Fatigue–creep interaction (FCI) responses of P92 steel are investigated experimentally and numerically. A series of isothermal FCI experiments with tensile dwell time ranging from 60 to 600 s were ...conducted at two temperatures under strain-controlled trapezoidal waveform. The experimental responses demonstrate that the peak stress is influenced by temperature and dwell time. In other words, creep-mechanism-influenced stress relaxation during dwell time influences the peak stress and fatigue life (Nf). In addition, effects of strain range on peak stress and fatigue life under fatigue–creep loading are evaluated. Towards developing a simulation-based design methodology for high temperature components, first a conventional unified constitutive model is evaluated against the P92 steel experimental responses. Based on the simulation deficiency of the conventional model, a modified static recovery term incorporated in the kinematic hardening rule is proposed and satisfactory simulations of the P92 steel FCI responses are demonstrated. The experimental responses of P92 steel and strengths and deficiencies of the conventional and modified Chaboche models are elaborated identifying the important FCI phenomena and progress in constitutive model development for FCI response simulation.
•A unified viscoplastic model is proposed for SiMo 4.06 between 20 °C and 650 °C.•Low-cycle fatigue-creep and thermo-mechanical fatigue tests are simulated.•The model is based on non-linear kinematic ...hardening and hyperbolic sine flow rule.•Mean stress evolution and effect of static recovery are incorporated.•Strain range dependency is suggested for isotropic hardening and for elastic modulus.
In this paper, a SiMo 4.06 cast iron is subjected to Low-Cycle Fatigue (LCF) loading and Thermo-Mechanical Fatigue (TMF) loading in the temperature range between 20 °C and 650 °C. Strain-controlled experiments were performed for various strain rates, with and without a tensile strain dwell. A modified unified viscoplastic material model is proposed for simulating the temperature- and time-dependent cyclic behaviour of the material. The constitutive model is based on the non-linear kinematic hardening rule of Chaboche, in which several features are added in order to incorporate strain rate sensitivity, the strain range dependency of the cyclic hardening and the elastic modulus, static recovery and mean stress evolution. A good correlation was achieved between the simulated results and the experimental results.
•Low-cycle fatigue-creep tests were performed for 42CrMo4 between 300 °C and 600 °C.•Effect of temperature, strain rate and strain-dwell on fatigue life is demonstrated.•Cyclic softening is present ...for all tested temperatures and loading conditions.•Increasing duration of dwell period leads to interaction between fatigue and creep.•Classical strain-life approach is modified by frequency dependent term.
A series of strain-controlled experiments was performed on quenched and tempered martensitic steel 42CrMo4, with and without a tensile peak strain-dwell. The experiments were performed for three different strain rates and for two different strain dwells in the temperature range between 300 and 600 °C. Scanning electron microscopy and optical microscopy were then used to study the microstructure evolution and the damage mechanisms. The experimental results show that tensile peak strain-dwell may lead to interaction between creep and fatigue. Finally, the effect of a decreasing strain rate and increasing duration of the strain-dwell on the observed fatigue lifetime is quantified.
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•Numerical modeling the isothermal and TMF conditions during tests for the crack growth rate requires the implementation of multiphysics computations.•The hysteresis loops area ...behavior was found to be directly related to morphology and changes in dominant failure mechanisms under isothermal and thermo-mechanical fatigue conditions.•In terms of ERR, the order of arrangement of isothermal and thermo-mechanical fatigue fracture diagrams of the XH73M nickel-based alloy is found.•For fatigue phase fields fracture problems, it is necessary to take into account the transitions of the ERR parameter from current to threshold values.
An experimental-computational study of the crack opening displacement (COD) was performed to evaluate the ability of this parameter through the hysteresis loop area and energy release rate (ERR) to characterise fatigue crack growth under isothermal and thermomechanical conditions. To this end, a range of experimental crack growth tests were carried out including isothermal fatigue (IF), creep-fatigue interaction (CFI), in-phase (IP), and out-of-phase (OOP) thermomechanical fatigue (TMF). The crack growth experimental results in terms of COD and ERR were interpreted based on multi-physics finite element analyses of the stress–strain and displacement fields performed by incorporating Maxwell 3D, Fluent, and the transient structural modules of ANSYS. As a result of measurements and computations, the COD on the outer surface of the SENT specimen at the location of the original notch was found to be directly related to the crack tip opening displacement (CTOD). To gain insight into the fatigue crack growth mechanism operating under isothermal and TMF conditions, the fracture surfaces of all the tested specimens were examined using a scanning electron microscope to compare the morphology and changes in the dominant failure mechanisms. It was found that propagating cracks under isothermal and thermomechanical conditions interact with the microstructure of the material, and supporting fractography evidenced subtle differences in fracture mechanisms in terms of COD and ERR parameters. The relationship between the experimentally determined energy release rate and degradation function in the fatigue phase-field fracture methodology is discussed.
•Effect of dwell time on fatigue and corrosion-fatigue is experimentally studied.•Tensile load hold time significantly reduces fatigue life.•Fatigue life of salt-coated samples is lower than that of ...the bare sample under 0 and 1 s holds.•The crack initiation sites of bare samples shift from internal defect to surface with increasing hold times.•Cracks in salt-coated samples are initiated on surface corrosion layer.
