Evaluation of Creep-Fatigue-Environment Interaction Based on Mechanical Response

Creep-Fatigue Tests were made with 304 stainless steel at 650°C and with 2 1/4Cr-lMo steel at 550°C in arr and vacuum of 0.1mPa and 0.1&lt；mu&gt；Pa under a wide variety of strain loading conditions.Test data of stress-strain response and fatigue properties were stored in a data base for further analyses.It was shown that the stress-strain response is independent of test environments for both steels.A measured stress is composed of the overstress and the internal stress.The internal stress of 304 steel is composed of the back stress.However，in2 1/4Cr-1Mo steel，the internal stress is composed of the drag component and back component of the stress.The overstress and back stress of304steel depend upon both inelastic strain rate and strain range。However，in2 1/4Cr-1Mo steel，the overstress is independent of these test variables。The drag stress of this steel depends upon only a strain range.The back stress depends upon the inelastic strain rate.It was shown with both steels that the back stress depends upon the cyclic strain hardening or softening behavior.The time-independent damage factor of both steels can be described with the overstress and a mean stress.Regarding the time-dependent damage factor，that of 304steel can be described with the overstress。But that of 2 1/4Cr-1Mo steel can be described with the internal stress.Based on these results，an evaluation@equation was derived for the pure creep-fatigue behavior of these two steels.Based on the above@equation，the environmental effect was seperated from the creep-fatigue interaction and was abalyzed with mechanical quantities of stress and strain。It was shown that the environmental effect is mainly affected by a strain rate。An evaluation oquation was obtained with respect ot the environmental damage described with mechanical parameters.

Creep-Fatigue Tests were made with 304 stainless steel at 650゜C and with 2 1/4Cr-lMo steel at 550゜C in arr and vacuum of 0.1 mPa and 0.1 <mu>Pa under a wide variety of strain loading conditions. Test data of stress-strain response and fatigue properties were stored in a data base for further analyses.It was shown that the stress-strain response is independent of test environments for both steels. A measured stress is composed of the overstress and the internal stress. The internal stress of 304 steel is composed of the back stress. However, in 2 1/4Cr-1Mo steel, the internal stress is composed of the drag component and back component of the stress.The overstress and back stress of 304 steel depend upon both inelastic strain rate and strain range. However, in 2 1/4Cr-1Mo steel, the overstress is independent of these test variables. The drag stress of this steel depends upon only a strain range. The back stress depends upon the inelastic strain rate. It was shown with both steels that the back stress depends upon the cyclic strain hardening or softening behavior.The time-independent damage factor of both steels can be described with the overstress and a mean stress. Regarding the time-dependent damage factor, that of 304 steel can be described with the overstress. But that of 2 1/4Cr-1Mo steel can be described with the internal stress. Based on these results, an evaluation equation was derived for the pure creep-fatigue behavior of these two steels.Based on the above equation, the environmental effect was seperated from the creep-fatigue interaction and was abalyzed with mechanical quantities of stress and strain. It was shown that the environmental effect is mainly affected by a strain rate. An evaluation oquation was obtained with respect ot the environmental damage described with mechanical parameters.