Consequence of contact local kinematics of sliding bodies on the surface temperatures generated

When studying contact with friction between two bodies, it is not possible to obtain data on real contact conditions on the basis of steady-state situations. Indeed, contacts with friction usually lead to dynamic instabilities generated at the contact interface. It is therefore necessary to take into account contact dynamics in order to better understand the phenomena involved during sliding with friction. The explicit dynamic finite element code PlastD in 2D is used to simulate the contact between two bodies. A constant Coulomb friction coefficient is imposed at the interface. The simulations carried out permitted identifying local contact conditions (kinematics, tribological state, stresses, etc.). They revealed that different instability regimes can be generated (stick–slip, slip–separation, stick–slip–separation, etc.). Local contact stresses and the sliding velocity oscillate through time when instabilities are generated and their maximum values can be much higher than those expected for steady-state conditions. The aim of this paper is to analyse the frictional instabilities and their consequences on the heat generated in the contact. First, the influence of the different instability regimes is studied on a simple contact. Then, an industrial mechanism is studied (wheel–rail contact) to investigate the influence of local contact conditions on the temperature of the rail surface.     

踏面制动尖叫噪声的有限元分析

利用ABAQUS软件建立铁路货车车轮踏面制动系统有限元模型,对其进行制动摩擦尖叫噪声的有限元复特征值分析.根据复特征值实部的正负判断系统发生尖叫噪声的可能性,如果有实部为正的特征值,则可判断系统有发生尖叫噪声的趋势.在ABAQUS建模方法中,闸瓦与车轮之间的法向力根据接触计算确定,不需假设接触弹簧,可以方便处理非平面滑动接触尖叫噪声问题.利用该模型,研究滑动摩擦因数、闸瓦压力角、闸瓦压力和转动方向对尖叫噪声的影响.研究结果显示,闸瓦压力角对制动尖叫噪声有重要影响,当闸瓦压力角α=5°时,制动系统发生尖叫噪声的影响.研究结果显示,摩擦因数越大,系统发生尖叫噪声的趋势就越大;闸瓦压力越大,尖叫噪声发生的趋势就越大.车轮逆时针转动比顺时针转动更容易引起尖叫噪声.     

Material behaviors and mechanical features in friction stir welding process

This paper presents the 3D material flows and mechanical features under different process parameters by using the finite element method based on solid mechanics. Experimental results are also given to study the effect of process parameters on joining properties of the friction stir welds. Numerical results indicate that the tangent flow constitutes the major part in the material flow. The shoulder can accelerate the material flow on the top half of the friction stir weld. The distribution of the equivalent plastic strain can correlate well with the microstructure zones. Increasing the angular velocity of the pin, the material in the nugget zone can be more fully mixed, which improves the joining quality of the two welding plates. The increase of speeds, including the rotational speed and the translational speed, can both accelerate the material flow, especially in front of the pin on the retreating side where the fastest material flow occurs. The contact pressure on the pin-plate interface is decreased with the increase of the angular velocity. An erratum to this article can be found at     

Non-uniform contact characteristics of the friction disc during the initial period of a braking process

Journal of Mechanical Science and Technology - To reveal the contact characteristics of the hydraulic brake friction pair during the initial period of braking process and improve its working...     

Viscoelastic and elastic–plastic behaviors of amorphous polymeric surfaces during scratch

In this study, we propose an analysis of the residual groove after contact between a spherical indenter and an amorphous polymeric surface (polymethylmethacrylate, PMMA) in scratch experiments. The geometrical shape of the residual groove was mathematically described using an exponential decay law. Finite element modeling (FEM) of scratch tests was compared to the corresponding experimental results. Assuming a two-segment simplified constitutive law with linear elastic behavior followed by linear strain hardening, the friction at the interface between the indenter and the material was modeled with a Coulomb's friction coefficient varying from 0 to 0.4, for computed ratios a/R between 0.1 and 0.4. The FEM results for elastic–plastic contact indicate that the shape of the residual groove is directly related to the plastic strain field in the deformation beneath the indenter during scratching. It is shown that the dimensions of the plastically deformed volume and the plastic strain gradient both depend on the ratio a/R and also on the friction coefficient.     

