A flat punch pressed against an elastic interlayer under conditions of slip and separation

The problem of a square-ended rigid punch bearing on an elastic half-plane through the medium of an elastically dissimilar interlayer is considered, under plane deformation. The interlayer is in frictional contact with both the punch and half-plane. Regions of slip and separation along the interface are found, and it is shown how these phenomena affect the stress state in the immediate neighbourhood of the contact. This problem is of direct, practical interest to the design of interfacial bearing pads employed in supporting glass plates in structural applications.     

弹性模量对水润滑轴承的性能影响

从材料的角度,研究弹性模量对水润滑塑料合金轴承的性能影响,包括轴承承载能力、水膜形状曲线、压力曲线和最小水膜厚度等。研究表明:在选择水润滑轴承材料时应根据其承载能力来确定轴承材料的弹性模量范围;在承载能力范围内,从弹性模量这个角度来说,应当尽可能选择弹性模量低并具有良好摩擦磨损性能的材料,因为选择较小弹性模量的材料有利于提高最小水膜厚度,从而改善水润滑轴承摩擦副的摩擦磨损性能。     

The effects of friction-induced vibration on friction and wear

Friction tests were carried out with an elastic system where the specimen was supported by the elastic plate spring with strain gauges and with a rigid system where the vibration of the specimen was restricted. The coefficient of friction was obtained by two different methods: a deflection method with an assembly that consisted of a spring and strain gauges and an inertia method with a flywheel. The friction and wear properties of steel-steel, white metal-steel, phosphor bronze-steel and bronze-steel pairs were examined. When the frictional operation was conducted in the elastic system accompanied by frictional vibration, the coefficient of friction measured by the deflection method was generally larger than by the inertia method. The relationships between the coefficient of friction and the driving velocity for steel-steel and white metal-steel pairs obtained by the inertia method in the rigid system were significantly different from those obtained by the same method in the elastic system. Frictional vibration increased the wear of white metal-steel, phosphor bronze-steel and bronze-steel pairs but decreased that of a steel-steel pair. Wear measurements from the elastic system exhibited more scatter than from the rigid system.     

Modeling of thermoelastic frictional contact

A model of thermoelastic frictional interaction based on nonideal thermal contact under friction of a corrugated elastic solid on an absolutely flat rigid solid is developed. The proposed model may be used for the estimation of the capability and durability of tribological conjunctions of the type frictional polymer material-metal.     

超声马达各向异性摩擦材料性能的试验研究

根据超声马达对摩擦材料切向和纵向性能的不同要求,采用在酚醛树脂胶中添加玻璃丝布和玻璃纤维增强的方法制备出不同弹性模量匹配的各向异性摩擦材料,并对其性能进行了测试。结果表明:当摩擦材料垂直方向与水平方向的弹性模量的比值为0.71~1.42时,摩擦材料受振动头的正压力和驱动力先增大后减小,比值为1.24时,马达的正压力和驱动力最大,驱动效果最好。     

Study of plastic shear deformation in surface layer at friction. Simulation results. Part I. Model description

A one-dimensional macroscopic model of friction is presented which considers changes in elastic characteristics of the material caused by frictional heating and strain hardening. A single act of contact between a microasperity made of a ductile hardenable material and the counterbody was calculated for various contact spot sizes. It is shown that the contact spot size affects considerably the temperature, plastic deformation, and changes in the material elastic characteristics. Shears of surface layers relatively to the base material observed experimentally are explained proceeding from the modeling results.     

Friction of lubricated soft surface layers

Elastohydrodynamic lubrication of solids covered by a thin layer of material of low elastic modulus is studied theoretically and experimentally. The pressure and film thickness lie between the limits set by rigid and bulk elastic solids. The sliding friction shows a marked difference from that of rigid solids.     

Collective Effects at Frictional Interfaces

We discussed the role of the long-range elastic interaction between the contacts inside an inhomogeneous frictional interface. The interaction produces a characteristic elastic correlation length ??_c?=?a ² E/k _c (where a is the distance between the contacts, k _c is the elastic constant of a contact, and E is the Young modulus of the sliding body), below which the slider may be considered as a rigid body. The strong inter-contact interaction leads to a narrowing of the effective threshold distribution for contact breaking and enhances the chances for an elastic instability to appear. Above the correlation length, r?>???_c,?the interaction leads to screening of local perturbations in the interface, or to appearance of collective modes??frictional cracks propagating as solitary waves.     

Postbuckling of an elastic plate in a rigid channel

The postbuckling of a thin, elastic plate in a frictional channel of rigid parallel walls is studied. Different modes of contact between the plate and the channel walls are identified and modelled using the theory of the elastica. Example results are presented which illustrate the sequence of deflections through which the postbuckled plate passes. Loading on the plate is also studied, and it is found that narrow channels arrest the initial buckling of the plate but permit a second stage of collapse at higher force levels. This work would have application to the design of automatic machinery for the handling of flexible elastic sheets such as film, cloth and paper.     

