粗糙表面接触问题的数值计算研究

基于Ausloos和Berman提出的推广的W-M函数对具有不同粗糙度参数的分形表面进行仿真模拟.同时,基于前人的理论研究,在模拟的分形粗糙表面基础上建立了微尺度粘着弹塑性接触模型.通过数值模拟,得到了给定条件下各个微凸体上的载荷、真实接触面积、接触斑点尺寸和平均接触压力的分布情况.建立的模型考虑各个微凸体具有不同的峰顶曲率半径,当变形足够大时考虑微凸体间的相互作用,因而较前人建立的模型更符合工程实际,同时也对与分形特征相关的摩擦学模型的进一步研究具有一定的指导作用.     

粗糙表面切向接触刚度概率分析方法

针对粗糙表面微凸体的多样性引起的接触切向刚度的变化,提出粗糙表面切向接触刚度概率分析方法.基于Hertz理论和GW模型,考虑微凸体接触半径与高度均服从正态分布,得到各自的概率密度函数.运用接触力学和统计学原理,计算粗糙表面微凸体的个数,建立粗糙表面切向接触刚度的概率统计模型.采用控制变量法,研究外界载荷、摩擦系数和统计参数对切向接触刚度的影响,与同种工况下分形模型的计算结果与实验结果进行对比.研究结果表明粗糙表面切向接触刚度与法向载荷和摩擦系数成正相关,与切向载荷成负相关;微凸体之间高度或接触半径的正态分布标准差越小,切向接触刚度越大.这与同条件下分形模型及实验得到的结果趋势相一致.     

微凸体切向滑移特性

为了从微观角度探究粗糙表面的切向滑移特性,构建一种具有回转抛物体形状的微凸体切向滑移有限元模型.该模型综合考虑了材料的弹性、弹塑性变形和塑性变形特征,分析整个切向滑移过程中切向、法向合力分别随切向、法向位移等参数的变化规律,并给出接触载荷的无量纲化拟合公式;推导了基于Kogut弹塑性(KE)模型的侧接触力学模型,并用该理论解释切向滑移特性.分析表明：KE模型侧接触模型仅仅适用于小变形范围;切向滑移位移越大,切向的塑性流动越大,KE模型侧接触模型与仿真模型的误差也越大;另外,载荷的最大值、最小值和均值都表现出了明显的变化规律.微凸体切向滑移特性的仿真分析以及侧接触理论研究为粗糙表面接触特性的研究奠定了基础.     

尺度相关的分形粗糙表面弹塑性接触力学模型

依据分形理论,研究了粗糙表面间的真实接触状况,建立了粗糙表面间的分形接触模型。考虑微凸体的等级,确定了弹性临界等级、第一弹塑性临界等级和第二弹塑性临界等级的表达式,研究了粗糙表面中单个微凸体的弹性、弹塑性及完全塑性变形的存在条件,推导出各个等级微凸体的临界接触面积的解析式。在此基础上应用微凸体的面积分布密度函数,获得了接触表面上接触载荷与真实接触面积之间的关系。计算结果表明:单个微凸体的临界接触面积是和微凸体的尺度相关,随着微凸体等级的增大而减小;微凸体的变形顺序为弹性变形、弹塑性变形和完全塑性变形,与传统的接触模型一致;在整个粗糙表面接触过程中,粗糙表面变形过程与单个微凸体的变形过程一致;最大微凸体所处的等级范围不同,粗糙表面所表现的力学性能也不相同。     

基于等温弹流润滑理论的滚珠丝杠副摩擦机理研究

针对滚珠丝杠副在低转速状态存在混合润滑导致丝杠滚道加速磨损的问题，基于等温弹流润滑理论建立了滚珠丝杠副弹流润滑接触模型研究滚珠与滚道间的摩擦机理和磨损形式.首先基于Frenet-Serret坐标转换方法建立丝杠与螺母的滚道曲面几何模型.其次基于Reynolds方程建立了滚珠丝杠副弹流润滑接触模型，求解了润滑油膜接触压力分布、油膜厚度分布和微凸体接触压力分布.最后从微观角度阐明了滚珠丝杠副的摩擦机理，其摩擦因数由微凸体接触摩擦因数和油膜润滑摩擦因数组成.实验结果表明：在低转速阶段丝杠滚道的磨损方式为黏着磨损和磨粒磨损，且模拟的摩擦因数曲线趋势和经典的Stribeck摩擦曲线趋势吻合较好，根据实测摩擦力矩准确识别了不同转速下滚珠丝杠副的摩擦因数和润滑状态.     

