Twin Disc Research Articles

Nanostructured Single-Crystalline Twin Disks of Zinc Oxide

Nanostructural twin disks of zinc oxide with high yield were fabricated by vapor-phase transport using a mixture of zinc oxide, indium oxide, and graphite powders as source material. The twin disk consisted of two identical hexagonal disks with a size of about 1.5 μm in diagonal and 100 nm in thickness. The scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction analysis demonstrated that the twin disk was a single crystal, enclosed by{0001}, {1010}, and {1011} surfaces. The growth process is discussed.

Investigating fluid penetration of rolling contact fatigue cracks in rails using a newly developed full-scale test facility

Fluid penetration of surface breaking rolling contact fatigue cracks in rails is believed to be a key factor in their growth to dangerous lengths. Fluid entry has been proved for surface breaking cracks in laboratory twin disc contact simulations, but the authors are not aware of direct evidence for fluid penetration of cracks in full-scale rail-wheel contacts. There is, however, a widely held view that the behaviour observed in the laboratory will translate to full-scale cases. To investigate fluid penetration of cracks in full-scale rail-wheel contacts, cracked rails with a range of rolling contact fatigue severity were removed from mainline railway track and re-installed at a newly developed test facility. Investigation was undertaken using two different water-based marker fluids, and the rails subjected to over 1000 wheel passes with a locomotive. The rails were subsequently removed and selected crack faces broken open for observation by placing the rail in four-point bending. Ultra-violet and visible light were used to assess the degree to which the marker fluids had entered the surface breaking cracks. Good evidence of fluid penetration was found for one of the marker fluids, but the second fluid was not observed inside the cracks. The possible reasons behind this difference in behaviour are discussed.

Transient Traction Characteristics between Rail and Wheel under Low Velocity and Low Slip Ratio (Variation of Traction Coefficient by Environmental Conditions)

This paper describes an experimental investigation on transient traction coefficient using a twin disc rolling-sliding frictional machine, which simulates the actual contact condition of rail and wheel under the low slip ratio and low velocity. The experimental results show the effects of temperature, relative humidity (RH), water, surface roughness and hardness, which have important influence, on the traction characteristics. As a result, with increasing temperature around test discs, traction coefficient decreased under constant RH condition. Increasing RH resulted in a decrease of traction coefficient under constant temperature condition. Furthermore, with increasing sliding distance under dry condition, traction coefficient increased linearly at first and reached to a peak, and then decreased a little and became steady. Up to traction coefficient reaches a peak after a test start, surface roughness and hardness decreased a little, and during traction coefficient decreased after the peak, surface roughness and hardness increased rapidly due to wear. On the other hand, as sliding distance increased under wet condition, traction coefficient increased linearly at first gradually and became steady. Until reaching maximum traction coefficient after a test start, surface roughness increased a little and hardness decreased a little. After then, surface roughness decreased a little, and hardness increased due to work-hardening. Moreover, the primary factors, which traction coefficient increased immediately after a test start, were associated with surface roughness, hardness, and such boundary films as oxide film, reaction film.

Experimental and theoretical investigation of railway wheel squeal

The tangential contact forces that arise at the interface between the wheel of a railway vehicle and the rail provide all the traction, braking, and guidance required by the vehicle. These forces are the result of microslip or creepage and can become unstable exciting vibration of the wheel, particularly at frequencies corresponding to the wheel's axial (and radial) modes. Although theories exist for predicting these creep forces and their relationship to creepages, most lack experimental verification in the characterization of the falling friction coefficient during unsteady squealing. This paper presents some new results from a project which aims to develop a complete, validated model of curve squeal noise generation accounting for friction characteristics, excitation due to unstable forces between the wheel and rail and vehicle dynamic behaviour. The model includes wheel and track dynamic response and acoustic radiation. As part of the project, a twin disc rig has been modified to provide experimental data for the validation of the model and measurements were made of the lateral force and dynamic response of the rollers due to varying amounts of lateral creepage during squealing. The main feature of the twin disc rig compared with previous research [1] is that the contact force measuring system measures the contact forces at the web of the rollers and therefore close to the contact patch and through a slipring arrangement enables the lateral vibration of both rollers in relation to squeal be measured with relative ease. In this paper, detailed descriptions of the twin disc rig and the test method developed are given. An outline of the squeal model is also presented. Results from the tests have been compared with the prediction from the squeal model and with available theories and showed good agreement.

