Processes of dynamics of surface layers during low-amplitude fretting

A model of dynamic effect during low-amplitude oscillations of surfaces comparable with the preliminary displacement and the first acts of sleep under the oscillating tangential loading of the contact is developed. Solving a system of differential force balance equations has yielded the main characteristics of the process of the dynamics of a nominally stationary contact depending on the governing parameters of the system. Regularities of the friction characteristic at low sliding velocity are determined with allowance for the preliminary displacement phenomenon, the Stribeck effect, as well as the parameter of the plasticity and viscosity of friction. The model allows one tracing the evolution of relative displacements and slip velocities, calculating phase diagrams, as well as deriving the friction characteristic and real slip amplitudes typical of the mixed friction mode and low-amplitude fretting.     

一种模拟热成形工况的高温摩擦试验机的设计及探讨

销-盘摩擦试验机是摩擦学领域的标准摩擦实验设备，但是面对热成形过程中的摩擦过程，销-盘摩擦试验机中的往复摩擦引起磨屑在轨道内快速堆积，与热成形实际工况并不一致。提出一种新的高温滑移摩擦试验机设计方案，该方案控制模具钢销在板料滑条上单向长距离滑移，用以实现对连续热成形摩擦过程的模拟。通过分析销-盘摩擦试验机和高温滑移摩擦试验机的实验结果，发现后者的实验过程更接近热成形的实际工况，利于展开对摩擦磨损机制的定性定量分析。     

Analysis of hydrofilm extrusion of three-dimensional shapes from round billets

A new method of die construction is proposed, which enables the exact geometrical control of die shape and ensures the initial sealing between billet and die. Using the modified method of die construction the internal metal deformation is analyzed with the aid of a special transformation combined with the upper-bound method. Based on a concept of equivalent friction factor the energy dissipation in fluid film is calculated approximately for three-dimensional hydrofilm extrusion. Computations are carried out for hydrofilm extrusion of various sections such as squares, rectangles, ellipses and clover shapes and various factors affecting the process are discussed. Experiments are performed for clover and square sections using the dies from NC manufacture based on the suggested theoretical design. A reliable sealing system is developed between punch and container. The experimental results are compared with the theoretical prediction and it is shown that the theory is in reasonable agreement with the experimental observation. Various aspects are discussed from the experiment.     

On the stress distribution in prestressed extrusion dies under non-uniform distribution of internal pressure

Cold extrusion die stress distribution is normally calculated on the assumption that there is a uniform distribution of internal pressure, e.g. by application of the Lamé equations. With FE-analysis this assumption can be overcome by the introduction of arbitrary boundary conditions. However, as little information existed about realistic distribution of radial and axial stresses in container and die this possibility was not used very much. Only after a recent investigation of Bay [1] into friction and pressure distribution in forward extrusion an FE-calculation of stresses in a prestressed extrusion die under non-uniform distribution of radial pressure seemed to be promising. The results show very clearly that no negative effects on the stress state—e.g. increase of stress peaks—may be expected in case of combined continuous decrease of radial pressure in the container and discontinuous increase at the die entry.     

Noise Effect on Ice Surface Softening During Friction

The ice surface softening by friction is investigated considering the additive non-correlated fluctuations of the shear strain and stress, and the temperature. The premelting is construed by the Kelvin–Voigt equation for shear strain and by the relaxation equations of Landau–Khalatnikov type for shear stress and temperature. Taking into account the noises in these equations, the Langevin and Fokker–Planck equations are derived. Their analysis is based on the investigation of extrema of the distribution function, i.e., steady-state values of the shear strain using the Stratonovich interpretation. The phase diagrams are constructed, where the noises intensities and thermostat temperature determine the regions of ice, softened ice and their mixture (stick–slip rubbing). We present that domain of ice friction is bounded by relatively small background sliding block temperatures and fluctuation intensities of the stress and temperature. The ice film softens with growth of the stress noise intensity even at small thermostat temperatures. The friction force time series for all rubbing modes are calculated and compared with experimentally observed ones.     

