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.     

金属滑动干摩擦条件下高频噪声的测试与分析

基于面-面接触旋转式滑动干摩擦系统,选用A3&45#钢为摩擦副材料,研究相对滑动速度,表面压力对高频摩擦噪声的影响和规律。运用ABAQUS有限元模态分析高频摩擦噪声产生的原因与机理。通过实验与模态分析发现,摩擦高频噪声易于发生在相对滑动速度较低处。A3&45#摩擦副,在面-面接触旋转式滑动干摩擦条件下,产生的高频摩擦噪声能量集中分布在7950 Hz,9200 Hz。验证了干摩擦条件下,摩擦高频噪声是由于摩擦界面形成类似"锤击效应"的微凸体之间的运动,使得摩擦力高频成分和系统固有频率耦合引起的系统不稳定现象。     

双粗糙接触模型的表面温度在滑动摩擦过程中变化分析

建立三维双粗糙体分形表面的热力耦合接触模型,在固定滑动速度工况下综合考虑了钛合金材料的磨损失效、界面粘着及接触过程中的热力耦合,动态探讨了粗糙体在滑动过程中接触表面的温度变化情况。运用有限元方法对滑动过程的温度场进行模拟仿真并得出:滑动摩擦初始时刻摩擦表面接触温度急剧上升,随着滑动距离的增加,最高接触温度处于波动状态;界面剪切强度越大,最高温度越高。通过研究接触表面的温度场分布情况,以探索滑动过程钛合金材料摩擦磨损的真正起因。将结果与相关文献实验进行比较,得出了模拟仿真的合理性。     

防爆起重机车轮在钢轨上全滑动摩擦温升分析

防爆起重机在钢轨上紧急制动或车轮卡死时可能产生全滑动工况,由于硬摩擦作用,该工况下起重机车轮和钢轨接触踏面温升显著,对工作在含有爆炸性气体环境中的起重机是一个潜在的危险源。建立起重机轮轨之间的接触模型,采用 Ansys 软件仿真分析起重机在钢轨上全滑动时的温升情况,探究摩擦因数、滑动速度对起重机轮轨温度场的影响。研究表明,全滑动工况下,起重机最高温升发生在车轮接触面上,轨道上的温升较低,其温度场分布为一条长轨迹;表面摩擦因数和滑动速度越大,轮轨踏面温升也越大,但对踏面温度场分布的影响不大。     

SiC/石墨配副密封端面织构低速磨损特性

采用端面上加工有倾斜椭圆微孔的SiC环和石墨环配副,实验研究椭圆微孔机械密封端面的低速摩擦磨损性能。实验测量干摩擦及油润滑条件下SiC环的磨损率和温升,分析表面织构对密封端面磨损特性的影响规律。结果表明:接触干摩擦条件下,与织构面配副的石墨环的磨损率明显高于光滑表面;油润滑条件下,转速相对较低时,织构面的温升高于光滑表面,表现出增磨效果;转速相对较高时,织构面的温升小于光滑表面,表现出减磨效果;并且干摩擦和油润滑条件下,表面织构均可减少磨屑的切削和犁削作用,起到表面研磨作用,使得石墨环表面更为光滑。     

滑动速度对钢轨表面接触应力的影响

利用ANSYS有限元软件建立轮轨系统弹性平面应变有限元简化模型,模拟轮轨滑动接触行为。研究机械载荷条件下滑动速度对钢轨接触表面接触应力的影响,以及热-机耦合载荷条件下滑动速度对钢轨表面接触应力、摩擦温升的影响,并对2种条件下的接触应力进行比较分析。结果表明:耦合载荷条件下的接触应力较机械载荷条件下显著增加,分布更集中于接触斑附近,即轮轨相对滑动产生了明显的摩擦热效应;滑动速度增加,摩擦热效应越明显,热影响层越浅,即滑动速度对接触应力有显著影响,钢轨接触应力分析时必须考虑滑动速度的影响。     

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.     

