Effect of Tangential Traction and Roughness on Crack Initiation/Propagation During Rolling Contact

Rolling-fatigue tests of 0.45 percent carbon steel rollers were carried out using a four-roller-type rolling-contact fatigue tester. Tangential traction and surface roughness of the harder mating rollers were varied and their effect was studied. The results of the study indicate that the fatigue life decreases when traction is applied in the same direction as that of rolling. When the direction of traction is reversed, the life increases over that obtained with zero traction. The roughness of harder mating rollers also has a marked influences on life. The smoother the mating roller, the longer the life. Microscopic observation of specimens revealed that the initiation of cracks during the early stages of life is more strongly influenced by the surface roughness, while the propagation of these cracks in the latter stages is affected mainly by the tangential traction.     

Wear and rolling contact fatigue of PEEK and PEEK composites

Wear and rolling contact fatigue were investigated on unfilled PEEK and on three PEEK short fiber reinforced composites. For this aim, roller-shaped specimens were subjected to rolling contact tests at different contact pressure levels. Wear rate-pv product and contact pressure-life diagrams were obtained, depending on material hardness. Microscopic observations of rolling contact surfaces and sections of the rollers highlighted damages at two different scales, surface and bulk, leading independently to wear and rolling contact fatigue. These damages were also of different typology for unfilled PEEK and composites: micro-pitting and deep transversal cracks occurred on unfilled PEEK rollers, while delamination and spalling phenomena where found on composite rollers.     

Experimental investigation on the effect of tangential force on wear and rolling contact fatigue behaviors of wheel material

The study aims to explore the effect of tangential force on wear and rolling contact fatigue (RCF) behaviors of wheel material using a JD-1 wheel/rail simulation facility. The normal, tangential and lateral forces between the wheel/rail rollers are controlled, and the magnetic power brake was used to generate the tangential forces (16–330 N). The results indicate that the surface hardness and wear loss of wheel rollers increase with the tangential force increasing. The surface cracks mouths are perpendicular to the resultant directions of the frictional forces. There are visible secondary cracks and multilayer cracks and the interlayer material of multilayer cracks are easy to break. The compositions of wear debris consist of Fe₂O₃, Fe₃O₄ and iron.     

Investigation on Wear Resistance and Fatigue Damage of Laser Cladding Coating on Wheel and Rail Materials under the Oil Lubrication Condition

The aim of this study is to investigate the wear resistance and fatigue spalling damage of wheel and rail materials with and without laser cladding coating under oil lubrication using a rolling–sliding machine. It illustrates that the laser cladding Co-based alloy coating improves the wear resistance of wheel and rail rollers. Serious spalling is dominant for untreated wheel and rail rollers. The wheel or rail roller undergoing laser cladding treatment takes on slight abrasive wear and visible ploughing. Furthermore, there are no cracks on the contact surface and subsurface. The laser cladding Co-based alloy coating exhibits outstanding resistance to wear and fatigue spalling damage due to its microstructure in the coating under oil lubrication.     

The effect of changing the rolling direction on the rolling contact fatigue lives of annealed and case-hardened steel rollers

The effects of changing the rolling direction and of repeated loading on the rolling contact fatigue lives of annealed 0.45% carbon steel rollers and case-hardened nickel-chromium steel rollers under conditions of sliding rolling contact were studied. The influence of plastic flow in the subsurface layer on the rolling fatigue life was examined. The increase in the rolling fatigue life of an annealed steel roller due to a change in the rolling direction was significant, especially when the rolling direction was changed just before the formation of macroscopic surface cracks and pits. The effect with case-hardened steel rollers was negligible. The varying effects of changing the rolling direction on the rolling fatigue life were due to differences in work-hardening and the extent of plastic flow in the rollers.     

Influence of coating thickness on rolling contact fatigue of alumina ceramics thermally sprayed on steel roller

The influence of the sprayed layer thickness on rolling contact fatigue of a thermally sprayed alumina ceramics with a nominal composition of Al₂O₃–2.3 mass% TiO₂ was investigated using a two-roller test machine under pure rolling contact condition with oil lubricant. The influence of undercoating of sprayed nickel-based alloy on rolling contact fatigue was also investigated. Thicknesses of the ceramics-sprayed layer of tested rollers were 0.2, 0.5 and 1.0 mm. The failure mode of sprayed rollers was spalling caused by subsurface cracking. In the case of sprayed rollers without undercoating, rolling contact fatigue strength of rollers with 0.2 mm thickness sprayed layer was the smallest. Rolling contact fatigue strength of sprayed roller with 0.2 mm thickness sprayed layer was improved by undercoating. In case the failure depth was small as compared with the thickness of sprayed layer, effect of undercoating on the rolling contact fatigue strength was little. The depths where the maximum values of subsurface shear stresses occurred, almost corresponded to the observed depth of spalling cracks.     

