Particle generation to minimize the computing time of the discrete element method for particle packing simulation

There are computation time constraints caused by the number and size of particles in the powder packing simulation using DEM. In this paper, newly suggested packing model transforms a general packing sequence —particle generation, stack, and compression-into particle generation and packing by growing particles. To verify the new packing model, it was compared using three contact models widely used in DEM, in terms of radial distribution function, porosity, and coordination number. As a result, contact between particles showed a similar trend, and the pore distribution was also similar. Using the new packing model can reduce simulation time by 400 % compared to the normal packing model without any other coarse graining methods. This model has only been applied to particle packing simulations in this paper, but it can be expanded to other simulations with complex domain based on DEM.

     

Third Particle Ejection Effects on Wear with Quenched and Tempered Steel Fretting Contact

The design and life prediction of fretting wear-sensitive mechanical components remain a challenge. In the present work, the role of wear particle movements under conditions of axisymmetric loading of an annular flat-on-flat contact were investigated using self-mated quenched and tempered steel specimens. Total fretting wear significantly increased when loose wear particles were periodically removed from the interface, and this effect increased as a function of the sliding amplitude. Additionally, increased wear was measured when grooves perpendicular to the sliding direction were added to the interface. Increasing the rate of wear debris ejection leads to increased wear rate because naturally occurring entrapped third-body particles significantly reduce the wear. The shape of fretting loops and values of the average and maximum coefficient of friction remained unaffected by the removal of entrapped wear debris and by the introduction of the grooves.     

Numerical Simulation of an Oil Particle in Emulsion Lubrication

The analogy of lateral particle migration in shear flows has been used to investigate the entrainment of oil particles in the inlet region where a Poiseuille flow is assumed. Previous researchers have studied the particle segregation positions and the particle size effect through two-dimensional simulations. In this paper, both two-dimensional and three-dimensional simulations were conducted. The two-dimensional analysis showed that there are two stable off-center segregation positions and one neutrally stable center segregation position for the small particles, while only one stable center segregation position was found for the large particles. The results are similar in three-dimensional simulations except that the center segregation position is more stable than in the two-dimensional case. It should be noted that all the segregation positions are within the backflow region, which means all the oil particles will be rejected from the contact region if the interparticle collisions are ignored. This is supported by experimental observations.     

Elevated Temperature Evaluation of Fretting and Metal Transfer between Coated Titanium Components

During fretting, small amplitude displacements and high normal surface loads combined with abrasive oxide particles cause surface damage that acts as initiation sites for fatigue cracks. Since these conditions are prevalent within the titanium dovetail joints of jet engines a wear mode analysis was performed on extended service jet engine disks and compressor blades. The results of the wear mode analysis indicated that titanium from the uncoated disk was transferred to the softer copper-nickel-indium coated dovetail surface of the blades. This transfer created titanium on titanium contact and eventually fretting wear. In order to simulate these conditions, a moderate displacement (125 μm), low cycle phase followed by a small displacement (25 μm), high cycle fretting phase utilizing a cylinder on flat configuration was developed. The analysis and test procedure developed during this study will ultimately aid in the selection and evaluation of a new coating capable of preventing fretting.     

Analysis of the Fretting Conditions in a Contact Between an Epoxy Thermoset and a Glass Counterface

The fretting conditions in a contact between an epoxy thermoset and a glass counterface have been investigated using a specific device which allows in situ observation of the contact area. The critical displacement for transition from partial slip to gross slip conditions was investigated by the in situ detection of the micro-displacements and by the analysis of the fretting loops. Experimental results were in good relation with the theoretical predictions derived from Mindlin's approach of incipient sliding. Depending on the loading conditions, a progressive change from gross slip to partial slip conditions was found to occur during the early stages of the fretting loading, i.e., before the appearance of any macroscopic damage such as cracking or particle detachment. These fretting conditions were synthesized in a fretting map giving the boundary between various fretting regimes as a function of the normal load, the imposed displacement and the number of cycles.     

