Near surface stress analysis strategies for axisymmetric fretting

Fretting is essentially a surface phenomenon, but bulk stresses and material properties contribute to subsequent failure. This feature of fretting demands a thorough understanding of near surface stresses under the joint action of normal, shear and thermal loading. Axisymmetric fretting is of great concern in piping and coupling design. In this paper, we develop design tools for Near Surface Analysis (NSA) for understanding axisymmetric fretting. Axisymmetric Fretting Analysis (AFA) becomes formidable owing to localised tractions that call for Fourier transform techniques. We develop two different NSA strategies based on two-dimensional plane strain models: 2D strip model (2DS) and half-plane Flamant model (2DF). We compare the results of 2DS and 2DF with the exact results for AFA obtained using Love’s stress function in conjunction with Fourier transform. There is a good correspondence between stress components obtained from 2D-models.     

Relationships between the fretting wear behavior and mechanical properties of thin carbon films

Mechanical load can drastically affect the properties of orthopedic implant materials. Damage of these materials usually occurs in contact surfaces, caused by abrasion, adhesion, fretting, delamination, pitting and fatigue depending on friction, lubrication, contact area, surface finish and level of loads (stresses).Carbon-based films are biocompatible with good bearing capacity, wear resistance, corrosion resistance and have a low coefficient of friction. However, great intrinsic stress prevents their wider application, mainly as implant coatings. To reduce this undesirable effect special deposition procedures are under development and/or the films are doped with suitable elements. It must be emphasized that DLC is not a material but a group of materials with a variety of properties. The relationships between the fretting wear behavior and mechanical properties of films based on carbon deposited by DC using the pulsed arc discharge PVD nitrogen doped (a-C) and the filtered pulsed arc discharge deposition system (ta-C) were tested.The composition of carbon films (sp³, sp²) was determined by Raman spectroscopy. Mechanical properties of elastic modulus and hardness were determined by a NanoTest apparatus with diamond Berkovich tip using the Oliver-Pharr procedure and adhesion was measured by nanoscratch tests. Tribological behavior was analyzed by fretting tests with a corundum ball under dry sliding lubricated conditions.The good performance of the hard carbon coatings is often discussed. Results from this study of fretting and the associated lubrication (bovine serum) show that ta-C coatings, despite their high hardness, have very low friction coefficients and low volume losses.     

The effect of interference-fit on fretting fatigue crack initiation and ΔK of a single pinned plate in 7075 Al-alloy

The effect of interference-fit on fretting fatigue crack initiation and ΔK was studied numerically for available experimental results in a single pinned plate in Al-alloy 7075-T6. The role of interference ratio was investigated alongside friction coefficient through finite element. Cyclic stress distributions in the plate ligament and fretting stresses on the contact interface were evaluated using 3-D elastic–plastic finite element models. Additionally a 3-D elastic finite element model was utilized to discuss ΔK of cracks emanating from interference fitted holes. Results demonstrate that fretting was the main reason for crack nucleation, and furthermore, the location was precisely predicted and fatigue life enhancement was explained.     

Fracture mechanics assessment of stress concentrations in incomplete fretting contacts

Life assessment of fretting fatigue has been studied for decades. Crack-analogy methods have been proposed for analyzing fretting fatigue of flat contact pairs. In the present work we re-consider the stress field near fretting contact pairs and study the feasibility of using known fracture parameters to assess incomplete fretting contact problems. Both analytical and FEM analysis reveal that the stress field near the discontinuous round corner of a friction pad, in which the round surface has been idealized without contacting the workpiece, is the same as that of crack tip. The stress field is described by the known stress intensity factors, K_I and K_II. For sticking contact these two fracture parameters are independent, whereas for the slipping contact K_II is linearly correlated with K_I. Therefore, the stress field around the slipping contact can be characterized only by one fracture parameter, together with friction coefficient. For the continuous contact pairs with finite round contact surface, the local stress concentrations near the contact edge are finite and can be characterized by K_I and K_II, either, in analogy to the blunting crack tip due to finite strains. Detailed computations confirm that using the fracture parameters to characterize the fretting contact failure is affected by both loading condition and friction pad geometry. The dominance zone around the pad corner decreases more significantly with vertical press load than the horizontal friction load. In the bi-material contact friction pair the stress field can be described by K_I and K_II in the same form.     

