粗糙体变形特性对接触过程的应力与应变的影响

运用W-M函数生成分形粗糙表面,建立一个新的双粗糙体接触模型,采用有限元方法模拟仿真了在粗糙体不同变形特性条件下的接触过程,并分析了接触表面的应力分布及不同接触位置的塑性应变随深度的变化规律．结果表明双粗糙接触表面的应力主要集中在个别的较高微凸体上,其应力最大值出现在微凸体肩部区域的位置;等效塑性应变在不同位置沿深度的变化,呈现出不同的规律,微凸体顶部区域沿深度方向的最大等效塑性应变均发生在次表层,材料表层下的塑性应变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,对比不同变形特性的模型,得出弹塑性一刚体模型的最大应力及应变值都大于弹塑性一弹塑性模型。     

弹塑性粗糙体/刚体平面滑动摩擦过程热力耦合分析

建立一具有三维分形特性的弹塑性粗糙表面与一理想刚性平面接触的热力耦合模型,充分考虑粗糙表面接触微凸体间的相互作用及接触界面摩擦热流耦合等的影响.运用大型有限元软件ANSYS中的非线性有限元多物理场方法,数值模拟并分析了滑动摩擦过程粗糙实体的热、力分布规律.发现由于速度的突变,滑动初始摩擦表面最高温度急剧升高,而匀速滑动中温升缓慢;整个滑动摩擦过程中粗糙表面的VonMises等效应力分布极其不均,且在接触凸点后方距表面一定深度处存在一拉应力区;在滑动瞬间及其闪点温度形成时,粗糙实体表面的VonMises等效应力发生明显变化,且最大拉应力值及其拉应力层厚度明显增大,此结果为材料裂纹萌生及扩展提供了一定的理论依据.     

基于分形理论的滑动摩擦表面接触力学模型

依据分形理论,考虑微凸体变形特征及摩擦作用的影响建立滑动摩擦表面接触力学模型。采用一个三次多项式来表达弹塑性变形微凸体的接触压力与接触面积的关系,从而满足在变形状态转变临界点处的微凸体接触面积与接触压力转化皆是连续和光滑的条件。推导出滑动摩擦表面临界弹性变形微接触面积、临界塑性变形微接触面积、量纲一真实接触面积的数学表达式。理论计算结果表明,表面形貌一定时,真实接触面积随着载荷的增大而增大;载荷一定时,真实接触面积随着特征尺度系数的增大而减小,随着分形维数的增大先增大后减小;当表面较粗糙时,摩擦因数对真实接触面积的影响很小;随着表面光滑程度的增大,摩擦因数对真实接触面积的影响增大,真实接触面积随着摩擦因数的增大而增大,特别是当摩擦因数较大时,真实接触面积增大的幅度也较大。接触力学模型的建立,为研究滑动摩擦表面间的摩擦磨损性能提供了依据。     

法向加-卸载过程中弹塑性微凸体侧向接触能耗研究

针对法向加-卸载作用下双粗糙表面上微凸体接触阻尼能耗问题,提出弹性、弹塑性、塑性微凸体侧向接触能耗计算方法。基于微凸体接触球形假设,根据微凸体侧向接触受力分析,将其分解为垂直于微凸体接触点公切面的法向分力和沿该面的切向分力。采用HERTZ,ETSION理论,分别建立了加-卸载过程中微凸体发生弹性、弹塑性、塑性变形时,法向分力与变形之间关系;依据CATTANEO-MINDLIN黏着-滑移理论,BKE模型,ERITEN模型理论,建立了加-卸载过程中三个变形阶段的切向分力与位移之间关系。利用法向分力-变形和切向分力-位移之间的关系,求得微凸体在法向、切向分力共同作用下产生的应变能耗以及摩擦能耗,进而求得微凸体侧向微观接触在三个阶段下的能耗。研究表明,微凸体侧向接触时耗能包括应变耗能和摩擦耗能,且法向变形量越大,应变耗能、摩擦耗能越大;接触角度越大,应变耗能越大,摩擦耗能越小。     

