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.     

Surface topography and properties of frictional contacts

The influence of surface topography on contacting solids is considered. The rough surface model is suggested and is used for the calculation of some tribological contact characteristics. A rough surface is modelled by a set of asperities of regular shape (wedge, cone, cylindrical, spherical segment), of differing height. A simple height distribution function and asperity shape function are used. These functions may be integrated analytically in further calculations.The surface model is used for calculation of one of the main contact parameters - real contact pressure (or real contact area) and other principal contact parameters, such as deformation, number of contact spots, average spot area, average distance between contact spots and intercontact gap.It is shown how the above parameters may be used for the calculation of such operational contact characteristics as friction coefficient, wear rate and electrical and thermal resistance.     

The Adhesion of Irregularly-shaped 8 μm Diameter Particles to Substrates: The Contributions of Electrostatic and van der Waals Interactions

The forces needed to remove irregularly-shaped, 8 μm diameter, polyester particles from a polyester substrate were measured using an ultracentrifuge. Measurements were also made on a second set of similar particles where nanometer-size silica clusters had been placed on their surfaces. These silica clusters acted as spacers, reducing direct contact between the particle and the substrate. It was found that the separation forces for the bare particles were consistent with predictions of the JKR theory of adhesion, but were much larger than could be accounted for from simple electrostatic interactions associated with either uniformly-charged particles or particles with localized charged patches. It was found, however, that the forces needed to effect separation decreased with increasing silica concentration. For particles with 2% by weight silica clusters on their surfaces, the separation force was only about 5% of the separation forces of the bare particles. At this concentration of silica, the estimates of the separation forces obtained from JKR theory, from the uniformly-charged model, and from the localized-charged-patch model are all about equal. The numerical estimates are consistent with the experimentally-obtained values.     

Computational micro- and macroscopic models of contact and friction: formulation, approach and applications

This paper is dedicated to new asperity-based constitutive models of contact interfaces. These models have been obtained through a combination of finite element analysis of surface asperities and statistical homogenization techniques, to predict macroscopic, phenomenological behavior of the interface. This new approach has generalized the existing asperity-based models of contact and friction by considering realistic, complex shapes and mechanical properties of surface asperities, as opposed to previous simplified analytical solutions. This has been achieved by application, at the stage of asperity modeling, of the finite element method, which takes into account arbitrary shapes of asperities, non-linear material properties, molecular-range adhesion forces, and sliding resistance on the contact surface. The h–p adaptive mesh refinement techniques, adaptive timestepping and other adaptive methods are used to assure high accuracy of the solution. The result of this development is a new family of constitutive interface models, consistent with surface micromechanics and applicable to studies of static and dynamic friction phenomena. They are also extendible to calculation of thermal or electrical resistances, wear modeling, and other applications. This paper presents the theoretical formulation, numerical methodology and sample models of contact, adhesion and friction obtained through these homogenization techniques.     

The Adhesion of Irregularly-shaped 8 μm Diameter Particles to Substrates: The Contributions of Electrostatic and van der Waals Interactions

The forces needed to remove irregularly-shaped, 8 μm diameter, polyester particles from a polyester substrate were measured using an ultracentrifuge. Measurements were also made on a second set of similar particles where nanometer-size silica clusters had been placed on their surfaces. These silica clusters acted as spacers, reducing direct contact between the particle and the substrate. It was found that the separation forces for the bare particles were consistent with predictions of the JKR theory of adhesion, but were much larger than could be accounted for from simple electrostatic interactions associated with either uniformly-charged particles or particles with localized charged patches. It was found, however, that the forces needed to effect separation decreased with increasing silica concentration. For particles with 2% by weight silica clusters on their surfaces, the separation force was only about 5% of the separation forces of the bare particles. At this concentration of silica, the estimates of the separation forces obtained from JKR theory, from the uniformly-charged model, and from the localized-charged-patch model are all about equal. The numerical estimates are consistent with the experimentally-obtained values.     

Modeling of a locking mechanism between two rough surfaces under cyclic loading

The work is motivated by experimental studies on energy dissipation due to micro-slip in mechanical joints. It has been observed that the loss of energy undergoes certain evolution under cyclic shear loading. It manifests itself in the form of the gradual decrease approaching a steady state as cycling progresses. This behavior has a repeatable character if contact is re-established and subjected to cycling again. In the present work, a simple multiple-asperity model is developed that suggests a physical hypothesis that when two rough surfaces are brought in contact and subjected to shear loads a certain locking mechanism comes into play at the interface. Such locking occurs due to the tendency of the interface to adapt its contact microstructure to the loading conditions. The adaptation mechanism is described in the present work through the contact inclination angles. The developed model is simple in application and it relates micro-characteristics of the contact to macro-behavior of the system exhibiting itself in energy dissipation.     

