A NEW EMPIRICAL LIFE PREDICTION EQUATION FOR STRESS-CONTROLLED FATIGUE-CREEP INTERACTION

A new empirical life prediction method is developed. The equivalent radius of cavities at grain boundary is adopted as the damage parameter. Similar with Nam's model, in this paper, it is also assumed that cavities only nucleate during fatigue cycles and further grow with the development of creep. Then the number of cavities nucleated in a cycle is proportional to the fatigue effect, i.e. the amplitude of loading stress. As the creep process is composed of the static creep and the cyclic creep during the fatigue-creep interaction, then the equivalent stress causing cavity growth should be proportional to the maximum hold stress (static creep) and the mean stress (cyclic creep). Therefore, this model is applicable to stress control mode and includes the effects of fatigue, static creep and cyclic creep during the fatigue-creep interaction. By employing this method, the fatigue-creep lives are assessed for 1.25Cr0.5Mo steel at 520 degrees C and 540 degrees C. The predicted lives are compared with the tested ones and a good agreement is found between them. Moreover, it is found that the coefficient of the mean stress is 3-order larger than that of the stress amplitude, which means the effect of static creep is much punier than that of cyclic creep. Considering the detailed test parameters, the short hold duration for peak load may be responsible for this.