Contact Creep Behavior of Polydimethylsiloxane and Influence of Load, Tip Size, and Crosslink Density

Time-dependent indentation creep behaviors of polydimethylsiloxane (PDMS) samples of different crosslink densities were studied through contact creep tests loaded with silica tips. Step loads from 0.1 to 10 mN were applied and held for 600 s. The data of penetration depth versus time were recorded during the holding period. A Hertz-type viscoelastic model was used to compute the creep compliance of the samples and the Johnson–Kendall–Roberts (JKR) theory was used to obtain the initial equivalent modulus, infinite equivalent modulus, and work of adhesion between the tested each pair of the PDMS and fused silica tip surfaces. The comparison between initial and infinite equivalent moduli obtained from the Hertz viscoelastic theory and the JKR theory shows that the adhesion between the tip and the sample surface plays an importance role in affecting the analysis results when the indentation strain is small. The influences of crosslink density, applied load, and tip size on the localized PDMS properties are discussed.     

Estimation of rubber sliding friction from asperity interaction modeling

Interaction between a soft rubber asperity and its hard counterpart is traced with the help of a finite element computation. The analysis is aimed to estimate the influence of adhesion between rubber and rigid surfaces and the energy losses arising from the deformation of rubber bulk to the sliding resistance. At the contact zone, interfacial bonds are formed due to adhesion and their resistance to sliding is represented by the shear strength of the contact interface. In the rubber bulk, the hysteresis loss is calculated using an appropriate model of the viscoelastic mechanical behavior of rubber for large strains. Dependence of friction on sliding speeds and temperature is hence detected. Influence of surface roughness and contact pressure on friction is also examined.     

Contact Problems for Rolling with Slip for Viscoelastic Solids

The three-dimensional contact problem for rolling of a rigid sphere over a base, which consists of viscoelastic layer bonded to a rigid half-space has been considered. The thin viscoelastic layer simulates the action of a friction modifier. The Kelvin model has been used to describe the layer properties. The method of calculating the distribution of normal and shear stresses within the contact interaction area has been presented, the boundaries of traction have been determined, and the slip subareas have been studied. In addition, the influence of the viscoelastic layer properties on the distributions of contact stresses has been analyzed.     

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.     

Deformational contact and friction on viscoelastic substrates

The deformation of a rigid slider of arbitrary shape sliding smoothly over the surface of a general linear viscoelastic layer is studied. A solution of the two-dimensional problem is presented in order to calculate rigorously the contact area, deformational friction and frictional heating. For a rigid cylinder moving on a standard linear viscoelastic substrate the distinctive effects of varying the size and shape on the contact area are calculated exactly as a special case of our general solution. The results, given in terms of the slider speed and the hysteresis properties of a viscoelastic solid, agree well with the excellent qualitative prediction of Moore's theory. More importantly, the calculation concludes that all the sudden changes in the size and shape of contact, the deformational friction and heat generation correspond to the maximum loss tangent of the viscoelastic material.     

Rolling Friction: Comparison of Analytical Theory with Exact Numerical Results

We study the contact mechanics of a smooth hard cylinder rolling on a flat surface of a linear viscoelastic solid. Using the measured viscoelastic modulus of unfilled and filled (with carbon black) nitrile rubber, we compare numerically exact results for the rolling friction with the prediction of a simple analytical theory. For the unfilled rubber, the two theories agree perfectly while some small difference exists for the filled rubber. The rolling friction coefficient depends nonlinearly on the normal load and the rolling velocity.     

纳米SiO2粒子链对硅橡胶的补强机理

介绍了纳米SiO2粒子链补强硅橡胶的机理。主要是因为纳米粒子链与聚硅氧烷分子链缠结和吸附，进行了无机与有机分子链水平的复合，复合体能随着基体形变而通过自身的屈服变形过程吸收大量的能量，从而补强硅橡胶。     

The lubricant retaining effect of micro-dimples on the sliding surface of PDMS

The frictional behavior of elastomers is characterized by compliance of asperities, special surface chemistry, and wetting properties. The objective of this paper is to investigate the coupled influence of surface texture and wettability on the lubrication of an elastomer contact. Patterns of micro-dimples were fabricated on disks of a polymer material, PDMS. An oxygen plasma treatment was used to hydrophilize the disk surfaces. Friction tests of the disks sliding against a spherical pin of PDMS were carried out. The experimental results indicate that the effect of surface texture is different for PDMS with different wetting properties.     