Effects of extrusion variables on temperature distribution in axisymmetric extrusion process

A numerical method was developed to simulate the non-steady-state temperature distributions during forward extrusion process. The velocity, strain rates, and strain fields within the deformation zones during extrusion were obtained, using upper bound method of analysis to obtain internal heat generations coupled to the necessary heat transfer conduction equations. The computer program written in C++ language essentially simulates the extrusion process and takes into account extrusion variables such as material properties, friction conditions, extrusion velocity, extrusion ratio, die preheat temperature, billet height, percentage reduction in area, and die land length. The effects of billet height and percentage reduction in area on the temperature distributions within the dead metal zone give good agreements with experimental results. It is found that the higher the billet's heights and higher the percentages reduction in areas, the higher the temperature rises during the extrusion process. The die land zone shows increasing temperature rise with increasing friction coefficient, while increasing friction coefficient has no effect on the dead zone temperature. Also, increasing speed of deformation shows an increasing dead zone temperature rise than a more gradual die land temperature rise. It can be stated that the extrusion temperature increases proportionally to the increase of the container temperature.     

考虑路面影响的车辆稳定性控制质心侧偏角动态边界控制

路面附着系数与车辆稳定性控制的效果紧密联系,因此有必要在考虑路面影响的情况下设计一种能够适用于多种路面的质心侧偏角控制策略。在7自由度非线性动力学模型的基础上,由车轮侧向力与路面附着的关系,分析不同路面对质心侧偏角控制的影响。根据路面附着系数的不同,通过定义极限边界和线性区域边界,设计变化的动态质心侧偏角安全边界。根据横摆角速度增益判断车辆是否处于非线性状态,并在有逼近安全边界的趋势时提前施加控制,以避免产生由车轮纵向力增加引起的侧向力减小所造成的加剧车辆侧滑的趋势。基于非线性输入的滑模控制算法设计质心侧偏角控制器。通过Matlab/Simulink仿真和实车试验验证了该控制方法能够在不同附着路面条件下的有效地保证汽车的行驶稳定性。     

Mechanical property changes in drawing/extrusion of hardening viscoplastic materials with damage

This paper is concerned with the mechanical property changes in drawing/extrusion operations of hardening viscoplastic materials with damage. The process model in this study includes two state variables, the hardness for strain hardening from slip dominated plastic distortion and the porosity for damage from growth of microvoids. The decrease in apparent density in strip drawing of aluminum for several die angles are computed and compared with that from experiments. Simulations of axisymmetric drawing/extrusion have shown that the accumulated porosity in drawing is much bigger than that in extrusion while the difference between the hardness distributions in these processes are insignificant. The effects of the process conditions, such as conical die angle, friction and drawing/extrusion speed, on the mechanical property changes are also examined.     

四轮轮毂电机驱动电动汽车纵侧向稳定性协调控制策略研究

结合四轮轮毂电机驱动电动汽车四轮转矩独立可控的特点,针对加速同时转向时地面附着力不足的情况,研究车辆纵向和侧向稳定性协调控制策略。针对未知和复杂多变的路面附着情况,设计对路面附着变化具有良好鲁棒性的滑转率自寻优驱动防滑控制策略,采用滑模控制方法实现了对路面最优滑转率的自适应追踪。在此基础上,构建稳定性协调控制策略,通过对车辆纵、侧向动力学目标进行优先级判断和多目标协调控制,有效提升了车辆纵向和侧向稳定性。通过CarSim-Simulink联合仿真验证了驱动防滑控制策略在未知路面附着情况下的有效性,提出的纵侧向稳定性协调控制策略能够有效提升车辆的纵向和侧向稳定性,控制效果优于直接横摆力矩控制。     

Analysis of rotation in the lubrication of a porous slider bearing: Small rotation

A mathematical model is developed to investigate the behaviour of a plane-inclined porous slider bearing under the effect of a uniform small rotation. The Beavers-Joseph slip condition was used for the slip velocity at the porous boundary. The expressions for dimensionless pressure distribution, load-carrying capacity and coefficient of friction are obtained. It is concluded that the load-carrying capacity and coefficient of friction depend on the direction and magnitude of rotation. For a negative rotation the load-carrying capacity increases and the coefficient of friction decreases. The load-carrying capacity also decreases with increase in the slip velocity parameter.     