含多粗糙峰涂层等效应力的有限元分析

研究刚性平面与含粗糙峰涂层在二维与三维模型下的弹性接触问题,采用有限元法分析涂层弹性模量比、涂层厚度、粗糙峰间距、刚性平面压下深度对涂层粗糙峰表面、涂层/基体界面分布及基体等效应力分布的影响。计算结果表明压下深度对三维涂层粗糙峰表面最大应力的影响最大,涂层厚度、涂层/基体弹性模量比、粗糙峰间距的变化对应力值影响逐渐减小;增大涂层厚度、减小压下深度和粗糙峰间距、降低弹性模量比会使得三维接触模型最大等效应力值显著降低;增加涂层粗糙峰数和涂层厚度、同时降低涂层弹性模量有助于提高涂层/基体界面结合强度。相对于二维接触模型来说三维接触模型在粗糙峰表面的等效应力增大,造成这种变化的主要原因是由于涂层表面粗糙峰之间的等效应力叠加引起的。该研究为涂层粗糙峰及涂层/基体界面强度的应力分析提供依据。     

裂缝内钻井液的漏失规律研究

为了揭示裂缝性漏失宏观规律和缝内微观流动特征的机理,通过室内实验研究了压差、缝宽、钻井液黏度变化时平板型光滑裂缝的漏失规律,结合CFD数值模拟分析了天然裂缝面的起伏粗糙特征对漏失的影响,讨论了影响漏失各个因素的特点。研究表明:缝宽和压差是决定漏失速率的关键因素,漏失速率对钻井液黏度的敏感程度相对较弱;裂缝面的整体起伏使其在局部对顶接触是一般裂缝形成有效流道的主要原因,而形成微裂缝有效流道的主导因素则是微凸体的分布与形态特征;天然裂缝面存在局部接触,从而引起液体流动阻力增大、流道变得更加曲折,漏失速率减小。该研究成果为裂缝性地层的漏失评价、堵漏材料优选、井漏控制提供了一定的理论依据。     

PREDICTING THE EFFECTS OF LOAD RATIO ON THE FATIGUE CRACK GROWTH RATE AND FATIGUE THRESHOLD

The effect of the load ratio, R, on fatigue crack growth behaviour is analysed on the basis of the recently proposed inelastic discrete asperities model. A wide range of load ratios, both positive and negative, are examined. Particular emphasis is placed on compressive excursions, i.e. negative R loadings. The inelastic discrete asperities model is a micro-mechanical analysis based on the plastic crushing of a single asperity (or multiple asperities) located on the crack face close to the crack tip and under dominantly plane strain conditions. Experimental data have indicated that the primary crack face contacts which obstruct closure are immediately adjacent to the crack tip, although segments of the crack face more distant from the crack tip are not neglected. However, the more distant asperities are a part of the past crack advance history which does not influence current behaviour. By use of this model, it is shown that the effect of the load ratio can be adequately predicted once some baseline information on mechanical material properties and surface roughness is provided. The model also provides useful trend information and explains many of the observed phenomena, e.g. the ‘saturation’ of the compressive underload effects. For a constant applied nominal stress intensity factor range, ΔK_nom , it is shown that the effective stress intensity factor range, ΔK_eff , initially decreases as the positive R decreases (corresponding to the increasing influence of closure), reaches a minimum around R = 0, and then starts increasing with negative R (corresponding to the plastic crushing of the asperities which reduces closure), eventually reaching a saturation level below ΔK_nom . Conversely, for an assumption of a constant ΔK_eff , the applied ΔK_nom increases as the positive load ratio decreases, reaching a maximum around R = 0, and then decreases with more negative R values, eventually reaching again a saturation level (above ΔK_eff ). It is also shown that the effect of material hardness can be directly analysed based on this model.     

A new diagnostic technique for asperity contact

Investigations into high frequency gear vibration have given a diagnostic technique which shows asperity interactions. Smith shocks are generated at the contacts and can be detected using high frequency accelerometers to give a measure of the number and intensity of the contacts.