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Copyright of Machine Tool & Hydraulics is the property of Guangzhou Mechanical Engineering Research Institute (GMERI) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The article explores the in-depth residual stress evolution of a surface-rolled titanium alloy subjected to fretting fatigue. The study aims to provide data for establishing an accurate fretting fatigue life prediction model by examining the surface residual stress distribution at different stages of fretting fatigue loading. The research highlights the complex stress distribution at the fretting wear zone and the impact of fretting wear on the evolution of residual stress, emphasizing the role of contact stress and fatigue stress in this process. The study suggests practical engineering approaches to mitigate residual stress release during fretting fatigue loading, such as using lubricants and employing techniques like induction hardening for surface strengthening. [Extracted from the article]

The high‐temperature fretting fatigue damage characteristics of dovetail specimens strengthened by nanosecond laser (NL) and nanosecond combined femtosecond laser (F‐NL) were investigated. The results show that the fretting fatigue life of the NL strengthened sample (NL sample) is improved by 211.2% compared to the base metal samples (BM sample). The lifetime of the nanosecond combined femtosecond strengthened sample (F‐NL sample) was increased by 319.6%. It was attributed to the strengthening introducing hardened layers, residual compressive stress field, and high density of dislocations. The combined strengthening process reduces the surface roughness of the NL strengthened surfaces, while the strengthening influence layer is further increased. The fracture morphology shows that the crack source of the strengthened specimen has changed from multi‐source sprouting to single‐source sprouting, and the crack source sprouted on the subsurface. The fatigue strip morphology similarly confirms that a reduction in the crack propagation rate occurs. Highlights: Dovetail specimens were subjected to fretting fatigue testing at 500°C.Composite laser peening reduces surface roughness and stress concentrations.Mechanism of fatigue property enhancement after combined laser peening revealed. [ABSTRACT FROM AUTHOR]

Fretting fatigue contributes significantly to the fatigue failure process in steel wire ropes at the wire‐to‐wire trellis contact region with partial slip conditions. In this respect, this work demonstrates a new damage‐based fretting fatigue model for the prediction of such a failure process. The model is based on Lemaitre's damage equations for quasi‐brittle material with a damageable micro‐inclusion embedded in an elastic meso‐element. It incorporates the cyclic degradation of the elastic modulus of the drawn steel wire material. The fatigue life model acknowledges the mean stress effect. The constitutive and damage equations are formulated into user material (UMAT) subroutine for integration with Abaqus finite element analysis code. The localized fretting fatigue damage mechanism is simulated with an isolated two‐wire model. The effect of the contact condition with the coefficient of friction of 0.2 and 0.8 on the contact mechanics of the drawn wires is considered. The fretting fatigue mechanism map is established for each simulation case. The simulated results of N0 (no of cycles to initiate damage) and damage variable, D, confirm the fretting fatigue condition as the damage occurs in the slip region of the contact area for both the frictional conditions. The results were found in agreement with the previously establish Ruiz fretting parameter. This study will provide a base for the onward reliability assessment of steel wire ropes. Highlights: A damage‐based Lemaitre two‐scale model has been modified for the fretting fatigue mechanism in the steel wire ropes.A unified curve of residual elastic modulus has been established through interrupted fatigue tests for different load ratios.A UMAT Subroutine is developed using ABAQUS software. The response of the damage model has been investigated using different parameters like Damage D, Residual Elastic Modulus E(N), Number of cycles to initiate damage N0, and von Mises stress at the contact region. [ABSTRACT FROM AUTHOR]

To reasonably forecast the crack initiation characteristics of fretting fatigue, this study evaluates the accuracy of several damage models. The existing experimental and simulation investigations are compared, noting the similarities and differences in the crack initiation location, crack orientation angle, and crack initiation life predicted by various damage models. It is crucial to employ a reasonable and reasonable damage prediction model of fretting crack initiation life, as fretting fatigue, a phenomenon of mechanically fastened connecting parts, can considerably reduce the fretting fatigue life of the components. This paper compares the fatigue damage prediction models of fretting fatigue specimens under partial slip conditions, and it will help researchers understand the benefits and drawbacks of various damage models and give them the knowledge and foundation to select the more reasonable fretting fatigue damage model. Highlights: A structured meshing technique and coupled constraint loading strategy are proposed.The cyclic characteristics of fretting contact parameters are analyzed.A normalized parameter for comparing the fatigue damage models is proposed.The accuracy of the different fretting fatigue damage models are compared. [ABSTRACT FROM AUTHOR]

Recently, a new dissimilar mechanical joining method, dimple spot welding (DSW), has been developed for joining steel sheets and aluminum alloys. In this study, tensile shear tests and fatigue tests were conducted on four types of dissimilar lap joints, with two types of steel sheets and two types of Al alloys joined by DSW. Both the tensile shear strength and fatigue strength of DSW joints were found equal to or greater than those of a self‐piercing riveting (SPR) joint. In all four types of DSW joints, fatigue cracks were initiated from the Al alloy and were associated with fretting induced by repeated contact and friction. A finite element analysis showed that the tangential stress was mainly governed by the friction coefficient and also affected by the steel sheet strength. Higher‐strength steel led to lower tangential stress, resulting in a longer fatigue life for the DSW joint. Highlights: A dimple spot welding (DSW) is developed for joining steel sheets and aluminum alloys.Tensile shear tests and fatigue tests are conducted on DSW joints and SPR joint.DSW joints shows greater tensile shear strength and longer fatigue lives than SPR joint.Fatigue cracks in DSW joints were initiated associated with fretting. [ABSTRACT FROM AUTHOR]

This paper evaluates the influence of contact pressure on the surface contact state, fretting damage degradation mechanism, crack initiation and propagation characteristics, and fretting fatigue life of A319‐T6 cast aluminum alloy in fretting fatigue test using bridge‐type fretting pad. For a certain range of contact pressure, the fretting fatigue life exhibits a trend of increasing and subsequently decreasing with increasing contact pressure. With finite element analysis, the fretting contact region can be divided into two characteristic zones according to the dynamic evolution of the contact status within one fatigue cycle. The contact pressure affects both the distribution of the two characteristic zones as well as the relative slip range, which results in distinct fretting damage mechanisms. The nucleation characteristics and propagation paths of fatigue cracks under three typical contact pressures were analyzed considering the effects of fretting damage and heterogeneous microstructures of the alloy, accounting for the contact pressure dependency of the fretting fatigue life. Highlights: The dynamic evolution of the contact status affected by contact pressure is considered.The fretting damage evolution with the variation of contact pressure is evaluated.The nucleation characteristics and propagation paths of fatigue cracks are elaborated.The contact pressure dependency of the fretting fatigue life is summarized. [ABSTRACT FROM AUTHOR]

