Radial fretting fatigue damage of surface coatings

Radial fretting tests with a 52100 steel ball-on-flat contact have been carried out under different normal loads. TiN, MoS₂ and TiN+MoS₂ coatings on a 1045 steel flat were examined. The normal loads amplitude used were 200, 400 and 800 N at speeds of 12 and 1.2 mm/min. Dynamic analysis in combination with microscopic examinations by SEM and EDX have been performed. It was observed that the vertical stiffness increased with the increase of loading speed and number of cycles. The metallographic examinations showed that little damage was observed for the MoS₂ coating, which exhibited excellent radial fretting fatigue resistance. For the TiN coating, micro-cracks appeared at the lower load while delamination occurred at the higher load. For the TiN+MoS₂ composite coating, the vertical stiffness increased but accompanied by some micro-cracks. As a result of the study, the radial fretting test is proposed as one possible new method to evaluate coating life.     

An investigation of fretting behaviour of several metallic materials under grease lubrication

Fretting wear tests under grease lubrication have been carried out on an aluminium alloy, 52100 steel and low-alloy steel. The sphere–flat contact configuration is used. The influence of the displacement amplitude and normal load is investigated. Comparison between dry and lubricated contact of aluminium alloy, between 52100/52100 steel and 52100/low-alloy steel contact with grease lubrication has been carried out. Results show that grease lubrication strongly affects fretting behaviour. Base oil that separated from the grease during friction may result in accelerated contact wear by fretting.     

An experimental study torsional fretting behaviors of LZ50 steel

Four simple fretting modes are defined according to relative motion: tangential, radial, rotational, and torsional fretting. This paper presents a new test rig that was developed from a low-speed reciprocating rotary system to show torsional fretting wear under ball-on-flat contact. Torsional fretting behavior was investigated for LZ50 steel flats against AISI52100 steel balls under various angular displacement amplitudes and normal loads. The friction torques and dissipation energy were analyzed in detail. Two types of T–θ curves in the shape of quasi-parallelograms and ellipticals were found that correspond to gross and partial slips, respectively. The experimental results showed that the dynamic behavior and damage processes depend strongly on the normal loads, angular displacement amplitudes, and cycles. In this paper, the debris and oxidation behaviors and detachment of particles in partial and gross slip regimes are also discussed. Debris and oxidation are shown to have important roles during the torsional fretting processes. The wear mechanism of torsional fretting was a combination of abrasive and oxidative wear and delamination before third-body bed formation. The mechanism was then transformed into third-body wear after a great amount of debris formed.     

不同水介质下GCr15/45#钢微动腐蚀行为研究

在洁净水、酸性水和海水3种不同水介质条件下，对GCr15／45^#钢进行了不同位移幅值的微动磨损试验。结果表明：洁净水和酸性水改变了微动区域，降低了摩擦系数和磨损量，具有润滑效果，磨损受水润滑和介质腐蚀双重作用影响,且润滑作用随位移幅值的增加而减弱；对海水，摩擦系数与干态相近,电化学腐蚀作用强烈，磨痕中可观察到大量点蚀坑。     

An experimental study on bending fretting fatigue characteristics of 316L austenitic stainless steel

Bending fretting fatigue tests of 316L austenitic stainless steel plates against 52100 steel cylinders have been carried out under same normal load and varied bending loads. Tests of plain bending fatigue were carried out as a control group. The S-N curves of the bending fatigue were made. The results indicated that there was an obvious drop of life under the condition of bending fretting fatigue due to higher local contact stress. A dislocation model of micro-crack nucleation mechanism, as a manner of zig-zag mode, was created to explain the nucleation of fretting fatigue cracks.     

Fretting behaviour of galvanised steel

The present work describes research conducted on the fretting behaviour of S 355 MC galvanised steel sheet. In order to study the influence of the normal load and the displacement effect, some of the specimens were galvanised by hot dipping and the rest were only polished before being tested. Fretting tests were carried out on a specially developed fretting rig prototype under ‘crossed-cylinders’ contact geometry. Tests were done during 0.72×10⁶ cycles in laboratory air conditions. The tangential force and the displacement were measured in order to establish the fretting cycles for each fretting condition. The fretted surfaces were analysed by means of optical and scanning electron microscopes to identify the main wear mechanisms. Three different fretting regimes were identified: the stick regime; the slip regime; and the mixed stick–slip regime, which depended mainly on the influence of the normal load and the stroke.     

