Creep-fatigue Life Prediction Research Articles

Creep fatigue life prediction of pipe joints under complex bending-torsional loading using large-scale nonlinear structural analysis

With the expansion of clean energy, thermal power plants will be used as a load-following power plant. Thus, temperature and strain variations will occur during load fluctuations including start and stop of operation, and large-diameter pipes will be damaged by fatigue in addition to creep due to internal pressure. Especially, in a bent pipe such as an elbow pipe, bending and torsion are simultaneously applied. Therefore, it is important to predict the creep fatigue life in a bending and torsional stress environment. In this study, analysis method for creep fatigue life using Norton’s law, ductile exhaustion law, and Manson-Coffin’s law is proposed. Next, the proposed method was applied to the prediction of creep fatigue life of the test samples for thermal power plant piping, and the validity of the proposed method was verified by comparing creep fatigue life obtained by proposed method and that obtained by an experiment. In addition, the proposed method is applied to the prediction of creep fatigue life with load fluctuations and investigation of the effects of load fluctuation interval on creep fatigue life.

Experimental study and life prediction for aero-engine turbine blade considering creep-fatigue interaction effect

As an important failure form of the turbine blade, creep-fatigue interaction damage affects the safe operation and maintenance strategy of aero-engine, and has been the focus of scientific research and the academic community. Firstly, based on the Kachanov-Rabotnov-Lemaitre continuum damage mechanics theory and the nonlinear symmetry of creep damage and fatigue damage, a creep-fatigue life prediction model is constructed considering the interaction effect in this paper. Then, based on the thermal-fluid–solid multi-physical field coupling numerical simulation of the turbine blade, the equivalent method of creep-fatigue load spectrum was explored according to the equal damage criterion and linear damage rule, and the creep-fatigue interaction test of smooth samples of the blade material was conducted to analyze the creep-fatigue fracture morphology. Finally, the stress term in the creep-fatigue life prediction model of blade material is modified by the correction factor α, and the modified creep-fatigue life prediction model of the turbine blade is constructed considering the interaction effect. The results show that the modified creep-fatigue life prediction model considering the interaction effect has a high life prediction ability with an error of 3.9%. The above research has important scientific research value for the life extension design of turbine blades and the improvement of aero-engine maintenance strategy.

Physics-informed machine learning framework for creep-fatigue life prediction of a Ni-based superalloy using ensemble learning

Under extreme environments, critical components of nuclear power equipment suffer creep-fatigue damage accumulation due to the complex load, threatening the safety operation of equipment. Therefore, creep-fatigue life assessment is crucial to ensure the structural integrity and performance requirement of nuclear power equipment during service. To address the poor prediction accuracy under small data sets, this paper developed a physics-informed machine learning (PIML) framework to model the creep-fatigue interaction behavior of a Ni-based superalloy. In this work, an ensemble learning approach is considered to embed physical information into machine learning (ML) models for the creep-fatigue life prediction. The comparison of each model shows that PIML is capable of utilizing the strengths of purely data-driven models and physics-based models to provide excellent prediction accuracy and promote the generalization ability.

Physics-informed neural network for creep-fatigue life prediction of Inconel 617 and interpretation of influencing factors

Providing a comprehensive assessment of the creep-fatigue life of critical structures in nuclear power facilities is important for structural integrity and performance requirements during service. The data-driven modeling method can set aside traditional mechanical theory and use experimental data to drive creep-fatigue performance modeling directly, avoiding complex parameter fitting. A sufficiently robust purely data-driven model must be supported by a large amount of training data, which means significant time and financial costs. To address these issues, a physics-informed neural network (PINN) is proposed in this work to predict the creep-fatigue life of Inconel 617 at high temperatures, where the physical constraints are introduced to enhance the physical fundamentals. The prediction results show that introduced physical constraints promote the stability of prediction performance and analysis based on the strength of physical constraints provides valuable guidance for creep-fatigue modeling to select the suitable architecture for building PINN. The SHapley Additive exPlanations (SHAP) method and dependency analysis are utilized to reveal the intrinsic mechanism of the properties and structure of PINN.