Fretting fatigue in elastic contacts due to tangential micro-motion

Stress analysis of two elastic bodies in contact, when one of them is subjected to an alternating stress, was carried out in this study. Surface and effective stresses for different values of applied alternating stress were determined within the area of contact. Critical zones where fretting fatigue cracks may initiate were found at the edge of contact where stresses reach a maximum. Displacement distribution due to various types of stresses was determined and was found to increase with increasing the applied alternating stress and the distance from the centre of contact. An expression to predict fatigue failure under fretting conditions was proposed. This expression includes most parameters which contribute to failure, such as: contact pressure, applied stresses, friction coefficient, displacement amplitude and elastic properties of both contacting materials. Initial predictions of the suggested expression provide good correlation with test results, and the expression could thus be adopted as an analytical tool for predicting strength under fretting conditions.     

Accelerated Fretting Wear Tests for Contacts Exposed to Atmosphere

Engineering components can be subjected to normal and/or rotational fretting wear with contacts that are intermittently exposed to the atmosphere. Exposure to the environment may lead to the alteration at the contact due to the changing role of third body particles such as hard oxides which can result in abrasion. The abrasion due to hard oxide particles differs for the closed contact and intermittently opened contact. In the former scenario, the oxides are compacted into tribo-film, while in the latter case they remain loose, leading to bigger role of abrasion. Standard fretting test setup employed to estimate the fretting wear characteristic operates with a constant load such that the contact remains closed between the counter surfaces and does not simulate the opening and closing of the contacts as observed in certain applications. The forceful interruptions to the experiments to simulate open and close condition of the contact require considerable amount of time and effort. In this paper, an accelerated test procedure is proposed and investigated to capture the effect of intermittent opening of the contact without stopping the experiments. A test rig is designed to simulate the opening and closing conditions, and tests were performed with abrasive diamond-like particles. Friction and wear results are compared with those of intermittently contact opening conditions along with operating wear mechanisms. Scanning electron microscope analysis showed that the wear mechanism observed in the case of fretting with intermittent opening of contact is similar to that of fretting with diamond-like abrasives at the contact.     

The influence of current load on fretting of electrical contacts

The fretting corrosion behavior of tin coated brass contacts is studied at various current loads (1, 2 and 3 A). The typical characteristics of the change in contact resistance with fretting cycles are explained. The fretted surface is examined using scanning electron microscope, laser scanning electron microscope and energy dispersive analysis of X-rays to assess the surface morphology, extent of fretting damage, extent of oxidation, surface profile and elemental distribution across the contact zone. The degradation of contacts at high and low values of current is explained with reference to the thermal and electrical phenomena occurring at the contact interface. The results showed that irrespective of the current loads under study, the contact resistance is maintained at 1.0±0.02 Ω where the oxide debris formation and the electrical breakdown of oxide particles competed with each other maintaining the equilibrium. The number of cycles to failure of the contacts is delayed at lower current. The fretting corrosion degradation of tin coated contacts occurs much faster at higher currents as it generates more accumulation of oxide wear debris at the contact zone. The observed surface morphology and the tin profile of the fretted surface are in agreement with the experimental results.     

微动接触应力的有限元分析

以方足微动桥,试样接触几何条件为研究对象,应用ANSYS有限元分析软件对其接触面上的应力分布进行弹性有限元分析,验证用ANSYS所建计算模型的正确性,分别计算不同名义接触压力和不同摩擦因数条件下接触状态（粘着区、滑动区、张开区）和接触面应力分布,选取不同水平的循环载荷进行计算,研究接触状态和应力分布随循环载荷的变化情况。结果表明,微动疲劳过程中接触表面拉应力与剪应力在接触面的粘,滑交界区存在突变,微动疲劳裂纹正是在这一区域内萌生并扩展,计算结果与实验结果吻合很好。     

The formation of spherical wear particles

A number of recent papers report the formation of spherical wear particles during sliding. In this paper a model is developed in which wear particles formed by adhesive wear processes are trapped in cavities in the sliding surfaces and become smoothed by burnishing processes. It is shown that the reported spherical particle diameters and lengths of sliding are consistent with this model. According to the model, spherical particles are only to be anticipated in slow uniaxial sliding, in fretting and within cracks of a material being fatigued.     

