Deformation characteristics at contact interface in ring compression

The main objective of this study is to provide the deformation characteristics in detail in ring compression, especially at the tool/workpiece interface. The surface flow patterns at the contact boundary are analyzed in terms of surface expansion, surface expansion velocity, pressure distributions exerted on the die surface, relative sliding velocity between die and workpiece, and the sliding distance along the die surface. Movement of neutral positions and folding phenomenon is also been investigated to see its effect on the flow pattern, that is geometrical change, which is important to measure the frictional condition at the interface using calibration curves.     

On the measurement of friction coefficient utilizing the ring compression test

The main objective of this research was to investigate whether generalized friction calibration curves, as recommended in the literature for use with ring compression tests, are applicable to all types of materials and test conditions. Specifically, the effects of material properties, strain-rate sensitivity, and “barreling” on the behavior of friction calibration curves were investigated. To this end, a series of ring compression tests were conducted in order to determine the magnitude of the friction coefficient, μ, as well as the corresponding calibration curves for two types of modeling materials, white and black Plasticine. The experiments were first conducted using the Physical Modeling Technique (PMT) and then simulated via an elastic–plastic finite element code (ABAQUS). In contrast to the results available in the literature, where the same friction calibration curves are recommended for all types of materials and test conditions, the results of this investigation showed that friction calibration curves are indeed affected by the material properties and test conditions and every material possesses its own distinctive friction calibration curve.     

Influence of a ring flat zone in the point contact surface on thermal elastohydrodynamic lubrication

This paper describes a geometrical profile, an elastohydrodynamically lubricated point contact surface with a ring flat zone, aimed at building up local line contact elastohydrodynamic lubrication (EHL) in point contact conjunctions to reduce the influence of side-leakage on the central film thickness. Effects of the ring flat zone on the thermal EHL characteristics are studied. A dimensionless coefficient, r_W, is defined to represent the relative half width of the ring flat zone in a point contact EHL surface. Thermal EHL numerical simulations have been performed to investigate the influence of r_W on the film thickness as well as pressure, temperature and friction coefficients under different operating conditions. In the range of 0≤r_W≤1.0 results show that the minimum film thickness decreases with increasing r_W and the central film thickness increases with increasing r_W, and the influence of r_W on the film thickness is more pronounced than those on the maximum pressure, the maximum temperature and the friction coefficients. It is revealed that the proposed ring flat zone with appropriate width is beneficial to the thermal lubrication.     

Analysis of the contact between cubic profiles

The contact between cubic profiles with continuous relative curvatures has been analyzed and solutions in closed form were obtained for both symmetric and non-symmetric conditions. Analytical solutions have allowed a complete parametric study to be performed, showing the effects of the gradient of relative curvature. The accuracy of approximate ‘equivalent’ Hertzian solutions has been verified.     

The digitization of surface profiles

The increasing dependence of surface roughness analysis upon digital computation causes a consequential need to specify conditions for digitizing the data. Measurement conditions should be related to the functional significance of the measurements, but there are occasions when the conditions must be specified without the necessary information being available. In this paper concern is with such occasions, with particular reference to the digitization sample interval of data from stylus-based profile-measuring systems. Aspects of the interaction of this interval and the finite stylus size with the surface are discussed and practical examples are given. It is unlikely that a general answer to the problem of sampling interval exists but plausible practical schemes can be suggested empirically.     

Statistical behaviour of surface profiles

Since the surface profiles obtained by stylus instruments may be considered as a random process, it is possible to represent these profiles by statistical parameters and functions. The usual geometrical parameters from surface profiles used for the specification of roughness do not completely give the nature of the profile. To obtain a better comparison of the profiles, their statistical similarities may be studied, but the number of such parameters used for the specification of surface profiles should, however, be limited for practical applications. Use of the autocorrelation functions allows the periodic and random nature of the profile to be established, but a simple quantitative value for these functions has yet to be developed. The paper analyses a few statistical parameters obtainable from surface profiles to determine their real significance. An analysis of the statistical behaviour of the profile features is also included.     

Mechanical filtration of surface profiles

This paper presents the analysis of factors affecting the small wavelength content of profile after measurement by stylus profilometer. Three following factors are considered: the shape and size of stylus tip sampling interval, and short-wavelength cut-off. The effect of mechanical filtration of replica measurement is considered. The influence of skid is also analysed. The real and computer generated random profiles of normal and asymmetrical shapes of the ordinate distribution are the objects of investigation. 2D profiles are analysed, but the effects of tip radius size as well as the skid effect were analysed in three-dimensional surface.     

Effective contact stress at the surface of barrel-shaped straight teeth on involute gears

In simulation of the three-dimensional contact problem for modified (barrel-shaped) gear teeth in cylindrical straight-tooth involute transmissions, skewing of the gear shafts due to manufacturing and assembly errors and to deformation of the drive components under load is taken into account. With considerable skewing, the limiting factor is not the normal stress but the effective contact stress.     