Fretting fatigue is a well-known and dangerous damage mode that occurs on the mating surfaces of mechanical components, mainly promoted by a combination of stress distribution, contact pressure distribution, and relative sliding (micro)motion between the surfaces. However, predicting this mechanism is challenging, necessitating specific studies for each assembly due to variable influences. This article presents a methodology for evaluating fretting fatigue damage at the contact between a titanium connecting rod big end and the bearing, adopting the Ruiz parameter as a quantifying damage index. For this purpose, a thermal-structural finite element model is prepared. In particular, the machining and assembly of the split conrod big end are simulated, considering thermal effects. A full engine cycle is first simulated, and results are used for identifying critical instants to be considered for accurate yet computationally efficient calculations. The dependence of fretting fatigue on three factors is studied: bearing crush, bolts tightening torque, and friction coefficient between the big end and the bearing. In summary, the damage increases with a higher crush and friction, while tightening torque has marginal effects. Following a 20% increase in crush height, a corresponding 10% rise in the Ruiz parameter is observed. Conversely, reducing the crush height by 20% leads to an approximately 8% decrease in the Ruiz parameter. When the influence of the bolt preload is taken into account, only a marginal 1% increase of the Ruiz parameter is recorded despite a 30% rise in preload. Evaluating the impact of the friction coefficient on the Ruiz parameter reveals an almost linear relationship. These findings suggest that adjusting the screw preload can enhance the hydrodynamic behavior of the bearing without exacerbating fretting. Furthermore, exploiting the linear correlation between Ruiz and the friction coefficient allows for the generalization of results obtained with specific coefficient values. This methodology can, therefore, serve as a valuable reference for adjusting different variables during the initial design phases of a four-stroke internal combustion engine's dismountable connecting rod. [ABSTRACT FROM AUTHOR]

The appearance of fretting fatigue cracks in bolted sheets limits their lifetime. Furthermore, repairing these failures requires much effort due to needing to replace the components instead of replacing just the bolt. To prevent such failures, the purpose of this study is to understand the failure mechanism and to identify the major influencing parameters. Therefore, a representative joint of a bolt of size M22 and sheet material were investigated by experiments and a finite element analysis. The experiments were conducted over a wide range of preloads from zero to maximum preload. It turned out that the failure mode changes at 50 kN. For this preload, the influence of the surface and the use of a lubricant was observed. A grinded surface as well as the use of lubricant showed a change in the failure mode. The accompanying simulation showed that an analysis of the stresses delivers no proper explanation for the observed effects in the experiment. Therefore, the contact status was analyzed for various preloads and friction coefficients. The results correlate with the change in the failure mode. The conclusion is that both the stress state and the tribological behavior influence the failure mode and have to be considered in a numerical analysis. [ABSTRACT FROM AUTHOR]

In this paper, a numerical model of fretting fatigue analysis of cable wire and the fretting fatigue damage constitutive model considering the multi-axis effect were established, and the user material subroutine UMAT was written. Then, the constitutive model of wear morphology evolution of cable wire and the constitutive model of pitting evolution considering the mechanical-electrochemical effect were established, respectively. The corresponding subroutines UMESHMOTION_Wear and UMESHMOTION_Wear_Corrosion were written, and the fretting fatigue life was further predicted. The results show that the numerical simulation life obtained by the program in this paper has the same trend as the tested one; the error is only about 0.7% in the medium life area; When the normal contact force increases from 120 to 240 N, the fretting life of cable wire decreases by 25%; When the evolution of wear morphology and corrosion effect are considered simultaneously, the depth of the wear zone exceeds 0.08mm after 600,000 loads, which is much larger than 0.04mmwhen only the evolution of wear morphology is considered. When the evolution of wear morphology and corrosion morphology is considered simultaneously, the damage covers the whole contact surface after 300,000 loads, and the penetrating damage zone forms after 450,000 loads, which is obviously faster than that when only the wear morphology evolution is considered. The method proposed in this paper can provide a feasible numerical simulation scheme for the visualization of the damage process and accurate life prediction of cable-supported bridges. [ABSTRACT FROM AUTHOR]

Micro relative sliding exists on the contact surface of the main primary equipment's surface structures, resulting in serious fretting fatigue. The plastic effect causes serious fatigue to the structure under alternating loads. Existing fatigue life prediction models fail to fully consider the shortcomings of fretting and plastic effects, which causes the prediction results to be significantly different to real-lifeworld in engineering situations. Therefore, it is urgent to establish a fretting damage fatigue life prediction model of contact structures which considers plastic effects. In this study, a plastic fretting fatigue life prediction model was established according to the standard structural contact theory. The location of dangerous points was evaluated according to a finite element simulation. The cyclic load maximum stress value was compared with the fretting fatigue test data to confirm the error value, and the error between the proposed fretting fatigue life model and the test value was within 15%. Concurrently, we combined this with mass data analysis and research, as it is known that the contact zone parameters have an impact on fretting fatigue and affect the structural lifespan. With the help of ABAQUS, the fretting numerical calculation of the dovetail tenon model was carried out to analyze the sensitive factors affecting the fretting fatigue life of the dovetail tenon structure. By keeping the fretting load unchanged, the contact area parameters such as contact surface form, contact area width and friction coefficient were changed in order to calculate the fretting stress value, σfretting and the dovetail structure was improved to extend its fretting fatigue life. Finally, it was concluded that fretting fatigue was most sensitive to the width and contact form of the contact area. In actual engineering design, multiple factors should be considered comprehensively to determine a more accurate and suitable width and form of the contact area. For the selection of friction coefficient, on the premise of saving costs and meeting the structural strength requirements, the friction coefficient should be as small as possible, and the problem can also be solved through lubrication during processing. [ABSTRACT FROM AUTHOR]

Copyright of Lubrication Engineering (0254-0150) is the property of Editorial Office of LUBRICATION ENGINEERING and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Lubrication Engineering (0254-0150) is the property of Editorial Office of LUBRICATION ENGINEERING and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Fretting wear enhances stress concentration at the bottom of the defect. • The new factor K considers the stress concentration introduced by fretting wear. • The modified MWCM criterion predicted fretting fatigue caused by fretting wear. Fretting fatigue cracks can occur in the wheel seat of railway axles, posing a risk to safe operation. This study investigates the critical crack depth that ensures the fatigue strength of railway axles remains within acceptable limits. Fretting fatigue tests were performed on scaled press-fitted axles with circumferential groove defects of varying depths. Results showed that as the defect depth increased, the fretting fatigue strength decreased. An elastoplastic FE simulation model was developed based on these experimental findings, analyzing both conditions with and without fretting wear. The analysis revealed that the failed specimen, compared to the non-failed one with the same defect depth, exhibited at least a 5 % increase in maximum stress at the crack initiation point and over a 10 % increase in the high-stress region due to higher nominal stress. When fretting wear-induced profile changes were considered, crack initiation could be predicted using a modified wöhler curve method (MWCM) and critical plane method. To simplify engineering simulations, an FE model without fretting wear was used, with a coefficient K = 1.133 ± 0.1 determined from published experimental data. Combining this coefficient with the MWCM criterion, the critical defect size under allowable stress conditions was predicted, yielding results in close agreement with experimental data. [ABSTRACT FROM AUTHOR]

