Creep-fatigue Life Research Articles

Creep-fatigue behavior of nickel-based single crystal superalloy with different orientations: Experimental characterization and multi-scale simulation

In order to clarify the creep-fatigue interaction properties of nickel-based single crystal superalloys with different orientations, mechanical tests of [001], [011] and [111] orientations were conducted at 980°Cunder various stress states and microstructural characterization was performed. The results demonstrated that the creep-fatigue life was longest for the [111] orientation, followed by the [001] orientation, and the [011] orientation exhibited the lowest life. Furthermore, as stress levels increased, the life of specimens with different orientations decreased. Damage accumulation in specimens with various orientations was primarily influenced by the formation and growth of material micropores. Fracture analysis revealed that the failure of [001] and [111] oriented specimens resulted from a combination of multi-slip systems and micropore aggregation. Conversely, the fracture of [011] oriented specimens occurred due to a single slip system. Molecular dynamics simulation was employed to analyze the evolution of defects under creep-fatigue loading for different orientation models, further supporting the fracture mechanism. In addition, the life of the specimens was predicted using the crystal plasticity theory based on energy dissipation, and the results were within ±1.5 error bands.

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.

Creep-Fatigue Life Property of P91 Welded Piping Subjected to Bending and Torsional Moments at High Temperature

Abstract In recent years, the role of thermal power plants has shifted from providing a baseload to providing supplemental supply to compensate for fluctuations in the output of renewable energy sources. Thus, the operation of these plants involves frequent startup and shutdown cycles, which lead to extensive damage caused by creep and fatigue interactions. In addition, the piping utilized in thermal plants is subjected to a combined stress state composed of bending and torsional moments. In this study, a high-temperature fatigue testing machine capable of generating such a bending-torsional loading was developed. Creep-fatigue tests were conducted on P91 steel piping with weldment. The results clarified that the creep-fatigue life was reduced by the superposition of the torsional and bending moments and that it was further reduced by a holding load. It was shown that the creep-fatigue life of piping with weldment can be estimated accurately using the equivalent bending moment, which is composed of the torsional and bending moments. It was also confirmed that crack occurred in the heat-affected zone (HAZ) of the welded part, which has been often observed in actual thermal power equipment. From the finite element analysis, it was identified that cracking was initiated in the HAZ due to the accumulation of creep strain and increase in the hydrostatic pressure component during a holding load.

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.

Microstructural evolutions and life evaluation of non-proportional creep-fatigue considering loading path and holding position effects

In this paper, strain-controlled non-proportional creep-fatigue tests under three non-proportional loading paths were performed for type 304 stainless steel. The strain holding period at the peak axial or peak shear strains was introduced to investigate the effect of the holding position on the creep damage accumulation. Compared with the other non-proportional loading paths, the circle loading path generated the most detrimental effect on the material life. In addition, the introduction of the holding period reduced the material life, in which axial holding resulted in a greater creep damage than that of the shear holding. A post-examination method through the electron backscattered diffraction (EBSD) observations was conducted to reveal the damage mechanisms under various loading paths. The variations of six EBSD-based misorientation parameters showed the significant effect of the loading path and holding position on the damage mechanisms. Subsequently, a multiaxial damage factor (MDF)-involved strain energy density exhaustion (SEDE) model and a non-proportional energy parameter (NPEP)-involved Ostergren model were collaborated to provide a precise prediction within a factor of 1.5 for non-proportional creep-fatigue life distributions.

Creep-fatigue life and damage evaluation under various strain waveforms for Ni-based Alloy 740H

This paper presents creep-fatigue life evaluation of a nickel-based Alloy 740H, for evaluating its performance as a material of advanced ultra-supercritical power plants. Strain controlled creep-fatigue tests were performed under six strain waveforms using solid bar specimens at 700 °C, and the effect of strain waveform on creep-fatigue life was discussed. Creep-fatigue lives were shorter than pure fatigue life depending on the strain waveform. The failure life in symmetrical waveform was reduced to about half as the strain rate decreased by one order of magnitude. The failure lives in the unsymmetrical waveforms were smaller than that in the symmetrical waveform and tensile slow straining was more detrimental to fatigue life than compressive slow straining. The creep-fatigue life decreased significantly due to strain hold, and tensile peak strain hold had more enhanced effect on fatigue life than compressive strain hold. Applicability of five existing approaches based on linear damage summation rule were discussed. In addition, a new approach based on modified ductility exhaustion model with several modifications was proposed in order to improve the life predictability of the classical approach. Time fraction rule with ASME failure envelope and new approach satisfactorily predicted the creep-fatigue lives almost within a scatter band of 2.

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.

Geometric discontinuity effect on creep-fatigue behaviors in a nickel-based superalloy hole structure considering ratcheting deformation

Multi-axial stress states are common in the most high-temperature components. In this paper, a ratcheting modified strain energy density exhaustion (RM-SEDE) model is developed to predict creep-fatigue life for hole structures. Afterwards, the numerical procedures are perfected to provide in-depth understandings in cyclic deformation, damage evolution and life prediction under multi-axial stress states. Some abnormal results contrary to uniaxial common sense, in terms of creep-fatigue life distributions and geometrically necessary dislocation accumulations, are well resolved based on the RM-SEDE. Among them, not only life prediction accuracy is improved, but also deterioration mechanism of ratcheting on creep-fatigue damage is explained.