Relative fatigue life estimation of cylindrical hollow rollers in general pure rolling contact

Solid and hollow cylindrical rollers in pure rolling contact have been modelled. The two rollers are subjected to a combined normal and tangential loading. The tangential loading is one‐third of the normal loading value. The finite element package, ABAQUS, is used to study the stress distribution and the resulting deformations in the bodies of the rollers. Then the Ioannides–Harris fatigue life model for rolling bearings is applied on the ABAQUS numerical results to investigate the fatigue life of the solid and hollow rollers. Using the fatigue life of the solid rollers as the reference fatigue life, the relative fatigue lives of hollow rollers are determined. Four main different hollowness percentages are been studied: 20, 40, 60 and 80%. The hollowness percentage is the ratio of the diameter of the hole to the outer diameter of the cylinder. For each of those hollowness percentages, two cases are studied – when the two rollers in contact are hollow and when one hollow roller is in contact with a solid roller. This study includes two main models: Model 1, where the two cylindrical rollers in contact are of the same size, and Model 2, where the two rollers in contact are not of the same size. The estimated relative fatigue lives of hollow rollers showed a great improvement of the fatigue life compared with solid rollers under the same loading conditions. This was a result of the redistribution of stresses in the contact zone in the case of hollow rollers. Redistribution of stresses over a larger volume of the roller body decreased the peak stress and reduced the volume under risk. Increasing the hollowness percentage from 20 to 60% increased the flexibility of the roller, and better stress distribution was achieved, which resulted in improving the fatigue life. Although 80% of hollowness rollers have more flexibility than 60% of hollowness rollers, the bending stresses (σ_b) on the inner surface of the rollers tend to decrease the fatigue life. Copyright © 2007 John Wiley & Sons, Ltd.     

Investigation of closure of internal cracks during rolling by FE model considering crack surface roughness

Internal cracks often appear in cast slabs, and their evolution during hot deformation directly affects the product quality. In this paper, the authors investigate the closure behavior of internal cracks during plate rolling using a finite element (FE) model that takes into account the roughness of the crack surface. Influences of the roughness and reduction ratio on the closure of cracks are analyzed. The simulated results show that the models with consideration of the initial crack roughness can be used to investigate the formation of residual voids around the crack after rolling. The simulation results are validated by experimental observations. Finally, we propose an explanation of the crack closure mechanism during rolling.     

Surface durability of powder-forged roller treated by shot peening

To investigate the influence of shot peening on the surface durability of powder-forged rollers, the case-hardened powder-forged rollers with a forging density of 7.5 g/cm³ treated by the single shot peening and the double shot peening were fatigue-tested under a sliding-rolling contact condition. The surface roughness, the surface hardness and the surface compressive residual stress of the rollers were increased by the shot peening. In addition, the pores near the roller surface were deformed by the shot peening. The failure mode of all the test rollers was spalling due to subsurface cracking. The fatigue lives of all the test rollers were improved by the shot peening, and that of the test roller S08, which was shot-peened with the hardest steel shots in this experimental range, was especially improved. The surface durability of the test roller S08 was also most improved by the shot peening. Cracks became difficult to occur and propagate under the roller surface since the pores near the roller surface were deformed by the stronger shot peening. In this study, double shot peening, which generally restrains the increase in surface roughness, was not particularly effective for the improvement in the surface durability of the powder-forged rollers, because the influence of tangential force on fatigue was not always great in a case of subsurface cracking.     

Integrated wear prediction model for cylindrical gear with variable hyperbolic circular arc tooth trace under mixed elastohydrodynamic lubrication

Cylindrical gear with variable hyperbolic circular arc tooth trace (VH-CATT) is a new type of gear. Sliding wear is the main mode of the surface failure of multiple mechanical parts. Both the lubrication state and contact temperature considerably influence wear characteristics, which may aggravate the transmission performance of gear pairs. Wear, contact temperature, as well as lubrication states are jointly explored. Therefore, an integrated wear prediction model was proposed through taking into account flash contact temperature and surface roughness of VH-CATT cylindrical gears in mixed elastohydrodynamic lubrication. According to the equivalent ellipse contact model of VH-CATT cylindrical gears and tooth surface equation, normal curvature and velocity relations for VH-CATT cylindrical gears were observed, and the normal meshing force was obtained through the consideration of load sharing coefficients and quality grades. Flash contact temperature was estimated by using the literature. This study proposes analytical solutions for investigating how various surface roughness, operation, and geometric parameters affect asperity contact ratio (ACR), asperity contact pressure (ACP), flash contact temperature (FCT), as well as wear depth (WD) related to driving gears. ACR, ACP, FCT, as well as WD initially decrease and then increase from engaging-in to engaging-out processes. The minimum occurs at the pitch point. The WD declines as module, cutter radius, and rotational velocity increase while augmenting when surface roughness and torques increase. The maximum and minimum wear depths in driving gears occur at the dedendum and pitch point, respectively. Its overall wear is reduced by 23.16 % compared to the wear of spur gears. The results are valuable for the studies of tooth pitting, wear resistance, and fatigue life improvement for VH-CATT cylindrical gear. These studies can provide verification data and references required for engineering designs and VH-CATT cylindrical gear operations.

