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This article predicts bending very high cycle fatigue (VHCF) life of three typical nickel‐based alloys SM2550, BG2532, and G3 used for completion strings. Fatigue tests were conducted on the three alloys using an ultrasonic fatigue system at a frequency of 20 kHz. The results showed that the fatigue strength ranges of the three alloys were markedly different, reflecting their different sensitivities to fatigue loading. Scanning electron microscope observations revealed numerous fatigue crack origins with internal decohesion in the fatigue source region. To achieve unified prediction of the fatigue life for the three alloys, a prediction model based on deep learning was built with inputs including fatigue initiation quantity, cleavage facet size, and other fatigue fracture characteristics. It was found that single source feature was insufficient to obtain satisfactory prediction accuracy for all alloys, while multifeature coupling integration could significantly improve the prediction precision, enabling reliable prediction of alloy fatigue life. This study provides new insights into bending VHCF life prediction. Highlights: This article predicts bending VHCF life for three completion strings.Bending VHCF life model utilizing deep wise separable convolution was established.Deep learning can effectively integrate with bending VHCF analyses. [ABSTRACT FROM AUTHOR]

Due to their distinct physical, chemical, and mechanical features, high-entropy alloys have significantly broadened the possibilities of designing metal materials, and are anticipated to hold a crucial position in key engineering domains such as aviation and aerospace. The fatigue performance of high-entropy alloys is a crucial aspect in assessing their applicability as a structural material with immense potential. This paper provides an overview of fatigue experiments conducted on high-entropy alloys in the past two decades, focusing on crack initiation behavior, crack propagation modes, and fatigue life prediction models. [ABSTRACT FROM AUTHOR]

During steam turbine rotor operation, the startup and showdown processes can cause asymmetric loading stress, leading to fatigue damage. To research the cyclic deformation behavior and fatigue failure mechanisms of 9% Cr‐based steel under asymmetric loading conditions, the strain ratios including −1, 0, and 0.1 are introduced into the low cyclic fatigue tests at 430°C. The results show the cyclic softening behavior is observed under three strain ratios at 430°C. As the strain ratio increases, the crack initiation sources, plastic strain, and consumed energy present an increasing trend. Comparing the microstructure morphology at different strain ratios, the coarsening lath and carbides affect fatigue crack initiation. Besides, the grain rotation and dynamic recrystallization can be observed, resulting in the cyclic softening behavior during the fatigue test. Finally, a new fatigue life prediction model is proposed at 430°C. Highlights: The low cycle fatigue tests are conducted under R = −1, 0, and 0.1 at 430°C.The dynamic recrystallization is observed, resulting in the cyclic softening behavior.A fatigue life prediction model considering the strain ratio is proposed at 430°C. [ABSTRACT FROM AUTHOR]

Symmetrical high‐temperature low‐cycle fatigue tests were performed in this study to examine the influence of temperature on the fatigue failure mechanism of the additively manufactured Inconel 625. According to the fracture analysis, cracks initiate from the crystallographic plane at room temperature. At 600°C, oxidation of carbides leads to crack initiation. The strengthening effect of γ″ precipitation prevents crack propagating within the matrix. At 750°C, the crack becomes transgranular propagating, as a consequence of γ″‐δ transformation. Geometrically necessary dislocations distribution on the fracture supports this result. Furthermore, a modified life prediction formula considering effect of temperature and strain amplitude was established. It was observed that the modified model predicted the fatigue life of the studied materials well in both test conditions. Highlights: Cracks initiate from favorable slip plane at room temperature.Precipitation of γ″ at 650°C prevents dislocation shearing into the matrix.Cyclic plastic deformation accelerates the γ″‐δ transformation at 700°C.Life prediction model considering strain amplitude and temperature shows good accuracy. [ABSTRACT FROM AUTHOR]

In this study, the main purpose is to analyze the fatigue failure of thermoplastic polyurethane (TPU) plate under tension-tension load control tests (frequency = 5 Hz, stress ratio = 0.1) and consider the change in hydrogen bond content. The results show that the S-N curve of TPU material shows a downward trend before reaching the fatigue limit (10.25 MPa), and the energy is continuously consumed during the cyclic creep process and undergoes three stages of the hard segment and the soft segment changes. The infrared spectrum study shows that the increase in fatigue life will lead to more physical crosslinking, resulting in the reduction of hydrogen bond content, and the increase in microphase separation, leading to the occurrence of fatigue fracture. In addition, the scanning electron microscope and three-dimensional confocal analysis showed that the crack originated from the aggregation of micropores on the surface of the material and was accompanied by the slip of the molecular chain, the crack propagation direction was at an angle of about 45°. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests were performed on Ti60 titanium alloy up to a very high cycle fatigue (VHCF) regime at various stress ratios to investigate the characteristics. The S-N curves showed continuous declining trends with fatigue limits of 400, 144 and 130 MPa at 109 cycles corresponding to stress ratios of R = −1, 0.1 and 0.3, respectively. Fatigue cracks found to be initiated from the subsurface of the specimens in the VHCF regime, especially at high stress ratios. Two modified fatigue life prediction models based on fatigue crack initiation mechanisms for Ti60 titanium alloy in the VHCF regime were developed which showed good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Notch and size effects show vital influence on fatigue performance of engineering components, which deserve particular attention for ensuring their structural integrity and reliability. In this study, the influences of size effect and stress gradient on effective stress as well as fatigue life were studied. Specifically, the effective stress methods were integrated with stress gradient, by which the relationship between stress gradient and size effect was discussed. In addition, a new stress gradient-based method for notch fatigue analysis was proposed by coupling the Weibull model with critical distance theory. Fatigue tests on Ni-based superalloy GH4169 notch specimens with different sizes were conducted at 650 °C, and experimental data of Al 2024-T351 alloy and low carbon steel En3B notch specimens were utilized for model validation and comparison. Results indicate that the proposed method provides more accurate fatigue life predictions than other effective stress methods. [ABSTRACT FROM AUTHOR]