The effect of grain flow orientation on the fretting fatigue characteristics of forged AMS6415 steel

The effect of grain flow orientation on the fretting fatigue response of AMS6415 steel specimens in contact with fretting pads of the same material obtained from a shaft-propeller flange forging was investigated statistically. Coupon specimens were machined from one forging. The specimens were obtained at two different orientations (radial and tangential) with respect to the forging axis. S-N (maximum stress versus the number of stress cycles to complete fracture) fretting fatigue tests were performed with a randomized paired comparison test matrix. The nominal maximum fatigue stress was +400 MPa, with an R ratio of +0.05, and the nominal normal contact pressure was ?40 MPa.Resultant test data were studied statistically. Student t tests, analysis of variance tests and Weibull analyses were performed. Test results suggested strongly, but not conclusively, with 90% confidence, that fretting fatigue lives of tangential specimens had a larger mean and a greater variance than those of the radial specimens. Metallographic and fractographic observations also were performed and are presented in this paper.     

Analysis of the Fretting Conditions in a Contact Between an Epoxy Thermoset and a Glass Counterface

The fretting conditions in a contact between an epoxy thermoset and a glass counterface have been investigated using a specific device which allows in situ observation of the contact area. The critical displacement for transition from partial slip to gross slip conditions was investigated by the in situ detection of the micro-displacements and by the analysis of the fretting loops. Experimental results were in good relation with the theoretical predictions derived from Mindlin's approach of incipient sliding. Depending on the loading conditions, a progressive change from gross slip to partial slip conditions was found to occur during the early stages of the fretting loading, i.e., before the appearance of any macroscopic damage such as cracking or particle detachment. These fretting conditions were synthesized in a fretting map giving the boundary between various fretting regimes as a function of the normal load, the imposed displacement and the number of cycles.     

沙尘环境对金属抗磨损特性的影响

通过向摩擦副所在隔离空间内鼓沙的方式实现对自然沙尘环境的模拟。在沙尘环境和干摩擦情况下,对比研究了随着载荷的增加,沙尘环境的存在对C52100铜、2124铝、AISI-1045钢、Z25铸铁、AISI-52100钢5种不同硬度的典型金属抗磨损行为的影响。结果表明,在沙尘条件下,5种金属的磨损率随载荷的变化呈现一定的变化规律,C52100铜和52100钢的磨损率均随载荷的增加而减小,2124铝和Z25铸铁的磨损率随载荷的增加而增大,而1045钢的磨损率则先增大后减小,但磨损率随载荷变化非常小。沙尘的存在使52100钢、1045钢和Z25铸铁的磨损率比干摩擦条件下有大幅度的减小,但是却加剧了2124铝在较高载荷下的磨损,这说明对不同的材料而言,沙尘所起的作用是不同的,它可能会加剧材料的磨损,但也有可能提高材料的抗磨损性能。     

The effect of variable amplitude loading on stress distribution within a cylindrical contact subjected to fretting fatigue

A finite element model of a cylindrical Hertzian contact on a test sample subjected to alternating shear loading has been developed. The model has been used to investigate shear stress distributions at the contact during variable amplitude fretting fatigue for a load configuration in which the sample cyclic stress is applied in phase with shear force on the cylindrical contact. It has been found that during constant amplitude cyclic loading, shear stress distributions and positions of the stick-slip boundary at load maxima and minima remain fixed. Application of overloads changes the stress distribution and the position of the stick-slip boundary attained by loading of subsequent cycles. The largest cycle maximum stress determines the position of the stick-slip boundary adopted by subsequent smaller amplitude cycles. In general variable amplitude fretting fatigue the position of the stick-slip boundary will be changing with each load cycle. Hence fatigue initiation processes will occur at locations dispersed over an extended region over the contact. The implications of this behaviour for models for fretting fatigue life calculation are explored.     

