Fretting Cycle Research Articles

Effect of corrosion on fretting wear morphology and mechanical behaviors of wires

Corrosion and fretting wear inevitably occur in wires, which are important parts of cables. To study the wear morphology and surface mechanical behavior of wires for different fretting and corrosion times, a numerical model of fretting wear of wire with different corrosion conditions is established using MATLAB and ABAQUS software’s. Firstly, according to the surface roughness of wires under different corrosion times, the node coordinates in the numerical model are modified to simulate different rough surfaces. Secondly, the wear characteristics of wires under different fretting cycles are obtained by using a cycle jump concept and an ABAQUS user subroutine. Finally, the numerical model is validated using experimental data. The results show that when the corrosion time is 0 h, the wear area has a regular oval shape. The wear volumes of wires with corrosion times of 120 h and 480 h are the largest and the smallest, respectively. As wear volume increases, both residual surface contact pressure and stress decrease. When the corrosion time is 0 h and 120 h, the residual surface contact pressure is maximum and minimum, respectively.

A comparative study on the electrical contact behavior of CuZn40 and AgCu10 alloys under fretting wear: Effect of current load

The current load is one of the most significant factors in the reliability of electrical contact under fretting conditions. In this paper, the fretting wear experiments are conducted for CuZn40 and AgCu10 alloys with varied current loads. The variations of electrical contact resistance (ECR), ECR endurance, temperature, and wear volume with the increasing current load of two alloy materials under fretting conditions are investigated, and the differences between their electrical contact behaviors are compared. The microstructures of the surface wear morphology are examined to reveal the mechanisms of fretting wear and electrical contact failure. In addition, the fretting wear volume evolution of both alloys under different fretting cycles is studied. The results show the current load impacts the wear morphology. The wear particles become smaller and the oxide layer becomes stable with the increase of current load. The effect of current load on ECR and temperature is more significant for CuZn40 alloy. The ECR endurance for both materials first increases and then decreases. This is attributed to two competitive mechanisms controlling the evolution of ECR endurance under varied current loads.

A Simple Semi-Analytic Contact Mechanical Model for Tangential and Torsional Fretting Wear of Axisymmetric Contacts

Fretting wear of axisymmetric contacts is considered within the framework of the Hertz–Mindlin approximation and the Archard law for the linear wear. If the characteristic time scale for the wear is much larger than the duration of a single fretting oscillation, the profile change due to wear during one fretting cycle can be neglected for the contact problem as a zero-order approximation. This allows to give an exact contact solution during each fretting cycle, depending on the current worn profile, and thus for the explicit statement of an ordinary integro-differential equation system for the time-evolution of the fretting profile, which can be easily solved numerically. The proposed method gives the same results as a known, contact mechanically more rigorous simulation procedure that also operates within the framework of the Hertz–Mindlin approximation, but works significantly faster than the latter one. Tangential and torsional fretting wear are considered in detail. A comparison of the numerical prediction for the evolution of the worn profile in partial slip torsional fretting of a rubber ball on abrasive paper shows good agreement with experimental results from the literature.

A Simple Semi-Analytic Contact Mechanical Model for Tangential and Torsional Fretting Wear of Axisymmetric Contacts

Fretting wear of axisymmetric contacts is considered within the framework of the Hertz–Mindlin approximation and the Archard law for the linear wear. If the characteristic time scale for the wear is much larger than the duration of a single fretting oscillation, the profile change due to wear during one fretting cycle can be neglected for the contact problem as a zero-order approximation. This allows to give an exact contact solution during each fretting cycle, depending on the current worn profile, and thus for the explicit statement of an ordinary integro-differential equation system for the time-evolution of the fretting profile, which can be easily solved numerically. The proposed method gives the same results as a known, contact mechanically more rigorous simulation procedure that also operates within the framework of the Hertz–Mindlin approximation, but works significantly faster than the latter one. Tangential and torsional fretting wear are considered in detail. A comparison of the numerical prediction for the evolution of the worn profile in partial slip torsional fretting of a rubber ball on abrasive paper shows good agreement with experimental results from the literature.