A turbine engine blade alloy (Ti‐8Al‐1Mo‐1V) under different heat treatment conditions was employed to conduct ultrasonic fatigue tests to investigate the influence of the colony content of bimodal titanium alloys on the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) behavior. As a result, the increase in the content of colonies or the decrease in the content of primary α grains (αp) can be achieved by a double‐annealed treatment below the α + β to β transformation temperature. A higher colony content will result in a higher HCF and VHCF properties. The crack initiation life has a lower sensitivity to the microstructure change in the αp/colony content than the fatigue crack growth life. With the fatigue life increasing from the HCF regime to the VHCF regime, the impact of the microstructure evolution on fatigue resistances gradually decreases. Irreversibly, slips mainly along the maximum shearing stress direction make the plastic strain accumulate within some favorably oriented αp grains and further cause the nucleation of fatigue micro‐cracks. The formation of the fine granular area is attributed to the comprehensive effects of the vacuum‐enhanced plasticity and the numerous cyclic pressing. [ABSTRACT FROM AUTHOR]

Purpose: Most supersonic aircraft were manufactured using 2A70 aluminum alloy. The purpose of this paper is to study the corrosion mechanism and fatigue behavior of an aircraft in a semi-industrial atmospheric corrosive environment, alternating effects of corrosion and fatigue were used to simulate the aircraft's ground parking corrosion and air flight fatigue. Design/methodology/approach: For this purpose, the aluminum alloy samples were subjected to pre-corrosion and alternating corrosion-fatigue experiments. The failure mechanisms of corrosion and corrosion fatigue were analyzed using microscopic characterization methods of electrochemical testing, X-ray diffraction and scanning electron microscopy. Miner's linear cumulative damage rule was used to predict the fatigue life of aluminum alloy and to obtain its safe fatigue life. Findings: The results showed that the corrosion damage caused by the corrosive environment was gradually connected by pitting pits to form denudation pits along grain boundaries. The deep excavation of chloride ions and the presence of intergranular copper-rich phases result in severe intergranular corrosion morphology. During cyclic loading, alternating hardening and softening occurred. The stress concentration caused by surface pitting pits and denudation pits initiated fatigue cracks at intergranular corrosion products. At the same time, the initiation of multiple fatigue crack sources was caused by the corrosion environment and the morphology of the transient fracture zone was also changed, but the crack propagation rate was not basically affected. The polarization curve and impedance analysis results showed that the corrosion rate increases first, decreases and then increases. Fatigue failure behavior was directly related to micro characteristics such as corrosion pits and microcracks. Originality/value: In this research, alternating effects of corrosion and fatigue were used to simulate the aircraft's ground parking corrosion and air flight fatigue. To study the corrosion mechanism and fatigue behavior of an aircraft in a semi-industrial atmospheric corrosive environment, the Miner's linear cumulative damage rule was used to predict the fatigue life of aluminum alloy and to obtain its safe fatigue life. [ABSTRACT FROM AUTHOR]

Purpose: In summary, it can be found that the current research on the simulation of natural atmospheric dry–wet alternating accelerated corrosion mainly focused on the study of electrochemical corrosion process and the study of corrosion rate; the micro-pre-corrosion mechanism of materials in this environment, especially for materials. The specific effects of fatigue and fracture performance still lack detailed research. Accordingly, this study aims to more realistically simulate the effect of natural atmospheric corrosion environment on the corrosion resistance and fatigue performance of aircraft skin. Design/methodology/approach: In this study, the uniaxial strain control method was used to test the fatigue performance of pre-corrosion samples under simulated natural atmospheric corrosion using MTS809 tensile-torque composite fatigue machine. Scanning electron microscopy, X-ray energy spectrum analysis, atomic force microscopy and X-ray diffraction analysis were used. Fatigue fracture, corrosion morphology and corrosion products were analyzed. Findings: The results show that the deep corrosion pit caused by pre-corrosion environment leads to multi-source initiation of crack; the fatigue life of pre-corroded sample decreases by about one-half, chloride ion invades the material and promotes intergranular corrosion; life prediction results show that the natural atmospheric corrosive environment mainly affects the plastic term in the Manson–Coffin formula resulting in a decrease in fatigue life. Originality/value: Innovative experimental schemes and materials are used and the test temperature and relative humidity are strictly controlled. The corrosion failure mechanism of 2A70-T6 aluminum alloy under alternating wet and dry accelerated corrosion environment and its influence on fatigue behavior were obtained. [ABSTRACT FROM AUTHOR]

When high‐strength steels are subjected to very high‐cycle fatigue loading, crack initiation site shifts from surfaces to the interior, and a fish‐eye forms on the fracture surface. Majority of the fatigue life is estimated to be associated with the formation of this internal crack morphology. In the present work, features of such internal cracks in two high‐strength steels are studied. Specifically, three initiation patterns are investigated. A general internal crack initiating scenario is proposed base on an understanding of dislocation slip in the materials. A simplified threshold is calculated from Young's modulus and interatomic spacing, defining the transition from the initiation stage to the crack propagation. The relationship between internal crack initiation and slower descending S‐N curves is discussed. [ABSTRACT FROM AUTHOR]

In the present work, a probabilistic framework for fatigue life prediction and reliability assessment of an engine high pressure turbine disc is proposed to incorporate the effects of load variations and mean stress, which provides a reference for engine structural design under a given target failure probability. Within this framework, a new probabilistic fatigue damage accumulation model under random loadings is elaborated based on a ductility exhaustion model, and probabilistic S-N curves for the high pressure turbine disc under different flight missions are derived based on experimental data of turbine disc alloy GH4169. The influence of random load variations on fatigue reliability of the high pressure turbine disc has been investigated and quantified by combining the engine load spectrum with finite element analysis. [ABSTRACT FROM AUTHOR]

The effect of temperature on tensile behavior, low cycle fatigue, and microstructure evolution considering the damage mechanisms of Cr-based heat-resisting steel was investigated. The results present that significant monotonic and cyclic softening was observed at both room temperature and 630 °C. However, the cyclic softening factor is independent of the strain amplitudes and temperatures. At room temperature, the steady fatigue behavior until 80% fatigue life was presented, by contrast, great changes were offered below 80% fatigue life at 630 °C. this is directly related to the microstructure properties during fatigue tests, where dislocation network, wall, and dynamic recovery were formed, resulting from the dislocation movement, interaction, and annihilation. The density of dislocation and intensity of slip is inversely proportional to the temperature. Besides, a prediction fatigue life model fitted by the quadratic function was established through the hysteresis energy, which can consider the effect of temperature on the different fitting parameters and offer the method to optimize the fatigue behavior of Cr-based steel at different temperatures Unlabelled Image • The low cycle fatigue of 9Cr at room temperature and 630 ℃ were performed to evaluate cyclic behavior and microstructure. • The softening behavior is observed at both temperatures. The higher the temperature, the great the softening behavior. • The dynamic recovery happened due to the dislocations annihilation and low angle boundary disappearance. • The higher the temperature, the lower the density of dislocation and the greater intensity of slip. • A prediction model of low cycle fatigue considering the temperature was constituted and confirmed. [ABSTRACT FROM AUTHOR]