On the motion of the structure varying multibody systems with two-dimensional dry friction

In the present paper the dynamics of the structure varying multibody systems caused by stick-slip motion with two-dimensional dry friction are analyzed. The methods to determine friction force both in stick and slip states are described. The direct method of considering the wagon bogie system as a structure varying system was used to consider two dimensional friction at the wheelset-side frame connection. The concept of friction direction angle used to determine the friction force components of two-dimensional dry friction both in the stick and slip motion states was used. A speed depended friction coefficient was used and described approximately by hyperbolic secant function. All switch conditions were derived and friction forces both for stick and slip states. Some simulation results are provided.     

湿滑状态下轮胎路面摩擦特性的数值分析方法

为研究湿滑状态下轮胎路面的摩擦特性,以胎面橡胶和沥青路面作为研究对象,利用谐波叠加法建立三维粗糙路面模型,采用“伪”流体动力轴承作用等效反映路面水膜“密封”作用,综合使用有限元软件ABAQUS和计算流体动力学软件Fluent得到湿滑状态下橡胶与路面滑动接触时的橡胶接触压力、滞后摩擦力及路面水膜承载力,由此形成了综合兼顾橡胶材料、接触压力、滑动速度、路面形貌和路面水膜等多因素的轮胎与湿路面摩擦特性的仿真方法。通过橡胶与干-湿路面摩擦特性的变化与公开的试验对比,证明本方法的合理性和可行性,并进一步分析滑动速度、接触压力和路面特征对湿滑状态下轮胎路面摩擦特性的影响规律。研究结果为轮胎和路面的抗湿滑性设计及优化提供理论依据。     

Metal cutting: Mechanics,surface physics and metallurgy

Orthogonal planing experiments have been carried out on 70/30 brass workpieces in the cutting speed range 0·3–150 ft/min. Measurements of chip radius, shear plane angle and the average coefficient of friction between chip and tool have been made and their inter-relationships are successfully explained in terms of a slip line field theory. For a non-hardening workpiece, chip shape is completely determined by the friction stress distribution between chip and tool, but three parameters are required to define this sufficiently, rather than the one, the coefficient of friction, of earlier theories.     

Numerical investigation of the combined effects of slip and texture on tribological performance of bearing

Slider bearings are used in many applications. An increase in the load support may allow for saving of energy. In this work, in order to enhance the load support and decrease the friction force, a combined textured surface bearing using boundary slip is discussed. A modified Reynolds equation with slip is adopted. With the main goal of evaluating the effects of slip and texture, a parametric analysis is performed. For the given operating conditions, texturing features as well as slip pattern are analysed in detail. The numerical analysis is undertaken under the condition of different gap ratio values and the slip-textured area. The results show that combined techniques of slip and texture have a significant effect on the improvement of the tribological performance of bearing, that is, a high load support but low friction force. The gap ratio of the bearing is shown to have a significant effect on the lubrication behaviour. It is found that even with a smallest gap ratio (parallel gap), a high load support can be produced. However, it is also shown that the gap ratio appears to contribute to the generated friction force and the volume flow rate more than the boundary slip. Further analysis indicates that the optimum slip-text zones for certain gap ratio are highlighted. These findings may provide references for designing hydrodynamic-textured slider bearing considering boundary slip.     

Occurrence limit of stick‐slip: dimensionless analysis for fundamental design of robust‐stable systems

In most practical mechanical systems, sliding surfaces are utilised under the assumption that they operate smoothly. Stick‐slip motion can therefore be a serious nuisance that interferes with achieving high performance in mechanical systems. The present paper describes the nature of stick‐slip based on an analysis of a 1‐DOF sliding system. The dimensionless parameters controlling the stick‐slip are clarified by deriving the dimensionless forms of the governing equations. For a friction model that considers the dependence of the kinetic friction coefficient on the relative velocity, we find three types of sliding systems with regard to stick‐slip: the unstable system, the stable system and the robust‐stable system. A criterion is proposed for the fundamental design of robust‐stable systems; if a sliding system is robust stable, no matter how large a disturbance is, the energy of the disturbance is dissipated perfectly, and steady sliding without any vibration is ensured. Copyright © 2009 John Wiley & Sons, Ltd.     