Investigation into relative slip during fretting fatigue under partial slip contact condition

A finite element analysis based methodology was developed to compute local relative slip on contact surface from the measured global relative slip away from contact surface. A set of springs was included in finite element model to simulate fretting fatigue test system. Compliance of springs was calibrated by comparing experimental and computed global relative slips. This methodology was then used to investigate local relative slip during fretting fatigue in cylinder‐on‐flat contact configuration under partial slip contact condition for unpeened and shot‐peened titanium alloy, Ti–6Al–4V. Relative slip on contact surface is significantly smaller (about one order) than the measured global relative slip by using a conventional extensometer near the contact surface. Effects of coefficient of friction, rigidity of fretting fatigue system and applied stress to specimen on the global and local relative slips were characterized. Coefficient of friction and contact load have considerable effect on local relative slip, and practically no effect on global relative slip. Gross slip condition can develop at some locations on contact surface in spite of overall partial slip condition. Increase in rigidity of fretting fatigue system increases local relative slip but decreases global relative slip. Finally, fatigue life diagrams based on relative slip on contact surface are established for both unpeened and shot‐peened titanium alloy. These show the same characteristics as of the conventional S–N diagram where fatigue life decreases with increase of relative slip.     

Cyclic plastic behavior of dovetail under fretting load

The high gradient stresses near the edge of contact zone in dovetail assemblies will result in the appearance of a small plastic zone, which makes it difficult to calculate the contact stresses accurately. These calculated stresses are the premise to evaluate the strength of dovetail assemblies, but the majority of previous research on the solution of contact stresses have been usually carried out under linear elastic conditions. Thus this study focuses on characterizing the plasticity occurred in the dovetail assemblies under cyclic loading. A viscoplastic constitutive model of the titanium alloy TC4 is established based on the Chaboche theory. Furthermore, the calculation of contact stresses is carried out by use of the finite element method (FEM) with the constitutive model. The distribution feature of the contact stresses along the contact surface and the depth direction is obtained. The results show that the cyclic stress characteristics i.e. stress gradient, cyclic peak stress, and stress ratio, are not the same at different locations, namely different nodes of the FE model along the contact surface and the depth direction. These differences would make it more difficult to select suitable parameters for life prediction and to identify the most important effect factor. It is meaningful that the results of the study could provide some useful data and an idea for obtaining suitable parameters in fretting fatigue life prediction. The life prediction method with the effect of stress gradient considered may become an efficient approach to predict the fretting fatigue life of dovetail assemblies.     

Experimental results in fretting fatigue with shot and laser peened Al 7075-T651 specimens

This work focuses on determining the effect of shot and laser peening on fretting fatigue in the Al 7075-T651 alloy. These surface treatments generate a residual compressive stress field near the treated surface where contact under fretting fatigue produces high stress levels. The fretting fatigue resistance of shot and laser peened specimens was assessed in a series of tests involving measurements of the residual stress field, residual stress relaxation under the action of cyclic loads, the friction coefficient, surface roughness and material hardness. The obtained results are compared with those for untreated specimens. The tests show the beneficial effect of the compressive residual stresses and the improvement that surface roughness causes in fretting fatigue life, especially in shot peened specimens. Another important effect observed, is the partial residual stress relaxation produced during the fretting fatigue tests.     