The electron beam welding process is widely used in the connection among titanium alloy material parts of aero-engines. Its mechanical properties need to meet the requirements of long life and high reliability. In this paper, the static strength and the fatigue failure behavior of the electron beam weldments of TC17 titanium alloy were investigated experimentally under low amplitude high frequency (20 kHz), and the mechanical response and failure mechanism under different external loading conditions were analyzed. In summary, the samples were found to have anisotropic microstructure. The tensile strength of the PWHT of TC17 EBW joint was ~4.5% lower than that of the base metal. Meanwhile, compared with the base metal, the fatigue strength was reduced by 45.5% at 109 cycles of fatigue life. The fracture analysis showed that the fatigue failure of the welded joint of TC17 alloy was caused by the welded pores and the fatigue cracks initiated from the welded pores. A fine granular area (FGA) was observed around the crack initiation region. The existence of pores caused the stress intensity factor of the fine granular area (KFGA) to be inversely proportional to the fatigue life. The KFGA calculation formula was modified and the fatigue crack propagation threshold of the welded joint of TC17 alloy was calculated (3.62 MPa·m1/2). Moreover, the influences of the effective size and the relative depth of the pores on the very long fatigue life of the electron beam welded joint of TC17 titanium alloy were discussed. [ABSTRACT FROM AUTHOR]

This paper presents the results of experimental investigations on fatigue behaviors of composite girders with corrugated steel webs (hereafter referred to as “composite girders”) subjected to bending based on the experimental work on the shear connector and corrugated steel beam. The experimental results indicated that failure started with cracks initiation and propagation at the concrete bottom plate, with the ultimate failure mode characterized as the shear failure on the bottom plate and interfacial slippage between the web and bottom plate. There was no obvious reduction in stiffness when the fatigue life approached 6 × 10 6 (equiamplitude load) or 2 × 10 6 (variable amplitude load) cycles, and the cracks grew slowly, which exhibited good fatigue resistance under the cyclic loading at quasi-static load. In addition, the cracks initiation location and fatigue life are predicted based on the fatigue damage theory, which shows good agreements with test results. The C degree of American standard AASHTO2004 is also suggested to be used in the fatigue life design of corrugated steel web in composite girders. The Palmgren-Miner cumulative linear damage theory is adopted in the partial fatigue life assessment, the fatigue life of box girder subjected to variable amplitude cyclic load is predicted when D exceeds 1. In addition, the fracture extension theory by Paris and amplitude theory of stress intensity factors are summarized to assess the fatigue life, which can be good evidence contributing to the engineering practice. [ABSTRACT FROM AUTHOR]

• A novel neural network model is designed to predict the low-cycle fatigue life under thermal–mechanical loading. • The model leverages the advantage of CNN and Transformer from taking binarized hysteresis images as input. • The proposed model is not sensitive to the crystal structure (BCC and FCC) and has a good accuracy. In this study, a novel neural network model is designed to predict the low-cycle fatigue (LCF) life under thermal–mechanical loading. The model is composed of three sub-networks: (1) a Convolutional Neural Network (CNN), (2) a Transformer, and (3) a Fully Connected Neural Network (FCNN). These three sub-networks function as the Backbone, Neck, and Head of the model, respectively. The model, named ConTrans (an abbreviation for Con volutional Neural Network- Trans former), utilizes binarized hysteresis images as input. The predictive capability of the ConTrans model for LCF life has been validated using experimental data from four different materials. The results demonstrate that almost all predictions fall within the 2-factor band, confirming the model's accuracy and effectiveness. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel fatigue life prediction model is established by combining critical distance theory and crystal plasticity theory. • Probabilistic fatigue life prediction of GH4169 notched specimens is conducted. • Microstructural and geometrical size effects on critical distance and fatigue life are investigated. Geometrical discontinuities are inevitable in engineering structures due to various functional requirements, which often yield complex stress–strain state and thereby significantly impact their fatigue performances. In this study, crystal plasticity theory is coupled with traditional critical distance theory for notch effect modelling considering microstructural heterogeneity. Specifically, microstructural and geometrical size effects on critical distance and fatigue performance are studied based on Sub-modelling approach. In addition, critical distance theory is coupled with Weibull distribution for probabilistic fatigue life evaluation of notched components. The developed method is verified and compared by using experimental data of GH4169 alloy. The findings demonstrate that the evaluated P-S-N curves exhibit strong agreement with tested data and geometrical size effect poses larger influence than the grain size effect on critical distance and fatigue life. [ABSTRACT FROM AUTHOR]

This study focuses on the fatigue durability of recycled concrete corrugated steel web composite box girders (RAC-CSWs). The parameters of recycled coarse aggregate (RCA) replacement ratios, i.e. 0%, 25%, 50%, 75%, 100%, and shear-to-span ratio (0.97, 1.46, 1.96, 2.43) were investigated on the nine specimens of RAC-CSWs subjected to four-point bending variable amplitude fatigue. The variable fatigue life, deformation properties, and fatigue damage mechanisms were explored. The results indicated that the shear-to-span ratio is the key parameter of the fatigue survival of RAC-CSWs. The RAC-CSWs exhibited a more complex crack distribution and a relatively larger deformation than the CSWs with natural coarse aggregate (NAC) during the test. For the specimens under variable amplitude fatigue life, the maximum fatigue life decrease of 31.4% was found on the RAC-CSWs with a 100% RCA replacement ratio, and a minimum of 10.2% was found on the specimens of RAC-CSWs with a 75% RCA replacement ratio. In addition, the effect of the shear-to-span ratio on the stress concentration of RAC-CSWs was investigated, which showed that a decrease of 57.5%, 79.4%, and 87.7% on fatigue life of RAC-CSWs was found when the shear-to-span ratio increased from 0.97 to 1.46, 1.96, and 2.43, respectively. Finally, the fatigue life of RAC-CSWs was evaluated based on the research of fatigue damage and finite element analysis (FEA), which shows a good agreement compared to the experimental results. • Proposed Recycled Concrete-Corrugated Steel Web Composite Girders, Pioneering Sustainable Use of Recycled Concrete. • Studied the Impact of Variable Amplitude Fatigue Loading on Recycled Concrete Web Girders. • Evaluated Fatigue Life through Combined Experimental, Finite Element Simulation, and Fatigue Damage Theory. • Emphasized the Necessity of Fatigue-Resistant Design in Green Infrastructure. [ABSTRACT FROM AUTHOR]

