A model for creep-fatigue interaction in terms of crack-tip stress relaxation

A model was developed to explain the mechanism of the degradation of fatigu e lives caused by the growth of transgranular crack without cavitational da mage in spite of the creep-fatigue loading condition for some type 304L sta inless steel and 1Cr-Mo-V steel. The model was developed by incorporating t he stress relaxation effect during tensile hold time into the purl fatigue crack growth model based on the crack-tip shearing process. In the crack-ti p region, the stress relaxation during hold time at the tensile peak stress reduces the maximum stress level but accumulates inelastic strain, which i nduces creep crack growth during hold time and enhances subsequent fatigue crack growth during subsequent loading by promoting the crack-tip shearing process. The predicted creep-fatigue lives by the model were in good agreem ent with the actual lives for type 304L stainless steel at 823 and 865 K an d for 1Cr-Mo-V rotor steel at 823 K. The model was further expanded to expl ain the degradation of the life under the conditions of compressive hold cy cling for 1Cr-Mo-V and 12Cr-Mo-V steels.