An Investigation into the Effect of Normal Load Frequency on Fretting Fatigue Behavior of Al7075-T6

Most previous studies on fretting fatigue have been accomplished under constant normal loading and less attention has been paid to cyclic normal loading. An innovative test apparatus was specially designed and manufactured for fretting fatigue tests under cyclic loading in this work and the fretting fatigue behavior of Al7075-T6 was investigated at different normal load frequencies. A finite element model was developed to study the effect of normal load frequency on the contact stress distribution. It was found that the cyclic normal load has a more damaging effect on fretting fatigue life compared to constant normal load, particularly at lower frequencies. The results showed that at the normal load frequency of f = 1 Hz, fatigue life decreased by 52% in the high cycle fatigue regime and 28% in the low cycle fatigue regime. The experimental results also indicated that at the normal load frequency of 80 Hz, the fretting fatigue life converged to its corresponding life under constant normal load condition. The fracture surface and the fretting area of the specimens were examined using both optical and scanning electron microscopy (SEM). The experimental observations showed that the dominant partial slip condition with a wider slip region compared to constant normal loading, severe delamination, and higher oxidation rate due to the normal load release at each cycle, are the most important reasons for significant reductions in fretting fatigue life, under cyclic normal loading, especially for low normal load frequencies.     

Fretting wear mechanisms of Zircaloy-4 and Inconel 600 contact in air

The fretting wear behavior of the contact between Zircaloy-4 tube and Inconel 600, which are used as the fuel rod cladding and grid, respectively, in PWR nuclear power plants was investigated in air. In this study, number of cycles, slip amplitude and normal load were selected as the main factors of fretting wear. The results indicated that wear increased with load, slip amplitude and number of cycles but was affected mainly by the slip amplitude. SEM micrographs revealed the characteristics of fretting wear features on the surface of the specimens such as stick, partial slip and gross slip which depended on the slip amplitude. It was found that fretting wear was caused by the crack generation along the stick-slip boundaries due to the accumulation of plastic flow at small slip amplitudes and by abrasive wear in the entire contact area at high slip amplitudes.     

Fretting damage in thin sheets: Analysis of an experimental configuration

A method for evaluating fretting damage in thin sheets was developed for AISI 301 stainless steel in full hard condition in contact with AISI 52100 steel and cast ANSI A356 aluminum. Samples were subjected to fretting and then were subsequently fatigue tested to determine the impact of the fretting damage on fatigue life. A finite element model of the experimental configuration was used to determine the response for the experimental conditions imposed. The values of Fatemi-Socie critical-plane fatigue damage parameter are shown to correspond to the trends in the observed residual fatigue life for contact with AISI 52100 steel.     

On the behaviour of an oil lubricated fretting contact

Although many engineering situations involving fretting damage are lubricated, comparatively little has been reported on this aspect of fretting wear. The viscosity of the lubricating oil and its boundary layer performance are expected to influence fretting behaviour, in addition to the normal fretting parameters, such as stroke and contact force.

This paper examines the effect of lubrication regime, oil viscosity and stroke on the behaviour of a ball-against-flat specimen arrangement. Ball and flat specimens were both manufactured from a bearing steel (SUJ2). Polybutane oils, without additives, covering a range of viscosities from 1 to 10 000 cSt, and fretting strokes up to 35 μm were investigated. The lubricating oil was added to the fretting interface after 0, 3 and 2000 fretting cycles had been completed. Lubrication regime, oil viscosity and stroke were all found to affect fretting behaviour in terms of both coefficient of friction (or traction coefficient) and wear. For strokes less than 9 μm, i.e. for conditions approaching almost complete ‘stick’, coefficient of friction values under oil lubrication were well in excess of double those observed without it. These high values suggest that the oil was unable to penetrate into the fretting contact region, but did maintain a shield around it, so that metal-on-metal contact was maintained under oxygen deprived conditions. The lowest values of steady state coefficient of friction (≈ 0.2) were observed when oil lubrication was applied after 2000 cycles had been completed, indicating that surface roughening and the presence of oxide films and oxidised debris assisted penetration of the lubricant into the fretting contact zone.     

Fretting cracking behaviour on pre-stressed aluminium alloy specimens

A new testing rig has been developed which enables fretting tests on pre-stressed specimens to be carried out. Three aluminium alloys, Al-Li 2091, Al-Cu 2024 and Al-Zn 7075, were used in the tests. The imposed amplitude D ranged from ± 10 to ± 75 μm and normal load F_n from 500 to 1000 N. The static external stress σ_S was set as σ_D/10, σ_D/2 and σ_D (σ_D is the fatigue limit). The tests were carried out with a frequency of 1 or 5 Hz up to 10⁶ cycles. In this paper, analysis of fretting behaviour has been carried out using the fretting map concept. The effect of the axial load (pre-stress) on fretting cracking is emphasized.     