The adhesion and friction of smooth rubber surfaces

In an earlier study of adhesion between smooth rubber and rigid surfaces time effects were apparent. It now appears that under non-equilibrium conditions these effects largely determine the magnitude of the force required to separate adhering surfaces. This paper presents an optical study of contact area time effects between such surfaces and shows in a simple way how these optical observations may be used to predict the rate of rolling of a ball bearing on smooth rubber, the time taken to detach itself under gravity and its resilience when bouncing on smooth rubber. The friction when a rigid surface slides over smooth rubber under conditions where Schallamach waves are generated is also shown to be quantitatively related to their mutual adhesion.     

粗糙表面弹性微动接触数值研究

由于实际工程表面多为粗糙表面,这里研究了粗糙表面对微动接触中压力和切向应力的影响。研究接触过程中法向载荷保持不变,切向载荷为周期性的交变载荷。首先,建立接触算法和模型,其算法核心是利用共轭梯度法(CGM)计算微动接触中的表面压力及切向应力并使用快速傅里叶变换(FFT)加快计算速度。然后,在验证算法正确的基础上,分析正弦和非高斯粗糙表面接触的压力和切向应力的分布,通过对光滑与粗糙表面的研究对比,表明:(1)在正弦表面接触切向应力分布呈现尺寸效应;(2)在非高斯表面接触中,切向应力分布跟光滑表面形状类似;同时由于粗糙峰存在,粗糙表面下的切向应力比光滑表面下的要大,研究粗糙表面微动接触对实际工程具有重要意义。     

封隔器胶筒大变形摩擦接触的有限元分析

关于封隔器胶筒接触压力的求解，目前文献所给出的计算公式均没有考虑摩擦因数对封隔器胶筒接触压力的影响，而摩擦因数对接触压力有较明显的影响。针对胶筒与套管之间的粘-滑摩擦接触问题，采用罚函数技术，结合橡胶大变形问题的增量分析过程，给出解决封隔器胶筒摩擦接触问题的数值方法，并在此基础上对胶筒与套管之间的摩擦接触进行有限元分析。计算结果表明，采用大变形非线性粘弹性理论和接触摩擦描述的有限元模拟技术，可以比较准确地模拟摩擦因数对封隔器胶筒接触压力的影响，所得的结果比经典理论公式的分析结果的精度更高，具有理论价值和工程应用价值，可为胶筒的优化设计提供一定的依据。     

实心轮胎滚动状态下与地面三维接触的有限元分析

考虑了轮胎的几何非线性、橡胶材料的非线性、橡胶物理体积的不可压缩性,建立了轮胎的三维有限元模型,分析了实心轮胎在滚动状态下的接触问题,考察了实心轮胎的变形、应力、应变、下沉量与载荷的关系.     

实心轮胎滚动状态下与地面的有限元分析

考虑了轮胎的几何非线性、橡胶材料的非线性、橡胶物理体积的不可压缩性，建立了轮胎的三譬有限元模型，分析了实心轮胎在滚动状态下的接触问题，考察了实心轮胎的变形、应力、应变、下沉量与载荷的关系。     

Role of Frictional Heating in Rubber Friction

The energy dissipation in the contact regions between solids in sliding contact can result in high local temperatures which may strongly affect the friction. This is the case for rubber sliding on road surfaces at speeds above 1 mm/s. I derive equations which describe the frictional heating for arbitrary (non-uniform) motion, taking into account that some of the frictional energy is produced inside the rubber due to the internal friction in rubber. Numerical results are presented for one limiting case for steady sliding.     

粗糙表面热弹塑性接触问题的无网格法

建立可考虑屈服应力温度相关的粗糙表面热弹塑性接触无网格法数值计算模型.研究摩擦力和不同热输入情况下圆柱体与弹塑性平面的接触力学特性,探讨摩擦热效应对表面温升、接触压力和接触面积的影响.结果表明在考虑剪切摩擦力作用后,弹塑性接触压力分布不再关于接触区域中轴线对称而出现了"塌陷"现象.通过无网格法解与有限元法解比较发现不恰当的有限元网格划分会造成接触压力的数值震荡,而无网格法可避免这一现象的发生.发现忽略温度相关效应将高估最大接触压力而低估相应外载荷下产生的接触面积.     