沟槽织构化表面影响摩擦振动噪声机理

在列车制动盘蠕墨铸铁材料表面上制备出平行间隔分布的沟槽表面织构,将其和光滑表面进行摩擦噪声对比试验,并利用有限元软件ABAQUS/Explicit(显式动态求解器)对试验进行数值模拟分析,研究沟槽织构化表面影响摩擦振动噪声的机理。结果表明,本试验条件下的光滑表面会产生较高强度的噪声而沟槽表面几乎不产生噪声,利用有限元软件ABAQUS/Explicit可以很好地模拟试验现象并揭示沟槽表面织构影响界面摩擦振动噪声的机理,即沟槽织构表面在对磨球滑过并碰击沟槽时引起的摩擦力波动能有效的打断摩擦界面的连续接触,作为不连续激励扰乱系统的自激振动,抑制界面摩擦力和振动加速度高频成分的形成并最终降低摩擦噪声。     

A new stress-based model of friction behavior in machining and its significant impact on residual stresses computed by finite element method

Friction modeling in metal cutting has been recognized as one of the most important and challenging tasks facing researchers engaged in modeling of machining operations. To address this issue from the perspective of predicting machining induced residual stresses, a new stress-based polynomial model of friction behavior in machining is proposed. The feasibility of this methodology is demonstrated by performing finite element analyses. A sensitivity study is performed by comparing the cutting force and residual stress predicted based on this new model with those based on a model using an average coefficient of friction deduced from cutting forces and a model using an average coefficient of friction deduced from stresses. The average coefficient of friction computed based on the measured cutting forces is the conventional approach and is still widely used. The average coefficient of friction due to stresses can be considered as a simplified version of the proposed model. Simulation results show significant difference among the predicted residual stresses. As the proposed model is able to capture the relationship between the normal stress and shear stress on the tool rake face better than the conventional approach can, it has a potential for improving the quality of the prediction of the residual stresses induced by machining.     

Thermo-elasto–viscoplastic numerical model for metal casting processes

The finite element method is used to model the thermomechanical behaviour of ductile cast iron using metallic moulds. Heat conduction is assumed for the heat transfer analysis and an elasto-viscoplastic model is employed to predict the development of thermal stresses and strains. Special finite elements with coincident nodes are used to model the heat transfer and the mechanical contact at the metal–mould interface. The local heat transfer coefficient between the casting and the mould may be dependent on the air-gap formation. The latent heat evolution effect is modelled by the use of the enthalpy method. An iterative procedure is required to take into account the material and the contact non-linearity. A real casting has been modelled and numerical results are compared with the experimental measurements.     

Surface stresses and flow modes on contact surface in a combined double cup extrusion process

The main goal of this study is to examine the influence of geometrical parameters in double cup extrusion process (DCEP) on the surface stress profiles such as surface expansion, contact pressure, sliding velocity and distance. The flow modes, sliding over or sticking onto the punch surface, were also investigated to see the possibility of the metal-to-metal contact or the lubricant film breakdown along the punch and workpiece interface. Two major design factors chosen for analysis include the reduction in area in backward direction (RAB) and the wall thickness ratio (TR). A sequential operation has been also simulated to compare the surface stress profiles and flow modes with those in a combined operation.     

Plastic deformation in rough surface line contacts—a finite element study

The paper describes an elastic-plastic finite element (EPFE) analysis of line contact between a cylinder and rigid plane using commercial software. The range of loading demonstrates the transition from purely elastic to fully plastic contact behaviour, revealing the residual deformations and stress fields upon unloading. A multiple contact configuration was analysed in the form of sinusoidal roughness. Results obtained under elastic conditions were validated by comparison with theoretical solutions. This model was extended by replacing the sinusoidal surface with a real roughness profile. Modelling multiple contacts indicates the influence of adjacent surface “asperities” on contact pressure and residual stress distributions.     