• Back stress hardening improves the fatigue performance at room temperature. • Stress-assisted second phase precipitation inhibiting the back stress hardening. • Life prediction model considering grain orientation shows high accuracy. During hot end components service, i.e., startup and shutdown processes, the cyclic stress caused by the asymmetry loading conditions is introduced. To study the effect of mean stress on the cyclic response, the low cycle fatigue, failure mechanism and life prediction model of additively manufactured (AMed) Inconel 625 superalloy at room temperature (RT) and service temperature (6 0 0 ° C) with strain ratio R = −1 and 0.1 are conducted. Experimental results indicate that the different cyclic softening and hardening behavior at RT and 600 ° C is the result of competition between dislocation slip, back stress hardening and second phase precipitation. According to the evaluation of the internal stress variables, the long-range back stress plays a crucial role to control the cyclic behavior of the alloy. At RT, the heterogeneous grain structure introduced by additive manufacturing causes the back stress hardening, making the material exhibit high fatigue performance. But this hetero-deformation induced (HDI) strengthening is inhibited at high temperatures due to the uniform distribution of dislocations. In addition, a low cycle fatigue life prediction model based on critical plane method is established which presents an easily assess to evaluate the low cyclic fatigue life of an AMed material under various test condition. [ABSTRACT FROM AUTHOR]

Preventing or mitigating very high cycle fatigue (VHCF) damage and failure of completion string is essential for ensuring their long-term stable operation. Therefore, a comprehensive understanding of the string's fatigue performance in tension, compression, and bending over an extended service life is necessary. This study investigated the evolution, microscopic mechanisms, fracture behavior, and life prediction of VHCF damage in completion string under axial and bending conditions. It was found that the intragranular twin structures on the critical interface of VHCF source directly participate in lattice reconstruction, enhancing the grain's strain coordination ability, and increasing the proportion of facet area on the fatigue fracture surface. Based on the fracture characteristics of VHCF, the Mayer empirical formula was improved, and a novel convolutional neural network (CNN) based VHCF life prediction model was proposed. Utilizing the CNN's powerful fracture feature extraction and nonlinear modeling capabilities, the VHCF fatigue life prediction accuracy was significantly enhanced, with prediction errors all within 2 times the experimental results' error range. Based on the axial and bending VHCF fracture mechanisms of completion string and CNN, a more advanced method for predicting axial and bending VHCF life is provided, offering theoretical supports for the engineering safety service and life assessment of completion string. • The structural reconstruction in VHCF fatigue source region will delay fatigue initiation and propagation. • Lifetime prediction of VHCF damage in completion string under axial and bending conditions was investigated. • Convolutional neural networks provide a more advanced VHCF lifetime prediction method. [ABSTRACT FROM AUTHOR]

• A dual-scale modelling approach combining sub-modelling approach with crystal plasticity constitutive model. • Efficient procedure for fatigue life prediction of anotch parts using the Tanaka-Mura-Wu model and Paris law. • The microstructural size effect on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Fatigue performances of critical structures are strongly affected by the microstructural (e.g. grain, defect, inclusion, etc.) size effect, and it is thus important to quantify their detrimental effect. In this work, a numerical procedure is constructed to quantify the influence of microstructure on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Specifically, by combining sub-modelling approach with crystal plasticity constitutive model, a dual-scale modelling approach is developed for studying grain-level mechanical behavior of Ni-based superalloy GH4169 notched components. In addition, the dislocation-based Tanaka-Mura-Wu model is applied for fatigue crack initiation life prediction. The Paris law is then utilized for fatigue crack propagation analysis based on the simulated short crack. To study the microstructural size effect on fatigue crack initiation and propagation behaviors, sub-models containing various grain orientation, grain size, defect size/shape are built and analysed. Finally, a series of fatigue tests on notched specimens of Ni-based superalloy GH4169 were carried out for method validation. Results indicate that the established dual-scale modelling approach and fatigue life prediction framework yields good agreement with experimental results. [ABSTRACT FROM AUTHOR]

In the present work, the fretting fatigue behaviors of GCr15 steel (Chinese GB/T-standard) have been studied up to the very high cycle fatigue (VHCF) regime by using an ultrasonic fatigue system. The main aims of the investigations were the analyses of S–N curves, worn morphology, as well as fracture morphology. As expected, the results of the fretting fatigue tests show a reduction in the lifetime compared to plain fatigue tests without fretting. 2-D and 3-D morphologies of the fretting worn surfaces are combined to study the evolution of the stick zone. With the increasing fatigue life, the stick zone will fully occupy the whole worn zone. Besides, fretting causes severe oxidation and debris in the worn surface. The crack initiation depth increases with the increase in fatigue life, especially in the VHCF regime, the depth increases drastically because of fretting delamination. • There exists no "traditional fatigue strength" for GCr15 steel in the vicinity of 107 cycles in fretting fatigue.. • Fretting fatigue strength is much lower than plain fatigue's and the strength reduction factor varied from HCF to VHCF. • Fatigue cracks nucleate in the stick zone in the VHCF regime, which is different to the previous two existing boundaries. • Fretting promote the crack initiation, especially in the VHCF regime. [ABSTRACT FROM AUTHOR]

