The high-temperature structure of aero-engine usually works in the environment of high temperature and fatigue load, which leads to a very complicated crack problem. This paper presents mixed-mode fatigue crack growth (FCG) experiments conducted on compact tension shear (CTS) specimens made of a nickel-based superalloy at 550 °C, with varying thicknesses. The influence of thickness on mixed-mode FCG behavior was investigated by experiments and finite element method (FEM) analysis. The expanded FCG model, which considers thickness effects based on closure theory, provides a more precise prediction of crack growth rates for mixed-mode loading conditions. In addition, it is observed that the mixed-mode FCG da/dN-ΔK curve for nickel-based superalloys shifts vertically as the specimen thickness varies, and the thickness has no effect on the mixed-mode FCG angle. This achievement has made a significant contribution to the damage tolerance design of aircraft engines.

A non-linear life prediction model considering the loading history is proposed based on isodamage curve. The stress-controlled low-high block loading at high temperature, with variously previous cycles, was performed on a nickel-based superalloy. The damage evolvement of the prior loading was revealed using scanning electron microscopy under the low-high block loading, especially the failure mechanism of coaxing effect. In addition, load history correlation factor was introduced to describe the influence of prior-cycle. Based on the Ni-based superalloy, the life prediction model of two-step loading, including the low-high block (LH) and high-low block loading (HL), was proposed, agreeing well with the experimental results for different metals. Comparing with other life prediction models, the proposed model demonstrated the higher prediction accuracy and wider applicability.

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.

The stress ratio ( R) effect, especially at elevated temperatures, is associated with the fatigue crack growth (FCG) prediction in aero-engine hot-end components under complex operation conditions. The FCG experiments with three R were conducted on Ni-based superalloy GH4169 at room temperature (RT), 550 °C, and 700 °C. The results indicate that the R-effect on the FCG of GH4196 is temperature-dependent. Therefore, efforts were made to identify the R-effect at various temperatures to describe the FCG. The concept of the crack-closure or the two-driving-force was examined to quantify the R-effect considering the temperature influence. Fractographic analyses on the fracture surface were performed to discuss the underlying mechanism responsible for the temperature influence. The study can contribute to the R-dependent FCG modeling at various temperatures.