An improved life prediction strategy at elevated temperature based on pure creep and fatigue data: Classical strain controlled and hybrid stress–strain controlled creep-fatigue test

Creep-fatigue (CF) interaction is an important issue for designing the next generation power plants. Thus, strain-controlled CF tests (SCCFTs) and hybrid stress–strain controlled CF tests (HCCFTs) of 9Cr3Co3W1CuVNbB steel were conducted. The cyclic responses and creep behaviors in SCCFTs and HCCFTs were investigated. In addition, an improved CF life prediction method was proposed, and only pure creep and fatigue data were needed. Then, the relaxed stress in SCCFTs and the accumulated creep strain in HCCFTs were predicted. Moreover, the oxidation damage was incorporated into the non-linear interactions. The optimally predictive results of the studied material, Grade 91 steel, and 304 stainless steel were in ±3 error band with strain energy density exhaustion model.

Effect of Flexible Operation on Residual Life of High-Temperature Components of Power Plants

Electricity generation from renewable energy sources is emerging as a result of global carbon emission reduction policies. However, most renewable energy sources are non-dispatchable and cannot be adjusted to meet the fluctuating electricity demands of society. A flexible operation process has been proposed as an effective solution to compensate for the unstable nature of renewable energy sources. Thermal load fluctuations during flexible operation may cause creep–fatigue damage to the high-temperature components of thermal power plants, as they are designed with a focus on creep damage under a constant power level. This study investigated the residual life of high-temperature components, such as a superheater tube and a reheater header, to failure under flexible operation conditions using finite element analysis and empirical models. First, we determined an analytical solution for the straightened superheater tube under thermal conditions and compared it with the numerical solution to verify the numerical models. Through the verified finite element model, the creep–fatigue life of the reheater header was estimated by considering flexible operation factors and employing the Coffin–Manson and Larson–Miller models. Although fatigue damage increases with decreasing minimum load and ramp rate, we confirmed that creep damage significantly affects the residual life during flexible operation. In addition, a surrogate model was proposed to evaluate the residual life of the reheater as a function of the flexible operation factors using the machine learning methodology, based on the results of finite element methods. It can be used to predict its residual life without performing complex thermo-structural analysis and relying on empirical models for fatigue and creep life. We expect our findings to contribute to the efficient operation of thermal power plants by optimizing the flexible operation factors.

Review on Creep Phenomenon and Its Model in Aircraft Engines

The aircraft is subjected to high-temperature and high-pressure conditions during flight, which renders it susceptible to the occurrence of creep phenomenon. Several academics have conducted extensive research on this issue. This research paper provides a comprehensive overview of the existing literature on creep phenomena in aircraft engines. First, several classical creep calculation models are enumerated and categorized as creep life calculation, creep-fatigue life calculation, and creep deformation calculation. Studies on creep phenomena are conducted in various components of aircraft engines, such as the engine’s turbine blades, turbine disks, and combustion chambers. The creep behavior of turbine blades in aircraft engines has been extensively researched. Furthermore, the protective measures aimed at mitigating creep are presented. Materials with high creep resistance can be used, and alternative fuels could be implemented. This paper provides an in-depth analysis of the advantages of creep in aircraft, presented in a favorable perspective. Finally, the prospective future research direction is discussed.

Discrepancy in low cycle fatigue and creep-fatigue life of 720Li alloy tested at 720 °C: Role of crystallographic texture evolution

Low cycle fatigue (LCF) and creep-fatigue (CF) interaction behavior of 720Li alloy were studied for the first time at 720 °C with different values of total strain amplitudes (Δεt/2) and hold time (HT). At Δεt/2 = 0.6%, CF life is less than LCF life. However, at Δεt/2 = 0.44, 0.48 and 0.55% CF life increased with increase in HT. Texture analysis of specimen tested at Δεt/2 = 0.44%, revealed that initial loading direction ǁ [11¯1] found to be rotated: (i) [21¯1] in LCF condition; and (ii) [11¯0] and [01¯1] in the CF condition predominantly. Notably, texture components developed in the CF specimens tested at Δεt/2 = 0.6% is quite similar to that in the LCF specimen tested at Δεt/2 = 0.44%. Development of [21¯1] texture component is due to multi planner slip (i.e. octahedral and cubic plane slip) under the influence of compressive load which is also confirmed by viscoplastic-simulation. Evolution of [21¯1] texture component leads to greater extent of inter crystalline cracks due to latent strain hardening and strain incompatibility between adjacent grains. Further, high static interfacial energy of annealing twin boundaries, confirmed through five-parameter stereological analyses, in LCF specimen tested at Δεt/2 = 0.44% and CF specimen tested at Δεt/2 = 0.6% resulted annealing twin boundary cracking. In contrast, evolution of low modulus texture components (i.e. [11¯0] & [01¯1]) has been inferred to be responsible for higher cyclic life in CF specimen tested at Δεt/2 = 0.44% and LCF specimen tested at Δεt/2 = 0.6%.