The Roles of Statics and Dynamics in Determining Transitions Between Atomic Friction Regimes

We introduce a model AFM tip/substrate system that includes full atomistic detail as well as system compliance to study the transitions between three regimes of atomic friction: smooth sliding, stick-single slip, and stick-multiple slip. We characterize these atomic friction regimes in terms of static and dynamic effects, and investigate how the slip modes affect the mean friction. Molecular statics calculations show that reduced-order model predictions of possible transitions between slip regimes are generally adequate for a fully atomistic system, even for complex reaction coordinates. However, molecular dynamics simulations demonstrate that, while static features of the system govern possible slip regimes, dynamic effects ultimately determine actual transitions between slip regimes.     

曲线半径及轴重对轮轨摩擦功影响的研究

基于非Hertz滚动接触理论利用数值计算方法详细分析了静态接触情况下,轴重和曲线半径对轮轨接触质点间等效应力、接触斑粘滑区的分布、总滑动量和摩擦功的影响。分析计算表明,轴重增加引起轮轨接触质点间等效应力,接触质点间粘滑区的面积以及总滑动量的变化,同时对轮轨接触质点阍的摩擦功的变化有重要影响;小曲线半径处轮轨接触质点间的总滑动量,接触斑滑移区的面积以及摩擦功都明显增大,导致曲线上钢轨磨损加剧。因此曲线半径和轴重是影响轮轨滚动接触磨损的重要因素。     

含摩擦和间隙直齿轮副的混沌与分叉研究

齿面摩擦、齿侧间隙及时变啮合刚度是导致齿轮副产生复杂非线性振动的主要因素。以齿轮副啮合点间沿啮合线的相对位移为广义坐标,建立了计及摩擦、间隙及时变刚度等因素的直齿轮副非线性动力学模型,用5~6阶变步长Runge—Kutta法求得系统的各类周期响应和混沌响应。通过对位移响应的FFT分析,发现在计及摩擦时,系统的超谐和次谐响应成分增多;比较不同参数下的Poincaré映射图得出,摩擦使混沌吸引子有所变大,但也可能使其转变为周期吸引子;由系统振动的分叉图及最大Lyapunov指数变化图看出,摩擦导致系统提前进入混沌,但混沌的程度有所降低。     

Representation and similarity assessment in case-based process planning and die design for manufacturing automotive panels

This work improves process planning and die design in automotive panel manufacturing using a novel case-based reasoning (CBR) methodology. An innovative indexing representation and retrieval approach are also addressed. The flat-bend graph, which is utilized to represent a panel model with a B-rep structure, retains geometric and topological data in the Standard for the Exchange of Product model data format. Flat-type faces collected into several groups are represented by graph nodes, and bend-type faces are represented by graph arcs. Based on the topological information between bend-type faces and flat-type faces, a graph is constructed. Additionally, the holes detected are considered another graph node types. Geometric information and stamping parameters are utilized as graph attributes. To retrieve an appropriate case for a potentially huge search space, independent maximal cliques detection is applied. All independent maximal cliques that represent the maximum number of features shared by models are identified. Based on the retrieval result, previous process plans and die sets can be acquired for use by new cases. Experimental results obtained using the CBR system integrated with the product data management system demonstrate the practicality of reusing previous designs to accelerate stamping process planning and die design.     

Boundary slip surface design for high speed water lubricated journal bearings

This paper presents the development of a numerical model for high speed and water lubricated journal bearings with different boundary slip arrangements. The effect of boundary slip and its possible mechanism are analyzed and discussed. The results suggest that a suitable combination of slip/no-slip surfaces on the sleeve of a journal bearing enables improvement of the tribological performance through (i) suppressing the occurrence of cavitation, (ii) enhancing the load bearing capacity, and (iii) reducing the interfacial friction between bearing sleeve and shaft. Such improvement becomes more significant for the bearings with smaller eccentricity ratio, smaller width and larger diameter.