5. CONCLUSION
In this paper, the effect of various creep-fatigue loading conditions on crack-tip fields are investigated via the FE debond analysis using experimental creep-fatigue test data of Grade 91 steel at 600oC. The following conclusions are drawn:
The crack-tip deformation and stress fields during crack growth under tension-compression creep-fatigue loading is overall different from those under the tension-tension creep fatigue and pure creep loading, due to crack closure. The crack closure under compressive loading initializes creep-redistributed stress states, re-activates transient creep deformation, promotes crack-tip blunting and reduces the stress triaxiality. This is the reason that the predicted crack growth rates using the C* parameter were overly conservative for the tension-compression creep-fatigue case.
For the tension-tension creep-fatigue case, the crack-tip stresses can be well characterized by the C* parameter both at crack initiation and at crack growth, whereas those under tension-compression creep-fatigue loading cannot be correlated either with C* parameter. To characterize the crack-tip stress field under tension-compression creep-fatigue loading, the effects of local creep-dominant zone and cyclic softening should be considered.