温度对湿式离合器摩擦副润滑及摩擦特性的影响

考虑局部温升的影响,建立了平面接触热混合润滑模型。分别对正弦粗糙表面、Gauss粗糙表面进行仿真计算,分析了温度对湿式离合器摩擦副间油膜厚度、局部压强分布、局部温升、油膜和微凸峰承压比的影响。结果表明,随着温度的升高,摩擦副间油膜厚度减小,微凸峰接触数量增多,局部温升增大,微突峰承担的载荷逐渐增大,而润滑油膜承担的载荷虽然有所减小,但依然非常显著,不可忽略。     

环境湿度对浸金属碳滑板磨损及温升的影响

为探究环境湿度对弓网摩擦副载流滑动过程中电弧放电能量、浸金属碳滑板温升及滑板磨损量的影响,采用环-块式高速载流摩擦磨损试验台,对比不同湿度条件下,电弧能量、滑板温升及滑板磨损量随滑动速度、电流强度、法向力的变化情况。试验结果表明：不同环境湿度下,滑动速度和电流强度的增大均会导致电弧能量及滑板温升急剧增大;电弧热是导致温升的主要热源;增大法向力对于抑制电弧放电、降低滑板温升均有显著效果,而对于滑板磨损量变化的影响,不同湿度情况则截然相反;高湿度环境下接触副附着的水膜改善了接触状况和散热情况,电弧能量及滑板温升都小于低湿度环境;低湿度环境下滑板表面受到更严重的机械摩擦,其表面状态相比高湿度更差;在平均湿度较高的夏季适当增加升弓压力,在平均湿度较低的冬季适当降低列车行驶速度可以减少浸金属碳滑板磨损。     

材料属性温度相关热弹塑性接触问题研究

应用有限元方法建立了可考虑屈服应力温度相关效应的粗糙表面热弹塑性接触模型。研究了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。求解了考虑屈服应力温度相关效应的粗糙表面热弹塑性接触问题,探讨了摩擦热效应对表面温升、接触压力、平均间隙及接触体应力分布的影响。提出了考虑热膨胀系数温度相关效应的热弹塑性接触模型。通过刚性圆柱体与半无限大平面的热弹塑性接触研究了热膨胀系数温度相关效应对接触体应力分布的影响。
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Model and computer simulation of friction

A model, consisting of the stochastic formulae which describe the relationship between the friction coefficient and the parameters known to be significant in dry friction, is proposed for dry sliding friction between two randomly rough surfaces. The model considers the frictional phenomena which dominate in energy dissipation and which may occur simultaneously between contacting elements. The kind of interaction in a given spot of asperity contact depends on the local energy flux density. The effect of several parameters on the friction coefficient, the real area of contact, and the number of contact spots can be calculated with the FRI-SIM program. A comparison of the simulation and experimental data shows a convergence of results.     

An experimental study on roughness noise of dry rough flat surfaces

This paper presents an experimental study of the friction noise, between two rough and dry flat surfaces. The domain of interest is the dry contact under light pressure where the roughness is the dominant cause of noise. The results show that, for sliding rough surfaces under light load, the fundamental mechanism of radiated noise is the presence of shocks occurring between antagonist asperities of sliding surfaces. The radiated roughness noise is controlled simultaneously by the detailed topography of the surfaces in contact, the sliding speed and the dynamics of the surfaces. In terms of topography and sliding speed, it was shown that the roughness noise is simultaneously an increasing linear function of the logarithm of the surface roughness and the sliding speed. In terms of dynamics, the roughness noise is generated for light dynamical coupling. Hence, the natural modes of samples are stiffer than the contact and therefore the resulting vibrations are not affected by the additional rigidity. The deformation of surfaces during contact is very light and its magnitude is negligible compared to the surface roughness.     