Investigations on the effects of friction modeling in finite element simulation of machining

Accurately predicting the physical cutting process variables, e.g. temperature, velocity, strain and stress fields, plays a pivotal role for predictive process engineering for machining processes. These predicted field variables, however, are highly influenced by workpiece constitutive material model (i.e. flow stress), thermo-mechanical properties and contact friction law at the tool-chip-workpiece interfaces. This paper aims to investigate effects of friction modeling at the tool-chip-workpiece interfaces on chip formation process in predicting forces, temperatures and other field variables such as normal stress and shear stress on the tool by using advanced finite element (FE) simulation techniques.For this purpose, two distinct FE models with Arbitrary Lagrangian Eulerian (ALE) fully coupled thermal-stress analyses are employed to study not only the effects of FE modeling with different ALE techniques but also to investigate the influence of limiting shear stress at the tool-chip contact on frictional conditions, which was never done before. A detailed friction modeling at the tool-chip and tool-work interfaces is also carried by coupling sticking and sliding frictions. Experiments and simulations have been performed for machining of AISI 4340 steel using tungsten carbide tooling and the simulation results under increasing limit shear stress have been compared to experiments. The influence of limiting shear stress on the tool-chip contact friction was explored and validity of friction modeling approaches was examined. The results presented in this work not only provide a clear understanding of friction in FEM modeling of machining but also advance the process knowledge in machining.     

Finite element analysis of the effect of sequential cuts and tool–chip friction on residual stresses in a machined layer

A thermo-elastic–viscoplastic model using explicit finite element code Abaqus was developed to investigate the effect of sequential cuts and tool–chip friction on residual stresses in a machined layer. Chip formation, cutting forces and temperature were also examined in the sequential cuts. The affected layer from the first cut slightly changes the chip thickness, cutting forces, residual strain and temperature of the machined layer, but significantly affects the residual stress distribution produced by the second cut. Residual stress is sensitive to friction condition of the tool–chip interface. Simulation results offer an insight into residual stresses induced in sequential cuts. Based on simulation results, characteristics of residual stress distribution can be controlled by optimizing the second cut.     

Effect of friction and adhesion on the load–displacement of a spherical probe in contact with a compliant incompressible elastic coating

The mechanical properties of thin films are extracted from the measured load displacement relation in a contact test conducted using micro or nano instruments. At this micro or nano force scale, the adhesion and friction operating between the test tip and thin film surface will contribute to the deformation. The well established Johnson–Kendall–Roberts (JKR) theory provided a relationship between the normal load and elastic central displacement for the adhesion contact. But because of its semi-infinite half-space hypothesis, the standard JKR theory is not applicable to thin film contact problem. Experimental verification demonstrates the numerical version of JKR theory is suitable for compliant thin film adhesion analysis, but it does not include the friction effect. In this paper, the load–displacement relation of totally bonded friction contact with adhesion is studied and compared with that of frictionless case. The practical thin film contact will lie in these two limits. The effect of friction to load and displacement seems very small except for the transition range from film to substrate response. Empirical expressions for the contact compliance are obtained from the detailed finite element study.     

Analytic and FE based estimations of the coefficient of friction of composite surfaces

In this work the coefficient of friction (COF) of a body with a composite surface layer (CSL) in sliding contact with a homogeneous counterbody is studied. With an analytic model the upper and lower bounds of the COF are estimated, whereas the material and surface properties of the CSL components are known and no information about their geometrical arrangement is at hand. An analogy with the Voigt and Reuss bounds is invoked. The analytic results are compared to FE computations. Additionally the influence of plastic deformation and geometric properties of the CSL are treated.     

Influence of testing method on the contact pressure distribution and its effect on coefficient of friction in polymeric bearings

Influence of contact pressure distribution on the coefficient of friction was investigated for some polymeric bearings in dry and lubricated conditions. Reciprocal pin on plate sliding test and Soda pendulum type sliding tests were carried out. It was found that the contact pressure distribution had a significant effect on the coefficient of friction in lubricated condition. The coefficient of friction became smaller when the pin was sliding with the lower contact pressure side ahead. The coefficient of friction in dry condition does not depend on sliding direction regardless of contact pressure distribution. Furthermore a parametric equation was derived to calculate contact pressure in polymeric journal bearings. The accuracy of the equation was confirmed by Finite Element Method (FEM).     