封隔器胶筒大变形的粘-滑摩擦接触分析

采用罚函数方法,结合橡胶大变形问题的增量分析过程,考虑封隔器胶筒与套管之间的粘-滑摩擦接触问题,研究摩擦因数变化对接触压力的影响规律。给出解决封隔器胶筒摩擦接触问题的数值方法,并在此基础上对胶筒与套管之间的摩擦接触进行有限元分析,分析得到采用大变形非线性粘弹性理论和接触摩擦描述的有限元模型,可以比较准确模拟封隔器在坐封和工作过程中胶筒接触压力和变彤的情况,结果表明,摩擦因数变化对封隔器胶筒的接触压力有着较明显的影响。     

Thermomechanical analysis of elastoplastic medium in sliding contact with fractal surface

The effects of mechanical and thermal surface loadings on deformation of elastic–plastic semi-infinite medium were analyzed simultaneously by using the finite element method. Rigid rough surface of a magnetic head and smooth surface of an elastic–plastic hard disk were chosen to perform a comprehensive thermo-elastic–plastic contact analysis at the head–disk interface (HDI). A two-dimensional finite element model of a rigid rough surface characterized by fractal geometry sliding over an elastic–plastic medium was then developed. The evolution of deformation in the semi-infinite medium due to thermomechanical surface loading is interpreted in terms of temperature, von Mises equivalent stress, and equivalent plastic strain. In addition to this, the effects of friction coefficient, sliding, and interference distance on deformation behavior were also analyzed. It is shown that frictional heating increases not only the contact area but also the contact pressure and stresses.     

热力耦合下微接触点塑性应变沿深度变化分析

在考虑粗糙实体弹塑性变形、热力耦合、微凸体间相互作用和摩擦热流耦合等影响下,运用有限元法数值模拟具有三维分形特性的粗糙面与刚性平面间滑动摩擦过程,分析了粗糙实体接触凸点塑性变形随深度变化情况。发现：在速度的突变和闪点温度形成时,摩擦接触表层等效塑性应变增大明显;在这一摩擦表层,过不同接触点的纵向剖面塑性应变沿深度分布不同：有的是接触表面塑性变形最大,有的是在接触微凸体表面下某一深度塑性变形最严重,而接触凸点表面的塑性应变稍小些。这与相关文献用SEM研究干摩擦后金属摩擦表层变形照片后发现的结果一致。滑动摩擦过程中,金属粗糙摩擦接触表层塑性变形的不断累积,将会导致材料表层中的夹杂或微观缺陷周围萌生微孔和裂纹源。     

Development of contact sliders with nanotextures by femtosecond laser processing

The information on the frictional resistance of a self-propelled robotic capsule endoscope moving inside the body is very important for the design and the performance enhancement of such parameters of the capsule endoscope as power consumption, motion control and positioning accuracy. Based on this motivation, the ultimate goal of this research was to develop an analytical model that can predict the frictional resistance of the capsule endoscope moving inside the living body. In this work, experimental investigations of the fundamental frictional characteristics and the viscoelastic behaviors of the small intestine were performed by using custom-built testers and various capsule dummies. The small intestine of a pig was used for the experiments. Experimental results showed that the average frictional force was 10–50 mN and higher moving speed of the capsule dummy resulted in larger frictional resistance of the capsule. In addition, the friction coefficient did not change significantly with respect to the apparent area of contact between the capsule dummy and the intestine, and also the friction coefficients decreased with an increase in the normal load and varied from 0.08 to 0.2. Such frictional behaviors could be explained by the lubrication characteristics of the intestine surface and typical viscoelastic characteristics of the small intestine material. Also, based on the experimental results, a viscoelasticity model for the stress relaxation of the small intestine could be derived.     

A study on mechanical and tribological properties of silicone rubber reinforced with white carbon black

This paper analysed the effect of different white carbon black (fumed silica and precipitated silica) on the mechanics and friction property of silicone rubber. The tensile strength, tearing strength, elongation rate and shore hardness of silicone rubber reinforced with different kinds or dosages of white carbon black were investigated in detail. The wear morphology, worn surface analysis and chemical compositions of the rubber sample after the friction test were analysed by means of scanning electron microscopy, three-dimensional profile metre and energy dispersive spectrometer, respectively. Results showed that the mechanics properties of silicone rubber were obviously enhanced with the increase in white carbon black content. Moreover, after the friction test, the vulcanised rubber reinforced with fumed silica has less wear surface protrusions and grinding than that of precipitated silica. The wear rubber filled with fumed silica surface is quite smooth and shows few furrows and grooves, which well corresponds to good anti-wear ability of the reinforcing fillers. In the course of practical application, it is necessary to take into account of the mechanical properties, wear resistance of rubber and the adding amount of filler to achieve the optimum coordination for the best mechanical and friction properties of silicone rubber.