An Analytical Solution for the Initiation and Early Progression of Fretting Wear in Spherical Contacts

Abstract This article derives analytical solutions to calculate the wear volume at the initiation of fretting motion and its early progression over the first few oscillation cycles. The Archard-based model considers a deformable hemisphere that is contact with a deformable flat block. The material pairs investigated are special alloys, the Inconel 617/Incoloy 800H, and Inconel 617/Inconel 617. The analytical study begins with a unidirectional frictional sliding contact, where the local interfacial sliding distance and the nominal sliding distance at the initiation of gross slip are derived. The obtained analytical expressions for unidirectional sliding are then used to derive the corresponding wear volume for the initiation and early progression of gross slip and the wear volume for a general fretting cycle under elastic conditions. These analytical derivations are all verified by the finite element analysis (FEA). The FEA method and the analytical solutions render virtually identical results for both similar and dissimilar material pairs. The effects of plasticity on the wear volume under elastic–plastic conditions are also investigated. It is found that the fretting wear volumes obtained from the FEA simulations, which include plasticity, are close to those obtained from the analytical expressions for purely elastic regimes. All the results are presented in normalized forms, which can be easily generalized and applied to three-dimensional fretting wear of other material pairs.

The Evolution of Fretting Wear Behavior and Damage Mechanism in Alloy 690TT with Cycle Number.

The evolution of fretting wear behavior and damage mechanism in Alloy 690TT with cycle number was investigated via laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), focus ion beam (FIB), and transmission electron microscopy (TEM). The results showed that the fretting running status underwent a transition from partial slip and mixed stick-slip to final gross slip with the transformation of Ft–D curves from the ellipse to the parallelogram. The coefficient of friction (COF) experienced three drops throughout the fretting process, which indicated the transformation from high-friction wear to low-friction wear. The first drop was due to the transition from two-body to three-body contact. The second and third drops were mainly related to the evolution of the glaze layer from a localized distribution to completely covering the whole contact surface. The competition between fretting induced fatigue cracking (FIF) and fretting induced wear (FIW) ran through the entire fretting wear process. Before the 1.2 × 104th cycle, the fatigue crack growth was faster than wear, and FIF won the competition. As the fretting cycle continued to increase, the wear velocity was obviously faster than that of FIF, which indicated that FIW defeated FIF. The tribologically transformed structure (TTS) participated in the competition between FIF and FIW. The gain boundaries and dislocations in the TTS were a suitable pathway for crack initiation and propagation and oxygen permeation.

Dynamic changes of mechanical properties induced by friction in the Archard wear model

Fretting is small-amplitude, oscillatory motion between two bodies leading to surface damage. During the fretting process, a tribologically transformed structure (TTS) which has different mechanical properties and microstructure than the starting material is formed on the surface. The commonly-used Archard wear equation relating wear volume to the hardness of the worn material does not account for changes in mechanical properties induced by friction in fretting. To investigate that effect, ball-on-plane fretting tests were conducted on three engineering materials (type 316 stainless steel, pure copper, and titanium alloy Ti-6Al-4V) against an alumina ball to generate TTS layers. The evolution of mechanical properties and microstructures of TTS layers were investigated using nanoindentation and focused ion beam-scanning electron microscope (FIB-SEM). Wear volumes after different fretting cycles were measured with a white light interference microscope. Results show that the mechanical properties of TTS layers evolve differently on different materials during the fretting process. Microstructures of TTS layers also vary from one material to the other. A modified wear model that accounts for friction-induced dynamic changes in mechanical properties is proposed. In these tests the modified model was able to predict the wear volume of 316 steel and pure copper more accurately than the classical Archard model, but it was less successful in predicting wear on Ti6Al4V where there is added complexity from changing microstructure, oxidation, porosity and cracking.

