Plastic deformation of rough rolling contact: An experimental and numerical investigation

Quantifying the surface roughness evolution in contacts is a crucial step in the fatigue prediction process. Surfaces are initially conditioned by the running-in process and later altered by surface fatigue. The aim of this study is to understand and predict the evolution of the micro-geometry in the first few over-rolling cycles. Numerical predictions are validated by experiments. A major difficulty in understanding surface degradation is the measurement of the surface roughness evolution at the relevant scales. A twin disc micro-test rig, called μMag, was specially designed for this kind of analysis. The μMag allows the “in situ” observation of changes in the disc surface during interrupted tests, thus avoiding dismounting the specimens, which is a major cause of inaccuracy. The new method also maintains the relative position of the two discs. The precision of the measurements allows one to use the initial surface micro-geometry as input for the numerical contact calculation. Thus, the plastic deformation of the surfaces can be measured during the first cycles and compared to the numerical prediction. Results show a very good agreement between numerical predictions and experimental measurements

Amplitude reduction of non-isotropic harmonic patterns in circular EHL contacts, under pure rolling

Surface roughness plays an important role in ElastoHydrodynamically Lubricated contacts, a role which is currently only partially understood. Recent work on waviness in EHL line contacts has shown and quantified the elastic deformation of the waviness inside the contact as a function of a single dimensionless parameter. In the present paper this work is extended to the circular contact problem. First it is shown that the amplitude reduction of an isotropic harmonic pattern can also be described as a function of a single dimensionless parameter. Subsequently, the effects of anisotropy are investigated varying from purely transverse to purely longitudinal. It is shown that one can create a single curve for the case varying from isotropic to longitudinal, and another for the case varying from isotropic to transverse. Both curves can be combined in a single formula

Boundary Layer Behaviour in Circular EHL contacts in the Elastic-Piezoviscous Regime

The solution of elastohydrodynamically lubricated contacts at high loads and/or low speeds can be described as a Hertzian pressure with inlet and outlet boundary layers: zones where significant pressure flow occurs. For the soft lubrication regime (elastic-isoviscous), a self-similar solution exists in the boundary layers satisfying localized equations. In this paper, the boundary layer behaviour in the elastic-piezoviscous regime is investigated. The lengthscale of the boundary layers and the scaling of pressure and film thickness are expressed in non-dimensional parameters. The boundary layer width scales as 1/M−−√ (equivalent to λ¯3/8 ), the maximum pressure difference relative to the Hertzian solution as 1/M−−√3 (equivalent to λ¯1/4 ) and the film thickness as 1/M−−√16 (equivalent to λ¯3/64 ) with M the Moes non-dimensional load and λ¯ a dimensionless speed parameter. The Moes dimensionless lubricant parameter L was fixed. These scalings differ from the isoviscous-elastic (soft lubrication) regime. With increasing load (decreasing speed), the solution exhibits an increasing degree of rotational symmetry. The pressure varies less than 10 % over an angle less than 45 degrees from the lubricant entrainment direction. The results provide additional fundamental understanding of the nature of elastohydrodynamic lubrication and give physical rationale to the finding of roughness deformation depending on the “inlet length”. The findings may contribute to more efficient numerical solutions and to improved semi-analytical prediction methods for engineering based on physically correct asymptotic behaviour

Analysis of the Influence of Indentations on Contact Life

Applying the relation between stresses and contact fatigue as postulated by Ioannides & Harris, an investigation is carried out to determine the influence of dent geometry on contact life. This analysis is performed using line contact and point contact models. The stresses are calculated using a dry contact model and the elastic half-space assumption. Results from calculations using the line contact assumption are compared with the results of elliptical contact calculations, which model the dents more exactly. The difference in contact pressure between the line and point contact case is analysed in detail. The dent geometry has been obtained from an FEM analysis of an indentation process and the dent is scaled with respect to depth and width to obtain a family of dents. The FEM calculations were compared to experimentally obtained dent profiles. As was done previously for the line contact analysis, the influence of the residual stress field from the plastic indentation process was investigated for the calculated dent geometry (without scaling). The life reduction compared to the smooth contact case has been obtained for three different load cases for a large number of different dent sizes and dent slopes, employing the point contact analysis. The life reductions are compared to the results of an extended line contact analysis and the influence of the position of the dent on life reduction is investigated. This analysis is aimed at establishing the reduction of contact life caused by indentations stemming from handling damage and the overrolling of debris. © 1992