Numerical and theoretical study of friction between atomically clean surfaces and effects of impurities

Sliding friction between atomically clean surfaces with and without impurities is studied numerically and theoretically. We show that the maximum static frictional force shows complicated behavior as a function of the elastic constant of the material, which results from various discommensurate structures. We also show that impurities can diminish the maximum static frictional force. Velocity dependences of the kinetic frictional force in the absence and presence of impurities are calculated. Transverse pinning, which yields static frictional force along the transverse direction to the sliding direction, shows peculiar behavior. It is also shown that the direction of the sliding motion can deviate from that of the external force.     

Characteristics of hydrostatic bearing/seal parts for water hydraulic pumps and motors. Part 1: Experiment and theory

In this paper, the load-carrying capacity, power losses and stiffness of disk-type hydrostatic thrust bearings including the case of eccentric loading are discussed theoretically. The numerical analysis method is established based on a two-dimensional elastohydrostatic problem with an elastic deformation model, which is extended to adapt it for a non-axisymmetric load acting on the thrust bearing. The bearing is made of a combination of stainless steel/stainless steel and stainless steel/plastics. For the elastic materials, the maximum stiffness derived from, i.e., minimum film thickness is, larger than that of the rigid material in the range of a large ratio of pocket pressure and a hydrostatic balance ratio of over unity, which is defined as the ratio of the load to the maximum hydrostatic load-carrying capacity. The maximum load-carrying capacity and minimum power loss can exist in the domain of the hydrostatic balance ratio over unity for the case of the bearing consisting of elastic/rigid materials, in comparison with that composed of the same rigid materials. For the case of water, the power loss due to leakage flow is slightly larger but that due to frictional torque is much smaller than that in the case of hydraulic oil. Then, the total power loss is much smaller than that of hydraulic oil.     

弹性材料表面织构对摩擦副润滑性能的影响

为了研究弹性材料表面微织构对摩擦副空化现象和润滑特性的影响,建立考虑空化效应的二维弹性织构计算模型,采用流固耦合方法计算润滑流场与材料变形之间的相互作用。对比刚性材料表面微织构,从弹性模量、滑动速度、微织构深度以及织构间距等方面分析弹性材料表面织构对摩擦副润滑性能的影响,通过实验验证模拟结果的准确性。结果表明：弹性织构摩擦副比刚性织构摩擦副摩擦因数更小,润滑性能更好;存在最优织构深度,使得弹性织构摩擦副的摩擦力最小且承载力最大;适当增大滑动速度以及织构间距可以提高弹性摩擦副的润滑性能;随着弹性模量的降低,弹性变形和油膜厚度增加,空化现象更为显著,摩擦副的润滑性能得到提升。     

Kinetic friction and dynamic elastic contact behaviour of polymers

Friction and contact between a glass hemisphere and plates of various polymers were studied under elastic contact. The contact at various sliding speeds was observed by means of Newton's rings. Although the frictional force was proportional to the area of contact observed, it was necessary to consider the increase in the shearing strength of the adhesive junctions with increasing load W. The area of dynamic contact of low density polyethylene (LDPE) was proportional to W^0.78 in a wide speed range, while the area of static contact was proportional to W^2/3 as expected from Hertz' equation. The shape of the dynamic contact was affected by the retarded elasticity of the polymer and frictional heating. The area of dynamic contact decreased and the shear strength increased in proportion to log v (where v is the speed) provided that frictional heating was negligible. The time dependence of elastic compliance 1/E of LDPE could be obtained by applying Hertz' equation to the dynamic contact observed at various speeds. However, the value of 1/E decreased with decreasing load in sliding. It was shown that the time dependence of elastic compliance plays an important role in determining the area of dynamic contact of polymers.     

Fabrication of micro-channel arrays on thin stainless steel sheets for proton exchange membrane fuel cells using micro-stamping technology

Bipolar plates are one of the crucial components of proton exchange membrane fuel cells. Because of the expensive production costs of traditional graphite bipolar plates, which require a few millimeters thickness over the space, the resulting metal bipolar plate reduces the cost, and the thickness can be reduced to the micron range. This study explored the application of micro-stamping technology to produce thin metal bipolar plates with the relevant process parameters. In this study, the channel design was 0.8?×0.75 mm for the use of a rigid punch on a 50-μm-thick stainless steel sheet (SUS 304) for micro-channel stamping processes. The finite element method and the experimental results were used to analyze the main parameters of the micro-stamping process. The traditional material model and the scale factor modified material model were used for simulation. The experimental results verified that the modified material model is more realistic for products and has superior similarity because the punch load is relatively small. This study used updated Lagrangian formulation concept to establish an elastic–plastic deformation finite element analysis model and scale factor to modify the calculation to effectively simulate the micro-stamping process for metal bipolar plates.     

