•Framework for fatigue reliability analysis under multi-source uncertainties.•Manufacturing errors/tolerances are included for fatigue reliability analysis.•Sensitivity analysis of a turbine bladed ...disk is conducted for fatigue design.•Geometrical uncertainty shows critical influences on fatigue reliability.
Turbine bladed disks normally operate under complex loadings coupling with uncertainties originate from multiple sources, including material variability, load variation and geometrical uncertainty. The influence of these uncertainties on mechanical response of engineering components are critical for their fatigue assessment and reliability evaluation. In this work, a general framework for fatigue reliability analysis is developed by coupling the Latin hypercube sampling with FE analysis to describe the combined effects of multi-source uncertainties. Fatigue reliability analysis of a full-scale bladed disk under multi-source uncertainties was performed as well as sensitivity analysis for fatigue design. In order to describe the manufacturing errors or tolerances, random dimensions are inputted. Comparing the predicted fatigue lifetime distributions with/without geometrical uncertainty, it shows that geometrical uncertainty matters in structural fatigue reliability. Particularly, sensitivity analysis indicates that the geometrical uncertainty exerts more critical influences on the fatigue lifetime and reliability of the turbine bladed disk than others. The sensitivity factors of three typical dimensions emerges the influence of designed sizes and dimensional tolerances on the failure probability, which provides a reference for engineering design.
•Expand the creep-fatigue life sample size by divide-and-conquer machine learning.•Probabilistic damage distributions are explored to depict the scatter characteristics.•Establish probabilistic ...creep-fatigue damage assessment diagram.
In order to investigate the probabilistic damage distribution under creep-fatigue interaction, machine learning framework with the divide-and-conquer methodology is proposed to expand the creep-fatigue life sample size of each load condition. The optimized deterministic life prediction model, strain energy density exhaustion model (SEDE), is selected to take material variability into account. Subsequently, random accumulated creep and fatigue damage are obtained by the combination of probabilistic SEDE life model and creep-fatigue life distributions through the Latin hypercube sampling (LHS) simulation. A relative scatter factor depicted in the creep-fatigue interaction diagram is introduced to reveal the dominance of scatter in creep/fatigue on life scatter. Consequently, a probabilistic creep-fatigue damage assessment diagram with involving probabilistic equipotential line for safety evaluations is established. Such probabilistic damage assessment may provide reference and has promising potential in the further creep-fatigue life design for reliability.
•Low cycle fatigue and creep-fatigue behaviors are systematically explored.•Cracking modes and damage mechanisms under different loading waveforms are investigated.•Σ3 CSLBs show great resistance of ...intergranular damage.•The present model addresses fatigue, creep and oxidation on life prediction.
The low cycle fatigue (LCF) and creep-fatigue behaviors of Ni-based GH4169 superalloy are investigated by uniaxial strain-controlled fully-reversed testing at 650 °C. Compared with LCF tests, the effects of tensile and compressive strain hold times on creep-fatigue lifetimes are experimentally explored with varying total strain ranges in the present work. In order to elucidate the damage mechanisms under complex loading waveforms, an additional series of tests with both tensile and compressive hold times are carried out at a given total strain range of 2.0%. Posterior to the cyclic tests, main-crack-failure modes and secondary cracking modes are studied via optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques. Main-crack failure mechanisms are examined by the fracture appearance observations. Cracking modes are explored through quantitative characterization on the distributions of secondary cracks in the longitudinal cross sections under different loading waveforms. Moreover, a generalized life model based on linear damage summation (LDS) framework and energy dissipation criterion (EDC) is elaborated to estimate the damage mechanisms of fatigue, creep and oxidation. The prediction results can well establish the correlations between the reductions of numbers of cycles to failure and the presences of different damage mechanisms under respective loading waveforms.
•A crystal plasticity-based approach is implemented to predict creep-fatigue life.•Creep and fatigue indicator parameters are introduced to describe damage evolution.•A potential methodology for ...conservative creep-fatigue life evaluation was provided.
A numerical process based on crystal plasticity finite element (CPFE) was implemented to predict creep-fatigue crack initiation life. CPFE-based model can describe the macroscopic cyclic deformation and reveal grain-level damage mechanism. A new life prediction approach was then constructed by introducing fatigue and creep indicator parameters. Furthermore, a series of strain-controlled creep-fatigue tests in GH4169 superalloy at 650℃ were used to validate predicted accuracy of this model, where most of the data points lied within ±1.5 error band. Finally, a potential methodology for conservative creep-fatigue life evaluation was provided by flexible creep-fatigue damage summation rule in engineering applications.
•Filling the gaps of creep-fatigue tests under non-proportional loading.•Enriching dataset with stress triaxiality in the range of 0 to 0.33.•Revealing interactions between addition hardening and ...creep.•Improving a viscoplasticity model for multiaxial constitutive behavior.•Providing reasonable life prediction results with an energy-based method.