• EUSBP establishes a 1250 μm hardened layer on the casting aluminum alloy. • EUSBP achieves surface micro defects healing of casting aluminum alloy. • EUSBP improves the fretting fatigue properties of casting aluminum alloy. Casting aluminum alloy faces challenges during service, such as micro defects, low surface hardness, and inadequate fretting fatigue performance. This study employs acousto-electropulsing-stress synergistic strengthening to construct a strengthened layer structure on the surface of casting aluminum alloy, characterized by "micro-defect healing + surface gradient nanostructure." The results demonstrate that, compared to surface burnishing processing, electro-ultrasonic surface burnishing processing (EUSBP) increases the thickness of the fine-grained layer by 75 % and enhances the amplitude of surface residual compressive stress by 39.3 % while simultaneously achieving micro-defect healing in the surface layer. Through fretting fatigue tests, it is discovered that the fretting fatigue life of EUSBP specimens is significantly higher than that of burnished and original specimens. Fracture surface analysis and damage zone characterization indicate that EUSBP specimens exhibit the best crack propagation resistance and fretting damage resistance. Molecular dynamics simulations reveal that EUSBP specimens enhance their resistance to fretting fatigue damage by utilizing the nano-gradient grain structure to inhibit dislocation motion and reduce the influence range of plastic deformation during the fretting fatigue process, resulting in a reduction of damage depth in the fretting fatigue damage zone by more than 25 %. [ABSTRACT FROM AUTHOR]

An experimental method that can quantify relative slip was developed using digital image correlation (DIC) in order to evaluate the sliding portion. The bridge-type test setup was designed to establish the fretting contact condition. The relative displacements between the contact surfaces were determined by DIC methods. Based on the evolution and distribution of relative slip, the transitions from gross slip to partial slip on the contact surface were found throughout all tests. This result indicated that the fretting scar was closely correlated to relative slip. The variation of relative slip corresponding to the stick-slip state was consistent with the tangential force coefficient. Besides, the load amplitude was an important factor for fretting fatigue damage, which can affect the stick-slip state. [ABSTRACT FROM AUTHOR]

This paper takes the dovetail structure of an aero‐engine compressor as the research object, designs a high‐temperature dovetail fretting fatigue test, and discusses the effect of high temperature on fretting fatigue damage. Based on the fretting fatigue continuous damage accumulation model, a high‐temperature fretting fatigue life prediction model of dovetail tenon considering the temperature effect is proposed and verified by experiments. The results show that the experimental data prove that the model has good prediction accuracy and its prediction error is less than 19.24%. Finally, the relative sliding distance, contact stress, Ruiz parameters, and friction work of the dovetail structure are selected, and the dovetail structure under different actual use conditions is calculated through simulation. The influence of friction coefficient, load, temperature, arc size, and other factors on the evaluation index is analyzed. The internal law of high‐temperature fretting fatigue life of dovetail structure under multi‐factor coupling conditions is studied. Highlights: The prediction error of high‐temperature fretting fatigue life is less than 20%.The contact area of the high‐temperature structure is severely worn.High temperature reduces structural fretting fatigue life. [ABSTRACT FROM AUTHOR]

Fretting fatigue is a fatigue damage process that occurs when two surfaces in contact with each other are subjected to relative micro-slip, causing a reduced fatigue life with respect to the plain fatigue case. Fretting has now been studied deeply for over 50 years, but still no univocal design approach has been universally accepted. This review presents a literature study that involves the three main types of mechanical joint affected by fretting fatigue (press-fitted shaft hub joints, dovetail joints, and bolted joints) aiming at giving insight into the progress made in terms of design guidelines for engineers. [ABSTRACT FROM AUTHOR]

The dovetail attachment between the turbine blade and disk for an aero-engine operates under varying centrifugal load and vibration at elevated temperatures. The fretting fatigue is prone to occur at the contact surface of the dovetail attachment. This paper investigated the fretting fatigue behavior of the dovetail specimen at 630 °C through experiment and numerical simulation, in which the blade-like dovetail specimen is nickel-based single crystal superalloy DD10 while two fretting pads in contact with the dovetail specimen simulating the mortise of the disk are made of powder metallurgy FGH99. It is revealed from all the tests that the fracture induced by the fretting wear occurs at the upper edge area of the contact surface. The contact surface near the upper edge is more severely worn; hence, the phenomenon of partition on the worn contact surface can be observed, which is consistent with the fretting fatigue mechanism. Moreover, the calculated area of maximum contact pressure gradient through finite element method is in good agreement with the experimental position of the initial fretting fatigue cracks. [ABSTRACT FROM AUTHOR]

• The model is stablished on X-Ray micro-computed tomography technology (μCT), combing SWT damage theory and Theory of Critical Distance (TCD) technique. • Inclusions can affect the crack initiation direction of fretting fatigue and reduce the initiation lifetime. The closer it is to the surface, the more dangerous it is. • The difference between the properties of inclusions and matrix materials leads to local mechanical response mismatch and concentration. • The aggregation distribution of inclusions results in the merging and connection of local regions with high stress, strain, and damage concentration. Fretting fatigue is a particularly destructive form of fatigue failure that has drawn significant attention from researchers. This paper proposes a numerical modelling method based on tomography scan to investigate the impact of inclusions on fretting fatigue crack initiation behaviour. The results reveal that inclusions can affect the crack initiation direction and reduce the initiation lifetime. The difference between the properties of inclusions and matrix leads to local mechanical response mismatch, resulting in local stress, strain, and damage concentration. The mismatch in strain and damage follows a gradient distribution along the thickness as the distribution in these mechanical responses. These could explain why particles closer to the surface are more likely to cause damage. Additionally, the aggregation distribution of inclusions results in merging and connecting local regions with high stress, strain, and damage concentration, ultimately promoting fracture behaviour. Finally, the numerical models are validated with experimental data from fretting fatigue tests. Compared to the homogeneous model, the numerical model that considers inclusions from topography data can more accurately analyse the material damage and provide a more comprehensive understanding of fretting fatigue behaviour. [ABSTRACT FROM AUTHOR]

Fretting fatigue pertains to the behaviour of engineering components undergoing cyclic loading while in contact with each other. This intricate contact-related phenomenon often results in premature failure compared to conventional fatigue issues. Moreover, when these components operate in high-temperature atmospheres and experience high wear conditions, such as the heat transfer tubes in the nuclear reactor, the application of Functionally Graded Material (FGM) coatings becomes essential. Consequently, it is imperative to investigate how FGM coatings influence the initiation and propagation behaviour of fretting fatigue cracks. This paper employs the Critical Plane (CP) method to calculate the damage parameter. Additionally, Linear Elastic Fracture Mechanics (LEFM), specifically the Extended Maximum Tangential Stress (E-MTS) criterion, is utilized to examine how FGM coatings affect the paths of fatigue crack propagation in the presence of fretting conditions. Meanwhile, due to the unavailability of material properties of FGM coatings, the damage parameter and stress intensity factors are utilized to indirectly assess the effect of FGM coatings on crack initiation and propagation lifetimes. The FGM coating significantly influences tangential stress, thereby affecting the length of the stick zone, while having minimal impact on the distribution of normal stress along the contact surface. Furthermore, it is observed that adjusting the variation in composition of FGM coatings and the elastic modulus of the first FGM coating layer provides a potential technique for enhancing both crack initiation and propagation lifetimes. [ABSTRACT FROM AUTHOR]