滑动摩擦中不同润滑条件对应力层的影响研究

真实工程表面接触时,微凸体将产生弹塑性变形。建立了在不同润滑条件下二维双粗糙弹塑性分形模型,分析了润滑对滑动摩擦中应力层的分布影响情况。在完全润滑和边界润滑时材料表层下方出现易断裂的循环应力区域,干摩擦时循环应力层消失。     

粗糙表面基于G-W接触的三维瞬态热结构耦合

工程表面是粗糙的,其对磨损有较大影响.为了研究磨损过程的热动力学,文中基于G-W (Greenwood-Williamson)接触模型,将两个粗糙表面简化为一规则形状微凸体与一理想平面,分析在移动热源作用下接触面的边界条件,着重考虑摩擦滑动过程中两物体的弹性变形以及摩擦接触温度与接触区域应力的耦合问题,利用热-结构顺序耦合建立三维瞬态有限元计算模型.从而揭示粗糙表面滑动摩擦副的温度和热应力分布规律,为进一步研究热-机械失效问题及磨损机理奠定理论基础.     

基于分形几何与接触力学理论的结合面法向接触刚度计算模型

基于分形几何理论和接触力学理论,用分形理论表征粗糙表面微凸体参数,考虑微凸体由弹性变形向弹塑性变形以至最终向完全塑性变形转化的过程,建立各变形阶段微凸体的接触刚度模型。在此基础上,提出机械结合面法向接触刚度计算模型,该模型揭示了在不同的塑性指数下,结合面法向接触载荷与法向接触刚度之间的关系。结果表明,在塑性指数较小时,微凸体的变形以弹性为主,法向接触载荷与接触刚度之间表现为近似线性关系;随着塑性指数的增加,微凸体变形主要以塑性为主,法向接触载荷与接触刚度之间表现为较强非线性关系。对已有的铣削加工和磨削加工情况下的结合面法向接触刚度试验结果,利用该模型进行数值计算、仿真和分析。结果表明:提出的模型更与试验曲线吻合。     

粗糙表面微凸体对液黏传动的影响

为研究液黏传动过程中粗糙表面的承载特性,将分形理论引入到两粗糙表面摩擦过程之中,分析传动过程中混合摩擦和边界摩擦两阶段的微凸体承载过程,考虑微凸体弹塑性变形,对M-B模型进行修正,建立修正的微凸体承载模型。建立基于修正M-B模型的微凸体承载模型。通过数值仿真得到有效面积系数、分形参数对液黏调速离合器传动过程的影响规律;对修正的微凸体承载模型的计算结果与M-B模型的计算结果进行对比分析。结果表明:微凸体接触载荷和传递转矩随着面积比的增大而增大,当有效面积系数与尺度系数增大时,接触载荷与传递转矩均有所增大;分形维数为1.5时,微凸体接触载荷与传递转矩最小且随面积比的变化最为缓慢;在整个接触区域内,弹性变形区域的面积、接触载荷以及传递转矩最大,其次是弹塑性变形区域,塑性变形区域最小;考虑弹塑性变形时,微凸体接触载荷与传递转矩均有所下降;修正M-B模型和M-B模型间的修正系数范围在25%以内,修正系数随着有效面积系数、尺度系数的增大而增大,随着分形维数的增大而减小。     