[Display omitted] • Combining the low-cycle fatigue life of nickel-based alloys, three types were selected for characterization. • Application of Pearson's correlation coefficient and Pearson's correlation coefficient for feature screening. • Combined fatigue dataset for multiple nickel-based alloys, and predictions for multiple types of nickel-based alloys. • The machine learning model was compared to the traditional Coffin-Manson model, and it yielded favorable results. A data-driven machine learning method for the low-cycle fatigue (LCF) life prediction of Nickel-based superalloys is proposed to overcome the limitations of empirical formulas. The method integrates three characteristics of the superalloys: chemical composition, heat treatment process and experimental parameters, and constructs a unified fatigue life prediction dataset. The relationship between many different microstructural parameters and LCF life is analyzed using the Pearson Correlation Coefficient (PCC) and Maximal Information Coefficient (MIC). The fatigue life dataset is then used to predict the LCF life with four machine-learning models: Random Forest (RF), Artificial Neural Network (ANN), Support Vector Machine (SVM), and a Genetic Algorithm-based Random Forest (GA-RF). The comparative study results demonstrate that the models accurately predict the LCF life, as evidenced by the respective R2 values of 0.8311, 0.8345, 0.7711, and 0.9272, with GA-RF performing the best and equally better in comparison with the Coffin-Manson model. The proposed method efficiently maps feature-life relationships for the LCF life of Nickel-based superalloys, reducing experimental time and cost and promising applications in the inverse design and manufacture of alloys. [ABSTRACT FROM AUTHOR]

• A new life prediction model based on the strain energy reformulated critical distance theory is proposed. • Experimental data of 4 materials verified the model predictions accuracy. • The influence of size effect on critical distance and fatigue life is quantified. Notch and size effects show significant impact on the fatigue performance of engineering components, which deserves special attention. In this work, strain energy reformulated critical distance theory was developed for fatigue life prediction of notched components under size effect. Experimental data of different notched specimens manufactured from GH4169, TC4, TC11 alloys and low carbon steel En3B were used for model validation and comparison. Results indicate the proposed model works better than the Yang's and the Shen's models. [ABSTRACT FROM AUTHOR]

• Two physics-guided machine learning (ML) frameworks for fatigue life prediction of AM materials; • The hybrid ML model based on the morphological details of critical defects for fatigue life prediction; • In serial physics-guided ML framework, ANN presents better prediction accuracy compared with SVR; • Under limited data condition, the serial physics-guided ML framework with ANN is recommended. Introducing random defects is a type of the dominant causes of fatigue scatter of additive manufacturing (AM) materials. The fracture mechanics-based models oversimplify the fatigue damage process and ignore the key influencing factors. Given data-driven nature of machine learning (ML), sufficient samples are required to train, which generally cannot be met in practice. In view of this, this work develops two physics-guided machine learning frameworks which combine physics-based models and ML algorithms, for improving the prediction ability. In the proposed framework, ML models can consider those factors ignored by physics-based models and physics-based models can ensure the consistency with physical results. Finally, fatigue data of three AM materials are used for model evaluation and comparison. The results show that, physics-based models lack sufficient explanatory power for scatter of fatigue life. Moreover, compared with the purely data-driven methods, the proposed framework can maintain high accuracy and alleviate the over fitting phenomenon under insufficient fatigue data sources. [ABSTRACT FROM AUTHOR]

• The high cyclic fatigue tests of 9% Cr steel are conducted under R = −1, 0.1, 0.5, and 0.7 at 630 °C. • The mean stress sensitivity factor (M R) is proposed and related to the stress ratio. • A new fatigue life empirical relationship considering the mean stress sensitivity factor (M R) is proposed with reasonable confidence. During the startup and shutdown processes of the steam turbine rotor, the fatigue failure behavior caused by asymmetric stress is introduced. To study the effect of mean stress on the fatigue failure behavior of 9% Cr steel at high temperatures, the high cyclic fatigue tests are conducted under different stress ratios (R = −1, 0.1, 0.5, and 0.7) at 630 °C. The results indicate three fatigue failure modes: surface crack initiation failure, sub-surface crack initiation failure, and necking induced failure. With the increasing stress ratio, the surface and sub-surface crack initiation failure modes gradually change to necking induced failure mode. Moreover, the oxide intrusive-extrusion mechanism primarily determines the surface crack initiation failure and promotes surface defect formation. Meanwhile, the inhomogeneous microstructure induces sub-surface crack initiation failure, wherein the crack propagates along the martensitic lath boundary or through the martensitic lath. The attenuation of mechanical properties and the increase in mean stress level are the critical factors affecting the necking induced failure under 630 °C. In addition, the mean stress sensitivity factor (M R) is proposed to explain the effect of the stress ratio on fatigue performance. Furthermore, the mean stress sensitivity factor (M R) can be used to quantitatively analyze the effect of mean stress on fatigue properties under different stress ratios. Finally, the fatigue life empirical relationship considering the mean stress sensitivity factor (M R) is constituted. [ABSTRACT FROM AUTHOR]

• A new definition of non-proportional degree on generalized plane for non-proportional loading; • A new model without additional material constants for fatigue life assessment under non-proportional loadings; • Experimental data of three materials under different loading paths verified model predictions accuracy. Engineering components often operate under complex loadings, in which the variable amplitude multiaxial stresses are raised by geometric discontinuities including holes, grooves, fillets and shoulders, etc. Besides, the non-proportional loading will lead to the rotation of maximum principal stress/strain and additional fatigue damage of structural elements in service. Consequently, the multiaxial and non-proportional loading have attracted increasing attentions. In this study, for distinguishing the effects of different load paths on fatigue life, a simple and applicable method to quantify the non-proportionality is defined, which comprehensively considers the influence of loadings on all material planes. A new equivalent strain damage parameter based on critical plane is proposed in this study. Specifically, to quantify the effect of non-proportional loading on fatigue damage, a non-proportional degree of loading on the generalized plane is developed. Coupling with the existing non-proportional coefficient, a novel fatigue damage parameter is derived by combining a non-proportional factor. Experimental data of 304 stainless steel, sintered porous iron and CuZn37 brass are utilized for model validation and comparison. Fatigue lives with different load paths are evaluated respectively. In comparison with the Fatemi-Socie (FS), Chen and Itoh models, proposed method more effectively evaluate the multiaxial fatigue life of materials under non-proportional loadings. [ABSTRACT FROM AUTHOR]

• A calibrated weakest-link theory considering the defects location and size effect for probabilistic fatigue assessment of defective materials; • A surface factor for fatigue modelling under surface and interior-defects; • A size sensitivity factor for characterizing the sensitivity of fatigue strength to the size/volume change. Fatigue life prediction is critical for the design of engineering components made from defective materials. Not only do these imperfections show a detrimental effect on the fatigue performances, but they also contribute to a considerable scatter of fatigue properties. In this work, a probabilistic fatigue life prediction model under size effect was proposed based on the calibrated weakest-link theory (WLT) considering the defect location and the size sensitivity. Specifically, a surface factor was defined to consider the superiority of the surface-defect. In addition, a size sensitivity factor was developed to capture the sensitivity of fatigue strength to the volume/size change. Model validation and comparison using two types of metallic materials indicated that the developed surface factor and size sensitivity factor improve the predictive ability of the traditional WLT. [ABSTRACT FROM AUTHOR]

