Performance of truncated surface asperities in elastohydrodynamic lubrication

The elastohydrodynamic lubrication in a sliding line contact between a flattened rough plane surface and a smooth plane surface is analytically studied. Both surfaces are assumed as elastic and parallel to one another. The flattened rough surface is treated as equivalent to a rough plane surface with uniformly distributed cylindrical asperities evenly truncated on the top. For understanding the elastohydrodynamic lubrication performance between these two plane surfaces, an inlet zone analysis is taken for the elastohydrodynamic lubrication formed between a truncated cylindrical asperity and the smooth plane surface. It is found from the obtained results that the asperity truncation reduces the elastohydrodynamic load-carrying capacity, and this effect is significant for low sliding speeds or/and heavy loads, while it is negligible for high sliding speeds and moderate loads. The asperity truncation increases the friction coefficient of the asperity contact especially at relatively light loads, while it only slightly increases the friction coefficient of the asperity contact at heavy loads. For heavy loads, the asperity truncation can have a significant effect on the reduction in the maximum surface temperature rise. It is recommended by the present study that the surface asperity be truncated in a certain degree in an elastohydrodynamic contact with high sliding speeds and relatively heavy loads because of giving the benefits of considerably reducing the maximum surface temperature rise while maintaining the elastohydrodynamic load-carrying capacity.     

Investigation into cylinder-on-flat adhesion elastic–plastic micro-contact under repeated loading and unloading condition

The evolution of microcontact induced deformation and stress states under repeated loading and unloading condition is of great interest for the scientific understanding as well as from the engineering design considerations of microelectromechanical systems (MEMS) based switches and similar other devices since they operate under cyclic condition. This study, therefore, investigated the microcontact interaction between deformable smooth cylindrical segment and deformable smooth flat using the finite element analysis. Elastic and elastic–plastic material behaviors and adhesion interaction are considered. The detailed information about contact area, displacement and stress state in the contact region under cycling condition are presented. Contact area varies nonlinearly even when the applied load is varying linearly in a cycle. Additionally, contact areas during loading and unloading portions of a cycle are not equal at the same load level. The deformation and contact area increases in the presence of adhesion force and also with elastic–plastic material behavior. However, the adhesion force is reduced while the contact area is increased during the microcontact between elastic–plastic bodies in comparison to those between elastic bodies. The maximum increase in stress and deformation states occurs during the first cycle, and then slowly with increasing number of cycles. Further, the maximum stress state is not at the contact surface and also not at instant of the maximum applied external force. Finally, adhesion force during microcontact interaction can be of the same order as the applied external force. Therefore, design and analysis of microcontacts in micromechanical switches and similar other devices should include the effects of adhesion force and cyclic effects. The views expressed in this article are those of the authors and do not reflect the official policy or position of the United State Air Force, Department of Defense, or the U.S. Government.     

探针尺寸对Si-DLC薄膜表面粘附和摩擦的影响

采用AFM尖头探针、球头探针和平头探针对Si-DLC膜进行摩擦实验,研究了薄膜的微观摩擦力学性能,探讨了不同接触尺度下薄膜表面粘附力及摩擦产生的机理,建立了不同探针与薄膜表面粗糙峰的接触模型,推导了表面粘附力与接触面积的关系表达式,表明在微观接触中,接触面积对粘附力起着主导作用。尖头探针与薄膜表面的微观摩擦系数取决于表面粗糙峰的斜率,与粗糙峰的高度相关不大;球头探针与薄膜表面的摩擦力主要取决于单位面积接触粗糙峰密度;平头探针与薄膜表面的摩擦力主要取决于外加载荷,表面形貌的微观尺寸效应可忽略。     