Nano-indentation mapping of fretting-induced surface layers

Tribologically modified surface layer results from the energy dissipated in the frictional contact area. This layer usually has a different elastic modulus and hardness from the substrate, and its structure corresponds to the intermediate stage between a material of the first-body and debris of the third-body. Even though, the existence of the tribologically transformed structure in the fretting contact has been well proven, the formation and mechanical transformation mechanisms are still not clear. Hence, in this paper, evolution of mechanical properties of four metallic materials (titanium alloy, stainless steel, carbon steel, copper alloy) induced by fretting was investigated using nano-indentation mapping of the fretting wear scars. It was shown that the tribologically transformed structure formed very quickly within the initial fretting cycles, and its mechanical properties remained almost constant during the entire test duration for tested materials. However, it was observed that all materials responded differently to the frictional energy, exhibiting particular rate of change of the H/E ratio before and after the fretting experiment. Modified XRD technique was used to probe the friction induced changes within the small spots of the fretting scars, and revealed distinctive structural modifications within the transformed layers. The approach proposed in this study can be used to inform the predictive wear models, by providing information about the evolution of the mechanical properties of the tribo-system with time.

Derivation of a wear scar geometry-independent coefficient of friction from fretting loops exhibiting non-Coulomb frictional behaviour

One source of variation of the sliding tractional force in a gross-slip fretting cycle is the geometrical interaction of the developing wear scars on the opposing specimens. An existing model has been developed to include the compliance of the fretting test apparatus. It has thus been demonstrated that through the influence on the tractional force, the geometrical development of the wear scars affects the slip amplitude and the dissipation of frictional energy in each loop. A method to determine a coefficient of friction which is independent of system stiffness and developments in the geometry of the wear scars is proposed.

Effects of frequency on the intermittent failure phenomena of gold-plated electrical contact materials under fretting conditions

The typical characteristics of the change in contact resistance and tangential force with fretting cycles are compared with different fretting frequencies. Intermittent phenomena or short duration discontinuities during each fretting cycle are recorded explicitly. The instantaneous higher contact resistance values that appeared at the contact zones continuously corresponded to the distorted tangential force between contacts. Fretting frequency was determined as the crucial parameter that influenced the intermittent contact resistance. Subsequently, the contact failure mechanism of frictionally heated electrical contacts during high-frequency fretting was explained by introducing the electrical contact theory and thermoelastic instability model.

Prediction of the electrical contact resistance endurance of silver-plated coatings subject to fretting wear, using a friction energy density approach

Electrical connectors need to display low and stable electrical contact resistance (ECR). However, subjected to vibrations (engines, heat, etc.), fretting wear damages promotes the formation of insulating oxide debris and a sharp ECR increase decaying the information transmission. Noble plated coatings like silver layers are usually applied to delay the ECR failure. However, to predict such ECR endurance, it appears essential to determine the fretting wear rate. In the present study, a homogeneous crossed-cylinders Ag/Ag interface was investigated under gross slip conditions imposing various loading parameters (±2µm<δg<±16µm, 1N<P<6N, RH=10% and f=30Hz) for Ag layers from 1.7µm to 4.8µm. A global chemical investigation of the fretting scars confirms that ECR failure is reached when most of the silver is removed from the interface and an oxide debris layer is trapped in the contact. ECR endurance (Nc: ΔR>4mΩ) can be formalized, using a power law function of the mean friction energy density dissipated during a fretting cycle taking into account the contact area extension. Finally, a simple ECR endurance expression is derived as a function of the fretting loading parameters and the thickness of the silver plated layer. A very good correlation between experimental and predicted ECR endurances is achieved.

Investigation of non-Coulomb friction behaviour in reciprocating sliding

A commonly observed phenomenon where the friction force increases during the gross slip phase of individual fretting cycles is investigated with the aim of identifying the physical origins of this type of frictional behaviour. Measurements of sliding friction from linear and torsional fretting tests, using the aerospace nickel alloy Udimet 720, and subsequent analysis of the post-test worn surfaces were used to investigate the phenomenon. It was found that this friction variation is due to wear–scar interaction effects. These interactions were primarily found to occur at sites distributed throughout the nominal contact area via the interference of local interlocking peaks and troughs on the worn surfaces. Cross-correlation and auto-correlation analysis of the worn surfaces was used to identify, and to show the approximate size of, these local features. Many of the features were found to be similar in size to the applied fretting stroke, but on average, the features were somewhat larger. A simple one degree-of-freedom model of the interaction of an idealised surface peak with a corresponding surface groove was developed to show how these interactions produce the type of friction variation which is commonly observed during the sliding phase.

