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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]

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]

Based on the isothermal fatigue and creep damage, a life prediction approach under multiaxial thermo-mechanical loading was proposed in this investigation. In the proposed method, the multiaxial thermo-mechanical fatigue damage during one cycle period was divided into the isothermal fatigue damage and the creep damage. In order to evaluate conservatively, the isothermal fatigue damage during one cycle period was calculated by using multiaxial fatigue damage model at the maximum temperature during the whole period, and the creep damage during one cycle period was calculated by accumulating the creep damage of all portions originated from the divided time history for the axial load component and temperature history. The life prediction results by the proposed model showed a good agreement with experimental data for nickel-base alloy GH4169 and cobalt-base Haynes188 under axial-torsional thermo-mechanical loading. [ABSTRACT FROM AUTHOR]

• A mechanism based multiaxial thermo-mechanical fatigue damage model is proposed. • The value of the scatter band of life prediction result is less than ± 2. • The mean relative error between predicted and experimental strain amplitudes is 6.54%. In this paper, a life prediction method based on damage mechanism for titanium alloy TC4 under multiaxial thermo-mechanical fatigue loading is proposed. Temperature and time dependent influence factors are proposed and introduced into the strain amplitude-life curve method to consider the temperature and time dependence of damage, respectively. The increase in fatigue and oxidation damages caused by tensile mean stress and non-proportional additional hardening is expressed by the mean stress in Macaulay brackets and by using a multiaxial damage parameter, respectively. A fast cracking influence factor is proposed to consider the influence of fast cracking of materials caused by the combined action of high temperature, tensile stress and shear stress on damage behavior. The material constants are identified by test data under uniaxial and proportional loadings, and are further used for prediction under non-proportional loading. The failure life results of uniaxial isothermal fatigue tests with and without dwell time, uniaxial and multiaxial thermo-mechanical fatigue tests are used to verify the proposed method. The value of the scatter band of life prediction result is less than ± 2, as well as the maximum and mean relative errors between predicted and experimental strain amplitudes are 18.49% and 6.54%, respectively. [ABSTRACT FROM AUTHOR]

• Multiaxial fatigue behavior at high temperature is experimentally characterized. • Fatigue-oxidation-creep damage behaviors are comprehensively captured. • The effect of non-proportional hardening on fatigue behavior is considered. • The notch stress–strain history is determined by the elastic-viscous-plastic analysis. • A new method is proposed to evaluate the fatigue life and applied to a notched bar. In this paper, a new life assessment framework is proposed based on the elastic-viscoplastic modeling and the damage behavior for the structural component under multiaxial non-proportional loading at high temperature. The viscoplastic constitutive model with the ability to capture the non-proportional hardening effect is used to obtain the stress–strain fields, in which a new method based on the rotation of strain axis is proposed to evaluate the notch rotation factor. The damage model, which can comprehensively capture the multiaxial fatigue-oxidation-creep behaviors, is used to assess the failure life. In addition, the proposed framework is evaluated by the life results under proportional and non-proportional fatigue loadings at 650 °C, and the errors are found to be within a factor of 2. [ABSTRACT FROM AUTHOR]

• The competition between protective and harmful effects of oxidation is revealed. • A high-temperature environmental damage model is proposed. • A mechanism based multiaxial thermo-mechanical fatigue life model is proposed. • The normal mean stress and cycle period are used to express competition mechanism. • The temperature dependence of material damage is quantitatively described. In this paper, the competition mechanism between protectiveness and harmfulness of oxidation, as well as the effects of cycle period, mean stress and temperature on the damage behavior of TA15 titanium alloy are revealed under multiaxial thermo-mechanical fatigue loading. Based on the damage mechanism, a high-temperature environmental damage model is proposed under multiaxial thermo-mechanical fatigue loading first, then a life prediction method is developed by combining a multiaxial fatigue damage model. The experimental verification results under uniaxial and multiaxial thermo-mechanical fatigue loadings showed that almost all of the prediction errors are within a factor of 2. [ABSTRACT FROM AUTHOR]

• Online fatigue-oxidation-creep damage evaluation is implemented. • Influence of temperature on the damage accumulation is considered. • Effect of non-proportional additional hardening on all damages is considered. • Changed oxidation damage is quantitatively calculated. A real-time multiaxial thermo-mechanical damage evaluation method was proposed under variable amplitude loading. In order to achieve real-time damage evaluation, a real-time multiaxial cycle counting method that can take into account temperature loading history was proposed. The proposed multiaxial cycle counting method can obtain the reversals one by one in order with the reading of the load-time history data, which can realize the damage calculation immediately as a single reversal is counted out. At the same time, to consider the effect of temperature loading on the damage accumulation, an equivalent temperature was determined for the time period of the counted reversal. A fatigue-oxidation-creep damage model that can consider the influence of the non-proportional additional hardening on the fatigue, oxidation and creep damages was used to predict the lives in cases of isothermal axial-torsional fatigue, uniaxial thermo-mechanical fatigue and axial-torsional thermo-mechanical fatigue under constant and variable amplitude loadings. The results showed that the prediction errors were within a factor of 2. [ABSTRACT FROM AUTHOR]

• Positive correlation between individual modulus and fatigue strength was identified. • Fatigue life model considering individual modulus was proposed. • Fatigue life prediction method under variable amplitude loading was proposed. Needled C/SiC composites are widely used in the aerospace field, making the evaluation of fatigue damage critically important. A fatigue damage model associated with the individual modulus is proposed. Compared to the fatigue damage model by S-N curve, the proposed fatigue damage model can better characterize the fatigue damage behavior of UN-C/SiC composites. By combining the rainflow counting method, the Goodman equation, and the Palmgren-Miner damage accumulation rule, a fatigue life prediction method considering the individual modulus is developed for UN-C/SiC composites under variable amplitude loading. Compared with the experimental life, the predicted fatigue life shows a good result. [ABSTRACT FROM AUTHOR]