A surface roughness parameter in Hertz contact

The influence of surface roughness on the accuracy with which the Hertz theory of elastic contact predicts the contact pressure and contact area between a sphere and a plane is examined theoretically and experimentally. Statistical theories of surface contact suggest that the influence of surface roughness is governed primarily by a single non-dimensional parameter α defined by $\text{α }\text{σR}\text{a}{}_0{}^2$ where σ is the combined roughness of the two surfaces, R is the radius of the sphere and a₀ is the contact radius for smooth surfaces given by the Hertz theory. Experimental measurements of contact area correlate well with this parameter. Provided that the value of α is less than about 0.05, errors in the application of the Hertz theory due to roughness of the surfaces are not likely to exceed about 7%.     

Point contact surface fatigue in zirconia ceramics

The surface deformation and fragmentation behaviour of three zirconia ceramics have been studied by using unlubricated metallic repeated point contact loading at room temperature to investigate the possibilities of cyclic fatigue effects. All tests were conducted on a purpose built computer-controlled apparatus. The zirconias studied were ceria stabilized tetragonal polycrystalline, magnesia partially stabilized, and single crystal calcia stabilized. 120° steel cones were cyclically loaded against the flat, polished zirconia counterfaces, and the damage was observed and analysed as a function of the number of cycles, up to a total of 5 × 10⁵ cycles, for loads of 19.6 ± 9.8 N. The ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the zirconia counterface. The contact pressures were in the range 4 to 10 GPa. In all cases plastic deformation was observed in the zirconias within, and adjacent to, the contact areas. The degree of plastic deformation increased with increasing number of cycles. After approximately 1 × 10⁴ cycles, localized cracking was induced at the peripheries of the contact zones, which gradually increased in extent until after 5 × 10⁵ cycles there was extensive fragmentation. No material transfer, i.e. metal onto ceramic, or vice versa, was generally observed until after the surface had become rough as a result of the fracturing.     

Surface roughness and material strength of tribo-pairs in ring compression tests

The quantification of friction by means of simple models such as Amonton-Coulomb’s and the law of constant friction is questionable because the influence of surface roughness and material strength, among other factors, is not considered.This paper provides a comprehensive analysis on the influence of surface roughness and material strength in friction and proposes a new operator based on a sigmoid function to incorporate the combined influence of both phenomena in a modified version of the Amonton-Coulomb’s law.The presentation is supported by thoroughly researched quantitative data obtained from experimentation and finite element modelling of the ring compression test.     

A study of the measurement of surface temperature at a cam/tappet contact

Cam/tappet wear is one of the most difficult reliability and durability problems to predict during the development of a prototype engine valve‐train system. In the present study, the cam/tappet surface temperature was measured and calculated to determine the effect of surface temperature on the lubricating conditions at a cam/tappet contact. The measurement method, sometimes called the dynamic thermocouple technique, was based on the Seebeck effect: by using different materials for the cam and tappet, the cam/tappet contact point becomes the hot junction of a ‘thermocouple’. The cam/tappet contact surface temperature can therefore be measured continuously. The measured temperature results show good agreement with predictions. Temperature spikes, however, were not found in the test using new oil, but were shown by the theoretical model. Such results indicate that the cam/tappet interface operates in effective lubricating conditions, even when the oil temperature is relatively high. Further research with degraded oil supplied at high temperature is needed for a thorough understanding of abnormal cam wear.     

化学机械抛光过程中护环对硅片表面接触压强分布和宏观轮廓的影响

为了获得单晶硅片化学机械抛光过程中护环对接触压强分布的影响规律,根据有护环化学机械抛光实际出发,建立了抛光过程的接触力学模型和边界条件,利用有限元的方法对有护环抛光接触状态接触压强分布进行了计算和分析,并利用抛光实验对计算获得结果进行了验证;获得了硅片与抛光垫间的接触表面压强分布形态,以及护环几何参数对压强分布的影响规律;结果表明护环抛光接触压强的分布也存在不均匀性,而且在硅片外径邻域内接触压强最大,这些也能导致被加工硅片产生平面度误差和塌边,选择合理地护环几何参量和负载比,可以改善接触压强场分布的均匀性。     

A test method to study the change in surface topography due to running‐in of a rolling contact

A method is proposed to determine the change in surface topography during running‐in of rolling contacts. Two types of experiments have been conducted in the present work to study the running‐in of the pure rolling contact situation: repeated moving and general free rolling using a high accuracy measurement setup. The results show that the surfaces run‐in with the proposed repeated moving contact method gives a fast running‐in when compared with the general free rolling contact method. The proposed repeated moving contact method appears to be a good method to study the ideal or pure rolling contact situation because wear, as present in the general free rolling contact method, is avoided. Copyright © 2013 John Wiley & Sons, Ltd.     