Fretting Wear Modeling of Coated and Uncoated Surfaces Using the Combined Finite-Discrete Element Method

Abstract A new approach for modeling fretting wear in a Hertzian line contact is presented. The combined finite-discrete element method (FDEM) in which multiple finite element bodies interact as distinct bodies is used to model a two-dimensional fretting contact with and without coatings. The normal force and sliding distance are used during each fretting cycle, and fretting wear is modeled by locally applying Archard’s wear equation to determine wear loss along the surface. The FDEM is validated by comparing the pressure and frictional shear stress results to the continuum mechanics solution for a Hertzian fretting contact. The dependence of the wear algorithm stability on the cycle increment of a fretting simulation is also investigated. The effects of friction coefficient, normal force, displacement amplitude, coating thickness, and coating modulus of elasticity on fretting wear are presented.

Friction and wear mechanisms of K 2O–B 2O 3–Al 2O 3–SiO 2–MgO–F glass-ceramics

Among various brittle materials, the mica-based glass-ceramics are of greater scientific interest, because of their machinability. Considering the potential of these materials as dental implants, an understanding of the wear behavior in oral environment is important. In the present investigation, K 2O–B 2O 3–Al 2O 3–SiO 2–MgO–F glass-ceramics containing about 70% crystals, heat treated at 1040 °C for 12 h was subjected to fretting against steel ball in artificial saliva (AS) environment. In order to elucidate the influence of environment on the friction and wear behavior, control experiments were also performed under dry ambient conditions. A systematic decrease in wear rate with test duration was recorded with a minimum wear rate of 10 −5 mm 3/Nm after 100,000 fretting cycles in AS medium. Scanning electron microscope–energy dispersive spectroscopy (SEM–EDS) analysis indicated the formation and brittle fracture of tribochemical layer in AS medium, whereas mica crystal pull-out was a dominant mechanism in dry conditions.

The relationship between AE and dissipation energy for fretting wear

This paper describes the behavior of acoustic emission (AE), especially the correlation between the AE output and the dissipation energy under the fretting conditions. Fretting tests are conducted with a ball contacting with a flat disc in air. The specimens used are a bearing steel for a ball, and a bearing steel or an aluminum alloy for a flat disc. The results show that the behavior of the AE output and the dissipation energy during each fretting cycle is not so similar to each other throughout the test, but the total AE root-mean-square and the total dissipated energy during the test have a good correlation between them.

Load dependent fretting wear properties of 3Y-TZP nanoceramics

Despite the fact that the processing of nanoceramics composites have been triggered to a great extent, the evaluation of their potential in engineering application is rather limited. To this end, the present work reports the results obtained with fretting wear (mode I, linear tangential displacement sliding) of Y-TZP nanoceramics, optimally densified using spark plasma sintering route. The tribological experiments were conducted by varying normal load (2–10N) for 100,000 fretting cycles with relative displacement stroke of 50 μm at a frequency of 8 Hz. Based on the investigation of the topographical features of worn surfaces, the mechanism of material removal has been proposed. An important finding is that TZP nanoceramic/steel tribocouple exhibit an increase in COF from 0.2 to 0.5 as well as severe wear damage with increase in load from 2N to 5N and above, under the selected experimental conditions.