考虑蠕变特性的橡胶微凸体与金属表面接触特性分析

金属-橡胶接触广泛存在于密封结构中,密封接触表面上微凸体间的相互作用会直接影响整个密封界面的接触特性,进而影响其密封性能。基于粗糙密封界面的单个微凸体,考虑橡胶的蠕变特性,采用理论分析和仿真研究相结合的方式研究橡胶微凸体与金属表面的接触特性。通过橡胶蠕变特性的实验结果,构建橡胶蠕变计算模型;构建半球微凸体与金属平板间的有限元模型,进行考虑蠕变特性的仿真,分析其接触特性,并与Hertz接触理论的计算值进行对比。结果表明：在蠕变阶段,接触半径、法向变形量和最大等效蠕变应变均随蠕变时间的增加而增大,最大接触压力随蠕变时间增大而减小,这均可能导致密封性能的下降;随压力载荷的增大,接触半径、法向变形量、最大接触压力和最大等效蠕变应变均增大,但增大的趋势逐渐减小;橡胶微凸体与金属表面间的等效模量随蠕变时间的增加而减小,随压力载荷增大而增大。     

基于真应力-应变关系的粗糙表面法向接触模型

提出一种粗糙表面的法向弹塑性接触分析的建模方法。基于微凸体的弹塑性有限元接触模型,分别研究了40Cr、45和Q235三种钢材料的微凸体与刚性平面的法向接触特性。有限元模型中采用三种材料的真应力-应变关系,考察了不同强化特性对微凸体接触性质的影响。建立了微凸体在弹性、弹塑性、塑性变形阶段统一的接触变量变化规律的表达式。在此基础上应用概率统计理论建立粗糙表面法向弹塑性接触模型。所建立的接触模型中微凸体接触变量的变化规律完全基于弹塑性有限元模型的计算结果,无需将微凸体的变形过程区分为不同的变形阶段,避免了接触变量在各阶段采用不同函数表达式带来的连续性和光滑性问题,以及在弹塑性阶段采用插值函数的随意性问题。通过与其他接触模型的计算结果相比较,证明了所提出接触模型的合理性。     

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.     

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.     

Analytical determination of friction resistance as a function of normal load and geometry of surface irregularities

A mathematical modeling and simulation of friction during steady state sliding of metals, based on the upper-bound approach, is demonstrated. The existence of wedge-shaped protrusions on the tool surface is assumed. Pressing these protrusions onto the workpiece and sliding the tool along the workpiece produces asperities on the surface of the workpiece. These asperities move in a wave-like motion along the surface layer and cause plastic deformation through a specified depth under the surface. This plastic deformation combines with local friction between the tool and the workpiece along the asperity interface to produce resistance to sliding. The relation between the normal pressure and the sliding resistance is established for the entire range of pressure levels from zero to infinity. The apparent Coulomb coefficient of friction for lower levels of normal pressure and the constant friction factor for excessive load levels are determined. The transition region from Coulomb coefficient of friction to constant friction factor also becomes clear. A mathematical determination is obtained by means of a force equilibrium considering the concept of a contact surface friction ratio. The force of resistance to sliding is related both to the geometry of the asperity of the surface of the tool and to the constant friction factor, which is used for measuring a local frictional force along the interface of each asperity.     

Effects of Friction on the Contact and Deformation Behavior in Sliding Asperity Contacts

Finite-element analyses are carried out to study the effects of friction on the contact and deformation behavior of sliding asperity contacts. In the analysis, on elastic-perfectly-plastic asperity is brought in contact with a rigid flat at a given normal approach. Two critical values of the normal approach are used to describe the asperity deformation. One is the approach corresponding to the point of initial plastic yielding, and the other at the point of full plastic flow. Additional variables used to characterize the deformation behavior include the shape and size of the plastic zone and the asperity contact size, pressure, and load capacity. Results from the finite-element analysis show that the two values of critical normal approach decrease significantly as the friction in the contact increases, particularly the approach that causes plastic flow of the asperity. The size of the plastically deformed zone is reduced by the friction when the contact becomes fully plastic. The reduction is very considerable with a high friction coefficient, and the plastic deformation is largely confined to a small thin surface layer. For a low friction coefficient, the contact size, pressure and load capacity of the asperity are not very sensitive to the friction coefficient. For a moderate friction coefficient, the contact pressure is reduced and the junction size increased; the load capacity of the asperity is not significantly affected due to the compensating effects of the pressure reduction and the junction growth. For a high friction coefficient, the pressure-junction compensation is not longer sufficient and the asperity load capacity is reduced. The degree of the friction effects on these contact variables depends on the applied force or the normal approach. Although the analyses are conducted using a line-contact model, the authors believe that the effects of friction in sliding asperity contacts of three-dimensional geometry are essentially the same and the same conclusions would have been reached. These results may provide some guidance to the modeling of rough surfaces in boundary lubrication, in which the asperity friction coefficient can be high and vary significantly both in time and from one micro-contact to another.     