• Stress distributions in notch fatigue and fretting fatigue are compared. • The general path of the Point Method is analyzed considering stress distributions. • A rig with an accurate alignment is designed for fretting fatigue experiments. • Fretting fatigue tests of complete contacts are carried out for Ti-6Al-4V alloy. • The minimum life path is proposed for the fretting fatigue life prediction. With the improvement and success of the Theory of Critical Distances (TCD) in notch fatigue problems, the TCD, especially the Point Method (PM), combined with multiaxial fatigue criteria, has been used over the last few years to predict the fretting fatigue life. However, little attention is paid to the effect of different damage mechanisms, especially stress distribution, between the two conditions on the application of the PM. This paper aims to select a suitable path for the PM, considering various stress distributions, for the fretting fatigue life prediction of complete contacts. A new method is developed to select the suitable path of the PM for fretting fatigue life prediction. To validate the analysis, corresponding tests were carried out using an improved proving ring-like rig in a Ti-6Al-4V alloy. Predicted lives on all paths were calculated using the PM combined with different multiaxial fatigue criteria, without incorporating wear effects, and compared with experimental lives using an error bar graph. Results indicate that when the fretting wear is not considered, the minimum predicted life path is more suitable for the PM than the perpendicular and maximum predicted life paths. This suitable path is compared to the path selection in the uniaxial symmetric V-notch fatigue condition. Analysis shows that the conventional path (the notch bisector) of the PM produces the maximum predicted life for the notch fatigue condition. The cause of this different path selection of the PM in the two fatigue conditions is assumed to be the effect of the asymmetric stress distribution and fretting wear damage in the fretting fatigue condition. [Display omitted] [ABSTRACT FROM AUTHOR]

A fretting fatigue numerical model of Ti-6Al-4V alloy is established, and the fretting fatigue crack initiation behaviors (crack initiation position and initiation direction) and initiation life are predicted. The predicted fretting fatigue crack initiation position based on three fatigue wear criteria of relative sliding amplitude, dissipated energy, and equivalent damage stress criteria are compared and analyzed. The fretting fatigue crack initiation direction is calculated using the maximum shear stress criterion as 56°, close to experimental measurement of 59.2°. Eventually, the local stress-strain model and continuous damage model are used to predict the fretting fatigue crack initiation life of Ti-6Al-4V alloy. The predicted fatigue life is in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

The objective of this work is to study the effects of contact parameters on the cracking parameters of a specimen and a pad assembly. These parameters have been studied and evaluated by the finite element method analysis in two dimensions fretting fatigue model through the Abaqus calculation code. Different values of the coefficient of friction of 0.1, 0.3 and 0.6 were applied on the various lengths in contact for a = 0.1, 0.5 and 1mm. Thus, on the various values of angle of orientation of the crack equal to 15 °, 30 ° and 45 °. In addition, elements of the type (CPE4R) and the criterion of maximum tangential stress were applied. The curves of the crack parameters such as the SIF coefficients and the integral J were obtained and discussed. [ABSTRACT FROM AUTHOR]

Salt layer preparation on the contact surface of Ti6Al4V dovetail joint specimens using solid NaCl and Na 2 SO 4 deposition, the effect of hot salt layer on fretting fatigue properties of Ti6Al4V alloy at high temperature was studied. The maximum stress load and temperature are 10 kN and 500 °C, respectively. The results show that the average fatigue life of the base metal dovetail joint sample (BM sample) is 9.93×104, and the average fatigue life of the hot salt dovetail joint sample (HS sample) is increased to 69.9×104. The increase in fatigue life stems from the fact that the hot salt layer promotes the generation of thicker loose oxides on the surface of the Ti6Al4V alloy, besides, the lubricating effect of the oxide with higher hardness and the wear debris alleviates the fretting damage between the dovetail joint sample and the contact surface of the fretting pad. Fracture analysis shows that the hot salt layer does not change the crack initiation mechanism, and all of them show single crack source propagation and ductile fracture mechanism. Analysis of crack formation zone by electron back-scattered diffraction (EBSD), it is found that the presence of less dislocation distribution in the HS sample relative to the BM sample. The dislocations are more concentrated on the crack propagation paths and the fretting fatigue contact surfaces. This indicates that the BM sample produced more significant plastic deformation in the crack formation zone, accelerating crack initiation. The contact surface plastic deformation of the HS sample is small and the fretting fatigue life is enhanced. • The salt-coated dovetail specimens designed for high-temperature fretting fatigue testing. • The salt-coated specimens had longer life under high temperature fretting fatigue test compared to the matrix specimens. • The mechanism of improved fretting fatigue life of salt-coated specimens relative to matrix specimens. [ABSTRACT FROM AUTHOR]

• Analysis of aluminum strand friction coefficient considering plastic effects. • Investigation of crack arrest using 3D FEM and SIF calculations under elastic and elastoplastic conditions in fretting fatigue. • Examination of plasticity's influence on crack arrest, for shear-driven cracks. • Reverse identification of friction coefficient between crack lips. • Crack geometry analysis based on SIF, with discussion on experimental results. This study investigates the fretting fatigue cracking process for wires made of aluminum alloy through elastic and elastoplastic simulations of former tests where both crack arrest and crack growth until failure were observed. A three-dimensional finite element analysis (FEA), incorporating 3D cracks within the contact, is conducted to replicate the experiments in the conditions of these former results. The present study demonstrates that an elastic assumption fails to capture the threshold crack extension, which would result in systematically underestimated lengths for crack arrest. When considering elastoplastic behavior of the aluminum strands, instead of elastic behavior, the SIF values evolve consistently with experimental observations. By this way, it is now possible to carry out a sounded FEA so as to account for friction in the crack lips in the presence of plasticity. First, this model clarifies the role of friction in crack arrest. Secondly, a reverse analysis is proposed to assess the associated coefficient of friction between crack lips, yielding a value consistent with measurements obtained from gross slip fretting experiments. [ABSTRACT FROM AUTHOR]

• The crack initiated vertical to the surface in the untreated specimen, while parallel in the treated specimen. • The debris would bring the crack away due to the parallel direction, which was one of the mechanisms of life enhancement. • The necessity of considering compression load effect on the crack propagation stage with the linear superposition model was confirmed for GH4169. In the present work, the fretting fatigue properties of GH4169 superalloy at 650℃ were investigated through numerical simulation considering the influences of the surface treatments. As a detrimental phenomenon, fretting fatigue involves fatigue and wear simultaneously, where the coefficient of friction and residual stress fields induced by the surface coating or modification techniques may affect the fretting fatigue lifetimes by changing the complex stress state between the two contacting surfaces. The numerical solutions were implemented to compare the crack initiation angles and paths under different surface conditions. The simulation results indicated that fretting fatigue cracks in residual stress fields were tendentiously parallel to the surface resulting in crack retardation and debris peeling, as verified by experimental observations. Additionally, three propagation models were compared to study the effect of compressive loading on the driving force of crack propagation rates, especially where compressive residual stresses was present. The combination of the linear superposition model with the Harter-T method was proven to be more accurate in the fretting fatigue life prediction, which may provide a modeling basis for service life prediction of engineering components in many practical applications. [ABSTRACT FROM AUTHOR]

