Fundamental investigations of electrical conductor fretting fatigue

Fretting is known to be the main factor leading to conductor individual wire breaks under aeolian vibration in the vicinity of a clamp. In this paper, previous studies on overhead electrical conductor bending fatigue are summarized. Results obtained with several conductor types and clamps are compared. A general fretting analysis as well as testing procedure are suggested. Influence of the main mechanical parameters on the occurrence of several types of degradation processes is discussed.     

A thermodynamic approach for predicting fretting fatigue life

This paper extends the application of the theory of continuum damage mechanics (CDM) to predict the occurrence of fretting fatigue failure. It is concerned with the application of thermodynamic theory of CDM for predicting crack formation in fretting. This is done by calculating subsurface shear stress distribution due to fretting loading conditions and obtaining corresponding principal plane location. For given fretting conditions such as the load, the coefficient of friction and the bulk material properties, the present model predicts the number of cycles to failure. The results are validated with published experimental data.     

Development of a test device for the evaluation of fretting in point contact

Fretting wear and fatigue may occur between any two contacting surfaces, wherever short‐amplitude reciprocating sliding is present for a large number of cycles. A test device has been developed for the evaluation of fretting fatigue and wear in partial and gross slip conditions. Three similar sphere‐on‐plane contacts run at the same time. Normal force, tangential force or displacement amplitude and constant bulk stress can be controlled and measured separately. Reciprocating tangential displacement is produced with rotational motion, the amplitude and frequency of which can be adjusted and controlled accurately by an electric shaker. The number of load cycles for crack initiation and growth is determined with strain‐gauge measurements near the fretting point of contact. The contact surfaces are measured with 3D optical profilometer before fretting measurements to determine actual contact geometry. The measurements were done with quenched and tempered steel. The initial results indicate that cracks are mostly formed in partial slip conditions, whereas fretting wear is more heavily involved in gross slip conditions. The initiation of a crack occurs near the edge of the contact in the slip direction, where the calculated cracking risk has its maximum value in partial slip conditions. The number of cracks increases as the displacement amplitude, i.e. friction force, increases in partial slip conditions. Copyright © 2008 John Wiley & Sons, Ltd.     

Thin layer activation for measuring fretting wear and material transfer in iron alloys

An important difference between fretting wear and macroscopic wear processes lies in the amounts of wear volume. Because of the small relative displacements in fretting wear the wear volumes are much smaller than in (classical) macroscopic wear processes. Classical wear measuring techniques such as weighing and 2D-profilometry often fail to determine the fretting wear volumes. Their sensitivity as well as their accuracy are inadequate. 3D-profilometry partially solves this problem but the obtained wear results do not take eventual material transfer between the test specimens into account. In this work thin layer activation (TLA) is presented as a potentially powerful tool for quantitative fretting studies, especially in relation to material transfer. In contrast with the other mentioned wear measuring technique TLA allows measurements of very small amounts of wear as well as material transfer between the test specimens. Preliminary results are reported for steel specimens. A surface zone of 7 mm² of one test specimen, in which the wearing zone during the fretting experiment is situated, is irradiated with protons from a cyclotron. As a result of a nuclear reaction the iron in the irradiated test specimen forms radionuclides, mainly cobalt-56, in a surface layer of 260 μm depth. The amount of cobalt-56 as compared to iron is negligible so that the physical and chemical properties of the irradiated zone remain unaffected. Measuring the gamma rays of cobalt-56 in the wear debris gives quantitative information about the wear volume and measuring the mating surface gives information on the transfer volume.     

An investigation of the fretting behaviour of low friction coatings on steel

Three different low friction coatings on steel (electrolytically deposited cadmium, PTFE solid lubricant in epoxy resin and PTFE solid lubricant in polyimide resin) were studied in order to relate their fretting behaviour with mechanical properties. Particular importance was given to adhesion which was measured using a scratch test. Fretting tests were carried out on the steel substrate and on the coatings under the same conditions. The major parameters of the tribological system were identified and then quantified. The values of the parameters obtained for each coating were compared with the corresponding values for uncoated steel. The mechanical characteristics of the coatings and their fretting parameters were plotted using a polar diagram in order to give an overview of the fretting behaviour of each coating. Differences were noted and the corresponding parameters were identified. The influence of the adherence of the coating and of the fretting test load on the lifetime of the coating was determined.     