Stress and strain and damage during frictional strengthening treatment of flat steel surface with a sliding cylindrical indenter

Frictional strengthening treatment of a flat steel surface with a cylindrical indenter is considered. A finite-element model of the process is developed. The parameters of stress and strain of the treated material are calculated, and the accumulated deformation in the surface layers is evaluated as a function of the number of frictional load cycles and the coefficient of friction. The type of changes in the target value of plastic deformation along the depth of the frictionally treated surface layer is compared to the experimentally calculated distribution of microhardness and density of dislocations in the surface layer. On the basis of evaluation of damage on the surface of frictional contact, restrictions on using the developed finite-element model for analyzing actual frictional treatment are considered.     

The significance of near surface microstructure in the wear process

The wear process in which flake-like debris is developed and removed from the surface of metals in sliding contact is the direct result of heavy plastic deformation of a thin surface layer. The repeated ploughing of asperity contacts over a mating surface can produce high dislocation densities and eventual change in the microstructure to a cell-type structure found in heavily deformed metals. Cell sizes depend on material characteristics such as stacking fault energy, the applied stress and the temperature. It is shown that a cell structure can present many suitable pathways for subsurface crack generation and the release of thin wear flakes without the benefit of asperity cold welding and shear. Depth of crack formation and severity of wear can be associated with stacking fault energy. Changes in the microstructure caused by frictional heating or change in strain rate can cause abrupt changes in wear mode.     

Contact analysis of three-dimensional rough surfaces under frictionless and frictional contact

A methodology for surface and sub-surface stress calculation of nominally flat on flat rough surface contact has been developed. This methodology is applicable for both large area contact (Hertzian contact) and small area of asperity contact (point load contact) with and without surface friction. A total of nine rough surfaces are generated by the computer with specified standard deviation of surface heights, σ, of 0.3, 1.0 and 3.0 nm, and correlation length, β^*, of 0.1, 0.5 and 0.9 μm. Under the typical applied load at the magnetic head slider-disk interface, small numbers of contact points are obtained and the deformation is purely elastic. Since these contact points are scattered and isolated, asperity contact behaves like point load contact. As β^* becomes larger, more adjacent points will be in contact at a certain contact spot and this is especially true at small σ. All the cases of flat on flat rough surface contact yield maximum von Mises stress on and near the surface at both frictionless and frictional contacts; no local maximum occurs in the sub-surface. In general, the friction effect in the vicinity of contact point is to increase the stress magnitude, while outside this region it also alters the stress distribution. For a surface of small β^* and large σ at high load of 1000 times of the nominal pressure at the head-disk interface, the contact pressure reaches the hardness at a few contact points and plastic deformation takes place in the near surface.     

梯度覆层体受粗糙面滑动作用时的弹塑性分析

运用大限元软件分别虽对单覆层体及梯度体受多微凸体粗糙面滑动使用时产生的应力在变进行了计算和研究，着重比较两种覆层体爱相同表面载荷作用下，在基体及表面出现部分塑性变形时，表面层、基体内及表面 基体界面处的应力、应变分北度层在防止其本产生塑性变形及改善界面应力等方面比单纯层体具有明显的优点。本文的研究结果表为表南 选一览表主加工提供参考。