In this paper, a series of strain-controlled fatigue and creep–fatigue tests under proportional/non-proportional loadings were performed for type 304 stainless steel at 873 K. Then, post-test metallographic observations were performed through the electron back scattered diffraction (EBSD) and transmission electron microscope (TEM) combinative characterizations. In this aspect, the wavy slip dominated deformation mechanism under non-proportional loadings was considered as the essence for additional hardening, while the introduction of creep resulted in further microstructure evolutions by facilitating recrystallization. Afterward, a unified viscoplasticity constitutive model was proposed to simulate the cyclic stress–strain responses, in which an additional hardening parameter combined with a loading-path parameter was used to describe the cyclic hardening curves. Concurrently, stress triaxiality was introduced to provide accurate descriptions for the stress relaxation behavior. Semi-physical continuum damage models involving multiaxial damage factor and non-proportional strain energy parameter was proposed to predict the multiaxial creep–fatigue damage evaluations. Good agreements between experimental data and simulated results were achieved with the help of the proposed numerical procedures.
•A viscoplastic constitutive model is developed to simulate stress-strain responses.•Multi-axial fatigue damage is predicted based on critical plane method.•Multi-axial creep damage is predicted by ...considering two important factors.•Crack initiation sites shifting from surface to subsurface are well described.
In this paper, a new numerical procedure based on a cycle-by-cycle analysis has been constructed for creep-fatigue behavior and life prediction of high-temperature structures under multi-axial stress states. Within this framework, a modified unified viscoplastic constitutive model with isotropic hardening and modified kinematic hardening rules is developed to simulate the cycle-by-cycle stress-strain responses. Moreover, the newly constructed creep-fatigue approach calculates fatigue and creep damage variables using the critical plane method (CPM) and the modified strain energy density exhaustion (SEDE) model, respectively. The multi-axial ductility factor and elastic follow-up factor are also introduced into the modified SEDE model to accommodate the special multi-axial and mixed controlled modes, which are widely existed in practical structures. In order to validate the feasibility of the proposed numerical procedure, a series of creep-fatigue tests of notched specimens made from nickel-based GH4169 superalloy were carried out at 650 °C. The predicted numbers of cycles to crack initiation agree well with the experimental data. Evidence of crack initiation under various loading conditions was observed via the electron backscatter diffraction (EBSD) technique, indicating location-dependent crack initiations depending on loading conditions. In detail, the crack initiation sites shifting from surface to subsurface with increasing hold times can be well simulated by the proposed numerical procedure due to a reasonable description of the creep-fatigue damage evolution.
•Explore degradation of material mechanical properties in creep-fatigue process.•A damage variable Dm is defined based on tensile plastic strain energy density.•Critical values of damage summation ...rules degrade as a power function of Dm.•3D damage diagram is constructed for evaluating creep-fatigue damage level.
Progressive degradation of material mechanical properties in the low cycle fatigue (LCF) and creep-fatigue (CF) interaction at high temperature affects the safe operation of in-service materials. By considering material degradation, the present work aims to establish a method for evaluating LCF and CF damage levels with wide applicability. Material-level data accumulations as well as theoretical foundations of LCF and CF are presented, including interrupted LCF and CF tests, subsequent tensile tests and energy-based damage models. A damage variable representing the degradation of material mechanical properties is then defined based on the tensile plastic strain energy density (TPSED), the physical mechanism of which is reflected in the microstructure evolution and fracture appearance. By taking into consideration the material degradation threshold in the traditional damage summation rule, a new three-dimensional (3D) damage interaction diagram is established, where the additional third axis indicates the material degradation level. Finally, taking GH4169 alloy and P92 steel as examples, this work demonstrates the implemented procedures of damage level evaluation, which has been validated via the experimental data.
•Energy density exhaustion model for creep-fatigue life prediction is proposed.•Mean stress effect is considered in the present model.•The present model is more accurate than the existing models.
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.
Damages caused by the effects of cyclic loading (fatigue) and high temperature (creep and oxidation) have been considered critical and need to be appropriately evaluated. A series of ...strain‐controlled fatigue and creep‐fatigue tests are performed on P92 at 873 K under oxygen‐containing environment. The creep‐fatigue life prediction results are summarized using models based on strain‐range partition, Manson–Coffin equation and linear damage summation (LDS) rule. Obviously, the models based on the LDS rule show relatively good performance with an error band of ±2.5. In view of the adverse effects of oxidation on creep‐fatigue endurance, this paper further develops a physically‐based oxidation damage equation, which is incorporated into LDS rule for the improvement of life assessment. The predicted and experimental results falling into ±1.5 error band proved the accuracy of the proposed oxidation damage equation in the LDS rule. Additionally, model selection criteria are recommended to evaluate the model prediction capabilities.
Highlights
A series of fatigue, creep‐fatigue, and oxidation tests are performed on P92 at 873 K.
Oxidation damage equation based on kinetics and concentration distribution is proposed.
Error band is reduced from ±2.5 to ±1.5 by using the proposed oxidation damage equation.
W‐SEDE‐O model performs best with the minimum error band & best statistical indicators.
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