• A calibrated weakest-link theory for probabilistic notch fatigue assessment of three alloys considering size effect; • A probabilistic framework for fatigue life prediction of notched components using Weibull model; • Experimental data of three alloys and a compressor disk verified model predictions accuracy. Notch and size effects exerts significant influence on the fatigue performances of engineering components. In the present study, a probabilistic framework for fatigue life prediction of the notched components is proposed based on the calibrated weakest link theory considering size effect. Specifically, a probabilistic method considering the notch effect is elaborated utilizing FE analysis, which inherently considers the size effect and can be used for fatigue life prediction of components with arbitrary shapes. Model validation and comparison are carried out using experimental data of GH4169, TC4 and TC11 alloys with various specimen geometries as well as a full-scale compressor disc. Results indicate that the proposed model presents better prediction accuracy than the other three traditional methods. [ABSTRACT FROM AUTHOR]

• Recent advances on stress gradient effect in metal fatigue are reviewed. • Fatigue methods considering stress gradient effect are compared using experimental data of two materials with different notch types. • Further directions on notch fatigue assessment under stress gradient effect are given. Under external loadings, notched components always show varying degrees of stress concentration. Accordingly, different kinds of approaches have been presented to characterize notch fatigue behaviours under complex loading histories. Among them, the considering of the stress gradient effect is a popular perspective, together with the fatigue damage zone and critical distance concepts. This work systematically reviews recent advances on notch fatigue analysis considering the stress gradient effect. Specifically, four approaches are summarized and explored, namely the local stress/strain-based approaches, critical distance-based approaches, critical plane-based approaches, and weighting control parameters-based approaches. This review aims to provide a systematic review and comparison on these methods, therefore to offer a reference for subsequent research on notch fatigue. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests were performed on friction stir welded AA6061 joint to investigate very high cycle fatigue (VHCF) behaviors. As a result, almost all the fatigue cracks are initiated from local plastic slip markings around the boundary between thermo-mechanically affected zone and heat affected zone. The fatigue strength decreases from the top to root of the welded joint, owing to the variation of plastic deformation history and temperature distribution through the thickness. In fractography, the fatigue crack initiation site is surrounded by a semicircular flat zone, of which the formation in VHCF regime accounts for more than 98% of the total fatigue life. [ABSTRACT FROM AUTHOR]

• A strain energy-based critical damage region is defined for fatigue modeling; • The effective strain energy density concept is utilized for fatigue analysis under notch and size effect; • A novel fatigue life prediction model combining weakest link theory with strain energy concept is proposed; • A general probabilistic procedure for notch fatigue modelling under size effect is presented; • Experimental data of GH4169, TC4 and TC11 alloys verified the model predictions accuracy. Notch fatigue analysis is vital for structural integrity design, while efficient fatigue models coupling notch and size effect are still lacking, which are highly desired for fatigue design of critical components. Accordingly, this study proposes a new probabilistic fatigue model for life assessment by combining the weakest link theory with strain energy concept. Specifically, a novel effective strain energy density concept is developed herein to build the connection between experimental data of smooth specimens and strain energy density of notched ones within the critical damage region. Experimental data of GH4169, TC4 and TC11 alloys with different geometries are utilized for model validation and comparison. Results indicate that the proposed model show higher accuracy than the other four models; in addition, the P – W a ¯ – N f curves describe the experimental data scatter effectively. [ABSTRACT FROM AUTHOR]

• Very long life fatigue behavior of TC17 welded joints at 400℃ was investigated. • Intergranular welded pores are responsible for fatigue failure. • The unique facets at 400℃ were discovered for TC17 welded joints. • The formation of facets is related to columnar grain and is driven by shear stress. The very long life fatigue failure mechanism of welded joints for titanium alloy at 400℃ has been investigated. The results show that the welded joint has a nonhomogeneous microstructure. The welded defects are mainly welded pores, and maximum pore sizes are evaluated by statistical methods. The increase in temperature reduces the fatigue strength and changes the shape of S-N curves for the welded joints. Crack initiation is attributed to the existence of welded pores. Crystallographic facets are found around welded pores on the crack initiation zone, and their formation is sensitive to columnar grains and is driven by shear stress at 400℃. [ABSTRACT FROM AUTHOR]

• Method for calculating the creep damage during LCF based on the equivalent creep stress was proposed. • Creep damage model considering a continuously changing stress was established. • Engineering application of this model was discussed. A method for calculating the creep damage during low cycle fatigue (LCF) (tensile plasticity reversed by compressive plasticity, PP) controlled by strain rate ε ̇ was proposed. According to the equivalent principle of creep damage with the same reduction in cross section, a creep damage model considering a continuously changing stress was established based on the equivalent creep stress σ en , which was determined by the total creep damage, the actual creep time (the total time of tensile loading stage in the whole process of fatigue), and the fatigue life during LCF (PP). For Sanicro 25 alloy, σ en first increases quickly, and then increases linearly and slowly with increasing ε ̇. Based on the creep damage model, the total creep damage and fatigue life under different ε ̇ were predicted accurately. To study the application of the method for calculating the creep damage, the creep damage model for 316 stainless steel was established and verified. At a constant ε ̇ , when γ decreases or K increases, the creep damage decreases and the fatigue life increases. Based on the variation trend, the engineering application of the creep damage model was discussed. [ABSTRACT FROM AUTHOR]

• The softening behavior has occurred under the strain and stress-controlled modes. • The tension-compression asymmetry under the stress-controlled mode is significant. • The dislocation density and pattern are related to the loading modes. • The prediction model of fatigue life under different loading modes is constituted. Low cycle fatigue response and damage mechanism of MarBN steel under symmetrical strain and stress-controlled modes is investigated at room temperature. Significant cyclic softening has been observed under both control modes, in which strain amplitude-independent and stress amplitude-dependent softening factors are presented, respectively. The tension-compression asymmetry is observed, which is the critical factor to control the cyclic fatigue resistance. The relationship between damage mechanism under both modes and microstructure is discussed. The effect of the tension-compression asymmetry on the prediction model of fatigue life is investigated based on an effective strain range-related model. [ABSTRACT FROM AUTHOR]