Wear characterization of Ti–6Al–4V under fretting–reciprocating sliding conditions

Wear affects life, vibrational characteristics, and dimensional stability of the components that slide against each other in small oscillatory movements. This study investigated the fretting–reciprocating wear behavior of a titanium alloy under cylinder-on-flat contact configuration (both Ti–6Al–4V) with bulk cyclic loading. Four series of tests were conducted to characterize the effects of sliding and loading parameters on the wear behavior. These were the relative slip, contact load, number of cycles, and bulk cyclic stress. The reciprocating wear produced “U-” and “W-”shaped (scar with multiple valleys) scars on the specimen as well as on the pad. Optical and scanning electron microscopy of the wear scars was conducted to study the wear mechanism(s). MATLAB-based computer programs were developed to determine the wear volume using the three-dimensional profilometry data, the product of contact load ( P) and relative slip ( δ), and the energy dissipated ( E d) per fretting cycle. Analysis of the experimental data showed that the cyclic stress increased the relative slip. The ratio of shear force to contact load ( Q/ P) increased with number of fretting cycles. The wear volume was proportional to the cumulative products of Σ Pδ n and the dissipated energy Σ E d. The microscopy of W-shaped scars showed that the valleys were covered by abrasive grooves, whiles the peaks showed signs of adhesive wear, thus suggesting that the wear was caused by both the adhesion and abrasion processes.

FATIGUE PERFORMANCE OF STEEL WIRES WITH FRETTING WORN NOTCH

A series of experiments on the fretting wear of steel wires are performed on self-made fretting wear test rig. The worn wires after fretting tests are put into fatigue test at different stress ratio and different stress amplitude on the servo-fatigue test machine. The research results demonstrate that the fretting wear depth of the steel wires increase with the increasing fretting cycles and contact loads. The fretting worn notch is the source of crack initiation because of stress concentration, which decrease the fatigue life of steel wires. The fatigue life of the steel wires with fretted damage is inverse proportional to the wear depth. The fretting fatigue mechanisms are analyzed through SEM morphologies of fracture sections, which indicate that fatigue are divided into crack initiation, crack propagation and crack break corresponding to different fatigue phases.

Development of a friction energy capacity approach to predict the surface coating endurance under complex oscillating sliding conditions

In the case of surface coatings application it is crucial to establish when the substrate is reached to prevent catastrophic consequences. In this study, a model based on local dissipated energy is developed and related to the friction process. Indeed, the friction dissipated energy is a unique parameter that takes into account the major loading variables which are the pressure, sliding distance and the friction coefficient. To illustrate the approach a sphere/plane (Alumina/TiC) contact is studied under gross slip fretting regime. Considering the contact area extension, the wear depth evolution can be predicted from the cumulated dissipated energy density. Nevertheless, some difference is observed between the predicted and detected surface coating endurance. This has been explained by a coating spalling phenomenon observed below a critical residual coating thickness. Introducing an effective wear coating parameter, the coating endurance is better quantified and finally an effective energy density threshold, associated to a friction energy capacity approach, is introduced to rationalize the coating endurance prediction. The surface treatment lifetime is then simply deduced from an energy ratio between this specific energy capacity and a mean energy density dissipated per fretting cycle. The stability of this approach has been validated under constant and variable sliding conditions and illustrated through an Energy Density–Coating Endurance chart.

Research on the fatigue and fracture behavior due to the fretting wear of steel wire in hoisting rope

Hoisting steel rope is an important component of the winding equipment in coalmines. Fretting wear and its induced fatigue and fracture of wires have been the major failure modes of the hoisting ropes. In this paper, a series of experiments on the fretting friction and wear of steel wires were performed on an elastic beam oscillation test rig. The worn wires after fretting tests were put into fatigue test on the servo-fatigue test machine. The research results demonstrated that the fretting wear depth of the steel wires increased with the increasing fretting cycles and contact loads. The fatigue life of the steel wires with fretted damage was inverse proportional to the wear depth, and then to the fretting cycles and contact loads. The fretting wear and fatigue mechanisms were analyzed through SEM morphologies of fretting wear scars and fracture sections. It was found that the wear mechanism of fretting wire depended on the contact loads. For low contact loads, light scratch modes dominated the fretting of wires. When contact loads increased, third body abrasion appeared on the worn surfaces.