Third body effects on friction and wear during fretting of steel contacts

Fretting wear proceeds through particle detachment from the contacting surfaces which, while trapped in the contact zone, can affect the frictional and wear response. Ball-on-flat fretting experiments were carried out between steel specimens under gross slip regime. A transition in the coefficient of friction was linked to a critical contact pressure. The microstructure and chemical composition of the third body evolve with the applied pressure. The evolution of the friction coefficient is strongly dependent on the third body properties. The wear is controlled by the applied load and thus the real contact area within the wear track.     

基于有限元分析的裂纹比拟法估算微动疲劳寿命

采用Ansys有限元软件对方足微动桥/平面试样接触状态进行分析,确定试样微动接触面上应力场分布和接触面三区(黏着区、滑移区、张开区)分布特征,分析接触面上接触状态随外加交变载荷的变化规律,在此基础上改进微动疲劳(fretting fatigue,FF)寿命估算的裂纹比拟法(crack analogue method,CAM)。选取不同水平的循环载荷及不同名义接触压力对Ti811钛合金试样在350℃下进行微动疲劳试验,验证改进裂纹比拟法(modified crack analogue method,MCAM)的准确性。结果表明,微动疲劳中接触面压应力与剪应力在黏/滑交界区存在突变,张应力幅在滑移/张开分界处达到最大值,裂纹易在此萌生并扩展。改进的裂纹比拟法估算值与实验结果取得良好的一致性。     

The use of nickel graphite composite coatings for the mitigation of gross slip fretting wear on Ti6Al4V interfaces

In metallic contacts, surface oxides, adhesion, and material transfer play a primary role in the initial stages of fretting wear degradation. Given this behavior, the focus of this study was to mitigate fretting wear within Ti6Al4V contacts at room temperature and 450 °C with the use of thermally sprayed nickel graphite composite coatings with 5–20% graphite. The results show that the embedded graphite particles reduced the friction of the nickel thermal sprayed coatings during both low and high temperature fretting wear experiments. Friction and wear mechanisms are discussed with correlations of contact chemistry, morphology, and mechanical performance. Wear on the mated Ti6Al4V surfaces was reduced by the formation of uniform transfer films that were identified as graphitic based at room temperature and NiO based at 450 °C.     

线接触条件下微动摩擦特性的研究

通过试验研究了线接触微动摩擦特性。研究表明：线接触微动摩擦因数变化与点接触形式相同；位移幅值在部分滑移区时，表面粗糙度对表面磨损有影响。磨痕分析表明，承载区域接触不均匀，存在虚接触区域，其大小与表面粗糙度有关，而接触区域没有不磨损的粘着区，不同于点接触形式。     

Behavior of alloy Ti–6Al–4V under pre-fretting and subsequent fatigue conditions

The mechanical behavior and microstructural changes in Ti–6Al–4V were determined in fretting tests, followed by axial fatigue tests. Prior to fatigue testing, specimens were subjected to fretting conditions over a range of contact stresses and fretting displacements. Fretting frequency was 100 Hz. High cycle fatigue (HCF) tests were run at 1000 Hz. The fretting test involved a flat-on-flat, bare Ti–6Al–4V/bare Ti–6Al–4V fretting system. The fretting process typically generated very shallow surface cracks at the ends of the wear scar. Subsequently, these shallow cracks were observed to propagate in axial fatigue tests, reducing the fatigue life significantly. Evidence of frictional heating during fretting was observed in the formation of scale-like oxide in the wear scar. Formation of oxides appeared to increase with increasing contact stress. Increased oxygen content was detected in the near surface regions of specimens. Large near surface deformation was typically observed within the wear scar. The contact geometry and slight tilting of the stationary fretting pad influenced the character of the fretting scar and the fretting-induced cracking. Fracture surfaces exhibited featureless, battered surfaces at the crack origins followed by (a) cleavage-type crack propagation, (b) formation of fatigue striations, and (c) final ductile tearing.     