A novel creep-fatigue life evaluation method for ceramic-composites components

A novel progressive creep-fatigue damage analysis method based on multiscale information was proposed to accurately predict the service life of SiC/SiC composites and their structural components under complex mechanical loads and external environments. First, the strength degradation of SiC fibers involving five damage mechanisms and degradation of the properties of the interface under high-temperature fatigue loading were described quantitatively based on experimental results. Subsequently, a micromechanical model incorporating the global load sharing model, two-parameter Weibull model, shear-lag model, property degradation models of various constituents, and the effect of the braiding angle was utilized to calculate the volume fraction of broken fibers. The fatigue life of 3D 4-directional SiC/SiC composites was predicted and exhibited a trend similar to that of the experimental data. The rupture life of the SiC/SiC composites was calculated considering the stress transfer among microscopic constituents and the influence of creep slow crack growth on the fiber strength. The volume fraction of broken fibers obtained in these micromechanical models was used to perform a gradual strength degradation in the novel progressive creep-fatigue damage analysis method. Microscopic constituents damage and macroscopic property degradation were quantitatively linked in this step. This infuses the micro information into the traditional phenomenological progressive damage method. The gradual stiffness degradation rules were derived from the fitting of the residual elastic modulus obtained from the fatigue and creep experiments. Finally, the damage evolution and service life of SiC/SiC turbine blades were simulated and evaluated as a preliminary validation of the entire process.

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.

Multiaxial creep-fatigue constitutive modeling and damage evaluation for type F82H steel under non-proportional loading conditions

In this study, the deformation behaviors of F82H steel at 823 K under uniaxial and non-proportional loading conditions were described using a unified viscoplasticity model. The multiaxial creep–fatigue life was predicted using a strain energy damage parameter in collaboration with cycle-by-cycle concept. The creep–fatigue tested conditions includes tension holding and slow strain rate strain waveforms. Our findings showed that the cyclic deformation behaviors were well described using the unified viscoplasticity model. The creep–fatigue life was well predicted by the proposed energy-based model within a factor of 2.

Effect of Load Changing on Creep fatigue Life of Pipe Joints

Effect of Load Changing on Creep fatigue Life of Pipe Joints

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.

Creep-ratcheting effect and lifetime prediction of advanced 9-12% Cr ferritic steel at 600 ℃

The creep-ratcheting behavior of advanced 9–12 % Cr ferritic steel at 600 °C under peak holding time of 0 min, 2 min, 5 min, 10 min, 20 min and 30 min were investigated and a creep-ratcheting constitutive model was developed. Results present that the anelastic strain during the whole creep-fatigue life for various holding time approximately keep constant. However, it decreases rapidly during the last several cycles. Moreover, the primary creep regeneration (PCR) behavior during repeated loading and unloading process was observed. It keeps almost the same with increasing number of cycles and changes slightly for different peak holding time under the same stress amplitude and stress ratio. Additionally, a modified plasticity-creep superposition model was established considering the combination effects of creep damage, ratcheting evolution, anelastic recovery and PCR. The comparison between tested and predicted data shows that the proposed model has high accuracy for the prediction of both creep-ratcheting strain and creep-fatigue lifetime. It is of interest that the proposed model can also well simulate the evolution of stress–strain during the whole lifetime, especially for the reproduction of anelastic creep recovery and PCR along the load history.

Effects of strain rate on the tensile and creep-fatigue properties of 316H stainless steel

Since the unstable start-ups and shutdown processes of nuclear power plants, the effect of strain rate on tensile and creep-fatigue (CF) properties has attracted attention. This paper aims to study the strain rate dependent tensile and CF behavior of 316H stainless steel. The tensile and CF tests were independently performed with different strain rates of 0.0001, 0.0002, 0.0005, 0.001 and 0.005 s−1 at 600 °C. Due to the dynamic strain aging (DSA), the tensile stress-strain curve appears serrated flow phenomenon and shows negative strain rate sensitivity (SRS), which means the tensile strength of the material does not increase significantly with an increase of the strain rate, and even decreases. Meanwhile, it was found that peak tensile stress and inelastic strain increase in CF tests under low strain rates. The CF life of 316H steel was predicted using the hysteresis energy method. At the same time, based on microstructure and fracture observation, different dominant damage mechanisms were discussed.

Study on Creep-Fatigue Mechanical Behavior and Life Prediction of Ti2AlNb-Based Alloy.

Low-cycle fatigue, creep and creep-fatigue tests of Ti2AlNb-based alloy were carried out at 550 °C. Compared with low-cycle fatigue, a creep-fatigue hysteresis loop has larger area and smaller average stress. The introduction of creep damage will greatly reduce the cycle life, and change the fatigue crack initiation point and failure mechanism. Based on the linear damage accumulation rule, the fatigue damage and creep damage were described by the life fraction method and the time fraction method, respectively, and the creep-fatigue life of the Ti2AlNb-based alloy is predicted within an error band of ±2 times.