Creep–fatigue life prediction of a titanium alloy deep-sea submersible using a continuum damage mechanics-informed BP neural network model

Deep-sea submersibles are subjected to trapezoidal wave loading during operation, which can cause creep–fatigue failure of pressure hull structures. To ensure structural safety, it is necessary to develop accurate and efficient creep–fatigue life prediction methods. This paper proposes a continuum damage mechanics-informed BPNN approach for predicting the creep–fatigue life of titanium alloy submersibles. To obtain a reliable sample set, the creep–fatigue life is calculated based on the titanium alloy room-temperature creep–fatigue damage model and numerical analysis methods proposed by the authors. The operating duration, operating pressure, axial stress, and circumferential stress were selected as the input variables, whereas the number of cycles and actual operating duration were chosen as the output variables. The proposed model fills the current gap of a lack of research on the creep‒fatigue life prediction of titanium alloy deep-sea submersibles via machine learning methods. The research results show that the neural network model established in this paper can quickly and accurately predict the creep–fatigue life of titanium alloy deep-sea submersibles. A sensitivity analysis of the input parameters revealed that the creep–fatigue life is more sensitive to the operational pressure than to the dwell time.

Experimental Study and Creep-Fatigue Life Prediction of Turbine Blade Material DZ125 Considering the Nonholding Effect and Coupling Effect of Stress and High Temperature

Abstract In response to the problem of creep-fatigue interaction damage failure of aero-engine turbine blade material, based on the modified damage evolution model of Kachanov-Rabotnov and Chaboche, a creep-fatigue life prediction model for nickel-based superalloy DZ125 is constructed considering the nonholding effect and coupling effect of stress and high temperature with the nonlinear interaction and superposition of creep damage and fatigue damage according to the continuum damage mechanics theory. Simultaneously, the microfracture morphology of DZ125 was analyzed using a scanning electron microscope, revealing the micromechanism of creep-fatigue interaction. The research results manifest that the creep-fatigue life prediction model has a high life prediction ability within ±2.0 times the dispersion band of the prediction results. Concurrently, a large number of intertwined tearing edges, microcracks, and microvoids appear in the fracture morphology, and creep and fatigue interact with each other in the form of effective stress. The above research can provide theoretical support for predicting the lifespan of mechanical structures in a high-temperature environment.

Deterioration effect of creep on fatigue properties in QCr0.8 alloy: Damage mechanism and fatigue-creep life evaluation

The QCr0.8 alloy used in the thrust chamber liner wall of the reusable liquid rocket engine is subjected to high-temperature fatigue and high-temperature creep loading. This work investigates the effect of fatigue-creep interaction (FCI) on the fatigue life of QCr0.8 alloy. Experimental results reveal accelerated cyclic softening and decelerated stress relaxation in the alloy under FCI conditions, attributed to creep deformation and stress amplitude growth. The damage evolution mechanism of FCI is revealed by the results of the energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). The decrease in fatigue-creep life of the QCr0.8 alloy is attributed to the fracture of the outer surface oxide layer, as well as the coupling development of creep voids and secondary cracks. Finally, a new fatigue-creep life prediction model has been developed, taking into account the increase in plastic strain energy resulting from stress relaxation and creep deformation behaviors. The study results provide substantial theoretical justification for the utilization of QCr0.8 alloy in the thrust chamber wall of reusable liquid rocket engines.

Creep-fatigue life prediction of notched structure after an advanced surface strengthening treatment in a nickel-based superalloy at 650°C

The elucidation of creep-fatigue damage mechanisms is still controversial for high-temperature structures after surface strengthening treatments, which serves as a critical foundation for the development of an accurate life prediction method. In this work, a numerical procedure is constructed for the prediction of creep-fatigue life improvement, where a dual-scale modeling approach is proposed to integrate important strengthening factors and microstructure features. The macro-scale finite element (FE) simulation aims to investigate the cyclic deformation behavior in a notched structure by using a viscoplastic constitutive model. The initial stress field is predetermined based on the experimental residual stress. The micro-scale FE analysis is employed to investigate the local damage evolution occurring at the notched root by combining size-dependent crystal plasticity with grain boundary cavity model. The cycle-by-cycle deformation histories are extracted from the macro-scale FE model and subsequently are utilized as boundary conditions in the micro-scale FE one. From the experimental perspective, the submerged micro-abrasive waterjet peening (SMA-WJP) process is carried out for creep-fatigue life improvement of the notched structure. Results shows that the notched structure treated by the SMA-WJP process forms an obvious plastic layer with the depth of 20 μm and residual stress with the maximum value of -926 MPa. The predicted numbers of cycles to crack initiation agree with the creep-fatigue experimental ones before and after SMA-WJP. In detail, the surface residual stress and plastic layer are unable to suppress the cavity nucleation on the grain boundaries of internal material. As a consequence, the creep-fatigue life improvement is diminished as the hold time increases, which can be accurately predicted by the developed numerical procedure.