     

Macroscopic and microscopic changes in the surface layers of annealed and case-hardened steel rollers due to rolling contact

Measurements of the residual stress and observations of the microstructure at the surface and in the subsurface of rollers were performed during rolling contact fatigue tests of annealed 0.45% carbon steel and case-hardened nickel-chromium steel rollers. Compressive residual stresses in annealed rollers were induced by the rolling contact. With case-hardened rollers they were induced by heat treatment prior to the rolling contact fatigue tests. After the rolling contact fatigue tests the compressive residual stresses on the surface of the annealed rollers and in the subsurface of the case-hardened rollers relaxed; a characteristic substructure was formed by the stress cycles, which caused surface failure. It was confirmed that the microcracks leading to surface failure initiate on the surfaces of annealed rollers and in the subsurface of case-hardened rollers.     

Surface fatigue failure of nitride-hardened aluminium-chromium-molybdenum steel rollers under pure rolling and sliding-rolling contacts

To study surface failures of nitride-hardened gears, rolling fatigue tests were performed using nitride-hardened steel rollers as gear-tooth surface models. Surface failure of nitride-hardened rollers was by subsurface-initiated spalling under all contact conditions, nitriding depths and Hertzian stresses used. Cracks leading to spalling initiated at the case/core boundaries. White etching areas (WEA) were observed along spalling cracks. Electron probe microanalysis revealed that the WEA material was carbide. It is considered that there is a relation between the initiation of spalling cracks and the formation of WEA.     

Experimental Study of the Smoothing Effect of a Ceramic Rolling Element on a Bearing Raceway in Contaminated Lubrication

The effects of steel and ceramic rolling elements on protrusions from the raceway of a bearing were experimentally investigated. Such protrusions, which are normally caused by solid contaminants in the lubricating oil, create stress concentrations and lead to a reduction in the rolling contact fatigue life of the bearing. To compare the over-rolling effects of steel and ceramic rollers, experiments with steel discs with artificial dents on the surfaces were performed using a modified twin-disc machine. The results show that ceramic rollers can reduce the height of the protruded edge of an artificial dent more than steel rollers, which means that they are more effective in smoothing a damaged surface. The stresses at the contact were calculated by finite element analysis based on the deformed profile of the dented surface. The reduction in the stress level due to the smoothing effect of ceramic rollers is greater than that of steel rollers. According to the Lundberg–Palmgren bearing fatigue model, that smoothing ensures a significantly longer rolling contact fatigue life for a bearing. To put the idea into practice, a rolling ball bearing with two of its nine steel balls replaced with silicon nitride balls (referred to as a “partial hybrid bearing”) was run, together with a full steel bearing of the same model, on a bearing tester in a highly contaminated lubrication condition. The results show that the partial hybrid bearing suffers from less damage in terms of wear. The post-experiment examination of the damaged surface of the bearing raceway found that the surface of the partial hybrid bearing was smoother than that of the full steel bearing. This reveals the smoothing effect of the rolling ceramic element on the contaminant-damaged bearing surface.     

Surface failure of soft and surface-hardened steel rollers in rolling contact

To study the mechanism of rolling contact fatigue failure, annealed thermally refined 0.43% carbon steel and case-hardened nickel chromium steel rollers were tested under conditions of pure rolling and sliding/rolling. The failure mechanism was examined by fractographic observation and by calculation of the amplitudes of the ratio of stress to strength.It was found that pitting cracks initiated on the roller surface and were induced by the normal stress in the circumferential direction of the roller or by the maximum principal stress. Spalling cracks initiated beneath the surface and were induced by the orthogonal shear stress.     

