A friction model for dry contacts during metal-forming processes

In a metal-working process, the friction between the material and the tools influences the process by modifying the strain distribution of the workpiece. This frictional behavior is often taken into account by using a constant coefficient of friction in the finite element simulations. However, friction coefficient varies in time and space with many parameters. This paper aims at modeling of friction in dry contacts which happens at metal-forming processes in the lack of lubricant. The coefficient of dry friction in this model is a function of contact area ratio (which is a function of surface contact characteristics) and strain hardening exponent (material property). The V-bending process of aluminum alloy 6061-T4 sheets was studied experimentally and numerically using ABAQUS/Standard with two kinds of friction models: Coulomb friction and newly developed dry friction models. The results clearly showed that the developed dry friction model has better results in predicting load-stroke curves and springback compared to traditional Coulomb friction model. The FE prediction error for 6061-T4 AA is 16.9% using Coulomb friction model and 9.2% using dry friction model.     

多层波纹钢板转子阻尼器能量耗散系数计算方法

多层波纹钢板转子阻尼器是利用干摩擦原理实现转子阻尼减振的支撑零件。这种阻尼器能在高低温等恶劣环境下工作,同时阻尼特性受转子偏心影响小,可以通过改变阻尼器钢板的厚度提高阻尼器的能量耗散性能。在有限元法和摩擦学原理的基础上进行理论分析,根据多层波纹钢板转子阻尼器的能量耗散分布,提出多层钢板阻尼器的能量耗散系数的计算方法。该方法首先通过有限元分析获得接触应力,然后采用Mindlin接触模型推导出两层干摩擦接触表面摩擦耗能的计算公式,最后得出转子阻尼器整体的能量耗散系数。试验证明了该计算方法是正确可行的,具有较高计算精度。     

The influence of surface roughness and the contact pressure distribution on friction in rolling/sliding contacts

A numerical contact model is used to study the influence of surface roughness and the pressure distribution on the frictional behaviour in rolling/sliding contacts. Double-crowned roller surfaces are measured and used as input for the contact analysis. The contact pressure distribution is calculated for dry static contacts and the results are compared with friction measurements in a lubricated rolling/sliding contact made with a rough friction test rig. The mean pressure is suggested as a parameter that can be used to predict the influence of surface roughness on the friction coefficient in such contacts. The results show two important properties of the friction coefficient for the friction regime studied in this paper: (1) there is a linear decrease in friction coefficient as a function of the slide-to-roll ratio, and (2) the friction coefficient increases linearly with increasing mean contact pressure up to a maximum limit above which the friction coefficient is constant. The absolute deviation of experimental results from the derived theory is for most cases within 0.005.     

An Adsorption Model for Friction in Boundary Lubrication

A model describing the relationship between friction coefficient and surface adsorption of a two-component lubricant (i.e. base lubricant and one additive) is developed for boundary lubrication. This model relates the coefficient of friction to the extent of metal-metal contact or, inversely, the fractional surface coverage of the lubricant. Using a suitable adsorption isotherm, an expression for friction coefficient is obtained, relating the friction coefficient to the additive concentration and the adsorption energy. The proposed model and the selected adsorption isotherm are experimentally verified by testing several model additives in a petroleum-base lubricant. An excellent correlation is obtained between the adsorption-free energies calculated from the friction coefficient data and the adsorption-free energies determined from adsorption studies. The ranking order of additives based on friction coefficient is explained in terms of the molecular structure and the structural effects on the adsorption-free energy.     

考虑边界膜强度的滑动摩擦副混合润滑模型研究

针对边界膜对摩擦副润滑状态的影响，提出一种能够综合反映压力及剪切速率对边界膜失效综合影响的边界膜强度模型，并基于润滑状态测试结果通过拟合获得模型参数；将该边界膜强度模型与流体动压润滑模型、粗糙表面接触模型耦合，建立考虑边界膜强度的混合润滑模型，并通过轴瓦摩擦实验机润滑测试结果对模拟结果进行验证。和现有典型混合润滑模型相比较，该混合润滑模型可以更准确地反映摩擦副的实际润滑状态以及摩擦因数变化规律。运用考虑边界膜强度的混合润滑模型分析轴瓦零件润滑状态转化特性和机制。结果表明：在存在边界润滑的混合润滑条件下，当加载力小于临界载荷，边界膜几乎未发生破裂，摩擦因数随载荷增加缓慢变大，其数值均较小；当加载力加至临界载荷，边界膜破裂，摩擦副微凸体接触区域出现干摩擦，摩擦因数出现突然增加，表明该摩擦副由边界润滑为主的混合润滑状态过渡到以干摩擦为主的润滑状态。     