Relative Slip on Contact Surface under Partial Slip Fretting Fatigue Condition

Abstract:  A combined experimental–numerical approach was utilized to characterize the relative slip along the contact surface and its features under the partial slip fretting fatigue condition. Relative displacements at two locations on the substrate (specimen) and fretting pad were measured in fretting fatigue tests. These measurements were then utilized to validate finite element analysis. Effects of the coefficient of friction on the relative slip and contact condition were investigated. The stress state along the contact surface was also investigated. Two contact geometries were analysed: cylinder-on-flat and flat-on-flat. There was no change in relative displacement between locations away from the contact surface because of the change in the coefficient of friction, while relative slip on the contact surface was affected by coefficient of friction. In addition, stick/slip sizes were affected by the change in coefficient of friction. Comparison between present and previous finite element models showed that stress state, as well as a critical plane-based crack-initiation parameter, was not much different between these approaches, while relative slip on the contact surface changed considerably.     

Thermal Residual Stresses in Multilayered Coatings

The mechanical integrity and reliability of coated devices are strongly affected by the residual stresses in thin films and coatings. However, due to the metallurgical complexity of materials, it is rather difficult to obtain a closed-form solution of residual stresses within multilayered coatings (e.g. functionally graded coatings, FGCs). In this paper, an analytical model is developed to predict the distribution of residual stresses within multilayered coatings. The advantage of this model is that the solution of residual stresses is independent of the number of layers. Specific results are obtained by calculating elastic thermal stresses in ZrO2/NiCoCrAIY FGCs, which consist of different material layers. Furthermore, the residual stress distribution near the edges and the stress-induced failure modes of coating are also analyzed. The topics discussed provide some insights into the development of a methodology for designing fail-safe coating systems.     

Effect of various surface conditions on fretting fatigue behavior of Ti–6Al–4V

An experimental investigation was conducted to explore the fretting fatigue behavior of Ti–6Al–4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti–6Al–4V with a highly polished finish, bare Ti–6Al–4V that was low-stress ground and polished to RMS #8 (designated as ‘as-received’), bare Ti–6Al–4V that was grit blasted to RMS #64 (designated as ‘roughened’) and stress relieved, and Cu–Ni plasma spray coated Ti–6Al–4V. Behavior against the Cu–Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 10⁷ cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu–Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements.

An increase in fretting fatigue strength of 20–25% was observed for specimens tested against Cu–Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ˜0.3 for the Cu–Ni coated pad against an as-received specimen and ˜0.7 for the bare as-received Ti–6Al–4V. Specimens tested against the polished Ti–6Al–4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu–Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.     

Analysis of Critical Stress for Subsurface Rolling Contact Fatigue Damage Assessment Under Roll/Slide Contact

As one of the main failure modes of component operated under rolling contact loading, the rolling contact fatigue is classified into two types: subsurface initiated and surface initiated. Different stresses such as orthogonal shear stress, maximum principle shear stress, and octahedral shear stress have been applied as the critical stresses for the assessment of the subsurface cracks’ initiation due to rolling contact fatigue. The influences of friction on distributions of the ranges of orthogonal shear stress, maximum principle shear stress, and octahedral shear stress in subsurface were analyzed with reference to the results of the reference articles. The results show that friction does influence the subsurface distributions of these stresses to a certain extent. However, the upper limits of both the maximum principle shear stress and octahedral shear stress are smaller than that of range of orthogonal shear stresses under the rolling contact conditions of usual steel components. Hence, it is more appropriate that the orthogonal shear stress be selected as the critical stress for the assessment of subsurface rolling contact fatigue.     

Tribological Performance of Titanium Alloy Ti–6Al–4V via CVD–diamond Coatings

In the present study, HFCVD nanocrystalline, microcrystalline and boron-doped nanocrystalline diamond coatings have been deposited on titanium alloy. The effect of boron doping on coefficient of friction and residual stresses of diamond coatings have been studied. The tribological characteristics of the aforementioned three coatings on Ti–6Al–4V substrates were studied using ball on disc micro-tribometer, the thickness of the coatings being 3 μm. The coated Ti–6Al–4V discs were slid against alumina (Al₂O₃) balls with normal load ranging from 1 to 10 N. The boron-doped NCD coated sample disc was found to possess the lowest average coefficient of friction ~ 0.0804 while the undoped NCD and MCD coated sample discs were found to possess the average coefficients of friction of ~ 0.143 and ~ 0.283, respectively. Raman spectroscopy studies revealed that the residual stresses in boron-doped nanocrystalline coatings were tensile in nature, while the residual stresses in undoped NCD and MCD were found to be of compressive nature.     