Strain-controlled creep-fatigue interaction property of Nb521 refractory alloy: Hold time effect and life prediction

Strain-controlled creep-fatigue interaction (CFI) tests were conducted for NbW alloy, i.e., Nb521 alloy, at 900 °C with a strain amplitude of ±0.3% under atmospheric conditions to study the effect of hold time on the CFI failure behaviour, with the results indicating that the creep-fatigue life of the Nb521 alloy decreased with an increase in the hold time. The decline rate gradually flattened, with no saturation effect observed. Furthermore, with an increase in the hold time, the hardening rate of the Nb521 alloy gradually decreased, but the softening rate remained high. Fatigue fractures can be divided into stable propagation, rapid propagation, and final fracture zones according to their morphological characteristics. Compared with the stable propagation zone of the pure fatigue fracture surface, the creep damage caused by the hold time resulted in numerous intergranular fracture features in the stable propagation zone of the CFI fracture surface, indicating that the crack propagation mode changed from a transgranular fracture to a mixed transgranular–intergranular fracture. The strain energy density exhaustion model under nonlinear damage summation better reflects the CFI damage behaviour, with the creep-fatigue life prediction result considered acceptable.

A kind of numerical model combined with genetic algorithm and back propagation neural network for creep-fatigue life prediction and optimization of double-layered annulus metal hydride reactor and verification of ASME-NH code

The safety and stability of metal hydride reactor (MHR) structure are significantly important for normal operations of hydrogen isotope storage and supply system. Moreover, considering that the experiments of studying the fatigue and creep properties of MHR structure are always time, money, difficulty and energy consuming. To further improve the service life of MHR as well as lower the failure risk, this paper proposes a system integration method based on ASME code validation, which combines genetic algorithm (GA) and back propagation neural network (BPNN) for rapid assessment of creep-fatigue life (CFL) and optimal design of structural parameters. First, numerical simulation is applied to perform thermal-mechanical coupled finite element analysis and verified by ASME code. Then, parametric sensitivity analysis and experimental design of parameter-based training set based on Taguchi method were performed, and response values are obtained from physical models combined with numerical simulations. Finally, a parametric optimization procedure combining BPNN and GA is developed based on the training data. In the workflow, parametric data flow is passed through the experimental design, numerical simulation, prediction and optimization processes. The results show that the proposed BPNN proxy model based on ASME code validation has good performance in predicting the life of MHR. Based on this, the life of the MHR is improved by 8% compared to the reference sample.

Study on creep-fatigue interaction mechanism and life prediction of aero-engine turbine blade

Aiming at the creep-fatigue interaction damage failure problem of turbine blades in aeronautical engineering, firstly, based on the modified Kachanov-Rabotnov-Chaboche (MKRC) damage mechanics theory, the creep-fatigue life prediction model of turbine blade material was constructed with the experimental verification completion of nickel-based superalloy DZ125. Meanwhile, the creep-fatigue interaction behavior was investigated with the mechanism revelation. Then, considering the shape, size and microscopic defect difference effect from the material to the structure, the creep-fatigue life prediction model of turbine blade structure is proposed by introducing the comprehensive correction factor with experimental verification. Finally, the research results show that there are a large number of interwoven tear edges, micro-cracks and micro-pores in the fracture morphology, and creep and fatigue interact with each other in the form of effective stress. Simultaneously, the creep-fatigue life prediction model has a high life prediction ability with an error of 3%, which can provide a theoretical reference for the damage tolerance design of the turbine blade.

A comparative study of creep-fatigue life prediction for complex geometrical specimens using supervised machine learning

This study proposes a supervised machine learning approach to predict the creep-fatigue life of complex geometrical specimens. Seven different specimens were tested under creep-fatigue loading, and finite element analysis and test results showed that the stress distribution and life of the specimens were significantly influenced by the diameters and arrangement of holes. Characteristic parameters were proposed to describe the specimens' features, and support vector regression (SVR) and artificial neural network (ANN) methods were utilized to predict their life. The results indicate that both methods are effective in predicting the life of the specimens, with the ANN showing better performance when input data is limited. This study offers valuable insights into the leading factors behind the failure of complex geometrical specimens under creep-fatigue loading.