Normal and sliding contact experiments on gneiss

The development of reliable discrete element models to simulate the mechanics of granular media requires knowledge of the grain-to-grain contact laws of the material in question. We have conducted a series of normal and sliding contact experiments on material used in laboratory triaxial experiments to obtain such contact laws for DEM simulations of the experiments. The contact experiments employed segments of 14.72 mm-diameter spherical grains from the triaxial specimens and flat specimens of the same material. The spherical grains had a uniform diameter with a smooth surface finish. Monotonic and cyclic loading paths were applied in both the normal and sliding modes, and both sphere-sphere and sphere-flat contact behavior were examined. Force-displacement behavior and frictional loss were measured in all cases. The behavior was generally Hertzian in the normal contact experiments, which involved forces up to approximately 100 N. The normal contact stiffness increased from ≈2 to 15MN m⁻¹ over the range of normal force examined. The sliding experiments employed several normal forces up to approximately 25 N, and produced a value of the coefficient of static friction of 0.28. The shear stiffness of the sliding contact increased with normal force, and ranged from 0.8 to 1.2MN m⁻¹ under normal loads ranging from ≈1 to 7.5 N, respectively, for virgin contacts. The shear stiffness observed for the sphere-flat contact decreased with wear. Surface roughness measurements were obtained on both tested and untested regions of the spheres and flat specimens. The average roughness (Ra) for untested regions of the sphere and flat specimens were 270 and 230 nm, respectively. Repeated testing in the sliding mode reduced these values by 29–45% for the flat surfaces, and by 20% for the spherical contact. Frictional losses were observed in both the normal and sliding modes. In the sliding mode, frictional loss decreased with increasing normal load. We observed stable sliding (associated with significant contact movement under an increasing shear force) at forces that were below the macroscopic frictional limit and resulted in permanent displacement of the contact. There was generally a distinct threshold in shear force for this permanent sliding. The extent of sliding increased significantly with wear for the sphere-flat contact and was accompanied by a substantial drop in shear stiffness.     

Hydroplaning Analysis for Tire Rolling over Water Film with Various Thicknesses Using the LS-DYNA Fluid-Structure Interactive Scheme

Current work studies the transient hydroplaning behavior of 200 kPa inflated pneumatic radial tires with various types of tread patterns. Tires were numerically loaded with a quarter car weight of 4 kN, and then accelerated from rest rolling over a water film with a thickness of 5, 10 and 15 mm on top of a flat pavement. Tire structure is composed of outer rubber tread and inner fiber reinforcing composite layers. The Mooney-Rivlin constitutive law and the classical laminated theory (CLT) were, respectively, used to describe the mechanical behavior of rubber material and composite reinforcing layers. The tire hydroplaning phenomenon was analyzed by the commercial finite element code - LS-DYNA. The Arbitrary Lagrangian & Eulerian (ALE) formulation was adopted to depict the fluid-structure interaction (FSI) behavior. Three different tire tread patterns, i.e. the smooth (blank) tread pattern and the 9 and 18 mm wide longitudinally-grooved tread patterns, were constructed to perform the current transient hydroplaning analysis. Simulated dynamic normal contact force and hydroplaning velocity of tire with a prescribed smooth tread pattern were obtained. The computed results were in good agreement with the numerical and test results given by Okano, et al. (2001) for tire running over 10 mm thick water fluid film. In addition, dynamic contact force of a smooth tread pattern tire rolling on a dry flat pavement was also found to be close to the result reported by Nakajima, et al. (2000). In addition, the effect of fluid water layer thickness on the hydroplaning velocity and normal contact force for tires with smooth tread pattern and longitudinally-grooved tread patterns rolling on a wet roadway analyzed by the LS-DYNA code are reported and discussed.     

润滑状态下线接触滑动粗糙界面的动摩擦特性研究

滑动粗糙界面的摩擦润滑特性对界面的润滑设计和润滑状态预测具有重要的理论和实际意义。本文通过建立不同润滑状态下的滑动粗糙界面模型,基于界面的法向载荷由润滑油膜和粗糙体共同承担的载荷分配思想,采用Greenwood-Williamson统计模型描述粗糙表面形貌,考虑界面润滑的时变效应和润滑油的粘-压特性,建立了线接触滑动粗糙界面的油膜厚度方程和粗糙体接触压力方程,获得了整个润滑区的润滑油膜载荷比例因子、油膜厚度和摩擦系数随滑动速度的变化关系,推导了界面由混合润滑过渡为液压润滑的临界速度关系表达式,分析了滑动粗糙界面的润滑承载机理,获得了界面油膜厚度、摩擦系数和临界速度随界面形貌参数、法向载荷、润滑油属性参数的变化规律,为机械结构的界面润滑设计、润滑状态预测和润滑优化提供理论和实验参考。     