Identification of wear process parameters in reciprocating ball-on-disc tests

Wear test results are presented for a spherical indenter in reciprocal motion interacting with soft (aluminium alloy) and hard (construction steel) substrates. The aim of this paper is to provide evidence of the evolution of the contact zone, specification of wear scar depth and surface roughness in the contact zone. Numerical analysis of the interaction of the sliding sphere with the wear track provided contact zone dimensions, although these differed from the experimental data. The incremental wear factor is also introduced in the paper, while the formulae for the wear volume of the sphere recommended by ASTM have been improved.     

Evaluation of temperature distribution in steel balls induced by friction generated during tribotest against diamond like carbon coatings

Temperature rise in carbon steel (SUJ2-ASTM E52100) and stainless steel (SUS440C-ASTM 440C) balls sliding against diamond like carbon was evaluated using thermal simulation. On the premise that most of the friction energy was consumed as friction heat, the temperature distribution in the steel balls was simulated by ANSYS thermal conduction analysis using the friction energy measured by the ball on disc test. The interior temperatures of the steel balls were also monitored by a thermocouple during the tribotest. The simulation data, calibrated by the heat partition rate based on the Peclet number, were compared to the experiment data, and good accordance of both data was demonstrated.     

Tribological testing of articular cartilage of the medial compartment of the knee using a friction simulator

The aim of this study was to develop a tribological simulation of a unicondylar natural knee, to measure the friction and wear of articular cartilage (AC) against itself (AC-vs-AC) and against stainless steel (SS) simulating a hemiarthroplasty (AC-vs-SS). AC-vs-AC produced low levels of friction and no wear was found. AC-vs-SS showed higher levels of friction and significant AC wear. Using AC-vs-SS with elevated loading, the wear of AC was accelerated and through to bone. This study demonstrated the importance of contact stress in the design of a spacer hemiarthroplasty for the medial compartmental knee. Initial results showed the importance of contact mechanics, stress and biomaterial type in determining short-term tribological function and long-term clinical outcome of hemiarthroplasty.     

An exploration of friction models for the chip–tool interface using an Arbitrary Lagrangian–Eulerian finite element model

A better understanding of friction modeling is required in order to produce more realistic finite element models of machining processes to support the goals of longer tool life and better surface quality. In this work an attempt has been made to explore and evaluate various friction models used in numerical metal cutting simulations. A finite element model, based on the ALE approach, was developed for orthogonal machining and used to study the conditions prevailing at the chip–tool interface for hardened steel. The ALE approach does not require any chip separation criteria and enables an approximate initial chip shape to smoothly evolve into a reasonable chip shape, while maintaining excellent mesh properties. The results, for a wide range of feed values, were obtained using different friction models and are compared to previously published experimental findings. A reasonable agreement was obtained between the measured and predicted forces with some discrepancy between the cutting and feed force depending on the friction model: if agreement with the cutting forces was good, then the feed force was underestimated; if the feed force agreed well, then the cutting force was overestimated. In all cases the chip thickness was well estimated but the chip–tool contact length was underestimated.     

Experimental study on the chaotic attractor evolvement of the friction vibration in a running-in process

Running-in wear experiments were conducted on a pin-on-disk tester. The attractors of friction vibration were investigated by applying the chaos theory, and the evolvement mechanism of the friction vibration chaotic attractors in the running-in process was analyzed. The experimental results indicate that the friction vibration has a chaotic nature, and the chaotic attractor in the phase space is an always open trajectory with a specific hierarchy and structure. The chaotic attractor of the friction vibration gradually converges and tends toward a balanced state in the running-in process. The evolvement of the friction vibration chaotic attractors indicated that the wear state changes from the running-in stage to the stable wear stage.