Finite‐Element analysis of a rolling contact model with anisotropic elastic material properties

The elastic properties of a material are an important factor for the designing and dimensioning of structural and functional components with respect to the dimensional stability, the spring‐back behavior or the fatigue properties. The present work is focused on the impact of the elastic anisotropy on a rolling contact loading of a severely deformed ferritic steel. Linear profiles produced by the newly developed forming process called linear flow splitting are considered to be well suitable as linear guides, due to their reduced surface roughness and highly increased surface hardness. Orientation data obtained by EBSD measurements are used for the calculation of the elastic tensors of the severely deformed parts applying the geometric mean in addition to the classical Voigt and Reuss approaches. The produced profiles exhibit a strong elastic anisotropy in the plane parallel to the desired rolling contact surface showing a strong gradient in forming rate. The evaluation of the stresses in the rolling contact area of the formed microstructure is estimated by the implementation of the determined compliance tensors into a finite element rolling‐contact model. In comparison to results with a homogeneous and isotropic material model the results are analyzed with respect to the rolling contact loading based on the von Mises failure criterion.     

Damage accumulation in light alloys on programmed loading. Part 2. Effects of fretting corrosion

A study has been made on how fretting corrosion affects damage accumulation in programmed loading with two-step blocks for D16AT, VT14, and AMg6 alloys. The accumulation is dependent on the relation between the stresses in the steps and the yield point for the surface layer of material. The hardening in the high step is due to the residual compressive stresses, which cause plastic strain with loading beyond the yield point of the surface layer but elastic strain in the internal layers.Translated from Problemy Prochnosti, No. 12, pp. 31–37, December, 1992.     

On propagation of short rolling contact fatigue cracks

Recent accidents involving railway rails have aroused demand for improved and more efficient rail maintenance strategies to reduce the risk of unexpected rail fracture. Numerical tools can aid in generating maintenance strategies: this investigation deals with the numerical modelling and analysis of short crack growth in rails. Factors that influence the fatigue propagation of short surface‐breaking cracks (head checks) in rails are assessed. A proposed numerical procedure incorporates finite element (FE) calculations to predict short crack growth conditions for rolling contact fatigue (RCF) loading. A parameterised FE model for the rolling‐sliding contact of a cylinder on a semi‐infinite half space, with a short surface breaking crack, presented here, is used in linear‐elastic and elastic–plastic FE calculations of short crack propagation, together with fracture mechanics theory. The crack length and orientation, crack face friction, and coefficient of surface friction near the contact load are varied. The FE model is verified for five examples in the literature. Comparison of results from linear‐elastic and elastic–plastic FE calculations, shows that the former cannot describe short RCF crack behaviour properly, in particular 0.1–0.2 mm long (head check) cracks with a shallow angle; elastic–plastic analysis is required instead.     