平流层飞艇气动阻力的参数分析

为了研究环境参数及外形布局对平流层飞艇气动阻力的影响,在验证CFD数值模拟方法的基础上,从气动阻力包括压差阻力与摩擦阻力的角度探讨了风速、动力粘度系数、空气密度、Re数、长细比及尾翼对飞艇气动阻力的影响规律及机理。结果表明:气动阻力系数随风速与空气密度的增加而减小,随动力粘度系数的增加而增加;气动阻力系数随Re数减小的趋势,取决于摩擦阻力系数随Re数的减小趋势;随长细比的增加,摩擦阻力系数呈现增加趋势,但气动阻力系数呈现先减小后增加的趋势;尾翼对气动阻力系数的影响主要体现在压差阻力系数的改变。     

The contact problem of a rigid stamp with friction on a functionally graded magneto-electro-elastic half-plane

We consider in this study the frictional sliding contact problem between a functionally graded magneto-electro-elastic material and a perfectly conducting rigid punch subjected to magneto-electro-mechanical loads. The problem is formulated under plane strain conditions. Using Fourier transform, the resulting plane magneto-electro-elasticity equations are converted analytically into three coupled singular integral equations in which the unknowns are the normal contact stress, the electric displacement and the magnetic induction. These integral equations are then solved numerically to obtain the distributions of the normal contact stress, electric displacement and magnetic induction at the surface of the graded medium. The main objective of this paper is to study the effect of the non-homogeneity parameter, the friction coefficient and the elastic, electric and magnetic coefficients on the surface contact pressure, electric displacement and magnetic induction distributions for the case of flat and circular punch profiles.     

Adhesion force during microcontact interaction between cylindrical-segment-on-flat and flat bodies

Adhesion force including its both attractive and repulsive components during microcontact interaction between cylindrical-segment-on-flat and flat bodies is formulated using the volumetric integration of interactions among the atoms. Thenceforth the corresponding formulation for cylinder-on-flat and flat-on-flat is developed and compared with the available counterpart attractive force and they are in excellent agreement with each other. However, the present study provides also the repulsive component of adhesion force for these contact geometries. The formulation is also extended for a cylindrical segment with two radii to model the elastic–plastic deformation. Several phenomena related to adhesion during microcontact are then studied leading to the following salient observations. The adhesion force during microcontact interaction can be of the same order as the applied force, and thus causes the plastic deformation. The attractive force from the cylindrical segment is the major component of adhesion force for its radius/height ratios smaller than 1000 while that from the bulk body is the major for the larger ratios, i.e. when the cylindrical segment is similar to an asperity. There is considerable difference in the attractive force between a cylinder and a cylindrical-segment-on-flat for the smaller values of radius. The difference between attractive and total adhesion force is significant (∼60%) when adhesion force is the maximum, but it rapidly decreases with increasing separation distance and can be neglected at the separation distance larger than the twice of inter-atomic equilibrium distance. The equilibrium separation distance is about 60% of the inter-atomic equilibrium distance and adhesion force is maximum at about 80% of the inter-atomic equilibrium separation distance for both deformed and undeformed configurations. The adhesion force also increased with the deformation of cylindrical segment. The views expressed in this article are those of the authors and do not reflect the official policy or position of the United State Air Force, Department of Defense, or the U.S. Government.     

Effect of asperity damage on shear behavior of single fracture

Experimental studies have shown that fractures often exhibit shear resistance softening and dilatation under shearing loads. From a mechanistic viewpoint, these phenomena are a consequence of the fracture surface roughness and the material mechanical properties. Consequently, this paper utilizes a micromechanical model of fractures that explicitly considers asperity interactions on fracture surfaces. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. A modified spherical harmonic expansion is used to model the orientation distribution of asperity contacts. Evolution laws for asperity heights and asperity contact orientations are introduced to account for the change in surface roughness resulting from asperity damage under shear. Results obtained from the model show that the asperity contact orientation evolution law is essential for correctly modeling the softening and dilatation behavior of fractures. The model results are compared with experimental data culled from the literature.     