Fretting corrosion of tin-plated contacts

The susceptibility of tin-plated contacts to fretting corrosion is a major limitation for its use in electrical connectors. The present paper evaluates the influence of a variety of factors, such as, fretting amplitude (track length), frequency, temperature, humidity, normal load and current load on the fretting corrosion behaviour of tin-plated contacts. This paper also addresses the development of fretting corrosion maps and lubrication as a preventive strategy to increase the life-time of tin-plated contacts. The fretting corrosion tests were carried out using a fretting apparatus in which a hemispherical rider and flat contacts (tin-plated copper alloy) were mated in sphere plane geometry and subjected to fretting under gross-slip conditions. The variation in contact resistance as a function of fretting cycles and the time to reach a threshold value (100 mΩ) of contact resistance enables a better understanding of the influence of various factors on the fretting corrosion behaviour of tin-plated contacts. Based on the change in surface profile and nature of changes in the contact zone assessed by laser scanning microscope (LSM) and surface analytical techniques, the mechanism of fretting corrosion of tin-plated contacts and fretting corrosion maps are proposed. Lubrication increases the life-time of tin-plated contacts by several folds and proved to be a useful preventive strategy.     

载荷对径向微动磨损的影响

为了研究在实际工况中较为常见的圆柱/平面接触副的径向微动磨损特性,分析载荷对径向微动磨损影响,本文通过ANSYS建立圆柱/平面的径向微动磨损模型,分析施加载荷过程,不同时间点的载荷对径向微动磨损的影响,并通过接触切应力和X方向应力的分析,提出径向微动磨损在粘滑过渡点以及X方向应力为零的点为裂纹萌生点.     

Radial fretting behaviours of dental ceramics

Radial fretting tests on a Si₃N₄ ceramics ball opposite to the two dental ceramics flats (Vita VMK95 and Cerec Vita Mark II) have been carried out. The test apparatus was developed from a tension-compression hydraulic machine. Maximum normal load (F_max) was varied from 100 to 800 N with a speed of 6 mm/min, and the number of cycles from 1 to 10⁵. The fretting scars were examined by optical microscope and laser confocal scanning microscope (LCSM). The results of kinetics behaviours showed that all loading and unloading curves of load (F)–displacement (D) curves were almost superposed in the whole fretting process for two dental ceramics under a lower normal load (), and all F–D curves opened, correspondingly some micro-cracks initiated and developed in contact area, when the normal load increased to a higher level (). Under lower normal load, the fretting scars displayed the worn zones in shape of annulus. The cracks in shape of homocentric circularity can be observed for Vita VMK95 at and for Cerec Vita Mark II at . Therefore, there is better radial fretting damage resistance for Cerec Vita Mark II. In the microslip zone, the microscopic analysis showed that the particles detached by the mechanism of delamination. It the meantime, the competing mechanisms of fatigue cracks and wear also were discussed in this paper.     

Dual-motion fretting behavior of mandibular cortical bone against pure titanium

Dual-motion fretting tests of flat cortical bone specimens from fresh human mandible against pure titanium (TA2) ball were carried out on a modified test rig with tilt angle of 45°. The imposed maximal loads varied from 100 to 200 N. Dynamic characteristics of dual-motion fretting tests were analyzed in combination with micro-examinations via optical microscopy (OM), laser confocal scanning microscopy (LCSM) and scanning electron microscopy (SEM) together with energy dispersive X-ray spectrum (EDX). Two types of F-D curves (the trapezoid and elliptic mode) were recorded during the tests. The examination showed that the wear scars of the dual-motion fretting were asymmetric, and the tangential component of dual-motion fretting was in the mixed fretting regime. Under the lower imposed load, only some detachment of particles and scratches without cracking were observed even after 5×10⁴ cycles. The main wear mechanisms of the dual-motion fretting damage were the abrasive and adhesive wear. Under higher imposed loads, the cracks initiated and propagated mainly at the high stress side of contact edges. The wear mechanisms of the dual-motion fretting of cortical bone under higher imposed loads were the combination of the adhesive wear, abrasive wear, cracking and lubrication of the human bone tissue debris. And the lubrication of the debris played an important role during the dual-motion fretting processes.