A rolling ball test for establishing contact fatique behaviour of bulk materials, surface layers and coatings

This paper describes a new method for characterising contact fatique behaviour of bulk materials, surface layers and coatings. It is based on contact between a tungsten-carbide ball and a cylinder. In principle most ‘pin-on-ring’ type apparatus can be adapted to meet the test requirements. The method seems particularly attractive for charaterising the performance of surface layers and coatings under dynamic loading conditions. Examples relate to carburised steel surfaces and to a series of six hard chromium coatings with different bond strengths, obtained by applying different plating conditions.The test with the chromium coatings were performed under conditions of substantial initial plastic deformation of the steel substrate material. They were supposed to yield information on the ‘dynamic bond strength’ of the coatings.It was found to be possible to distinguish four quality classes, in agreement with expectations expressed by a plating expert.     

Effects of surface roughness and surface films on contact resistance

An analysis of the electrical contact resistance between two metals with a tarnish film was carried out assuming the asperities to be represented by randomly distributed cones with base angles which vary with the surface roughness, and assuming that the radius of the broken area of the film at the interface of each contact asperity is constant beyond a critical depth of penetration of an asperity.The validity of the proposed theory was confirmed by experimental data of the electrical contact resistance between a silver cone and a silver flat on which carbon films were deposited, and between a silver flat with an Ag₂S tarnish and a palladium flat without tarnish. Comparison of theoretical and experimental data shows that the critical depth of penetration U_e of an asperity varies mainly with the surface roughness, the thickness of the tarnish film and the amount of plastic deformation of the contact asperities with films, i.e.$\text{U}{}_{\mathrm{e}}\text{σ}\text{= kR}{}^{\mathrm{j}}{}_{\max }$, where R_max and σ are the maximum height and the standard deviation (r.m.s. roughness) of the profile ordinates, and j and k are constants dependent on the type of finish, the thickness of the film and the difference in the hardness of the mating surfaces.     

The contact pressure distribution during plastic compression of lead spheres

Pressure-sensitive film has been used to examine the form of the contact pressure distribution for lead spheres plastically compressed between hard steel platens. The mean contact pressure attained a maximum value of 0.6–0.7 times the Vickers hardness of the undeformed sphere when the sphere had been deformed to , where a is the contact radius and R is the radius of the undeformed sphere. The form of the pressure distribution changed markedly as the contact area increased in size, with an increasingly larger proportion of the load being supported towards the contact perimeter. The results are compared with theoretical pressure distributions proposed by Matthews [Acta Metall.28, 311–318 (1980)] for contact between spheres. Fair agreement is obtained if the sphere is assumed to deform by power-law creep, which is thought to reflect the observed time-dependent plastic deformation of the lead.     

The real area of contact and compliance of rough cylinders in compression

Bearing area analysis has been used to study the real area of contact and compliance of rough turned steel cylinders in compression. Calculations show that the elastic real area of contact is very small compared to the plastic real area of contact, and that local compliance due to flattening of asperity tips is a small proportion of the total compliance obtained from experiments. The fact that increased load brings more and more new asperities under load rather than enlarging the contact spots leads to a rather simple load-compliance relation for a rough cylinder, viz., W' = N_h · K₁δⁿ, where W₀ = K₁δⁿ defines the load-compliance relation of the individual asperities, and N_h represents the number of asperities bearing the load.     

Evaluation of non-metallic inclusions of steels used for rolling bearings from fracture surface by rotating ring fatigue fracture test

Recent improvements in steel-making techniques have much decreased the content of non-metallic inclusions in steels. Inclusions are known to be detrimental to the rolling contact fatigue life of bearings. While bearing life has improved because of advances in steel-making techniques, the conventional method for evaluating non-metallic inclusions is losing its effectiveness. A new method for evaluating the non-metallic inclusion content of steels used for rolling bearings has been developed. In ring-type rotating fatigue fracture tests on quench-hardened specimens a crack usually initiates on the inside surface of the ring specimen. However, a crack initiates near the outside surface when the specimen has been treated, to produce a large residual tensile stress on the outside surface before the test. In this case, the fractured surface remains protected from damage caused by rubbing of the mating fracture surfaces until the test rig stops. A fish eye is observed on the fracture surface and there is a non-metallic inclusion at the centre of the fish eye. There is a correlation between the size of the non-metallic inclusion in the fish eye and the rolling contact fatigue life of the steel.     

Criterion for surface contact deformation of metals

In most engineering applications, bulk plastic deformation of the surface is avoided. There is, however, no criterion for determining whether or not bulk plastic deformation occurs during the contact between rough surfaces. This paper presents a criterion for predicting the deformation behaviour of the contact between a real rough surface against a curved body. In order to verify the proposed criterion, experimental investigations were carried out for three different materials. It was found that the experimental results confirmed the proposed criterion very well. For a given operational contact condition, a contact system where bulk deformation does not occur can be designed using this criterion. If the criterion value C_m is less than 0.52 plastic deformation occurs on asperity level only and there is no bulk deformation, but if C_m is greater than 0.52 there are two possible deformation modes, i.e. plastic deformation of the bulk and plastic deformation in both the asperity and the bulk. Copyright © 2006 John Wiley & Sons, Ltd.