Optical observations of “junction growth” in asperities of copper,aluminium, PTFE and nylon under combined normal and tangential stresses

An experimental investigation is described in which the phenomenon of junction growth in model conical and spherical asperities of copper, aluminium, PTFE and nylon has been examined optically. The test asperity was first normally pressed against a smooth flat surface of soda-lime glass and the contact observed through the glass surface. In the presence of the normal load the asperity deformed plastically. When a tangential force was applied, the size of the real area of contact between the two solids, as measured in situ, was found to increase for all the asperities used. Depending upon the geometry and material of the test asperity, the increase in the size of the contact area was up to 40%. The behaviour was compared with an analytical expression for junction growth in the case of a right circular cylindrical asperity. The closest agreement between the measurements and the theory was found for the 60° conical asperities of work-hardened copper and for nylon spheres. The sliding of the asperity on the glass plate caused transfer of the material of the former to the latter. This occurred for all the asperities used. Moreover, the sliding of the metallic asperities resulted in up to 5 m deep grooves on them as well as on the glass plate. These observations have also been briefly discussed.     

A finite sliding model of two identical spheres under displacement and force control. Part II: dynamic analysis

In Part I of the present study, the static analysis for the sliding of two identical spheres under displacement and force control was carried out. For linear and circular sliding trajectories, the contact traction evolution was analytically specified for both monotonic and reciprocal sliding regimes. Similarly, for the specified gravity loading, the driving force evolution and the sliding path were also determined. In the present Part II of the analysis, the dynamic response for the same sliding modes is presented. The contact traction and velocity evolutions are considered in detail. The analytical formulae are proposed for prediction of the tangential restitution coefficient, critical velocity, and time of contact for the displacement and load-controlled motions. The effects of loading and reloading in reciprocal sliding are also considered with account for the slip and sliding regimes. The generated results have practical aspects and can be implemented in modeling of the asperity and rough surface interaction, wear analysis and also in the development of the numerical discrete element method.     

A new approach of using Lorentz force to study single-asperity friction inside TEM

Taking advantage of the magnetic field inside transmission electron microscope(TEM),a unique Lorentz-force-actuated method for quantitative friction tests was developed via a commercial electromechanical holder.With this approach,a submicron-sized silver asperity sliding on a tungsten flat punch was actu-ated by Lorentz force due to electrical current through the punch,with the normal force imposed by the built-in transducer of the holder.The friction force was determined by tracking the elastic deflec-tion of the fabricated cantilever from in situ video.Through correlating the friction behavior with the microstructural evolution near the silver-tungsten interface,we revealed that even when the relative motion commenced with the plastic deformation of the silver asperity,the interface can still sustain the further increasing static friction force.Exactly following the arrival of the maximum static friction force,the sliding occurred at the interface,indicating the transition from static to dynamic friction.This work enriches our understanding of the underlying physics of the dynamic friction process for metallic friction behavior.     

Fracture mechanics analysis of asperity cracking due to adhesive normal contact

A two-dimensional linear-elastic fracture mechanics analysis of asperity cracking induced by adhesive normal contact was performed with the finite element method. Normal contact between two elastic asperities was analyzed with the equivalent contact system of an elastic asperity with equivalent radius of curvature and effective elastic modulus compressed by a rigid plane. Surface adhesion was modeled by nonlinear springs obeying a constitutive force-distance law derived from the Lennard–Jones potential. The maximum ranges of the tensile and shear stress intensity factors were used to determine the crack-growth direction and the dominant mode of asperity fracture in terms of the Maugis parameter (a function of the equivalent radius of curvature, work of adhesion, effective elastic modulus, and intermolecular equilibrium distance), friction coefficient at the crack interface, maximum surface interference, and crack position. Finite element simulation results indicate that the direction and the rate of crack growth are mostly affected by the Maugis parameter and the maximum surface interference. A transition from shear to tensile dominant mode of crack growth is encountered with the increase of the Maugis parameter and/or the decrease of the maximum surface interference. Opening, slip, and stick between the crack faces during loading and unloading are discussed in the context of crack mechanism maps.     