The use of notch and short crack approaches to fretting fatigue threshold prediction: Theory and experimental validation

In this paper the similarities between fretted and notched components in terms of stress gradient and the consequent “size effect” are discussed. Critical distance and short crack arrest approaches for the prediction of fretting fatigue thresholds are then presented and the predictions are compared with experimental results. Two geometry and alloy combinations are considered in order to validate the prediction by the means of experimental results. In particular, both Hertzian fretting tests performed on Al4%Cu alloys and experiments carried out employing ‘flat and rounded’ contact pads, made of Ti6Al4V alloy, are used for the comparison. It is shown that both criteria provide good predictive capabilities. However, the short crack arrest method is less empirical and can be adapted to a wider range of applications (e.g. surface treated components).     

双梁U型门式起重机轨道下方疲劳裂纹分析与修复处理

针对某站场双梁U型门式起重机轨道下方出现疲劳裂纹的问题,分析了形成原因,指出应力集中、焊接残余应力和小车轮压的作用是产生疲劳裂纹的主要原因。给出了上盖板疲劳裂纹的具体修复措施和建议,对于安全生产具有重要意义。     

The metallographic characteristics of fatigue cracks in the titanium alloy IMI829 at 400°C and 600°C

High temperature fatigue tests under fluctuating axial tension were carried out on the alloy in the fully heat treated condition, the microstructure consisting of a β matrix containing colonies of aligned α-platelets. Fatigue cracks were initiated by fretting at temperatures of 400°C and 600°C. A detailed metallographic examination has been made of the fretted region and the resulting fatigue crack and these characteristics have been related to the microstructure of the alloy. At test temperatures of 400°C the fatigue cracks initially propagated at right angles to the α-platelets but where their orientation was unfavourable, propagation also occurred parallel to the α-platelets. At test temperatures of 600°C the initial direction of crack propagation was independent of the microstructure.     

A deterministic model for line-contact partial elastohydrodynamic lubrication

A deterministic model for partial elastohydrodynamic lubrication (EHL) is presented in this paper. The modelling methodology adopts some of the concepts used in the stochastic modelling of partial EHL and some of the procedures for deterministic calculation of asperity pressures. The model is shown to be capable of simulating the basic process of asperity interaction and solid-to-solid contact within an EHL conjunction of rough surfaces. Deterministic results of transient partial EHL in line contacts are obtained when one pair or multiple pairs of asperities collide. The model may help to gain a fundamental understanding of the transient behaviour of asperity interactions in lubricated concentrated contacts of rough surfaces. Asperity pressures may be calculated more accurately than the conventional analyses under dry and static contact conditions. The work represents a first attempt in deterministic modelling of tribo-contacts operating in the mixed regime of micro-EHL and boundary lubrication. Future work will aim at developing more realistic models incorporating factors such as three-dimensional asperity contacts, asperity plastic deformation, thermal effects and the effect of tribo-chemistry.     

Effect of oil environments on the initiation of fatigue cracks and propagation of small cracks in plain carbon steel specimens

In fatigue tests of plain carbon steel specimens in air, fatigue life is taken up mainly by the life in which a crack propagates from its initial size up to about 1 mm. This means that the behaviour of a small crack in the oil environment must be known in order to evaluate the effect of oils on fatigue life. In this paper, using a series of base oils of different viscosity grades, the effects of oils on fatigue damage are investigated in rotating bending fatigue tests of annealed 0.34% carbon steel plain specimens. Successive detailed observations of the specimen surface are made in order to study the fatigue processes of micro-crack initiation and small crack propagation. The physical background of the effect of oil environments on fatigue behaviour is shown, and a method for predicting fatigue life in oil environments is discussed.     

Investigation of high cycle and low cycle fatigue interaction on fretting behavior

Fretting-fatigue behavior and damage accumulation under a variable-amplitude cycling load is investigated in a configuration involving a cylindrical indenter in contact with finite width plate. Relative magnitudes of cyclic tangential and bulk loads not only affect the contact conditions, but also their relative positions with respect to each other. Several stick–slip conditions on the contact surface may develop during the application of variable-amplitude fatigue load, and these are secondary and tertiary slips as well as shake-down. Further, residual shear traction develops during the application of cyclic load. The appropriate characterization of fretting-fatigue behavior or life should, therefore, include the complete history of applied cyclic tangential and bulk loads. Furthermore, experiments from a previous study conducted under a variable-amplitude fatigue loading condition are analyzed to characterize the damage accrual from its individual components involving constant-amplitude fatigue load by incorporating the contact mechanics and a multi-axial fatigue critical plane parameter. This analysis shows that there is nonlinear damage accumulation during variable-amplitude fretting-fatigue load.     