• A modified weight function considering the inhibition effect was proposed. • A novel strategy for determine the radius of fatigue failure region was explored. • The influence of size effect on fatigue failure region radius of notch specimens was characterized. • Experimental data of GH4169 superalloy, En3B steel and Al 2024-T351 alloy verified the model predictions accuracy. Notch features have shown significant effects on fatigue performance of engineering components, which deserve more attentions during structural integrity design and assessment. This work firstly revisits the stress field intensity (SFI) approach as well as its recent advances. Then, an improved weight function is developed to account for the inhibition effect, and yields a modified SFI approach for notch fatigue analysis considering size effect. In addition, the relationship between fatigue failure region (FFR) radius and fatigue life was explored and quantified. Fatigue tests on Ni-based superalloy GH4169 notch specimens with different sizes were conducted at 650 °C, and experimental data of En3B steels and Al 2024-T351 alloys were used for model validation and comparison. Results indicate that the modified SFI approach provides better predictions than the other two approaches. [ABSTRACT FROM AUTHOR]

• Low cycle fatigue and microstructure are investigated at different temperatures. • Single and multiple crack initiation sources are dependent on the temperature. • Softening behavior is associated with laths and dynamic recrystallization. • Fatigue life model is constituted based on the hysteresis energy model. Low cycle fatigue, tensile tests, and microstructure of MarBN steel at different temperatures are conducted. The results show that tensile strength decreases with increasing temperature. Continuous cyclic softening has occurred as increasing the temperature, which is associated with the laths and dynamic recrystallization at room temperature, and grain rotation at high temperature, respectively. The relationship between cyclic softening and microstructure at different temperatures is discussed. Besides, the fatigue life prediction model with hysteresis energy method considering the temperature is fitted by the quadratic function, which presents a good method and model to optimize the low cyclic fatigue life and properties. [ABSTRACT FROM AUTHOR]

When turbine steel services, i.e., startup and shutdown processes, the cyclic mechanical stress caused by the asymmetry loading conditions is introduced. To study the effect of mean strain on the cycle response and microstructure-sensitive at room temperature, the low cycle fatigue under two strain ratios was used. Experimental results indicate that significant cyclic softening was observed for both modes. The cyclic strain ratio is independent of strain amplitude and strain ratio. The plastic strain, plastic strain energy density, and cumulative strain energy density at strain ratio of 0.1 are higher than that at strain ratio of −1. Therefore, applied stress amplitude and fatigue life fitted by Manson-Coffin formula under strain ratio of 0.1 is lower than that at strain ratio of −1. According to the evaluation of the internal stress variables, i.e., back stress, isotropic stress and viscous stress under both modes, the back stress is a critical part of the cyclic stress, which can directly control the cycle softening behavior. Besides, the fatigue life prediction model considering the strain ratio is constituted through hysteresis energy method. Finally, the mechanical results are linked to the microstructure observed by transmission electron microscopy. The microstructure sensitive was evaluated and deduced. Unlabelled Image • The cyclic softening behavior exhibits at both strain ratios in low cycle fatigue. • The intragranular back stress is the key factor to control the cyclic softening behavior under different strain ratios. • A transition process of dislocation pattern occurs, resulting from incompatibilities of plastic strain due to mean strain. • Low cycle fatigue life prediction model considering the strain ratio has been constituted and confirmed. [ABSTRACT FROM AUTHOR]

It is suspected early that the ultra-slow crack propagation in fine-granular-area (FGA) may be sensitive to the internal vacuum-like environment. In other words, fatigue failure in the very-high-cycle regime may be related to the internal vacuum-like environment. Since magnesium alloy is more susceptible to the environmental effect, it is selected as the research carrier in this study. A magnesium alloy is investigated in ambient air and vacuum environments comparatively, and the vacuum loading environment offers a valuable reference to investigate the possible environmental effect. The results show that plastic deformation mechanism in different loading environments keeps the same at crack initiation site. However, the different loading environments not only change the fatigue life, but also the failure mode. At higher stress levels, surface failure dominates in air loading environment, whereas internal failure dominates in vacuum loading environment and presents longer fatigue life. It is considered that failure mode in vacuum loading environment is mainly affected by the critical grains, whereas that in air loading environment is mainly affected by the competition between the surface air acceleration effect and the critical grains. Fatigue life is mainly affected by the internal vacuum retarding effect and the surface air accelerating effect. The different loading environments not only change the fatigue life, but also change the failure mode. Unlabelled Image • The intrinsic fatigue behavior in vacuum loading environment. • Air acceleration effect promotes premature surface failure. • Different loading environments change the failure mode. [ABSTRACT FROM AUTHOR]

• Microstructural evolution along the depth direction presents a gradient feature. • Four types of deformation twins are introduced into primary α phases. • High-angle grain boundaries are increased by deformation twins (~13%). • Strengthening mechanisms of extra-long life fatigue resistance are revealed. Ultrasonic fatigue tests were performed on notched specimens to investigate high and very high cycle fatigue behaviors in the laser shock peened region of a compressor blade titanium alloy (Ti–8Al–1Mo–1V). The plastic deformation and compressive residual stress, induced by the laser shock peening (LSP), present gradient distributions along the depth direction. High-angle grain boundaries (HAGB, >15°) are increased by deformation twins, which are introduced into primary α phases during the plastic deformation. The enhancement in fatigue resistance is ascribed to the comprehensive retardation effects of the compressive residual stress, high-density dislocations, and HAGBs. [ABSTRACT FROM AUTHOR]

• The very high cycle fatigue strength of titanium alloy fabricated by additive manufacturing is much lower than that of the traditional process. • Combining with the weak zone, Δ K FGA is analyzed, and it is found that the size of the FGA plastic zone was similar to the width of the martensitic laths. • Failure critical dimension is obtained and a new fatigue life relationship is established based on defect characteristics. • Based on probability statistics, the influence of maximum defect size on fatigue behavior is discussed. Tensile and fatigue behaviors of selective laser melted titanium alloy were investigated in the as-built condition. The alloy has a high defect density, and its tensile properties are comparable to those of wrought alloys, but the fatigue strength is significantly reduced. The FGA plastic zone is similar to the martensite laths width. The critical size of fatigue failure is analyzed by the K-T diagram. A new value is obtained based on the defect characteristics. Using the probability statistical, the maximum effective size of the defect considering the shape was calculated, and the competitive relationship of the defects was analyzed by volume density. [ABSTRACT FROM AUTHOR]