基于修正Iwan模型的螺栓结合面非线性建模研究

准确建立高保真的螺栓结合面非线性力学模型是分析高端装备的前提和基础。针对螺栓结合面的迟滞非线性力学问题,提出了一种修正Iwan模型的力学建模方法,使得传统唯象的Iwan模型各参数物理意义更加明确。首先,根据多尺度理论和数理统计方法,建立了具有连续光滑接触特性的结合面法向接触模型;然后,通过考虑动态和静态摩擦因数的差异并利用库仑摩擦定律,将修正后的Iwan唯象模型与具体的法向接触模型联系起来,提出了新的螺栓结合面切向响应模型;最后,基于Matlab仿真和已有的试验数据,验证了所建模型的正确性,并探究了加载条件、塑性指数和动静摩擦因数比对结合面接触特性的影响。研究表明：一个周期加、卸载的能量耗散是随着位移幅值的增加而递增;相比于塑性指数,动静摩擦因数比的改变对结合面切向载荷的影响更为显著,在后续研究中需重点考虑;微凸体临界滑移力分布受其他因素的影响主要体现在接触微凸体的数目、峰值点的位置以及曲线收敛速度的改变。     

The dynamics of the dry friction oscillator taking into account pre-displacement and tangential contact stiffness

A one-dimensional model of the dry friction oscillator is considered taking into account the phenomena of pre-displacement and tangential contact stiffness. The dynamic parameters and characteristics of the basic mode of stabilized vibrations are determined for the cases of force and kinematic excitation. The obtained results are compared with the Coulomb model of the force of friction, which gives evidence that this model is inapplicable to calculating low-amplitude oscillations. The effect of the tribological contact parameters on the resonance characteristics of the oscillator and the damping properties of the friction contact are investigated.     

An analytical model of dissipated viscous and hysteretic energy due to interaction forces in a pneumatic tire: Theory and experiments

When in use, a tire dissipates energy according to various mechanisms: rolling resistance, viscosity, hysteresis, friction energy, etc. This dissipation of energy contributes to influencing tire temperature, contact conditions and the resulting friction coefficient.This research project deals with viscoelastic and hysteretic mechanisms, and presents an explicit expression of the energy dissipated by tire-road interactions caused by these mechanisms. It is based on the Dahl model with regard to the hysteretic force together with a spring and a frequency variable damping coefficient with regard to the viscoelastic one. The energy expression found in this way can be used in tire thermal models to determine one of the heat flows needed to estimate the contact temperature and to find out the actual friction coefficient to be used in real time tire-road interaction models.Experimental tests were carried out, for longitudinal interaction only, in order to evaluate the effectiveness of the proposed expression by identifying the parameters and validating the results.     

Effects of restitution coefficient and material characteristics on dynamic response of planar multi-body systems with revolute clearance joint

To study the effects of restitution coefficient and material parameters on the dynamic characteristics of planar multi-body systems with revolute clearance joints, a comprehensive methodology for clearance revolute joints in a planar multibody system is presented and analyzed. A new nonlinear contact force model, which accounts for the axial length of bearing and has a wide suitable range of impact situation, is presented to evaluate the contact force between journal and bearing. Furthermore, a modified Coulomb friction force model also presented to consider the effect of friction in clearance joint. Then, a typical journal-bearing model and a planar slider-crank mechanism has revolute clearance joint are used as examples. From the main numerical results under the different restitution coefficient and material parameters, it can be concluded that coefficient of restitution has significant effects on dynamic response of multi-body systems with revolute clearance joint. Additionally, materials of journal and bearing with smaller Young’s modulus will cause obvious delay on dynamic response of multi-body systems.     

考虑摩擦的圆柱面切向接触刚度分形模型研究*

为了更准确地计算圆柱面切向接触刚度,本文考虑摩擦因素的影响,在圆柱面分形接触模型的基础上,引入存在摩擦时弹塑性变形的临界面积公式,并利用切向接触刚度的基本理论,推导考虑摩擦的圆柱面切向接触刚度分形模型,并通过Matlab对上述模型进行仿真,研究不同参数(摩擦因数、分形维数、粗糙度幅值 、材料的特性参数、曲率半径)以及接触的形式对切向接触刚度的影响。仿真结果表明：切向接触刚度与法向载荷成正比关系,但随分形维数取值范围的变化分别呈现指数与线性规律。摩擦因数与切向接触刚度成反比关系;材料的特性参数对切向刚度的影响,不仅与分形维数有关,还与自身取值关联;分形维数,粗糙度幅值与切向刚度的关系,受分形维数和材料特性参数的影响呈现正比或反比趋势。另外,内接触比外接触时的切向刚度大;随着曲率半径的变大,切向刚度增加。该研究为后续开展高副结合面动力学分析提供理论 基础。     