Fretting fatigue is a phenomenon that occurs when two surfaces come into contact and experience oscillatory stresses and displacements of small amplitude. This can lead to a significant reduction in mechanical parts' lifetime. The complexity of this type of problems relies on the many variables that can affect the results. One of its most important variables is the Coefficient of Friction (CoF), which has been observed to vary throughout a single fretting test. However, this variation is rarely considered in fretting fatigue finite element analyses. In order to analyse the effects of a Variable Coefficient of Friction (VCoF) on nucleation lifetime, a cylinder-on-plane finite element model is developed. To predict nucleation lifetime, two critical plane damage parameters, Fatemi-Socie (FS) and Findley parameter (FP), are used along with an evolving CoF during the first stress cycles. These parameters are evaluated by calculating initiation lifetime where the maximum value of the damage parameter occurs (hotspot method) and by the quadrant averaging method, because of its capability of predicting lifetime with relative accuracy and negative crack angles. To simulate the evolution of the CoF, a Python script that changes the value of CoF before each simulation is developed. This coefficient varies between 0.3633 and 0.65, following arbitrary points on the coefficient's evolution curve. Once the simulation is concluded, the script evaluates the damage parameters and their equivalent lifetime throughout the contact surface and saves those values in an accumulation parameter that follows Miner's rule. This process is repeated until CoF reaches its maximum value. The lifetime to nucleation is statistically analysed for all methodologies. The results suggest that, regarding the hotspot method, FP values remain within ± 3 N deviations from the experimental results, while FS results are nearer, within ± 2 N deviations. In addition, the quadrant method results remain at ± 3 N deviations, but providing more accurate results for the FP parameter. The application of a VCoF to these methods leads to less conservative and less deviated results, providing more accuracy for conservative approaches and more consistency for predictions in general. It is concluded that VCoF analysis is an interesting tool to determine more accurately nucleation lifetime for conservative approaches. Additionally, it reduces the predictions variation for differently loaded fretting setups regardless the desired approach. [ABSTRACT FROM AUTHOR]

In order to reveal the behaviors of crack initiation and short fatigue crack propagation in laser cladding IN718 under high temperature conditions, very high cycle fretting fatigue (VHCFF) experiments were conducted at 650 ℃ with a transverse clamping force of P = 400 N and different axial stress loading conditions to obtain S_N data. The surface wear morphology and fracture morphology characteristics were analyzed by scanning electron microscope (SEM) and white light interferometer. Additionally, the microstructure characteristics of grains in the short fatigue crack region were examined through electron back scatter diffraction (EBSD). Parameters such as equivalent grain size, grain elastic modulus, Schmid factor, geometric compatibility factor, twist angle, and tilt angle were considered in the analysis of the characteristics of fatigue crack initiation and propagation. The results indicate that fretting fatigue cracks tend to initiate on grains with higher Schmid factors, larger equivalent grain size, and lower grain elastic modulus on the fretting contact surface. Moreover, the short fatigue cracks tend to propagate along the grains with the microstructural features of the high Schmid factor, the geometric compatibility factor of m ′ > 0.4 , the twist angle of α < 50 ° , and the tilt angle of β < 50 °. [ABSTRACT FROM AUTHOR]

This study assesses the generalization capacity of artificial neural networks (ANNs) for predicting fretting fatigue of mechanical contacts using different materials and geometries. These ANN utilize as inputs, material properties and stress quantities that have been physically related to the fatigue crack initiation mechanism under multiaxial loading. Initially trained and validated on aeronautical aluminum alloys data, one tests their generalization performance by applying them to fretting fatigue data for Ti‐6Al‐4V, ASTM A743 CA6NM steel, and Al 7050-T745, employing both cylindrical and spherical pads under various loading conditions, including out-of-phase loading. The ANNs adeptly predict fatigue lives across this extensive dataset, surpassing classical multiaxial fatigue criteria in accuracy. This underscores the effectiveness of ANN-based methodologies in diverse fretting fatigue scenarios. • ANN models trained on 132 aluminum FF data to predict fatigue life. • Inputs are based on established multiaxial fatigue criteria stress parameters. • They provided accurate life estimates for more than 200 FF tests. • The ANN models outperformed multiaxial fatigue models based on experimental values. • Demonstrated excellent generalization capability for new materials. [ABSTRACT FROM AUTHOR]

The fretting fatigue crack formation and propagation behaviors of Ni‐based single‐crystal (NBSX) superalloys are investigated in this paper. Subsurface crack formation process is revealed by in situ fretting fatigue experiment. The crack is observed to form on subsurface area, then propagates to the contact surface. Inclusions in materials are found to have obvious effects on crack propagation, and slip lines are closely related to the crack propagation direction. Crystal plastic finite element method (CPFEM) simulation is used to simulate crack formation position. The accumulative plastic strain peaks at the edge of contact zone and the subsurface area. The results show that the CPFEM simulation and in situ observation achieve good agreements. [ABSTRACT FROM AUTHOR]

The effect of crystal orientation on fretting fatigue induced crack initiation and dislocation distribution is studied by in-situ SEM observation and electron back-scattered diffraction (EBSD) in this paper. Cracks and slip lines are observed in the fretting contact area of Ni-based single-crystal (NBSX) superalloys. The in-situ SEM observation captures different crack and slip line behaviors under different crystal orientations. The EBSD analysis results show obvious misorientation and orientation deviation in the fretting contact area. For both crystal orientations, the geometrically necessary dislocation (GND) density distributions in the contact area are obtained by using Hough-based EBSD methods. The peak position of grain reference orientation deviation (GROD) and GND density matches with the fretting fatigue crack formation position. EBSD analysis shows that the dislocation density distribution on each slip system is closely related to the crack initiation direction. The direction of slip system with the maximum dislocation density agrees with the crack initiation direction obtained by in-situ observation. Image 1 • In-situ observation of fretting fatigue shows different crack and slip line behaviors of different crystal orientations. • EBSD results show obvious misorientation in the fretting contact area. • The peak position of GND density matches with the fretting fatigue crack formation position. • The direction of slip system with the maximum dislocation density agrees with the crack direction. [ABSTRACT FROM AUTHOR]