Notch and size effects generally show great influence on the fatigue behavior of engineering structures, which plays a vital role during their structural integrity and reliability evaluations. In this study, the influence of notch size effect on fatigue life and critical distance values were investigated. Particularly, a novel method for fatigue life distribution assessment of notched specimens was proposed based on Weibull model and critical distance theory. Experimental data of Al 2024-T351 smooth plate specimens and center hole plate (CHP) specimens with four different notch radii were utilized for model validation and comparison. Results show that all predicted lives of Al 2024-T351 CHP specimens are within the ±2 life scatter bands of experimental ones. Unlabelled Image • Weibull model describes well the dispersity of fatigue lives of Al 2024-T351 alloys. • Influence of notch size effect on critical distance and fatigue strength were characterized. • Proposed model by coupling Weibull with critical distance theories for considering notch size effect. • Proposed model predictions of Al 2024-T351 center hole plate specimens are within ±2 life scatter bands of experimental ones. [ABSTRACT FROM AUTHOR]

• A modified generalized local model considering multiaxial stress and stress gradient is developed. • A novel probabilistic framework for notch fatigue analysis considering size effects is established. • Probabilistic fatigue life prediction of TA19 notched specimens with different scales are conducted. Structural integrity assessments with discontinuities are critical for ensuring operational life and reliability of engineering components. In this work, through combining with the generalized local model, a probabilistic framework is proposed for fatigue life assessment of notched components under size effects, in which the Smith–Watson–Topper damage parameter is utilized to characterize the multiaxial stress state at the notch tip. In particular, an effective stress concept is introduced to characterize the inhomogeneous stress distribution within the notch region. Finally, experimental data of TA19 notched specimens with different scales are utilized for model validation and comparison, results show that the proposed framework yields acceptable correlations of predicted fatigue lives with experimental ones. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Microstructure inhomogeneity governs the fatigue crack initiation mechanism. • Fatigue damages prefer to concentrate in the few larger grains due to cyclic slip deformations. • Internal failures present longer fatigue lives than surface failures due to the environment effect. The VHCF behaviors of an extruded magnesium alloy are investigated. Due to the microstructure inhomogeneity, fatigue damages prefer to concentrate in the few larger grains. Basal slip deformations are supposed to be responsible for the transgranular crack initiations, and form the similar faceted morphologies. Under same stress amplitude, internal failures present longer fatigue lives than surface failures due to the requirement of larger initial crack and the deceleration rate in the vacuum-like environment at specimen interior. [ABSTRACT FROM AUTHOR]

For local stress-strain behavior modeling at the notch root, both the Neuber's rule and equivalent strain energy density (ESED) method are commonly utilized in engineering practice, in which the former overestimates notch elasto-plastic responses while the latter gives conservative predictions. In this work, a correction coefficient related to the effective Poisson's ratio v eff is proposed to quantify the weight of dissipated heat energy term with the modified ESED model. Notch-root strains predicted by the proposed model are compared with nonlinear FE calculations of GH4169 and TC4 V-shaped notched-bars, in which the proposed solution provides good agreement with FE results for both cases. Then, consulting the idea of local stress-strain approaches, a fatigue lifing model is derived assuming that the notched specimens fail once reach the critical total strain energy as same as that of smooth ones. Results indicated that prediction accuracy of local equivalent strains at notch roots is improved, and yields better life predictions than the Ellyin's model for the two alloys. • A proposed solution to quantify the weight of heat energy term based on the modified ESED method. • Better correlations between the proposed model predictions and FEA results for TC4 and GH4169 alloys. • A modified energy-based model for life prediction considering the notch support effect. • Two types of V-shaped notch specimens with different K t for model validation. [ABSTRACT FROM AUTHOR]

Supersaturated coarsening β phase and submicron acicular martensitic α′ phase originated by the high cooling rate during the welding process resulted in the electron beam welded TC17 titanium weldment to be poor in strength and ductility. Post-weld heat treatment was performed at 630 °C for 2 h to improve the mechanical properties of electron beam welded TC17 titanium alloy joint through modifying the microstructures of the weldment. Results showed that undesired α′ martensites in the fusion zone and heat affected zone, which inhibited the mechanical response of the weldment, were completely eliminated. The supersaturated β phase with martensitic α′ structure in the as-welded joint was recrystallized into α lamella with a width in nanoscale embedded in the β matrix after the post-weld heat treatment, which conspicuously enhanced the mechanical performances of welded joints. Micro-hardness of the FZ presented to be more stable around HV410. Tensile strength was regained from 868 MPa to 1138 MPa, which was very closed to the raw material. Intrinsic metallic deficiency inducing fatigue failure of the as-welded joint was eliminated after the post-weld heat treatment accompanied with the fatigue strength enhancing from about 140 MPa of the as-welded joint to 240 MPa with an equal fatigue life of 107 cycles under the stress ratio of 0.1. Image 1 • Post-weld heat treatment was performed on the electron beam welded TC17 titanium alloy joint. • Martensitic α′ phase was sufficiently disintegrated and recrystallized into β phase with nano α lamellas. • The joint showed great micro-hardness stability after post-weld heat treatment. • Tensile and fatigue strength was significantly enhanced. [ABSTRACT FROM AUTHOR]

• Fretting fatigue behavior of bearing steel is investigated up to VHCF cycles. • Fretting fatigue strength reduction is higher in VHCF regime. • Fretting fatigue life in VHCF regime depends on the fretting damage size growth. • Higher fretting fatigue life can be achieved with lower wear rate and harder surface. The fretting fatigue behaviour of SAE 52100 bearing steel has been investigated up to very high fatigue cycles. The fretting fatigue strength was found to be markedly lower than the plain fatigue strength of the alloy. The fatigue strength reduction is higher in very high cycle fatigue regime which varies with fretting contact load. The fretting fatigue life is restricted in high cycle fatigue regime when the contact load is higher than the critical one. The rate of fretting damage growth influences the fretting fatigue life in very high cycle fatigue regime. The specimens with lower surface wear rate acquire higher fretting fatigue life by impeding the increase of fretting damage size. [ABSTRACT FROM AUTHOR]