Contributions of adhesion and hysteresis to coefficient of friction between shoe and floor surfaces: effects of floor roughness and sliding speed

Abstract

Improving shoe–floor friction in order to reduce slip and fall accidents requires thorough understanding of the factors that contribute to friction. The friction between a sliding viscoelastic material (shoe) and a hard surface (floor) has two major components: adhesion and hysteresis. This study aimed to quantify the effects of floor roughness and sliding speed on adhesion and hysteresis to determine how each component contributes to the coefficient of friction. Experiments were conducted on a pin on disc tribometer using ceramic tiles with three levels of roughness, six sliding speeds, two common shoe materials and four liquid lubricants. Hysteresis was measured using a lubricant that minimised adhesion. Dry and lubricated adhesion was measured by subtracting hysteresis from the coefficient of friction. Analysis of variance regression models were used to determine the contributions of hysteresis, dry adhesion, sliding speed and fluid to lubricated coefficient of friction. Increased floor roughness led to increased hysteresis, while increased sliding speed reduced both adhesion and hysteresis. These findings are consistent with theory that states that larger asperities increase hysteretic deformation and that sliding speed affects deformation and real area of contact between a viscoelastic material and a hard surface. The model correctly predicted 83% of variation in coefficient of friction based on dry adhesion, hysteresis and fluid dependent constants. The sensitivity of hysteresis friction to shoe material and floor roughness indicates that optimising these parameters may be effective at reducing slip accidents on oily floor surfaces.     

热力变形下渣浆泵机械密封干摩擦磨损分析

机械密封在起停阶段或操作失误时常处于干摩擦状态,由此导致的热损伤与磨损将影响其密封性能。以某YWN8合金接触式机械密封为研究对象,建立基于硬度及磨损系数的磨损数值模型,试验测定摩擦副密封环的硬度、磨损系数、干摩擦因数,验证磨损数值模型的准确性;对机械密封磨损进行仿真模拟,研究摩擦副密封环在干摩擦运转时单力场及热力变形下的磨损深度,并用磨损理论值进行验证。结果表明：干摩擦运转时密封环端面温升较低,温度不是其失效的主要原因;热力变形后密封环内外径间隙增大,造成端面粗糙峰接触面积减小,黏着磨损较变形前呈下降趋势,导致多物理场下的磨损深度与理论值不符。     

钢-钢摩擦副在不同润滑和接触形式下的高温减摩性能的研究

用多功能SRV试验机评价了在干摩擦和油润滑条件下,试验参数对钢-钢摩擦副在点接触和线接触形式下的高温减摩性能的影响。结果表明,在试验范围内,随着试验负荷的增大,钢-钢摩擦副在干摩擦条件下线接触摩擦因数和点接触摩擦因数呈逐渐下降的趋势,这种下降趋势在线接触时尤为明显;但在油润滑条件下,试验负荷的增大对钢-钢摩擦副之间的线接触和点接触摩擦因数的影响不显著。在干摩擦条件下,不论是在点接触还是线接触形式下,试验速度对钢-钢摩擦副之间的高温点接触和线接触摩擦因数的影响并不显著;但在油润滑条件下,试验速度对钢-钢摩擦副的高温线接触和点接触摩擦因数的影响都比较显著,特别是在较高的试验速度时影响更为明显。     

Simulation of a nonstationary contact in a sliding bearing with allowance for frictional heating and convective heat transfer

The serviceability of polymer dry sliding bearings is governed by a number of parameters, such as the rotational velocity of the shaft, the friction coefficient, еру thermal and mechanical properties of bearing members, and hence, the resulting contact temperatures. This work was aimed at developing a calculation model of the operation of a two-layer polymer dry sliding bearing, determining the temperatures distributions and the equivalent and contact stresses in bearing members, and choosing the optimum parameters of the bearing system for which heat balance is achieved.     

考虑摩擦因数与滑动速度相关时的轮轨滚动接触有限元分析

使用与滑动速度相关的摩擦因数替代库伦摩擦定律中的常系数,结合mixed Lagrangian/Eulerian方法建立轮轨滚动接触有限元模型,分析牵引力主导的蠕滑工况下的干燥状态的轮轨滚动接触特性。通过与摩擦因数取值为常数的轮轨滚动接触分析结果对比发现:与滑动速度相关的摩擦因数对轮轨滚动接触最大接触应力和接触斑面积影响不大,均在1%以内;但是对轮轨接触斑内最大Mises应力、最大纵向切应力、最大横向切应力和最大等效塑性应变影响较大,特别是对最大纵向切应力影响幅度近20%;更需要引起注意的是对轮轨滚动接触摩擦力矢量分布和切向塑性应变分布影响明显,这对轮轨滚动接触疲劳损伤分析非常重要。     