带鳍状凸肩结构汽轮机叶片的接触分析

为了研究大型汽轮机叶片旋转过程中鳍状凸肩间的接触状态,利用拉格朗日增广算法,采用三维有限元模型,计算了考虑摩擦情况下的凸肩间接触压力的变化情况,得出了接触压力与转速之间的关系,并计算了3000r/min下整圈叶片的固有频率．计算表明:当叶片转速达到400r/min时,鳍状凸肩开始接触,此后随着转速的增加,鳍状凸肩间的接触压力逐渐增大,接触面积不断扩大;当转速为1825r/min时,接触压力增加程度明显趋缓;转速为3000r/min时接触压力最大(700MPa),鳍状凸肩间的实际接触面积为设计接触面积的1/4左右．     

刀具-岩石摩擦过程中的闪温分析

摩擦表面闪温对刀具材料性能产生着重要影响。针对刀具摩擦表面的微凸体建立了导热微分方程,系统分析了影响微凸体温度的影响因素。在提出摩擦表面接触系数的概念的基础上推导出了干摩擦条件下刀具闪温的解析计算模型并分析了水射流条件对闪温计算的影响。给出了基于均匀分布模型和概率分布模型的摩擦表面接触系数和水射流参与下满足动力润滑条件的摩擦系数等计算参数的确定方法。计算分析与红外测温实验的结果表明：刀具与岩石的摩擦闪温严重依赖两摩擦面的接触系数和各自表面微凸体分布情况;给出的摩擦面闪温计算模型比传统计算方法更接近实验测量值,更适用与工程中对材料安全性要求较高的场合应用。     

基于分形理论的机械密封干摩擦时端面接触特性的研究

针对碳石墨与碳化钨配对机械密封在干摩擦时端面的接触特性进行研究,在考虑动、静环材料属性,端面微凸体之间的相互作用,摩擦运动方向以及摩擦热流的基础上,建立了三维粗糙实体与理想光滑刚体的转动摩擦热-力耦合模型,采用ABAQUS有限元分析软件对其瞬时干摩擦过程进行数值模拟。研究结果表明：粗糙端面的真实接触面积随外载荷的增大近似线性增加;随着外载荷与滑动速度的增加,粗糙端面最大接触压力保持在415～432MPa范围内,表现出“自限性”;粗糙端面温度分布不均匀,外载荷与速度增加会导致端面整体温度增加。端面最高温度的位置受到局部接触压力和局部滑动速度以及局部热传递的共同影响;API682标准中提出抑制机械密封的介质压力≤0.07MPa是有必要的,可以防止干摩擦时密封环温升过高。本研究可对探索机械密封摩擦端面热损伤、摩擦磨损研究以及机械密封设计提供借鉴。     

干气密封摩擦副启停阶段摩擦特性的仿真研究

针对干气密封非稳态下摩擦特性对密封性能的影响进行研究,考虑动、静环材料属性,微凸体之间的相互作用以及摩擦热流耦合,建立了三维粗糙实体与理想光滑刚体滑动摩擦热力耦合模型。运用ANSYS软件数值模拟了摩擦热以及应力变化规律。研究发现,粗糙表面最高接触温度随滑动时间增加呈逐步上升趋势,并且温升呈 现了一定的波动性;粗糙表面的VonMises等效应力分布极其不均匀呈非线性变化;同时,还发现最大x 方向应力分量σxx 并未出现在最高接触微凸体上;在沿三维粗糙实体厚度方向存在一拉应力区,随着滑动时间的持续,拉应力区有一定程度扩大。从而说明两端面间的温升和波动性以热传导为主要影响因素,应力的变化是由于微凸体发生了弹塑性变形。研究成果为今后干气密封启停阶段特性研究以及参数优化奠定了基础。     

关节轴承中微凸体的热分析

为研究关节轴承内外圈摩擦副上微凸体在相对滑动过程中的摩擦热问题,建立半球状微凸体相对光滑平面滑动的模型,对点热源导致的温升进行积分,计算关节轴承内外圈在微凸体接触面处的稳态温升分布. 分别计算微凸体在弹性接触与塑性接触状态下接触面的稳态温升分布,并研究了在不同角速度和载荷下接触面沿速度方向的温升变化. 给定在一定范围内变化的载荷及速度,分别计算绘制出了两种不同型号关节轴承中的微凸体在接触区的最大闪温图. 计算结果表明:内圈微凸体在接触区的温升是对称分布的,最大温升位于接触面的中心点,外圈接触区后沿的温升大于前沿的温升,最大温升出现在中心点偏后沿的位置. 轴承角速度或微凸体载荷越大,接触区的温升越大. 在低速重载工况下,微凸体的最大闪温值较小;而在高速情况下,微凸体的最大闪温值较大. 关节轴承在工作时应注意载荷和角速度的控制,防止因微凸体摩擦生热过多从而造成轴承使用性能受损.     