Low cycle fatigue and fatigue-creep interaction effects on softening behavior and life prediction of cast aluminum alloy at elevated temperature

In this study, a series of strain-controlled low-cycle-fatigue (LCF) and low-cycle-fatigue-creep (LCFC) experiments were conducted on a cast Al-Si-Cu alloy. LCF and LCFC experiments were performed at 250 ℃, 300 ℃ and 350 ℃ with three different strain amplitudes. The results showed significant softening behavior, with temperature affecting both the softening behavior and fatigue life. Interestingly, coupling creep at 250 ℃ was found to increase fatigue life. Microstructure evolution was investigated by using scanning electron microscopy, and it was found that the introduction of strain-dwell at the tensile peak caused the interaction of fatigue and creep, resulting in a large amount of plastic deformation that affected fatigue life. Two fracture models were proposed for LCF and LCFC tests at different temperatures, based on the microstructure evolution. Using the experimental data, a modified life prediction model was proposed, which demonstrated a better prediction ability than other traditional models in fatigue-creep life prediction under different temperature and strain amplitudes.

Crystal plasticity constitutive model and thermodynamics informed creep-fatigue life prediction model for Ni-based single crystal superalloy

Ni-based single crystal superalloys are the main constituent materials for aeroengine turbine blades. They are subjected to extensive in-service plastic deformation and creep-fatigue interaction, which can cause damage and failure and hence limit the turbine blade durability. In this study, a novel crystal plasticity-based constitutive model is proposed to predict the cyclic inelastic deformation of Ni-based single crystal superalloy under creep-fatigue loads, and the key aspects examined include cyclic strain hardening, ratcheting and stress relaxation behavior. The novelty of the model lies in the introduction of a dislocation density parameter in the kinematic hardening rule to describe the evolutionary characteristics of hysteresis loops. The constitutive model is implemented via the crystal plasticity finite element method and the predictions are in good agreement with experimental results. Furthermore, thermodynamic entropy generation is innovatively adopted as an indicator parameter for analysis of Ni-based single crystal creep-fatigue failure, and the corresponding creep and fatigue damage models are developed to evaluate the degree of damage. The half-life concept associated with the steady-state hysteresis loop is employed in the failure model to predict the creep-fatigue life without being limited by the computational efficiency of the crystal plasticity finite element method. The proposed model can well capture the characteristics of Ni-based single crystal creep-fatigue life, and the prediction falls within a scatter band of factor 2.0 compared to experimental results. The proposed creep-fatigue life prediction model is underpinned by deformation and failure mechanisms, which would provide a basis for accurate analysis and robust assessment of Ni-based single crystal superalloy performance and life.

High Temperature Strength Property and Creep-Fatigue Life Prediction of a Temper Bead Welded CrMo Casting Steel

Cr-Mo casting steels are widely used as casings and valves in thermal power plants. Creep fatigue damage gradually proceeds in these high-temperature components during operation resulting in creep voids and micro crack initiation in which repair welding is considered to be applied for life extension. In this study, a temper bead welded joint of Cr-Mo casting steel as well as fine and coarse grain simulated materials were prepared. Creep and fatigue, creep-fatigue tests were performed using specimens taken from base and weld metals of the welded joint, the simulated materials and cross welded specimens. Material constants of creep deformation and cyclic stress-strain properties for constituted materials of the welded joint were obtained from the creep and fatigue tests. Minimum creep strain rate of the base metal at the same stress was the highest than that of other materials. Plastic deformation resistance is higher in the order of the base metal, fine grain, coarse and weld metal. Creep-fatigue life of the welded joint, which failed at the base metal 2mm away from the boundary between the base metal and fine grain, was shorter than that of the uniform base metal specimen indicating that the temper bead welding may reduce the creep-fatigue life. Finite element analysis of the cross welded specimen showed that elastic-plastic and creep strains accumulated at the base metal near the boundary are higher than those at other regions causing life reduction and failure at the base metal. The creep-fatigue life prediction by using the analysis results indicated that the nonlinear damage accumulation model gave more accurate life prediction results than the time fraction and ductility exhaustion rules.