In this work, a fretting fatigue experimental campaign on Al 7075-T651 alloy specimens is simulated by using both analytical and numerical approaches. According to one of the classifications available in the literature, the theoretical approaches for fretting fatigue assessment may be divided into stress-based models or Stress-Intensity Factor (SIF) models. The analytical approach here applied falls in the group of the stress-based models, whereas two numerical methods fall in the group of the SIF-based models. The fatigue assessment is performed by evaluating both crack path and lifetime. • Fretting fatigue tests are here simulated by analytical and numerical approaches. • The analytical approach falls in the group of the stress-based models. • The numerical methods fall in the group of the SIF-based models. • In this work, both crack path and fatigue life are evaluated. [ABSTRACT FROM AUTHOR]

In this paper, the influence of the heterogeneity in the predicted crack initiation lifetime under fretting fatigue conditions is analysed for a regular and a random distribution of micro-voids. A critical plane analysis with two multiaxial damage criteria is performed to assess the crack initiation lifetime. The predicted initiation lifetime in the heterogeneous material is compared with the results obtained in the homogeneous case. The numerical results show that the heterogeneity has a noticeable influence on the predicted initiation lifetime. Furthermore, the numerical model suggests that a crack may firstly initiate at the upper edge of the micro-voids located close to the contact edge, leading to a mean reduction of the predicted crack initiation lifetime. However, in some cases, the introduction of micro-voids reduces the stress intensity at the contact edge and thus decreasing the predicted crack initiation lifetime. • Effect of heterogeneity on fretting fatigue crack initiation. • Numerical analysis using Finite Element Method. • Regular and random distribution of micro-voids. • Critical plane analysis using multiaxial damage criteria. [ABSTRACT FROM AUTHOR]

A numerical lattice-based approach equipped with a brittle erosion algorithm and material forces in the context of LEFM and configurational mechanics is introduced to analyze contact fatigue life and crack propagation of brittle solids under low amplitude cyclic loading using the classical Paris's law. Material force vectors at lattice nodes were employed in identifying potential crack path in high-cycle fatigue applications. A 2D plane strain pad-substrate system with contact formulations under constant compression and cyclic sinusoidal shear with constant amplitude was considered to investigate fatigue crack's growth rate and its shape in the substrate, made of hydrided brittle Zircaloy-4 used in nuclear industry. The capability of the lattice in predicting the crack extension path using the material force vectors at the crack tip is verified by comparing with analytical solutions for a centered crack domain under pure shear and direct tension loading, and also for an initial small surface crack for the contact fatigue pad-substrate system. Obtaining the total fatigue life by assuming a failure value for the crack length and confirming the Paris' law crack growth, the lattice results demonstrate that the fatigue crack path in the substrate is a curved trajectory which gradually reduces as the fatigue crack tip grows deeper into the substrate. Having simple constitutive formulation and straightforward erosion algorithm, this approach together with configurational mechanics implications can be employed in analyzing fretting fatigue problems. [ABSTRACT FROM AUTHOR]

The bending configuration of shrink-fitted connection fretting fatigue tests causes a fluctuation in the contact pressure and in turn a more favourable condition for shear, rather than tensile, crack initiation. According to the classification type I (shear) and type II (tensile) fretting cracks, type II was only observed in the tests with no surface enhancement, which experienced the lowest fretting strength and thus a reduced pressure fluctuation effect. The crack initial direction was predicted as the critical plane orientation undergoing the maximum Fatemi-Socie (FS) stress parameter, for the shallow type I shear cracks. A significant result is that if the critical plane is searched at the fretting hot-spot, an incorrect outward direction is obtained, since the orthogonal direction, with the same shear stress amplitude, experiences a more compressive maximum normal stress. Conversely, with the application of the Point Method, taking the stress location at half the critical distance and by following the potential critical plane, a higher shear stress along the inward direction is obtained. The more compressive stress below the contact can therefore be compensated, and consequently the inward shallow direction is correctly predicted. [ABSTRACT FROM AUTHOR]

• Competitive mechanisms between plain and fretting fatigue were clarified. • The effect of the stress relief groove depth on plain and fretting fatigue was studied. • A valid methodology for evaluating plain and fretting fatigue damage was developed. • The role of fretting wear on fretting fatigue of axles with stress relief grooves was studied. To investigate the competitive relationship between plain and fretting fatigue in press-fitted railway axles, the plain fatigue limit and fretting fatigue strength were tested, respectively, by changing the depth of the stress relief groove. Next, detailed information on fretting and plain fatigue, as well as on fretting wear, was gathered. Finally, an evaluation methodology integrating the finite element (FE) simulation with the Modified Wöhler Curve Method (MWCM) was established to evaluate the damage in both fretting and plain fatigue scenarios. It was found that as the groove depth increased, the plain fatigue limit decreased while the fretting fatigue strength increased. The optimal groove depth, h opt , that equalized the anti-fatigue capabilities of the groove and wheel seat, was affected by the number of test cycles. For this press-fitted railway axle, h opt was 0.2 mm < h opt < 0.4 mm for 107 test cycles, and h opt > 0.6 mm for 108 cycles. The calculated critical local stress condition, σ Cri , required for initiating a crack, was 161.8 MPa, which almost agreed with the theoretical value. The calculated σ MWCM values can be used to characterise the fretting and plain fatigue damage. The local stress condition for determining an optimal groove depth was: σ MWCM in the wheel seat and groove root simultaneously reached σ Cri. [ABSTRACT FROM AUTHOR]

Abstract A dovetail fixture for fretting fatigue test was designed. Fretting fatigue experiments were performed for Ni-based single crystal (NBSX) superalloys at 600 and 700 °C in two secondary orientations, [110] and [010]. The influence of secondary orientation and temperature on fretting fatigue behavior was investigated. The crystal plasticity finite element (CPFE) simulation was used to study the stress field at the contact region. The result indicates that the stress distribution is correlated to fretting damage, which is consistent with the crack initiation site. Fretting fatigue life is predicted based on critical plane method, which is in good agreement with the experimental result. Highlights • A dovetail fixture for fretting fatigue life test at elevated temperature was designed. • The effect of secondary orientation and temperature on fretting fatigue was studied. • The simulation agreed with experimental observation in fretting fatigue behavior. • Life prediction by critical plane method was consistent with experimental result. [ABSTRACT FROM AUTHOR]

Highlights • A sub-scaled specimen and its fixture were designed and tested under cyclic loading. • XFEM was successfully implemented in determining crack growth path. • Fretting-contact-induced crack closure was introduced into life prediction. • Life prediction based on the total life model fit well with experiments. Abstract The fretting fatigue in dovetail assembly was investigated by experiments and finite element method. A sub-scaled specimen and its fixture were designed and tested under cyclic loading of three load levels. The stress distribution in the contact zone was calculated and several fatigue parameters including SWT and FS parameter were employed to predict the direction and the location of the crack initiation and predict the initiation life. The crack growth path was determined by extended finite element method and crack growth life was predicted after introducing the effect of fretting-contact-induced crack closure. Then based on a total fatigue life prediction model, the total life was obtained by summing up the crack initiation life and the crack growth life using a fixed initial crack length, and the predicted results agreed well with experiments. [ABSTRACT FROM AUTHOR]

