are observed to decrease with increasing hold time at a fixed test temperature, and the reason for life reduction is reported to be due to the creep effect of stress relaxation which makes additional plastic strain to enlarge the hysteresis loop during hold time. Until recently, many studies have aimed at creep-fatigue interactions, but there is no precise explanation in terms of active deformation during hold time in which stress relaxation occurs.
A quantitative analysis of activation process and stress dependence on stress relaxation creep rate during hold time under creep-fatigue interaction conditions have been interpreted for 1Cr-Mo-V steel. The apparent activation energy for stress relaxation at the saturated stage was the same for the lattice diffusion activation energy of iron i.e., 251kJ/mole independent of the total strain range. It was shown that the creep mechanism is identical with that of the steady state in monotonic creep after a long enough hold time, that is the dislocation climb which is controlled by self diffusion. Also the creep behavior analyzed from the stress relaxation is in good agreement with that from the typical monotonic creep, showing the stress exponent values of 17 in the stress dependence on stress relaxation creep rate during hold time.
Therefore, it is known that the creep mechanisms of stress relaxation which are the main reasons for fatigue life reduction under creep-fatigue interactions after a long enough time are the same as that of long-term monotonic creep.
From the concept that the life reduction with hold is due to the creep effect of the stress relaxation, a new life evaluation function based on a model for the creep-fatigue life prediction in terms of stress relaxation during hold time is suggested, and the relation between the relaxed stress and creep-fatigue lives shows the normalized trend regardless of test conditions. Also the life evaluation function which is the relaxed stress can be expressed by the combination of the experimental variables and the operating conditions, which have the factors affecting the damage formation of a materials such as strain range, wave shape, hold time and temperature.
The creep-fatigue data from the present and other investigators are used to check the validity of the proposed model, and the proposed life prediction model is verified to normalize all the experimental data under various test conditions. Therefore, it can be generally used for the more accurate life prediction with hold time by estimating the life function, and it can give the design and operational parameters such as stress level, duration of working period and temperature if the expected life is known.