Fretting fatigue crack analysis in Ti–6Al–4V

A study was conducted to verify the efficacy of a fracture mechanics methodology to model the crack growth behavior of fretting fatigue-nucleated cracks obtained under test conditions similar to those found in turbine engine blade attachments. Experiments were performed to produce cracked samples, and fretting fatigue crack propagation lives were calculated for each sample. Cracks were generated at 10⁶ cycles (10%-of-life) under applied stress conditions previously identified as the fretting fatigue limit conditions for a 10⁷ cycle fatigue life. Resulting cracks, ranging in size from 30 to 1200 μm, were identified and measured using scanning electron microscopy. Uniaxial fatigue limit stresses were determined experimentally for the fretting fatigue-cracked samples, using a step loading technique, for R=0.5 at 300 Hz. Fracture surfaces were inspected to characterize the fretting fatigue crack front indicated by heat tinting. The shape and size of the crack front were then used in calculating ΔKth values for each crack. The resulting uniaxial fatigue limit and ΔKth values compared favorably with the baseline fatigue strength (660 MPa) for this material and the ΔKth value (2.9 MPa√m) for naturally initiated cracks tested at R=0.5 on a Kitagawa diagram.Crack propagation lives were calculated using stress results of FEM analysis of the contact conditions and a weight function method for determination of ΔK. Resulting lives were compared with the nine million-cycle propagation life that would have been expected in the experiments, if the contact conditions had not been removed. Scatter in the experimental results for fatigue limit stresses and fatigue lives had to be considered as part of an explanation why the fatigue life calculations were unable to match the experiments that were modeled. Analytical life prediction results for the case where propagation life is observed to be very short experimentally were most accurate when using a coefficient of friction, μ=1.0, rather than for the calculations using μ=0.3     

Effect of microarc oxidised layer thickness on plain fatigue and fretting fatigue behaviour of Al–Mg–Si alloy

The objective of this work was to investigate the performance of microarc oxide coatings of two different thicknesses (40 and 100 μm) on Al–Mg–Si alloy samples under plain fatigue and fretting fatigue loadings. Tensile residual stress present in the substrate of 40 μm thick coated samples induced early crack initiation in the substrate and so their plain fatigue lives were shorter than those of untreated specimens. Presence of more pores and tensile surface residual stress in 100 μm thick coated samples caused early crack initiation at the surface leading to their inferior plain fatigue lives compared with 40 μm thick coated samples. While the differences between the lives of coated and uncoated specimens were significant under plain fatigue loading, this was not the case under fretting fatigue loading. This may be attributed to relatively higher surface hardness of coated specimens. The performance of 40 μm thick coated samples was better than that of 100 μm thick coated specimens under both plain fatigue and fretting fatigue loadings.     

椭圆形路径载荷下的微动疲劳失效特征

为了解微动疲劳失效机理,通过柱面对柱面的接触方式,研究了60Si2Mn钢在椭圆形路径、拉扭耦合作用下的多轴低周微动疲劳特性,深入分析讨论了不同轴向循环拉伸应力幅值对摩擦磨损表面和断口形貌的影响.结果认为：磨损区产生的氧化物磨屑对微动区磨擦损伤行为具有显著影响;微动摩擦磨损对试样表面的影响深度只有数十微米;微动疲劳裂纹源...     

Pressure with friction of a perfectly rigid die upon an elastic half space with cracks

We study the interaction of a rigid die with a base of any shape and the surface of an elastic half space containing cracks in the presence of friction in the contact zone. The solution of the plane contact problem of the theory of elasticity is obtained by the method of singular integral equations. The detailed analysis of the problem is performed for the case where the base of the die is parabolic and a crack is rectilinear and appears on the surface of half space. We also investigate the effects of the friction coefficient, crack length, its orientation, and location on stress intensity factors KI and KII at the crack tip and the distribution of contact stresses under the die.     

THE EFFECT OF FINAL FRICTION COEFFICIENT ON FRETTING FATIGUE WAVEFORMS

Abstract— The waveforms of the frictional forces and the relationship between the frictional forces and the applied forces are derived for a fretting fatigue model in which the coefficient of friction is not constant. This study is a continuation of a previous one in which it was assumed that the coefficient of friction was a constant. In this new model it is assumed that the coefficient of friction between the rubbing surfaces is initially zero and slowly increases to a constant value during the early cycles of the fatigue life. The consequences of this change are examined and the results from the two models compared. It is observed that the two models give different frictional forces if the deformation of the surfaces is elastic, but are identical when non-linear macroslip occurs on both loading and unloading.