Creep-fatigue reliability assessment for high-temperature components fusing on-line monitoring data and physics-of-failure by engineering damage mechanics approach

A new framework for creep-fatigue reliability assessment of high-temperature components is proposed in this paper, during which the monitoring data and physics-of-failure are integrated into the natural degradation process on the basis of engineering damage mechanics approach. Firstly, the surrogate model is constructed to establish the mapping from on-line monitoring data to elastic loads spectrum. Then, the plastic stress is corrected by Neuber rule, and thus the creep-fatigue life prediction models are combined with the linear damage summation rule to calculate the damage accumulations of components at dangerous positions in service. Secondly, uncertainty quantification is performed to reveal the probabilistic damage accumulations and modeled by stochastic process. Finally, the component-level reliability assessment of steam turbine rotor considering the correlation of damage accumulations at dangerous positions is explored by using the copula function. It shows that the combination of inverse Gaussian process model and Gumbel copula are appropriate to describe the damage accumulations and to analyze the dependencies at dangerous positions. Furthermore, the comparative research is also performed with respect to the independent assumption and hotspot method. The independent treatment tends to be conservative and hotspot method may be non-conservative in reliability assessment, which demonstrates the applicability of the proposed framework and is expected to be promoted in other high-temperature components.

Establishment of unified creep–fatigue life prediction under various temperatures and investigation of failure physical mechanism for Type 304 stainless steel

AbstractInvestigations into creep–fatigue life and the corresponding failure physical mechanism are crucial for guaranteeing the structural integrity of components. In this work, a series of strain‐controlled fatigue and creep–fatigue tests were performed at different temperatures. Then, the EBSD‐TEM combinative analysis was performed to reveal the microstructure evolution. The creep failure parameter dependence derived from standard creep experimental data and their importance in further creep–fatigue employment were discussed. Results show that strain energy density has better relevance than ductility in connecting with creep failure. The temperature‐dependent critical strain energy density and an equivalent failure strain energy density, considering geometric effect, were incorporated with the current energy‐based model, which enables creep–fatigue life scatter within a factor of 1.5. Moreover, multi‐slip activations and severe slip interactions under creep–fatigue conditions were responsible for the ultimately lifetime reductions based on microstructure observations.

A non-unified viscoplastic constitutive model based on irreversible thermodynamics and creep-fatigue life prediction for Type 316 stainless

In this work, to describe the cycle behavior considering fatigue-creep interaction, a non-unified viscoplastic constitutive model for 316 stainless steel is derived within the irreversible thermodynamic framework. The internal variables considering kinematic and isotropic hardening properties are selected to construct the evolution equation of visco-plastic and creep terms. The proposed constitutive model was validated by the comparison with the existing literature. It was manifested that this constitutive model could successfully predict the hardening behavior and stress relaxation process under the cyclic loading. During the dwell period, the increment of the inelastic strain is decomposed into the viscoplastic and creep term. The viscoplastic deformation dominates first stage of the stress relaxation, while the stable stage is controlled by the creep term. Finally, the predicted values of mean stress are taken into the Manson-Coffin law, the low cycle fatigue life prediction are carried out based on the numerical model, which showed robust correlation with experimental results.

A modified energy model including mean stress and creep threshold stress effect for creep–fatigue life prediction

AbstractCreep–fatigue interaction plays an important role in the failure of high‐temperature components. However, most of the existing life prediction limits materials and testing conditions. The present study was conducted to modify the strain energy density exhaustion model systematically to predict the creep–fatigue life under a wide range of materials, hold times, and total strain ranges. Mean stress effect was considered by the creep–fatigue net tensile hysteretic energy to eliminate the overestimation of fatigue damage in the traditional method, while creep threshold stress was introduced to estimate the creep damage. In addition, a novel failure strain energy model was proposed to describe the correlation among failure strain energy, creep failure mechanisms, and temperatures. Based on the tension‐hold creep–fatigue data of Grade 91 steel, 304 SS, Alloy 617, and 1Cr1Mo0.25V steel, the modified model predicted reasonable lives within a ±1.5 scatter band.

A creep-fatigue life prediction model considering multi-factor coupling effect

Abstract A creep-fatigue life prediction model based on a novel creep damage evaluation method (NCDEM) considering the multi-factor coupling effect is presented in this paper. Further, to verify the validity and practicability, the creep-fatigue life of GH4169 at 650 °C is calculated to compare with the experimental results. Ultimately, the prediction results are respectively compared with those of the creep-fatigue life prediction models based on the time fraction method (TFM), ductility exhaustion method (DEM), and strain energy density exhaustion method (SEDEM). Consequently, the prediction results are distributed in ±1.5 times dispersion band, which elucidates the creep-fatigue life prediction model proposed based on the NCDEM has the best ability.