Fatigue-creep tests were carried out on bare and salt-coated of DZ125 superalloys samples at 850 °C. The fatigue lifetime of a corroded alloy was reduced by more than 80% as the hold time was increased to 120 s. However, the influence of hot corrosion on fatigue lifetime became weaker when the dwell time was further increased to 240 s. It was found that hot corrosion influences the lifetime of the alloy by controlling the crack initiation location. The cracks on salt-coated samples were initiated on the corrosion layer. These findings enable an accurate assessment of the hot-corrosion-induced fracture behavior of superalloys.
► The constant amplitude fatigue behavior of adhesively-bonded joints loading is investigated. ► The characteristics of the CLD developed for the examined joints are discussed. ► A creep–fatigue ...interaction under certain fatigue conditions was revealed. ► A phenomenological formulation is proposed and validated by the experimental data.
The effect of mean load on the fatigue behavior of adhesively-bonded pultruded GFRP double-lap joints under constant amplitude loading was experimentally investigated. The joints were examined under nine different stress ratios (R) representing pure tension, compression, and combined tension–compression fatigue loading. A transition of the failure mode, from tensile to compressive, was observed as the mean load decreased from positive to negative values. The slope of the S–N curves derived for R-ratios with positive or negative mean load consistently decreased with increasing mean load. The highest load amplitude corresponded to the R-ratio where the transition of the fatigue failure mode occurred (R=−2). The characteristics of the constant life diagram developed for the examined bonded joints are thoroughly discussed. A phenomenological formulation was proposed and its accuracy evaluated by comparisons with the derived experimental data. The comparison of the new formulation with models commonly used for composite materials proved its higher accuracy that is achieved with less implementation effort.
•A micro-macro confined compressive fatigue creep solid model is proposed.•Recoverable plastic strain caused by crack recovery at static unloading is studied.•Effects of cyclic stress path on fatigue ...creep failure behaviors are discussed.
Fatigue properties under cyclic static compressive loading and unloading (i.e., compressive fatigue/cyclic creep properties) have an important meaning for evaluation of lifetime in brittle solids containing numerous microcracks. The total deformation during cyclic static compression consists of time-independent elastic, time-dependent viscoelastic, time-dependent viscoplastic and time-independent plastic deformations. In this study, the viscoplastic and plastic deformations are collectively called as plastic deformation below. The plastic deformation dominates the lifetime of brittle solids. The rebound of elastic and viscoelastic deformations at static unloading is widely studied. However, the rebound of plastic deformation at static unloading is rarely studied under constant lateral confining pressure, and the total time-dependent deformation during cyclic static compressive failure also is rarely studied in theory. Furthermore, the rebound of plastic deformation has a great significance for cyclic static compressive failure, and microcrack variable influences seriously plastic mechanical properties of brittle solids. However, the theoretical relationship between plastic deformation rebound and microcrack variable in brittle solids during a cyclic static compressive failure is rarely established. In this study, an analytical solution is proposed by coupling the confined cyclic static loading and unloading path, the Hooke-Kelvin viscoelastic model and the formulated micro-macro model, which explains the total time-dependent visco-elastic-plastic deformation caused by microcracks variable during cyclic static compressive failure. Effects of residual static axial stress after unloading and confining pressure on cyclic creep failure are studied. Irreversible elastic and viscoelastic deformations caused by residual static stress after unloading are illustrated. An interesting rebound phenomenon of plastic deformation induced by microcrack recovery is found. A critical value of axial stress causing a rebound of plastic deformation is also found. Effects of parameters Δσ1a, Δσ1b, and Δt in cyclic static loading and unloading path on the mechanical properties of cyclic static compressive failure are discussed. The proposed theoretical model provides a certain help for evaluation and prediction of fatigue lifetime in brittle solid engineering.
Modified 9Cr-1Mo steel is used for high temperature components such as steam turbine rotors because of its superior high-temperature property. Creep-fatigue damage progresses preferentially at stress ...concentration portions in the steam turbine rotors during cyclic start up- shut down operation. Therefore development of an accurate creep-fatigue failure life assessment method under multiaxial stress states at the stress concentration portions is necessary to maintain reliable operation. In this study, creep-fatigue tests using three kinds of round notch bar specimen on a modified 9Cr-1Mo steel and finite element(FE) creep analyses of the specimens have been conducted to clarify effect of multiaxial stress conditions on failure life. Larger life reduction of all kinds of notch specimen under the creep-fatigue tests than the fatigue lives was observed at lower strain range. Observation of notch root of the creep-fatigue damage specimens by a scanning electron microscope suggested that most of the creep-fatigue life was occupied by crack propagation up to 1mm from the notch root surface. Appropriate life prediction results were not obtained in the case of using the FE analysis results at the notch root surface. On the other hand, creep-fatigue lives of notch specimens were accurately predicted by applying the damage area rate concept, in which average creep-fatigue damage value was calculated within 1mm from the notch root surface, with both nonlinear damage accumulation model and ductility exhaustion theory.
Based on quick access recorder（QAR）data generated by engine operation, high pressure turbine（HPT）speed ratio of the right engine N2（the ratio of actual working speed to design speed of HPT）was ...extracted to compile turbine blade load spectrum. A fluid-thermal-solid coupling model was established, and the thermal boundary conditions required for calculation were established by combining QAR data and thermal analysis. The temperature, stress and strain distributions of HPT blade under different working conditions were obtained by solving the fluid-thermal-solid coupling problem with finite element software. The Manson-Coffin model and Larson-Miller model were used to predict the fatigue and creep life of the blade respectively, the influence of cooling on the life of blade was mainly analyzed. Finally, the fatigue-creep life of the blade was obtained by the linear damage accumulation theory. The results show that the fatigue life of blade is improved and the creep life is significantly improved after considering
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