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A new creep–fatigue damage cumulative model is proposed under multiaxial thermomechanical random loading, in which the damage at high temperature can be divided into the pure fatigue damage and the equivalent fatigue damage from creep. During the damage accumulation process, the elementary percentage of the equivalent fatigue damage increment is proportional to that of the creep damage increment, and the creep damage is converted to the equivalent fatigue damage. Moreover, combined with a multiaxial cyclic counting method, a life prediction method is developed based on the proposed creep–fatigue damage cumulative model. In the developed life prediction method, the effects of nonproportional hardening on the fatigue and creep damages are considered, and the influence of mean stress on damage is also taken into account. The thermomechanical fatigue experimental data for thin‐walled tubular specimen of superalloy GH4169 under multiaxial constant amplitude and variable amplitude loadings were used to verify the proposed model. The results showed that the proposed method can obtain satisfactory life prediction results. [ABSTRACT FROM AUTHOR]

In this paper, a notch stress correction method based on the structural yield surface is proposed, in which the actual stress increment at the notch root can be obtained by multiplying two quantities, that is, the pseudo stress increment and the ratio of the slopes of the material and structural stress-strain curves. In the proposed method, the influence of temperature on the notch stress correction can be taken into account, and no iteration is required during the calculation process. Moreover, based on the proposed notch correction method and a unified viscoplastic constitutive model at high temperature condition, a notch stress-strain estimation method is developed under multiaxial thermo-mechanical cyclic loading. In order to assess the prediction accuracy of the proposed method, the calculated results are correlated to those from thermal-structural non-linear finite element analysis under various proportional and non-proportional mechanical loadings at non-isothermal high temperature condition. The comparison between the calculated and analyzed results shows that the proposed method can precisely estimate the notch stress and strain under multiaxial thermo-mechanical cyclic loading. [ABSTRACT FROM AUTHOR]

A fatigue–oxidation–creep damage model that can take into account the effect of multiaxial cyclic feature on the damage mechanism is proposed under axial-torsional thermo-mechanical fatigue loading. In the proposed model, the effects of non-proportional additional hardening on fatigue, oxidation, and creep damages are considered, and the variation of oxidation damage under different high temperature loading conditions is also described. Moreover, the intergranular creep damage needs to be equivalent to the transgranular damage before accumulating with the fatigue and oxidation damages. The fatigue, oxidation, and creep damages can be expressed as the fractions of fatigue life, critical crack length, and creep rupture time, respectively, which allows the linear accumulation of different types of damages on the basis of life fraction rule. In addition, the proposed model is validated by various fatigue experimental results, including uniaxial thermo-mechanical fatigue, axial-torsional thermo-mechanical fatigue, and isothermal axial-torsional fatigue under proportional and non-proportional loadings. The results showed that the errors are within a factor of 2. [ABSTRACT FROM AUTHOR]

• A multiaxial cycle counting algorithm was proposed for thermomechanical random loading. • An interaction modulus B Ti varying with temperature was proposed. • A prediction model was proposed under multiaxial thermomechanical variable amplitude loading. Based on the Wang-Brown method, a time-dependent multiaxial cycle counting algorithm was proposed under multiaxial thermomechanical variable amplitude loading. By using the proposed multiaxial cycle counting algorithm, a new thermo-mechanical fatigue life prediction method was proposed, in which the pure fatigue damage, creep damage, and the interaction damage taking into account the interaction modulus varying with temperature were calculated for each reversal counted by the cycle counting algorithm. Three types of damage from all the counted reversals for the whole strain-time history were accumulated, and the thermo-mechanical multiaxial fatigue life can be predicted. The thermo-mechanical fatigue experimental data for thin-walled tubular specimen of superalloy GH4169 under multiaxial constant amplitude and variable amplitude loadings were used to verify the proposed method. The results demonstrated that the proposed method can provide very satisfactory prediction results. [ABSTRACT FROM AUTHOR]

• An equivalent energy-based critical plane damage parameter is proposed. • No other additional material constants are needed for the proposed method. • Non-proportional cycle hardening and mean stress can be reflected more perfectly. • The method shows great potential under multiaxial random fatigue loading. Combining the concept of energy on the critical plane, a multiaxial fatigue damage parameter without any weight coefficient was proposed and the effects of non-proportional additional hardening and mean stress on fatigue damage can be reflected more perfectly using an equivalent form of shear strain energy. Furthermore, two sets of experimental data are employed to verify the proposed method, and the sensitiveness to fatigue life of the proposed equivalent stress correction factor is discussed as well. The results show that satisfactory and conservative assessments can be obtained by the proposed method, especially under multiaxial irregular loading with great randomness. [ABSTRACT FROM AUTHOR]