Tribological and vibroacoustic behavior of a contact between rubber and glass (application to wiper blade)

The wiping of the windshield of a car is carried out with the reciprocating motion of a rubber blade on glass that removes the water from glass. This function is realized by a contact dimension between the rubber and glass of a few tens of micrometers. A good wiping is characterized by a homogeneous disposal of the water, without noise generation and by limiting as much as possible the phenomenon of wear (loss of wiping or noise presence). This wiping is only possible by a good understanding of the tribological, mechanical and vibroacoustic parameters that control completely the contact. Our article proposes to approach four typical phenomena occurring on rubber wiper blades. Also, we will describe the evolution of dry friction coefficient with temperature. Later, we will approach the influence of velocity on the friction coefficient of a wiper rubber blade by water. Then we will talk about a phenomenon called tacky friction where the friction coefficient reaches a very important value. And in conclusion, we try to give a relationship between the stick–slip and the squeal noise.     

Modeling and analysis of dry friction in micro-forming of metals

The Tabor friction model was modified by introducing the friction factor. Based on the multi-region model, the micro-uniaxial compression process was simulated to study the size effect considering dry friction. It was found that the grain-boundary has little effect on the dry friction behavior. The mechanical properties of individual grains either in the contact area or in the non-contact area have a significant influence on the friction behavior. With decrease of specimen's size, the average friction coefficient did not vary much but the value of friction coefficient of each random grain orientation distribution becomes more dispersive.     

Enhanced friction model for high-speed right-angle gear dynamics

The modeling of elastohydrodynamic lubrication friction and the analysis of its dynamic effect on right-angle gears, such as hypoid and spiral bevel types are performed in the present study. Unlike the classically applied empirical constant coefficient of friction at the contacting tooth surfaces, the enhanced physics-based gear mesh friction model is both spatial and time-varying. The underlying formulation assumes mixed elastohydrodynamic lubrication (EHL) condition in which the division and load distribution between the full film and asperity contact zones are determined by the film thickness ratio and load sharing coefficient. In the proposed time-varying friction model, the calculation of friction coefficient is performed at each contact grid inside the instantaneous contact area that is being subjected to mineral oil lubrication. The effective friction coefficient and directional parameters synthesized from the net frictional and normal contact forces are then incorporated into a nonlinear time-varying right-angle gear dynamic model. Using this model, the effect of friction on the gear dynamic response due to the transmission error and mesh excitations is analyzed. Also, parametric studies are performed by varying torque, surface roughness and lubrication properties to understand the salient role of tooth sliding friction in gear dynamics. The simulation results are included. But experimental verification is needed.     

A Load-Dependent Model to Investigate the Velocity Dependence of Friction Coefficient: ZrO2/Polymer Nanocomposite

Using dynamic finite simulations, we investigate how the friction coefficient of ZrO₂/polymer nanocomposite depends on the sliding speed. The load-dependent model we developed corresponds to common friction systems, where the friction couples are sliding under fixed load for various speeds. Here, we study the effect of the sliding speed on the contact distance between two contacting bodies. In accordance with experimental observations and theoretical arguments, we find the contact distance increased with the sliding speed. We show that the dependence of the reaction force on sliding speed can be rationalized by assuming that the frequency dependence of the polymer chains relaxation times is affected by the damping effects of contact stress. By investigating the energy dissipation, we show how the friction coefficient is affected by the sliding speed. The deformation volume and relaxation times decreased with the increasing sliding speed, which result in the decreasing of energy dissipation. Then, the work in pushing the top cylinder across the bottom plateau decreases, results in a reduction of the mean horizontal reaction force and friction coefficient.     

Effect of Porosity and Normal Load on Dry Sliding Friction of Polymer Foam Blocks

In this study, the effect of porosity on the dry sliding fiction of ethylene-vinyl acetate (EVA) foams was investigated under different normal load conditions. EVA foam blocks with varying porosities were slid against a smooth stainless steel plate under dry conditions. The friction coefficient increased with increasing porosity under all of the normal load conditions. In addition, the contact area was estimated using a contact model considering elastic buckling of the cell walls (elastic collapse). The elastic collapse area in the anterior portion of the EVA foam block increased with increasing normal load and porosity, which resulted in an increased contact area. Furthermore, the friction coefficient was positively correlated with the estimated contact area divided by the normal load, indicating that adhesion friction increases with increasing porosity of polymer foams. These results may contribute to the design of high-friction, lightweight shoe sole tread blocks prepared using polymer foam blocks.