Study of damage mechanisms and failure analysis of sintered and hardened steels under rolling–sliding contact conditions

Due to continual improvements of the powder material processing, it is possible for powder metal alloys to compete even in some of the most demanding applications. The main lure of powder metallurgy (PM) components amongst the several challenging mass production methods plays an imperative role, as saving material, energy and also provides good fatigue properties, which are equally good and substitute for the conventional methods. The investigation on the lubricated rolling–sliding contact fatigue damage mechanisms in sintered and hardened steel (FLC 4608-110HT) containing combination of closed and interconnected pores has been carried out. Rolling–sliding contact conditions are simulated in twin disc type experimental set up for various contact pressure and slide/roll ratio. The damaged surface of sintered and hardened steel is examined under image analyzer for different sliding conditions. It is observed that the peel of material and surface crack opening is seen for the higher slide/roll ratio. Wear debris analysis is also carried out by dual-ferrograph analyzer for sintered and wrought steels for comparison study. The characteristics of wear morphology and the size, shape and concentration of worn particles are also analysed for different sliding contact conditions.

Wear at the wheel/rail interface when sanding is used to increase adhesion

Sanding is used in train operation to improve adhesion at the wheel/rail interface during both braking and traction. An experimental study has been carried out to determine the effect of sanding on the wear of wheel and rail materials. Static tests were performed using actual wheel and rail sections. Dynamic tests were carried out with and without sand on a twin disc machine, where wheel and rail steel discs are loaded together and driven under controlled conditions of rolling and sliding. Sand was fed into the disc contact through a standard compressed air sanding valve. In both static and dynamic tests, sand caused severe surface damage. During the dynamic tests, because of the application of sand, wear increased by factors between 2 and 10. The wheel steel wear rates showed the largest increases. Wear in wet conditions was higher than that in dry conditions because the wet discs entrained a larger amount of sand through the contact that otherwise was ejected when the discs were run dry. The mechanisms of sanding wear have been investigated. Severe plastic flow as well as a high material removal rate and surface corrugation takes place. The rail discs are scored by sand particles that embed and stick in the softer wheel disc. As well as wear by abrasion, the discs were subject to a rapid fatigue process and large chunks of material fractured from the surface. A simple abrasive wear model has been developed to predict wear of rail material caused by sand in the wheel/rail contact, which shows good correlation with test results. There are a number of idealizations inherent in the test simulations that lead to increased severity over the actual wheel/rail contact. These include the amount of sand entering the contacts in both types of test and the disc geometry and motion in the dynamic tests. While the twin disc specimens have been scaled down, the sand is as used on the railway network, which may lead to surface damage appearing more severe than it would be in an actual wheel/rail contact. Results are therefore only to be taken as a guide to what happens in the full size wheel/rail interface.

Decarburisation and rolling contact fatigue of a rail steel

Decarburisation is present on all new rails and unless ground off, will be on the running surface once installed. This paper presents a detailed investigation into the effect of decarburisation of rolling contact fatigue and wear. Twin disc testing has been used to study the phenomenon under water-lubricated rolling/sliding contact. Rail discs have been cut from grade 220 rail and heat-treated to give samples with varying depths of decarburisation. It was found that as the depth of decarburisation increased the wear rate of the rail disc increased while that of the wheel disc decreased. The effect of decarburisation on rolling contact fatigue was to increase the crack growth rate of samples with increasing depth. Cracks under rolling contact fatigue were propagated by contact stresses via the fluid pressure mechanism and influenced by microstructure. The effect of decarburisation was predicted to have little effect on rail/wheel contact when scaled from the laboratory tests using the depth of maximum shear stress.