Modelling third body flows with a discrete element method—a tool for understanding wear with adhesive particles

This work presents a fundamental approach to the study of the wear process by considering the detachment of particles, their flow in the contact and their ejection. A numerical model is constructed in order to visualize and to accurately measure these phenomena. Moreover, the adhesion between the detached particles and with the rubbing surfaces can be simulated and controlled. This physicochemical parameter plays an antagonistic role on the wear process. The greater the particle adhesion the thicker the interfacial layer will be, though with a corresponding reduction in the ejection of particles. Finally, emphasis is placed on the influence of the interfacial layer on the wear process.     

轧机油膜轴承锥套接触疲劳损伤的机理分析

研究了油膜轴承锥套与辊颈接触表面在轧制载荷作用下发生微动疲劳损伤的力学机理。在分析油膜轴承油膜压力分布特征的基础上,采用有限元法给出了锥套与辊颈接触区边缘产生接触应力集中和微滑移的分布规律,阐明了弹性结合连接的锥套与辊颈接触表面产生微动疲劳损伤的原因。利用断裂力学裂纹尖端应力场与接触边缘区域应力局部渐近场的等效原则,给出了疲劳裂纹自点蚀处萌生与扩展的力学条件。上述结果为锥套和辊颈的实际损伤状态及模拟试验所验证。     

Unlubricated fretting wear of mild steel surfaces in air at room temperature: II electrical contact resistance measurements and the effect on wear of intermittent loading

Long-duration fretting wear tests, involving continuous monitoring of the electrical contact resistance of the wearing surfaces, and/or intermittent loading, are described and discussed. A correlation between the number of low resistance metal-metal contacts and the amount of wear suggests that, in air at room temperature at least, wear proceeds by the continuing formation of metallic wear particles, even after long fretting times.     

Finite Element Analysis of Localized Plasticity in Al 2024-T3 Subjected to Fretting Fatigue

Fretting fatigue is a combination of two complex mechanical phenomena, namely, fretting and fatigue. Fretting appears between components that are subjected to small relative oscillatory motion. Once these components undergo cyclic fatigue load at the same time, fretting fatigue occurs. Fretting fatigue is an important issue in aerospace structural design. Many studies have investigated fretting fatigue behavior; however, the majority have assumed elastic deformation and very few have considered the effect of plasticity. The main goal of this study is to monitor the effect of different fretting fatigue primary variables on localized plasticity in an aluminum alloy (Al 2024-T3) test specimen. In order to extract the stress distribution at the contact interface under elasto-plastic conditions, a modified finite element contact model was used. The contact model was verified through comparison with an elastic analytical solution. Then, a bilinear elasto-plastic isotropic hardening model with a von Mises yield surface was implemented to simulate the material behavior of the aluminum alloy. The effect of different fretting fatigue primary variables, such as axial stress, contact geometry, and coefficient of friction, on localized plasticity was investigated. Finally, the relationship between the location of maximum localized plasticity and Ruiz fretting damage parameter with the crack initiation site is discussed.     

Effect of contact pressure on fretting fatigue of austenitic stainless steel

The effect of contact pressure on fretting fatigue in solution-treated austenitic stainless steel was studied. With an increase in contact pressure, fretting fatigue life was almost unchanged at low contact pressures, however it decreased drastically at high contact pressures. At low contact pressures, stress concentration due to fretting damage occurred at the middle portion of the fretted area and the main crack responsible for failure was initiated there. At high contact pressures, concavity was formed at the fretted area without accompanying heavy wear. The main crack was initiated at the outer edge corner of the concavity which probably acted as a notch. Plain fatigue prior to the fretting fatigue test increased the fretting fatigue life at high contact pressures since the concavity formation was suppressed by the cyclic strain hardening.