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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]

The multiaxial thermomechanical fatigue properties for nickel-based superalloy GH4169 in aeroengine turbine discs are investigated in this paper. Four types of axial–torsional thermomechanical fatigue experiments were performed to identify the cyclic deformation behavior and the damage mechanism. The experimental results showed that the creep damage can be generated under thermally in-phase loading while it can be ignored under thermally out-of-phase loading, and the responded stress increasing phenomenon, that is, non-proportional hardening, can be shown under the mechanically out-of-phase strain loading. Based on the analysis of cyclic deformation behavior and damage mechanism, a life prediction method was proposed for multiaxial thermomechanical fatigue, in which the pure fatigue damage, the creep damage, and the interaction between them were simultaneously considered. The pure fatigue damage can be calculated by the isothermal fatigue parameters corresponding to the temperature without creep; the creep damage can be calculated by the principle of subdivision, and the creep–fatigue interaction can be determined by creep damage, fatigue damage, and an interaction coefficient which is used to reflect the creep–fatigue interaction strength. The predicted results showed that the proposed method is reasonable. [ABSTRACT FROM AUTHOR]

Abstract This paper was concerned with an approach to predict fatigue life based on a multi-scale crack propagation model. The expressions of crack propagation rates in microstructurally small crack (MSC), physically small crack (PSC) and long crack (LC) stages were unified in the multi-scale crack propagation model. Its integral form presented a fatigue life prediction approach. The nickel-based GH4169 alloy was employed to validate the prediction capacity of present approach. The prediction results of lives of specimens with different initial defects sizes were compared with the experimental data. [ABSTRACT FROM AUTHOR]

The proposed deflecting abrasive waterjet peening (DAWJP) process can be used for surface strengthening of metallic parts with confined space features, which has great potential to improve the surface integrity and fretting fatigue (FF) life of dovetail slots in turbine discs. In this paper, the nickel-based superalloy GH4169 dovetail slot is strengthened by DAWJP and shot peening (SP), and the effects of DAWJP and SP on surface integrity and FF performance of the dovetail slot are investigated. First, the DAWJP with three different processes (water pressure of 205 MPa, traverse speed of 5 mm/s, standoff distance of 2 mm, and path interval of 0.05, 0.1, and 0.15 mm) and the SP with one process (peening intensity of 0.25 mmA with coverage of 100%) were conducted on the contour surface of the dovetail slot. The surface integrity of the dovetail slot before and after the DAWJP and SP treatment was investigated, including microstructure, surface roughness and microscopic morphology, microhardness, and residual stress. Results showed that the dovetail slot treated by the DAWJP with different path intervals formed a plastic deformation layer with a thickness of 22–43 μm and surface roughness Ra and Sa of 0.391–0.501 μm and 0.763–0.907 μm. The surface microhardness and CRS of the DAWJP-treated dovetail slot were approximately 541–591 HV and 1005–1169 MPa, 16%–27% and 178%–224% times higher than the as-received dovetail slot. The SP-treated dovetail slot produced relatively large surface roughness, slight plastic deformation, and low microhardness and CRS compared with the DAWJP-treated dovetail slot. Finally, FF tests were carried out to study the FF behavior of the dovetail slot before and after DAWJP and SP treatment. The FF lives of the dovetail slot treated by DAWJP were 4.82 and 3.91 times that of the as-received dovetail slot at room temperature and a high temperature of 650 °C. In contrast, the FF lives of the SP-treated dovetail slot were 2.96 and 1.96 times that of the as-received specimen at room temperature and a high temperature of 650 °C, which verified that DAWJP has a more significant effect on improving the FF life of dovetail slots than the SP treatment used for comparison. The FF fracture and fretting contact surface and its cross-sectional microstructure were examined, and the results showed that the extreme grain refinement, large CRS, and high work-hardening induced by DAWJP could be the key to greatly improving the FF life of dovetail slots. This work takes the lead in the surface treatment of dovetail slots through the proposed DAWJP process, which provides a feasible reference plan for effectively and economically improving the FF resistance of dovetail slots in turbine discs. [ABSTRACT FROM AUTHOR]

The purposes of the present work are to extend the previous energy-based model on the basis of strain energy density exhaustion and to estimate creep-fatigue endurance and accumulated damage using cycle-by-cycle concept in tension-hold-only tests. The nickel-based GH4169 superalloy at 650 °C is employed to fit material constants and to verify the prediction capacity of the present model under various loading conditions. The present model exhibits a higher accuracy than some existing models, especially for certain loading waveforms, where the half-life cycle cannot be considered as a representative one. In addition, the trajectory for time-dependent accumulated damage during the operating period can be monitored in the creep-fatigue interaction diagram. [ABSTRACT FROM AUTHOR]

The accumulated creep–fatigue damage is expected to be an important failure mechanism for lots of high-temperature components. The aim of this paper is to propose a modified strain energy density exhaustion model to predict the tension-hold-only creep–fatigue life. This model exhibits high accuracy due to the reasonable evaluation of creep damage. The proposed model elaborates the determinations of mean stress, stress relaxation rate and creep damage. A few existing experimental data sets of Grade 91 steel, Alloy 617 and 304 stainless steel are used to verify the prediction capacity of the present model under different temperatures and loading conditions. Results show that most of the experimental data falls into a range within a scatter band of ±1.5 on life. [ABSTRACT FROM AUTHOR]

• Damage mechanisms under creep-fatigue combined with high-low cycle were revealed. • Damage evolution under various cyclic loadings was described by energy dissipation. • Multi-damage model was established by using crystal plasticity finite element. Many high-temperature rotating components are always subjected to complex loading waveforms, arising the concerns for multi-damage driving crack initiation. In this paper, a series of strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests as well as the novel creep-fatigue combined with high-low cycle (CF-HL) tests were performed in a nickel-based superalloy at 650 ℃. Then, post-test microstructure observations were carried out to reveal the damage mechanisms under the multi-damaged CF-HL loading based on the EBSD-TEM combinative characterizations. In this aspect, the single-slip-dominated deformation mechanism under CFI loadings transformed to double-slip-dominated one under CF-HL loading was revealed. Computationally, a numerical procedure with a combination of crystal plasticity theory and finite element implementation was constructed for predicting the CF-HL crack initiation life and quantifying the crack initiation mechanisms. With the help of the developed fatigue and creep indicator parameters represented by accumulated energy dissipation, good agreements between experimental data and simulated results were achieved within a scatter band of ± 2 on life prediction. In addition, the simulation results indicate that the combined effects of grain orientation and multiple slip system activation showed great influence on the CF-HL crack initiation. [ABSTRACT FROM AUTHOR]