05/00916 Transient and stability analysis in single-phase natural circulation

The paper presents a ratchetting model, based on the non-linear kinematic-isotropic hardening law of Leimatre and Chaboche, able to predict the shear strain accumulation during rolling contact loading and including wear as a competitive phenomenon. A procedure was proposed to calibrate the model with the material constants suitable for rolling contact problems, obtaining them from bi-disk contact tests.For this aim and to study the damage evolution at the wheel–rail interface, some twin disk rolling contact tests were carried out on a common wheel–rail material couple under a dry rolling–sliding condition typical of normal service. The tests were stopped at progressive cycles numbers for different couples of specimens, which were then cut and observed with optical and electron microscope in order to analyse the damage evolution in the zone near to the contact interface. A wear-rolling contact fatigue competition was observed with a progressive crack tip advance following a shear band cracking mechanism and crack tail removal due to wear.The numerical model allowed predicting the experimental strain profile along the depth as a function of contact cycles number, also demonstrating through the critical strain approach that in the analysed conditions wear was able to prevent deep crack formation.

Influences on the tribological behaviour of slip-rolling DLC-coatings

Diamond like carbon (DLC)-coatings are applied very successfully on computer discs. Sliding friction tests confirm their high performance as dry lubricants with friction coefficients (COFs) below 0.1. In contrast to sliding tests, until very recently most of the DLC-coatings failed in slip-rolling test after fewer cycles than uncoated samples. However, present tests with DLC-coatings of seven different suppliers show a more promising tribological behaviour under slip-rolling conditions. The DLC-coatings were deposited onto steel 100Cr6, HRC 60, with a thickness of 2–3 μm. The counter bodies were uncoated, grinded or polished 100Cr6 discs with 30 mm radius of curvature. The tests were performed on a twin disc testing rig (Amsler type) with paraffin oil under boundary/mixed lubrication and rolling with 10% slip. The initial, average Hertzian contact pressure was adjusted to P m = 1.0, 1.25, and 1.5 GPa. The failure criterion was defined as the occurrence of a single damaged area larger than 1 mm 2, which was controlled by optical microscopy (OM). An acoustic emission (AE) measurement system was installed as an additional online control for coating failure. The stressed coatings and the chipping areas were analyzed by OM, SEM, EDX, AFM and Raman spectroscopy. Until now, the results reveal that a large scatter in lifetime of coatings occurs not only between samples of different coaters but also between the samples of one single batch. The highest lifetime reached by a DLC-coating under these testing conditions is 10 × 10 6 cycles. That is the highest life time for a coating tested under the testing conditions above in our laboratory so far. The following factors were identified to influence the life time of the coatings: coating thickness, interlayer type, topography of the coatings, counter body roughness and mechanical properties. But the most important factor determining the life time of the coatings is that the coating process does produce homogenous layers free of faults.

Assessment of the factors influencing micropitting in rolling/sliding contacts

Micropitting is currently a significant failure mechanism in carburised steel gears, but the detailed mechanisms of crack initiation and propagation are not well understood. Experiments have been carried out using a twin disc machine, according to a factorial experimental design in order to assess the influence of seven factors: material, surface finish, lubricant, load, temperature, speed and, slide-to-roll ratio. In order to minimise time, the design adopted was a fractional factorial design 2 (7-4). It has been found that load has the biggest effect on micropitting initiation whereas speed and slide-to-roll ratio have the biggest effects on micropitting propagation. Finally it is shown that micropitting is related to the phenomenon of martensite decay.

Shear mode growth of short surface-breaking RCF cracks

Surface-breaking cracks occur in railheads due to unidirectional material flow caused by ratchetting from repeated contact loading. This type of crack is frequently observed on the gauge corner of high rails in curved track, where wheel sliding and high tangential forces occur. The present investigation examines the growth of a similar type of crack in a twin disc test specimen, using elastic–plastic finite element calculations and fracture mechanics theory. A two-dimensional finite element (FE) model was used to simulate the rolling contact fatigue twin disc test and crack behaviour for four short cracks: 0.05, 0.1, 0.2 and 0.4 mm. The results from the FE calculations were used in a Hobson-based, elastic–plastic fracture mechanics model. The Pineau criterion was employed to determine the governing crack growth mechanism, either shear or tensile. Net crack growth rate was defined as the difference between the rate of advance at the crack tip and crack mouth truncation by wear. Shear crack growth dominated for all four crack lengths and the net crack growth rate was greater than zero. The paths of crack advance were calculated for each crack; they show that the three shortest continue to grow parallel to the surface of the disc. The 0.4 mm crack, however, grows upward; thus, it may lead to spalling failure. The results are compared with others from similar twin disc tests and discussed in terms of their application to real train traffic situations where crack coalescence may occur.