基于Persson接触理论的转子轮盘接触刚度的计算

目前对于分布式拉杆转子轮盘接触刚度的研究较多使用的是Hertz接触理论。Hertz接触理论把轮盘间的接触等效为微观微凸体弹性半球之间的接触过于理想化,不能精确地模拟轮盘接触的实际情况;且Hertz接触理论中弹性半球接触角大于30°时计算结果就会出现很大误差。基于Persson接触理论提出一种新的接触模型,优化了分布式拉杆转子轮盘接触刚度的求解方法。     

电镀锌板摩擦系数的影响因素分析

采用销盘摩擦试验研究了正压力和速度对电镀锌板摩擦系数的影响规律,运用正交试验及方差分析的方法研究了正压力、速度、镀锌层对摩擦系数的影响水平.结果表明:当速度在5654.86～9424.78 mm/min,正压力在0.3～5 N的范围内,随着正压力的增大,镀锌板的摩擦系数逐渐降低,随着速度的增大,摩擦系数也逐渐增大,镀锌层对镀锌板摩擦系数影响最大.     

Steady state thermoelastic contact problem in a functionally graded material

This paper is concerned with the stationary plane contact of a functionally graded heat conducting punch and a rigid insulated half-space. The frictional heat generation inside the contact region due to sliding of the punch over the half-space surface and the heat radiation outside the contact region are taken into account. Elastic coefficient μ, thermal expansion coefficient α_t and coefficient of thermal conductivity k are assumed to vary along the normal to the plane of contact. With the help of Fourier integral transform the problem is reduced to a system of two singular integral equations. The equations are solved numerically. The effects of nonhomogeneity parameters in FGMs and thermal effect are discussed and shown graphically.     

Dynamic rigid indentation induced by sliding frictionless contact

Dynamic rigid indentation induced by sliding frictionless contact is studied by considering a rigid smooth half-wedge which simultaneously translates tangentially and normally at constant speeds with respect to the surface of an elastic half-space. A more general problem is treated by homogeneous function techniques and the solutions are then applied. Among other results, it is found that if no tangential speed is imposed, the half-wedge must approach a flat punch configuration and a maximum normal force must be applied. The normal half-space surface displacements are also plotted.     

大厚度无元素稀释表面保护层的摩擦焊敷工艺

采用20钢，1Cr18Ni9Ti不锈钢在低碳钢线材表面上进行了大厚度无元素稀释表面保护层的耗材摩擦焊敷工艺试验研究。探讨了摩擦界面的摩擦行为，述了耗材金属过渡的机理，分析了焊敷层的形貌，微观组织，显微硬度和组分的分布。研究表明，应用耗材摩擦焊敷工艺可以获得大厚度，无元素稀释，显微硬度分布均匀，固相连接的表面保护层；摩擦系统的热物理不对称性是耗材金属过渡的驱动力；耗材金属通过真实接触面向母材表面过渡，其行为遵循金属摩擦的焊合－剪切－犁削理论；耗材磨擦焊敷工艺具有细化焊敷层和热影响区组织的工艺特性。     

A semi‐implicit time‐stepping model for frictional compliant contact problems

In this paper, we formulate a semi‐implicit time‐stepping model for multibody mechanical systems with frictional, distributed compliant contacts. Employing a polyhedral pyramid model for the friction law and a distributed, linear, viscoelastic model for the contact, we obtain mixed linear complementarity formulations for the discrete‐time, compliant contact problem. We establish the existence and finite multiplicity of solutions, demonstrating that such solutions can be computed by Lemke's algorithm. In addition, we obtain limiting results of the model as the contact stiffness tends to infinity. The limit analysis elucidates the convergence of the dynamic models with compliance to the corresponding dynamic models with rigid contacts within the computational time‐stepping framework. Finally, we report numerical simulation results with an example of a planar mechanical system with a frictional contact that is modelled using a distributed, linear viscoelastic model and Coulomb's frictional law, verifying empirically that the solution trajectories converge to those obtained by the more traditional rigid‐body dynamic model. Copyright © 2004 John Wiley Sons, Ltd.