Numerical Investigation into the Effect of Air Voids on the Anisotropy of Asphalt Mixtures

The aim of this study is to investigate the asphalt mixture anisotropy of both the modulus and Poisson's ratio due to air voids using a discrete element modeling simulation method. Three three-dimensional cubic digital samples of asphalt mixture with different shapes of single air void were built using discrete element software PFC~(3D). The aggregate gradation, air voids and mastic included in the digital samples were modeled using different contact models, with due consideration of the volumetric fractions of the different phases. Laboratory uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling. Simulation of the uniaxial cyclic compressive tests was performed on the three cubic samples loaded in three different directions. Dynamic modulus in three directions and Poisson's ratio in six directions were calculated from the compression stress-strain responses. Results show that both the modulus and Poisson's ratio are dependent on the preferential orientation of air voids. The anisotropy of the modulus and Poisson's ratio increases as the pressure loading on the asphalt mixture increases. Compared to the modulus, Poisson's ratio due to air voids has been shown to be more anisotropic. The maximum of Poisson's ratio and modulus is shown to be up to 80% and 11% higher than the minimum, respectively.     

Associated rules between microstructure characterization parameters and contact characteristic parameters of two cylinders

The contact strength calculation of two curved rough surfaces is a forefront issue of Hertz contact theory and method. Associated rules between rough surface characterization parameters(correlation length, and root mean square deviation) and contact characteristic parameters(contact area, maximum contact pressure, contact number, and contact width) of two rough cylinders are mainly studied. The contact model of rough cylinders is deduced based on GW model. As there is no analytical solution for the pressure distribution equation, an approximate iterative solution method for the pressure distribution is adopted. Furthermore, the quantitative relationships among the correlation length, the root mean square deviation, the asperity radius of curvature and the asperity density are also obtained based on a numerical simulation method. The maximum contact pressure and the contact number decrease with the increase of correlation length, while the contact width and the contact area are on the contrary. The contact width increases with the increase of root mean square deviation while the maximum contact pressure, the contact area and the contact number decrease.     

弹塑性微峰接触的部分膜点EHL数值分析

在部分膜EHL润滑下,磨擦副表面微峰接触状况对零件表面应力及失效形式有直接的影响,为此,在部分膜点接触EHL模型中引入更符合实际的表面微峰弹塑性接触模型来考察微峰接触载荷;并应用多重网格法针对多种表面微峰结构参数的影响进行了完全数值分析。结果表明,粗糙度对油膜的影响和表面纹理方向有关;相同粗糙度情况下,纵向纹理表面的接触载荷＞各向同性纹理表面的接触载荷＞横向纹理表面的接触载荷,表面粗糙度大时,峰顶参数对油膜压力和微峰接触压力有一定的影响,而对油膜形状影响甚微;表面越粗糙、表面凸峰越尖锐,微峰接触载荷则越大。     

同步器同步机理建模与结构影响因素分析

针对同步器结合过程,利用平均雷诺方程和微凸体摩擦原理,建立了油膜压力、微凸体接触力、同步环轴向力、同步力矩4个数学模型,运用4-Runge-Kutta法对油膜厚度和转速差进行耦合数值求解,分析了同步器结合过程油膜厚度、转速差、粘性剪切转矩、粗糙摩擦转矩以及总转矩的变化规律.对同步器结合过程数学模型进行试验验证后,利用所建模型研究了同步环宽度、同步环半径、摩擦锥角以及摩擦材料厚度等因素对同步器结合过程的影响规律.结果表明:同步环宽度增大,粘性转矩和粗糙接触转矩增大,油膜厚度下降速率减缓,粗糙接触转矩响应延迟,同步时间增加;同步环半径增大,粘性转矩和粗糙接触转矩增大,油膜厚度下降速率加快,同步时间缩短;同步环摩擦锥角增大,粘性转矩增大,粗糙接触转矩减小,转速差下降速率变缓,同步时间增加;摩擦材料厚度增大,粗糙接触转矩相应加快,油膜厚度下降速率增大,最小油膜厚度减小,同步时间缩短.     