偏心运动双平面超精研抛圆柱面研究

对偏心运动双平面超精研抛圆柱面的加工技术进行了理论和试验研究。基于几何运动学理论建立了加工系统数学模型,应用速度矢量法求解圆柱工件各运动参数,进而实现了工件圆柱面加工轨迹的仿真,分别分析了工件中心至夹具中心距离与夹具中心至研磨盘中心距离的比值、夹具自转转速与夹具公转转速的比值对加工轨迹形态和轨迹交叉角度的影响规律。在自制试验装置上对轴承钢GCr15圆柱滚子进行了超精研磨和抛光试验,改善了一批工件圆柱面的圆度和表面粗糙度及其偏差。仿真结果和试验结果的比较分析说明,仿真结果可反映实际工件表面微观加工痕迹的相互交叉的几何特征。     

Influence of soft surface modification on rolling contact fatigue strength of machine element

Various surface modification methods have been employed in order to improve the tribological performance of machine elements. In this work, electroless Ni–P alloy plating and sulfurizing treatments were employed, and the surface modified steel rollers and ball bearings were fatigue-tested under a pure or free rolling contact condition. The fatigue lives of both rollers and bearings were improved by these surface modifications. The contact pressure and subsurface stresses of the surface modified rollers and bearings were analyzed. The reason why the rolling fatigue strengths of surface modified rollers and bearings were higher than those of the non-coated ones would be due to the smaller contact pressure and subsurface stresses by the smaller elasticity as well as the conformity of the plated layer.     

Experimental and theoretical investigation of running-in

In this paper, the running-in and steady-state wear in the mixed-EHL line contact problem are analytically and experimentally studied. An experimental apparatus is designed and built to investigate the transient wear process during the initial stage of contact of two rollers with fresh surfaces as well as the contact of broken-in surfaces, when wear rate becomes steady. Seven experiments are conducted with five running-in experiments on fresh rollers and two steady-state experiments on the run-in rollers. The results of experimental tests and a series of simulations of an analytical model that uses the load-sharing concept and accounts for plastic deformation of asperities during the running-in stage are presented. The comparison of experimentally measured wear weight, wear depth, surface roughness, friction coefficient, and surface temperature of these seven experiments are shown to be in good agreement with the simulation results.     

High-speed imaging for online micrographs of polymer composites in tribological investigation

Studying wear process by effective means avoids undesirable situation such as frequent replacement of components, breakdown-maintenance and catastrophic accidents. In tribological investigations, dimensional changes due to wear processes are assessed using offline and online techniques. However, traditional postmortem analysis cannot display intermediate wear mechanisms. Visible textures as a direct measure of wear is a valuable evidence to understand tribological characteristics. In recent years, online-monitoring through computer vision has been used to study surface roughness. Nevertheless, online-monitoring for topographical and microstructural changes during wear could provide a clear insight of wear process. In this work, attempts are made to monitor rolling/sliding contact using a high-speed camera in conjunction with a microscope. Optical and imaging parameters were optimised to obtain clear images. Tests were conducted using polymer composite for understanding the contact surface topography of the wear process. Micrographs of the composite contact surface were acquired online using 35,000 fps and 0.375 ns shutterspeed. Periodical changes in wear mechanism are evident with resin back transfer, surface cracks and partial exposure of fibres. Thus, effective characterisation through online-monitoring gives a clear insight of the fundamental mechanisms behind wear process. The newly developed high speed imaging system stands as an effective tool in tribological investigations.     

高寒地区列车遭遇暖湿气流后轮轨界面黏着与车轮损伤响应行为研究

利用轮轨滚动试验机模拟了-40℃环境工况下不同湿度（10%～99%）暖湿气流对高速轮轨界面黏着与车轮表面损伤的响应行为。低温环境下轮轨界面遭遇暖湿气流时,黏着系数会迅速减小,且随着气流湿度的增大,黏着系数减小的幅度和恢复时间均增加;同时,与未遭遇暖湿的轮轨界面相比,黏着系数、磨损量、塑性变形层厚度均明显增大,且随着气流湿度的增大,平均黏着系数减小,磨损量和塑性变形层厚度增大。在低温无湿气作用工况下,车轮磨损机制主要为疲劳磨损,剖面裂纹以表层裂纹为主;低温间歇暖湿气流作用下,车轮磨损机制主要以氧化磨损和黏着磨损为主,磨损表面出现氧化磨屑堆积而成的第三体层,剖面裂纹出现了多层裂纹和次表层裂纹。低温环境下,暖湿气流对列车的轮轨界面黏着和车轮损伤影响显著,主要体现在黏着系数的瞬时大幅减小以及车轮材料更为严重的磨耗和疲劳损伤。因此,高寒地区应特别注意暖湿气流对列车轮轨损伤和黏着的影响,以保证列车的安全运行。     

Surface morphology in engineering applications: Influence of roughness on sliding and wear in dry fretting

Influence of initial surface roughness on friction and wear processes under fretting conditions was investigated experimentally. Rough surfaces (Ra=0.15-2.52 μm) were prepared on two materials: carbon alloy (AISI 1034) and titanium alloy (Ti-6Al-4V). Strong influence of initial surface roughness on friction and wear processes is reported for both tested materials. Lower coefficient of friction and increase in wear rate was observed for rough surfaces. Wear activation energy is increasing for smoother surfaces. Lower initial roughness of surface subjected to gross slip fretting can delay activation of wear process and reduce wear rate; however, it can slightly increase the coefficient of friction.