采用Mindlin理论预测分析滚动微动疲劳裂纹萌生

针对滚动柱面过盈配合面的滚动微动失效形式,建立了预测微动裂纹萌生位置的控制模型,该模型应用控制参数W,综合考虑了接触面上的拉应力、剪切应力、摩擦系数、分布区域等主要裂纹萌生影响参数,对微动裂纹的萌生位置进行了预测。应用该预测模型对实际失效轴承裂纹萌生位置进行了预测,并与实际轴承失效的微观特征进行了对比,验证了预测模型的正确性。     

A simple datum for measurement of the Abbott curve of a profile and its first derivative

The methods used in some recent work to predict boundary friction from surface slopes require that the variation of rms slope with bearing area be calculated: an Abbott curve of the first derivative of the profile. It is shown that this curve can be distorted if a conventional filter is used to set the datum, in the same way as the Abbott curve itself. Better results can be obtained for both the standard Abbott curve and the Abbott curve of slopes if an envelope through the peaks of the profile is used as the datum, rather than a digital filter.     

Numerical analysis of grease thermal elastohydrodynamic lubrication problems using the Herschel-Bulkley model

Grease thermal elastohydrodynamic lubrication (TEHL) problems of line contacts are analysed numerically. The effects of temperature and rheological parameters on grease TEHL are investigated using the Herschel-Bulkley model as a rheological model of greases. The pressure distribution, the shape of grease film, mean film temperature and surface temperature of solid wall in line contacts are obtained. It is found that thermal effects on the minimum film thickness become remarkable at high rolling speeds. The effect of yield stress of the Herschel-Bulkley model on minimum film thickness is negligible, while the flow index and viscosity parameter have significant effects on minimum film thickness.     

Effects of humidity and contact material on fretting fatigue behavior of an extruded AZ61 magnesium alloy

Fretting fatigue tests of the extruded AZ61 magnesium alloy with the same contact material under low and high humidity were carried out to investigate basic fretting fatigue characteristics and effect of humidity on fretting fatigue behavior. Influence of contact material was also studied by using JIS S45C carbon steel contact material. Degradation of fatigue strength due to fretting was much more significant than that due to corrosion under high humidity condition. Therefore, no effect of humidity on fretting fatigue strength was found. Reduction rate of fatigue strength due to fretting for the magnesium alloy was between those of aluminum alloys and titanium alloys. Tangential force coefficient of the magnesium alloy was rather low compared to other materials such as steels, aluminum alloys and titanium alloys. Fretting fatigue strength with the S45C contact material was inferior compared to that with the same contact material. This is mainly due to higher tangential force in AZ61/S45C contact. Fretting fatigue cracks at the edge of fretting contact region were observed to nucleate in the very early stage of fatigue life, similar to other structural materials.     

A new method for the quantification of nucleation of fretting fatigue cracks using asymptotic contact solutions

This paper introduces a recently developed family of local, self-contained contact solutions which can be used to compare rigorously the states of stress in the microslip region of a range of contacts. This provides (a) a rigorous means of designing experimental fretting fatigue test geometries to match those in complex prototypes rigorously, and (b) gives the possibility of developing a reliable database of generalszed stress intensity thresholds for design against crack nucleation.     

On the application of a micromechanical small fatigue crack growth model to predict fretting fatigue life in AA7075-T6 under spherical contact

This work presents a method for assessing the fretting fatigue life by estimating the fatigue crack growth rate from the regime of microcracks to the final failure, which is achieved using a two-threshold small fatigue crack growth model. The propagation thresholds are associated with the interaction of the "monotonic plastic zone" and the "cyclic plastic zone" with the microstructure of the material. The predicted fatigue life and the estimated non-propagating cracks agree very well with the experimental fretting fatigue tests with spherical contact in 7075-T6 aluminium alloy.     

Modelling and evaluation of the fretting fatigue cracking risk in smooth spherical contacts

A numerical model for the calculation of fretting fatigue crack initiation is presented and compared with experiments. The model is focused on smooth sphere-on-plane contact in partial and gross slip conditions. It is based on Hamilton’s explicit stress equations and the multi-axial Dang Van and Findley fatigue criteria enhanced with a statistical size factor concept. Promising correlation was found between the model and the experimental results with quenched and tempered steel 34CrNiMo6. The model assumptions, limitations and general application are also discussed.