Mapping rail wear regimes and transitions

This paper outlines work carried out to produce maps of rail material wear coefficients taken from laboratory tests run on twin disc and pin-on-disc machines as well as those derived from measurements taken in the field. Wear regimes and transitions are identified using the maps and defined in terms of slip and contact pressure. Wear regimes are related to expected wheel/rail contact conditions and contact points (rail head/wheel tread and rail gauge/wheel flange). Surface morphologies are discussed and comparisons are made between field and laboratory data.

Prevention of RCF damage in curved track through development of the INFRA‐STAR two‐material rail

ABSTRACTResults from the European 5th frame research project ‘INFRA‐STAR’ are presented. The goal of the project is to prevent rolling contact fatigue (RCF) and to reduce squeal noise in curves by applying an additional surface layer material on the top of the railhead, resulting in a two‐material rail. In INFRA‐STAR, a dynamic train–track interaction model is used to provide the contact forces. Wheel‐rail profiles, wheel‐rail friction, vehicle data, track data and operating conditions are included to calculate the wheel‐rail contact forces and spin moments, contact positions and load distributions in the contact patch. The contact pressure, friction coefficient, coating thickness, material properties of the coating and the rail material are used in finite element calculations and shakedown theory to calculate shakedown limits, which are then used to predict the RCF performance of the system. The paper details the work on theoretical modelling, twin disc testing, metallurgical research and field testing completed to date (August 2002, the project just passed midterm). The development of the surface layer application methods that are used, and the further objectives of the INFRA‐STAR project, are discussed.

Use of the inverse method to check the stress predictions in rough elastohydrodynamic lubrication contacts lubricated with a non-Newtonian fluid for a range of asperity geometries

The inverse method enables a check to be made on the pressures and stresses calculated in rough elastohydrodynamic lubrication (EHL) contacts. Essentially, a rough, soft surface is run against a smooth, hard counterface in a twin disc machine for a limited time. This tends to deform the asperities on the soft disc and, at the end of the run, the profile is measured and used as input to an EHL solver to determine the hydrodynamic pressures. From these the stresses are calculated. If the material has deformed and deformation has ceased, the maximum stress should be equal to the yield strength of the soft disc. This comparison provides a quantitative check on the accuracy of the EHL analysis and on the assumptions made about the fluid rheology. Allowance has, of course, to be made for the build-up of residual stress in the disc material. The method is applied here to surfaces in which defects with a range of sizes have been manufactured. A non-Newtonian fluid with reasonably well-established characteristics was used under conditions of moderate slip. Good agreement is found between the predicted stresses and the material strength for all defect geometries, suggesting that the fluid properties are sufficiently well defined for accurate predictions to be made of the pressures and stresses in EHL conjunctions.

3209 低速域のレール/車輪間の粘着力特性に及ぼす水の影響 : 二円筒試験機による錆の考察

3209 低速域のレール/車輪間の粘着力特性に及ぼす水の影響 : 二円筒試験機による錆の考察

Wear testing and measurement techniques for polymer composite gears

Injection-moulded composite gears using fibre-reinforced thermoplastics often exhibit increased performance compared with those made from unreinforced polymers. In general, materials are chosen on cost or simple performance grounds (such as, nominal flexural modulus or heat deflection temperature). Aspects of the performance of polymeric gears have been studied by a number of workers and efforts have been made to simulate the contact conditions during gear running. However, until now the wear performance of gears made from polymer matrix composites has not been studied systematically. While such materials have been studied using pin-on-disc or twin disc roll/slide wear techniques, no attempt has ever been made to directly compare the results from such studies with those from gear tests. This paper attempts to explain the comparative methods of measurement of various polymer matrix composite gear materials and to relate their performance to results obtained in contact simulation experiments by other workers. Methods of wear testing are compared including direct gear testing and disc testing, together with electronic (displacement) measurement, weight loss and direct measurement. A new method of characterising the wear of gears is presented, which relates actual contact conditions and gear tooth wear. This co-ordinate measurement technique provides wear rates as a function of roll angle, and hence load, sliding speed and slip ratio. It allows comparisons between gear tests and a simpler test configuration, the twin disc roll/slide test. The conclusions reached have interesting commercial implications for the producers and end users of polymer composite gears and for those involved in testing them.