Thermoelastic Analysis of Grooved Friction Clutches Using Finite Element Method

High temperatures appear on friction clutch surfaces due to the heat generated between the contacting surfaces during the slipping period. In some cases the maximum temperatures will exceed the allowable working temperature; friction clutches will fail rapidly when working under these conditions for a long time. Therefore, it is necessary to study the effect of design parameters (e.g., grooves) on the thermoelastic behavior of friction clutches to avoid these kinds of failures or at least increase the lifetime of friction clutch. A finite element method will be used to investigate the effect of the circumferential groove on the thermoelastic behavior of a single-disc clutch during a sliding phase. Axisymmetric models have been developed in this analysis to simulate the dry clutch systems during the sliding period. The effect of the groove area ratio G.R (defined as the groove area divided by the nominal contact area) on thermal and elastic behaviors is investigated. The results showed that the groove size has a significant effect on the magnitude and distribution of contact pressure, temperature field and heat generated along the frictional surfaces.     

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.     

双粗糙面滑动摩擦热力耦合有限元分析

建立了双粗糙分形表面滑动摩擦的热力耦合模型,综合考虑了随温度变化的材料性能、材料的弹塑性变形及摩擦副的磨损失效等因素,以摩擦材料的性能参数及设定的材料损伤参数为实例对双粗糙分形表面滑动摩擦全过程的温度场、应力场及磨损进行了数值模拟,分析得到了滑动摩擦过程中摩擦界面最高接触温度、接触应力的分布、磨损率及其变化规律,实现了对双粗糙面摩擦磨损情况的模拟及预测。     

Friction characteristics of nanoscale sliding contacts between multi-asperity tips and textured surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.     

Dissipative function of coulomb friction on elliptical contact area

The problem of calculation of the moduli of the resultant vector and the basic moment of the forces of the dry rolling friction of a solid body against a rough plane surface in the presence of spinning around a vertical axis is solved in quadratures. The elementary forces of sliding friction are directed opposite to the points of the elliptical contact area fulfilling plane motion. The friction coefficient is accepted to be independent of velocity. The proposed method of computing the resultant vector and the basic moment of friction forces on the elliptical contact area makes it possible to find with a preset accuracy the forces and the friction torque in the contact problems by using numerical procedures. In particular, it is used to determine the tangential forces arising during the motion of a rail vehicle. With increasing vertical load on the wheels, the contact spot area in which the friction forces of sliding play the main role becomes larger. The proposed method can also be used with some modifications, e.g., by introducing a corrective factor, to calculate the contact spots having a configuration different from elliptical but restricted by a closed curve.     

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。     

Surface Roughness Effects on Metallic Contact and Friction

A study was made of surface roughness effects on metallic contact and friction in the transition zone between hydrodynamic and boundary lubrication. The system used was one of pure sliding and relatively high contact stress, namely a fixed steel ball riding on a rotating steel cylinder.

It was found that very smooth and very rough surfaces gave less metallic contact than surfaces of intermediate roughness; very smooth surfaces also gave less friction.

Four different types of antiwear/antifriction additives (including tricresyl phosphate) were studied and although they were found to reduce metallic contact and friction, they had little effect in reducing surface roughness. Rather, the additives merely slowed down the wearing-in process of the base oil. Thus, the “chemical polishing” mechanism advanced for the antiwear behavior of tricresyl phosphate appears to be incorrect.

With rough surfaces, the improvement in load-carrying capacity with increasing viscosity was less than that shown previously with smooth surfaces. Also, oils with a large pressure-viscosity coefficient did not show the expected beneficial effect with rougher surfaces.     

Roughness and Lubricant Effect on 3D Atomic Asperity Contact

Large-scale molecular dynamics simulations were performed to study the sliding process of rough surfaces with and without lubricant. In the dry contact, a linear relationship has been observed between the load and the contact area for surfaces with large root mean square (RMS) roughness. However, it becomes nonlinear when the RMS is small. In the presence of adhesion, small roughness results in a large friction force when the surfaces are flattened and the contact area reaches 60 %. In order to confirm this observation, nonadhesive models have been established with an observation that under the combined influence from roughness and adhesion, the contact area plays a crucial role to determine whether the dry sliding is under the domination of roughness or adhesion. In the lubricated sliding, an increase in friction force has been found for the partially lubricated condition because the asperity contact still accounts for a great deal of resisting force. Besides, the lubricant exerts a comparable resisting force to the sliding.