Frequencies of circular plate with concentric ring and elastic edge support

Exact solutions for the flexural vibrations of circular plates having elastic edge conditions along with rigid concentric ring support have been presented in this paper. Values of frequency parameter for the considered circular plate are computed for different sets of values of elastic rotational and translation restraints and the radius of internal rigid ring support. The results for the first three modes of plate vibrations are computed and are presented in tabular form. The effects of rotational and linear restraints and the radius of the rigid ring support on the vibration behavior of circular plates are studied over a wide range of non-dimensional parametric values. The values of the exact frequency parameter presented in this paper for varying values of restraint parameters and the radius of the rigid ring support can better serve in design and as benchmark solutions to validate the numerical methods obtained by using other methods of solution.     

Viscoplastic analysis of plane-strain rolling using different friction models

Plane-strain rolling is analyzed numerically. The nonlinearities arising from the large plastic deformations, the strain-rate dependence of the stress and the influence of the roll velocity are described by using a finite-strain elastic-viscoplastic material model. The stress-strain variation with temperature is included by approximating the isothermal behaviour at a given temperature and strain rate. The combined contact and frictional problem at the interface between the rolls and the plate is modelled by using elastic springs with high stiffnesses and by assuming two different friction models. The influence of different rolling and material parameters on the variation of the frictional stress and the normal stress along the contact length is shown for one of the friction models. Furthermore, the difference between the results obtained by the two different friction models is emphasized.     

线性缓冲材料对质量块撞击下的弹性杆动态屈曲影响研究

针对刚性质量块撞击情况,对弹性直杆在线性缓冲材料防护下的动态屈曲进行分析,得到撞击条件下杆体屈曲发生的临界弹簧刚度。以杆体整个长度发生轴向应变的最小弹性屈曲载荷为判据,分析了临界弹簧刚度与载荷作用时间、载荷幅值、缓冲材料及杆体长度的关系,以及缓冲材料位置对于杆体屈曲防护的影响等。结果表明：撞击初速和刚性块质量是引起临界弹簧刚度改变的两个因素,缓冲材料的安放位置在一定条件下影响着弹性杆屈曲防护。所得结论对于在刚性质量块作用下弹性直杆的动态屈曲防护问题提供了一定的理论依据。     

Energy Loss in the Impact of Elastic Spheres on a Rigid Half-Space in Presence of Adhesion

Adhesion can cause energy losses in asperities or particles coming into dynamic contact resulting in frictional dissipation, even if the deformation occurring is purely elastic. Such losses are of special significance in impact of nanoparticles and friction between surfaces under low contact pressure to hardness ratio. The objective of this work is to study the effect of adhesion during the normal impact of elastic spheres on a rigid half-space, with an emphasis on understanding the mechanism of energy loss. We use finite element method for modeling the impact phenomenon, with the adhesion due to van der Waals force and the short-range repulsion included as body forces distributed over the volume of the sphere. This approach, in contrast with commonly used surface force approximation, helps to model the interactions in a more precise way. We find that the energy loss in impact of elastic spheres is negligible unless there are adhesion-induced instabilities. Significant energy loss through elastic stress waves occurs due to jump-to-contact and jump-out-of-contact instabilities and can even result in capture of the elastic sphere on the half-space.     

Temperature Analysis in Lubricated Simple Sliding Rough Contacts

A temperature analysis of dry sliding fully plastic contact is extended to calculate the asperity temperatures between a sliding lubricated rigid smooth plane and a stationary elastic rough surface. First, surface roughness is generated numerically to have a Gaussian height distribution and a bilinear autocorrelation function. Lai and Cheng's elastic rough contact computer program is then used to determine the asperity contact loads and geometries of real contact areas. Assuming different frictional coefficients for shearing the lubricant film at the noncontact areas, shearing the surface film at the asperity contacts and shearing the oxide film as the asperity temperature exceeds a critical temperature, asperity temperature distributions can be calculated. Eight cases in Durkee and Cheng's scuffing tests of lubricated simple sliding rough contacts are simulated by using 20 computer-generated rough surfaces. The results show that scuffing is correlated to high-temperature asperities which are above the material-softening temperature.     

Contact Problem for a Viscoelastic Half-Space and a Rough Rigid Cylinder under the Conditions of Hydrodynamic Lubrication

A model has been proposed for use in studying the combined effect of the roughness of a rigid punch and the viscous properties of a base separated by a thin lubricant layer exerted on the characteristics of contact interactions and the sliding friction force. The problem of the motion of a thin a lubricant layer between a fixed rigid cylinder with a regular relief, as well as the surface of a moving viscoelastic half-space, the rheological properties of which are described by an integral operator with an exponential creep kernel, has been considered. The pressure and thickness of the lubricating layer, as well as the deformation component of the frictional force depending on the sliding velocity, have been analyzed. A comparison of the results of solutions of contact problems for viscoelastic and elastic rough bodies in the presence of a lubricant has been presented.