The influence of preloading on the residual fatigue life of nickel-based superalloys under elevated temperatures was investigated experimentally. A powder metallurgy nickel-based superalloy FGH96 was preloaded with different number of cycles, and the residual fatigue life was tested under subsequent high-amplitude loads. The test results show that the fatigue life of the material was increased when the preloading cycle number was within a specific range. At the same time, the fatigue life of virgin specimens under high-amplitude loads shows little scatter, which provides the possibility to consider the strengthening effect of low-amplitude loads. A novel damage accumulation model was proposed to incorporate the strengthening effect of low-amplitude loads into the life prediction framework. The proposed model provides better life prediction than some existing models. Finally, the proposed model was validated using experimental data for various materials under the low-high loading sequence.

This paper proposes a novel combined cycle fatigue life prediction method of a blade-like specimen considering the effect of multiple damage modes and complex stress state. Combined tensile and bending fatigue (CTBF) experiments were performed on a blade-like specimen at 850°C, using a novel biaxial tension-vibration test system. To account for the effect of high stress ratio related creep on bending fatigue damage, a creep damage factor was proposed and explicitly introduced into Lemaitre-Chaboche model. In the framework of continuum damage mechanics (CDM), the CTBF life was assessed considering the geometry-stress gradient feature factor and multiple damage modes, including the damage of LCF, high stress ratio bending fatigue and oxidation. Finally, a novel CTBF life prediction model was proposed, agreeing well with the experimental results. Compared with the existing classical models, the proposed model provides the significantly higher prediction accuracy.