Highlights • The progress in fretting fatigue understanding and predictability is reviewed. • Applicability to engineering applications is especially considered. • Asymptotic approaches for fretting fatigue assessments are reviewed. • The use of multiaxial fatigue criteria to determine fretting fatigue crack nucleation life is considered. Abstract The progress in fretting fatigue understanding and predictability is reviewed, with engineering applications in mind. While industrial assessments often relies on simple empirical parameters, research in fretting fatigue should allow the design engineer to improve confidence in the fretting fatigue analysis. Fretting fatigue cracks often form in multiaxial stress fields with severe gradients under the contact area, and are inherently difficult to predict. By describing the fretting stress gradients using comparisons with the mechanical fields surrounding cracks and notches, crack nucleation threshold conditions and finite life can efficiently be determined. Also, non-local stress intensity multipliers provide promising tools for the industrial finite element analysis, often involving complex geometries and loading conditions. The use of multiaxial fatigue criteria to determine fretting fatigue nucleation life is also reviewed. Researchers have shown that critical plane calculations with some stress-averaging method can predict fretting fatigue crack initiation. However, the frictional interface causes non-proportional loading paths, and the application of critical plane methods is not straight forward. [ABSTRACT FROM AUTHOR]

The effect of crystal orientation on fretting fatigue crack formation behaviors of Ni-based single crystal (NBSX) superalloys is investigated in this paper. Differences of slip lines and crack formation patterns between two crystal orientations are revealed by in-situ fretting fatigue experiments. The cracks are found to initiate along the direction of slip lines for both crystal orientations, which indicates the crack formation has close relations with crystallographic slip. Crystal plastic finite element method (CPFEM) simulation is implemented and shows the difference of slip system activations between two crystal orientations. Equivalent plastic strain is proved effective to predict crack formation sites and directions. The predicted slip lines and cracks obtained from CPFEM simulations are in good agreements with the experimental observations. [ABSTRACT FROM AUTHOR]

Failure analyses and root cause determination were carried out on the rotor of a high-power generator of gas-diesel dual fuel which presented cracking due to torsional fatigue in its end (region of section change and coupling), after 30,000 h of service. The generator of 307 MW-3000 rpm has a rotor (shaft of 400 mm Ø) manufactured in a proprietary steel grade equivalent to ASTM A470 type, Class 7 of high hardenability. It was reported that the equipment control system showed, in service, a high level of vibrations, not admissible for continuing the operation. First, and during the equipment shutdown for inspection, the presence of cracks, slant to the rotor shaft, was detected by means of visual inspection and dye penetrant test. The failure region corresponds to the zone of coupling-shaft joint, linked by means of fixation by interference, whereas the cracking spread on two fracture planes at 45° with respect to the rotor shaft. On this zone, where cracking started, a severe fretting corrosion damage was evidenced. The characterization and identification of present damage mechanisms were conducted through macrographic, fractographic, SEM, EDS, chemical analyses, and mechanical tests. It was recognized that from the damage by fretting corrosion, fatigue micro-cracks were produced that spread due to service tensions by a mechanism of fretting fatigue. The fatigue fracture propagation was developed into two orthogonal planes at 45° from the longitudinal shaft, which reveals an inversion in the loading condition, only justifiable by torsional vibrations that were assigned to a torsional resonance typical of the system dynamics. It was considered that the torsional vibrations cause micro-movements between components, promoting fretting corrosion and the subsequent fretting fatigue that finally induced the failure by high-cycle torsional fatigue with low-stress amplitude. [ABSTRACT FROM AUTHOR]

There are several short crack methodologies available in the literature to estimate crack arrest under fretting loading conditions. Some of them claim that the most appropriate threshold stress intensity factor curve for short crack behaviour is better described by the El-Haddad curve, others assume that the approximation of the threshold curve by two straight lines in the Kitagawa-Takahashi diagram (in logarithm scale) would be a simpler and adequate threshold. Despite the non-proportional multiaxial stress field under the contact, the crack driving force in such methodologies has either been computed considering just mode I or mixed mode (I and II). The aim of this work is to conduct a review of these short crack arrest models and to propose a new and more conclusive approach. Plain fretting and fretting fatigue data available in the literature for AISI 1034, Al4%Cu and Ti-6Al-4V were considered to validate the analysis. The results showed that the accuracy of the short crack arrest methodology is strongly dependent of the threshold stress intensity factor curve considered and that the introduction of the Bazant’s parameter in the analysis is preferred to obtain more sensible estimates. It was also observed that the introduction of the mode II for the computation of an effective stress intensity factor did not significantly improve the results for the data here assessed. [ABSTRACT FROM AUTHOR]

This study aims to provide an overview of numerical and experimental work, related to crack nucleation under fretting fatigue conditions. In fretting fatigue, multiaxial loads and severe stress gradients are present at the contact interface, which can lead to failure. The damage process, in general, is considered as a two-phase phenomenon, namely, nucleation and propagation. Various damage models and approaches are available in literature to model each phase. In the present work, different criteria, related to nucleation phase, are classified based on the approach used to define failure. These approaches include, critical plane approach, stress invariant approach, fretting specific parameters and continuum damage mechanics. Apart from theoretical background, the work related to the applications of these approaches to fretting fatigue problems is also presented. It is observed that, to analyse various aspects, intricate details near the contact interface and mechanisms involved in fretting fatigue, the strength of finite element method can be employed. In the light of numerical and experimental observations, comparison between different approaches, common sources of errors in prediction and generalized conclusions are presented. [ABSTRACT FROM AUTHOR]

• Crystal plasticity theory is conducted to predict the crack initiation lifetime of fretting fatigue in AA2024-T351 alloy. • Both accumulated plastic slip and accumulated energy dissipation can predict the FFCI lifetime. • Fretting fatigue crack nucleation hotpots are determined by the difference of maximum value of maximum principal strain. • Multiple grain orientations affect the total cumulative shear strain distribution during fretting fatigue. • The macroscopic mechanical behavior of fretting fatigue remains consistent across different grain orientations. AA2024-T351 alloy finds extensive use as a component material in the fields of civil engineering, machinery and aerospace, which are prone to long-term contact loading during service that may cause catastrophic failure such as fretting fatigue. In this work, a crystal plasticity finite element (CPFE) model has been established to predict the fretting fatigue crack initiation (FFCI) lifetime of AA2024-T351 alloy. Submodel methods for frictional contact parts are adopted, and crystal plasticity theory is utilized to obtain more accurate simulation results. The hotspots for fretting fatigue crack nucleation are determined by the difference of maximum value of maximum principal strain. The accumulated plastic slip and accumulated energy dissipation are utilized as fatigue indicator parameters (FIPs) to accurately predict FFCI lifetime. The results show that both FIPs predicted crack initiation lifetime are within twice the scatter band. Furthermore, the submodel is employed to generate diverse grain orientation sets while maintaining a constant total of 800 grains. The results indicate that different sets have little effect on the prediction of FFCI lifetime, but has a significant impact on the cumulative shear strain distribution between each individual grain at the mesoscale. [ABSTRACT FROM AUTHOR]