Finite element analyses of rolling contact fatigue crack initiation in railheads

A finite element (FE) tool was developed for the analysis of rolling contact fatigue crack initiation in railheads. An elastic-plastic non-linear kinematic hardening material model is included in the tool, which can model decaying ratcheting rate. The FE tool was applied to rails used in commuter train traffic at a Swedish test site. Two rolling contact conditions were compared, with respect to fatigue crack initiation, by modelling the contact loads differently. The two conditions were newly manufactured rails (Hertzian distribution) and rails that have been in service for 16 months (non-Hertzian distribution). A strategy proposed for fatigue analysis comprises a critical plane approach used together with a multiaxial fatigue model; a damage accumulation rule is also proposed that takes into account low-cycle fatigue and ratcheting damage mechanisms. Together with a two-dimensional FE analysis of a twin disc test, the fatigue evaluation strategy was validated by a set of twin disc data with good agreement. A fatigue analysis of the results from the FE tool show good agreement with observations and measurements made at the Swedish test site with respect to position for fatigue crack initiation, angle of the crack plane and the calculated number of wheel passages to initiation of head checks. Agreement was particularly good for the Hertzian distribution.

Life prediction of rolling contact fatigue crack initiation

A strategy developed for fatigue life prediction of rolling contact fatigue (RCF) crack initiation is presented. It combines elastic–plastic finite element (FE) analyses, multiaxial fatigue crack initiation models used together with the critical plane concept, fatigue damage summation calculations, and comparison of results from numerical analyses and experiments. The strategy presented is utilised and evaluated for two RCF examples: (i) a twin disc test, and (ii) a railway wheel–rail rolling contact. The results from both of the examples verify that the strategy and evaluation methodology presented can be used for fatigue life predictions of RCF crack initiation caused by low-cycle fatigue and ratchetting failure.

Prediction of fatigue crack initiation for rolling contact fatigue

In finite element (FE) simulations of a twin disc test of a wheel/rail contact, fatigue crack initiation criteria for elastic shakedown, plastic shakedown and ratchetting material responses were evaluated for a pearlitic rail steel BS11 normal grade. The Chaboche material model for nonlinear isotropic and kinematic hardening was used in the FE simulations. The ratchetting material response results were compared with a constitutive ratchetting model, and there was good agreement with respect to the number of cycles to crack initiation and shear strain distribution below the contact surface. In addition, angles for critical planes for crack initiation were calculated for both plastic shakedown and ratchetting material responses. Results from simulations with the ratchetting model at constant contact pressures and varying friction coefficient showed asymptotic values of the friction coefficient at which crack initiation due to ratchetting will not occur.

Pressure-Viscosity Behavior of Lubricants to 1.4 GPa and Its Relation to EHD Traction

A new falling body viscometer was developed to extend the measurement of the limiting low shear viscosity to 1.4 GPa in pressure and 105 Pas in viscosity for temperatures above ambient. Rather complete characterizations of the temperature-pressure-viscosity behavior of three synthetic base oils were performed. Free volume models can reproduce and provide a physical explanation for the inflection observed in pressure viscosity isotherms and provide a relationship between viscosity and state behavior. Examples of traction calculations indicate that used alone, traction curves cannot be used to deduce flow curves. The interpretation of a traction curve in terms of shear rheology is ambiguous without knowledge of at least the pressure-viscosity behavior up to the maximum Hertz pressure. A pressure-viscosity correlation capable of providing accurate values for α at these pressures may be very different from a correlation which has proven useful for film thickness analysis. A need exists to extend the flow curves obtained from rheometers to the highest EHD pressures in order to model friction losses in machines and the tractive performance of continuously variable transmissions. It should be possible to do this with viscosity data and correlations of the kind presented here, together with traction obtained from say a twin disc machine. A well-proven rheological model such as the Carreau Yasuda equation could then be used in a regression analysis to obtain the shear rheological response to very high pressure. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999