塑性接触对表面形貌的影响

针对工程中许多接触表面(如机床静结合面)的接触特性(如接触面刚度等)都决定于塑性接触后的表面形貌的情况,本文研究了塑性接触对表面形貌的影响,提出了相应的计算模型,并考虑了法向载荷与切向载荷的影响。该模型不仅为研究静结合面(这种结合面塑性接触后的表面形貌是无法测定的)接触特性提供了有效方法,而且对于合理选择精整加工的最佳工艺参数具有指导意义。     

Parametric analysis of mixed lubrication characteristics in work zone of strip rolling

A theoretical model for mixed lubrication with more accurate contact length has been developed based on the average volume flow model and asperity flattening model,and the lubricant volume flow rate and outlet speed ratio are determined by integrating differential equations based on rolling parameters.The lubrication characteristics at the roll-strip interface with different surface roughness,rolling speed,reduction and lubricant viscosity are analyzed respectively.Additionally,the average volume flow rates of lubricant under different rolling conditions are calculated and used to explain the change rule of lubrication characteristics.The developed scheme is able to determine the total pressure,lubricant pressure,film thickness and real contact area at any point within the work zone.The prediction and analysis of mixed lubrication characteristics at the interface is meaningful to better control the surface quality and optimize the rolling process.     

Lateral Pressure of RC Silos with Static and Dynamic Granular Materials

This paper aims at analyzing material-induced lateral pressure of RC cylinder silo in both static and dynamic condition using the finite element method (FEM).In the finite element software ABAQUS, concrete material is modeled by concrete damaged plasticity model, and stored materials in silo is modeled by the hypoplastic theory.In terms of numerical model, shell elements (S4R) and solid elements (C3D8) are applied for model silo wall and stored materials respectively. The interaction between silo wall and stored materials is simulated by Coulomb friction model and penalty contact constrain provided by ABAQUS.The numerical results are verified with the existing experimental data that are designed to ensure the validation of such numerical model using FEM and it obtains good agreements between numerical results and experimental data.Then the material parameters are analyzed in both static and dynamic condition.According to the analysis, it is clear that critical friction angle, initial void ratio and minimum void ratio have an obvious effect on static lateral pressure while all the material parameters affect dynamic lateral pressure at different levels. In addition, differences of silo wall between elastic and plastic state are analyzed in dynamic condition.The numerical results show that it contributes to increasing dynamic pressure when silo wall enters into the plastic state.Finally, this paper discusses the time-history lateral pressure at different heights along silo wall, and analytical results indicate that larger acceleration values play main roles in producing the maximum lateral pressure at higher part of the silo wall.     

An analytical model for the prediction of cross-section profile and mean roll radius in alloy bar rolling

In a round-oval-round pass rolling sequence,the cross-section profile of an outgoing workpiece was predicted fast after getting the maximum spread.The concept"critical point on the contact boundary"was proposed and the coordinates of the critical point were solved.The equivalent contact section area was represented and the mean roll radius was determined.The validity of this model was examined by alloy bar rolling experiment and rigid-plastic FEM simulation.Compared with the existing models,the mean roll radius obtained by this model is similar to experiment data.     

粗糙节理的改进形貌表征方法及采样点距效应

为了定量表征粗糙岩石节理的三维形貌特征,采用三维工业扫描仪X3对3组天然红砂岩节理进行三维激光扫描试验. 基于Delaunay点云离散算法重构岩石节理面,分析单个微单元体与剪切方向的几何关系,提出改进模型来描述潜在接触部分随表面有效倾角门槛值的变化特征,并研究节理粗糙度随采样点距的变化规律. 结果表明：改进模型与试验结果较吻合,精度优于传统的Grasselli模型. 基于改进模型提出新的粗糙度评估参数,该指标满足量纲分析的要求,能准确捕捉节理粗糙度的各向异性特征,可以用于表征天然岩石节理的粗糙度. 发现节理形貌表征存在采样点距效应. 随着采样点距的增大,节理粗糙度不断减小. 在特殊情形下,采样点距增大会导致粗糙度增大,主要是因为放大了节理的一阶粗糙度特征.