• A new prediction method for fretting fatigue crack initiation lifetime. • A combination of deep neural network, Findley parameter and finite element method. • Accurate prediction of fretting fatigue lifetime. In the aviation field, the crack initiation stage could be the dominant one of the whole damage failure process of components subjecting to fretting fatigue. Thus, getting an accurate prediction of the crack initiation lifetime become an important topic in research of fretting fatigue problems. In this work, a new prediction method for fretting fatigue crack initiation lifetime is proposed, which is based on Deep Neural Network (DNN), Findley Parameter (FP) and Finite Element Method (FEM). Input features are shear stress amplitude and maximum normal stress in the critical plane, and crack initiation lifetime is set as the output target. Dropout and bootstrapping are considered to improve the performance of DNN. Through comparative analysis, it is shown that this new proposed approach can effectively predict the fretting fatigue crack initiation lifetime. The prediction accuracy and stability are greatly improved compared with the theoretical formulation based on FP. [ABSTRACT FROM AUTHOR]

• The modified crack analogue incorporates a novel shear type cohesive zone at the contact edges of the original crack analogue model casted in the form of Mode II stress intensity factor. • The cohesive zone model introduces a new approach regarding the intimate synergy between friction and mild wear through plastic ratcheting of the surface asperities, allowing relative slip between the contacting surfaces. • A key internal variable has been recognized to be the normalized surface roughness that evolves with fretting micro-slip and affects the friction coefficient. • The cohesive zone model can be easily incorporated in most finite element codes and other contact mechanics schemes. • The cohesive zone model suggests that the local surface interactions be handled in a way that the surface topology and the asperity plastic shakedown are interconnected. Fretting fatigue may appear in mechanical components that are in contact under vibrational loading, inducing oscillatory frictional stresses, often combined with alternating bulk stresses. Fretting fatigue cracking is important, with its initiation being difficult to estimate. This work extends a well-known fretting fatigue model, the crack analogue model that examines fretting fatigue induced by incomplete contacts of metallic surfaces. This is done by inserting a mode II type cohesive zone at the contact edges. In our previous work on fretting fatigue of complete contacts, we proposed mixed mode I and II cohesive zones because of the nature of the stress fields around the contact edges. The proposed cohesive zones address several issues related to the mechanics of friction and wear, by combining them in the case of mild wear where asperity plastic shakedown is the dominant inelastic mechanism. Thus, the micromechanical behaviour of the surface topology and its evolution are naturally introduced into the analysis. The surface roughness becomes an internal variable that evolves with the cyclic loading, establishing a steady state friction coefficient. Alternatively, friction evolution can be established pointwise by monitoring the accumulated micro-slip. The work of friction dissipates the mechanical work and at the same time becomes the precursor of large scale wear. A predictive methodology for fretting crack nucleation is proposed through the fatigue threshold stress intensity factor used in ordinary fatigue. [ABSTRACT FROM AUTHOR]

Fretting fatigue may occur on surfaces that are in contact, subjected to a normal load, and where there is a cyclic small relative movement between them. It could occur in press-fitted shafts, bolted and dovetail joints, and many other engineering components. It is recognized that the coefficient of friction of the contacting bodies is a primary variable in the fretting process. Also, a recent study indicates that its behavior might be greatly affected by the possible rotation of the pad. Consequently, the aim of this investigation is to evaluate the impact of diverse angles of rotation of the pad and different coefficients of friction on crack initiation angle, location, and life. For this purpose, a numerical model based on a finite element model of a cylinder-plane test configuration is constructed. The Fatemi-Socie damage parameter for multiaxial fatigue is used to evaluate initiation behavior. Firstly, different angles of rotation are applied to the pad, in order to model a lack of stiffness of the test rig, while the coefficient of friction is kept constant and equal to 0.75. Secondly, several coefficients of friction between the pad and the specimen are utilized, while considering a cyclic rotation of the pad between 0.19° and – 0.19°. For the case studied, the results indicate that an increase of the coefficient of friction (μ) may cause a substantial reduction of crack initiation life. However, a variation of μ in the range 0.75 ≤ μ ≤ 0.85 does not produce a significant change in life estimation. This suggests that it cannot be stated that an increase in the coefficient of friction always leads to a reduction in nucleation life. Regarding the angle of initiation of the crack, the results show that an increase of μ causes an approximately linear reduction of the angle. In the case of an increase of the pad angle of rotation, this causes a longer nucleation life and a smaller crack nucleation angle. Even the rotation angle of 0.23° produces an increase of initiation life from 16.6 to 154 thousand cycles. It is concluded that there is a major impact on initiation behavior of the coefficient of friction and the rotation of the pad, which may be due to the lack of rigidity of the test setup. Therefore, controlling the stiffness of the test rig and measuring accurately the coefficient of friction are important factors to consider in fretting fatigue research. [ABSTRACT FROM AUTHOR]

• IN718 reinforced by IN625/20 % WC laser cladding is used to complete the VHCFF experiment in high temperature. • The optimal process parameters of IN625/20 % WC cladding material are generated. • WC particles can enhance the contact surface's anti-fatigue performance, which is proportional to the contact area. • The predicted crack based on the critical plane method matches the physical crack in the same load condition. The objective of this study is to evaluate the very high cycle fretting fatigue (VHCFF) behavior of laser cladding (LC) Inconel 718 (IN718) metal matrix composites reinforced with IN625/20 % WC. Experiments and simulations were conducted to analyze the fretting fatigue behavior of the cladding layer. The wear and fracture morphologies were obtained by using a confocal laser scanning microscope (CLSM) and a scanning electron microscope (SEM). The multiaxial fatigue damage parameters of the FS and SWT models were used to predict fatigue life through the fitting curve, and the critical plane method was adopted to predict the crack initiation and propagation path. Results indicate that the predicted crack corresponds well with the real crack of the specimen. WC particles can improve the anti-fatigue performance of the contact surface, which is proportional to the contact area between the WC and fretting pad. [ABSTRACT FROM AUTHOR]

In this paper, we present for the first time the application of a fretting fatigue crack propagation predictive technique to a railway axle subjected to bending fatigue loading condition. We also present the first numerical study on predicting fatigue crack trajectory in an axle and wheel model of a railway axle. The technique is based on combining the eXtended Finite Element Method (XFEM) with two fatigue crack growth criteria, namely Maximum Tangential Stress (MTS) and minimum shear stress range. In the implementation of XFEM, enrichment functions, shifted formulation and overlay elements are adopted. The element crack closure is taken into account using the punctual restriction criterion. To calculate MTS criterion, the stress intensity factors are extracted using interaction integral method, in which mode I and mode II can be separated. The implementation is validated using a complete contact problem and experimental data from literature. It is found the minimum shear stress range criterion provides crack paths closer to the experimental results than those obtained using MTS criterion. The technique is further applied to a Chinese railway axle and the results are compared to the available experimental data. Good agreement between the numerically predicted and experimentally measured crack trajectory in the railway axle is found. [ABSTRACT FROM AUTHOR]