Twin-disc Test Research Articles

Study on the optimization of solid particle additives in top-of-rail friction modifiers based on a twin-disc testing apparatus

ABSTRACT Top-of-rail friction modifiers are used to manage the friction on the top-of-rail and help alleviate corrugation, reduce noise, decrease material wear, etc. In this paper, five series of FM samples were prepared and tested using a twin-disc testing apparatus to optimize the solid particle parameters in the FM, aiming to achieve an intermediate adhesion level and a positive creep curve at the wheel-rail interface. The roles of every composition were explored and further the mass content of solid particles, the mass content ratio of lubrication to modifying particles, and the hardness and size of modifying particles were optimized. The possible influence mechanism of FM third body layer shear strength on the wheel-rail adhesion behaviour was discussed based on the Coulomb-Mohr theory. The FM sample containing 76.31 wt% of water, 2.77 wt% of carboxymethyl cellulose (CMC), 10.46 wt% of resin, 3.04 wt% of graphite particles, and 9.13 wt% of kaolin particles can reduce adhesion coefficient to 0.129 and generate an obvious positive creep curve in the wheel-rail interface.

Evaluation of polygonization resistance of two wheel steels from full-scale railway wheels and small test discs

Polygonization of railway wheels poses a significant challenge for the wheel-rail system. Despite the evidence from engineering practices highlighting the substantial impact of material factors on the polygonization resistance of railway wheels, research in this field remains limited. This study investigates two types of railway wheels, namely non-alloyed and alloyed wheels, which demonstrate different polygonization resistance under identical operating conditions. It involves microstructural analysis and hardness testing for the matrix materials and plastic deformation layer (PDL) of full-scale wheel, twin-disc testing for wheel steels, followed by analyzing the PDL of test wheel discs. The findings demonstrate that the full-scale wheel and test wheel disc exhibit consistency in terms of polygonization resistance, PDL microstructure, and microhardness, suggesting that the twin-disc test can be employed for material comparison regarding polygonization resistance. In addition, the alloyed wheel was found to exhibit better polygonization resistance at the external region of the wheel rim due to its higher resistance to plastic deformation and wear, while it demonstrates poorer polygonization resistance at the internal region of the wheel rim due to its lower wear resistance. Furthermore, the wave troughs display significant plastic deformation, dynamic recrystallization, and strain hardening near the contact surface, indicating a higher slip ratio at these locations. In contrast, the wave crests predominantly exhibit these characteristics in the subsurface region, indicating that the wave crests experience impact loading.

Optimizing Railway Tribology: A Systematic Review and Predictive Modeling of Twin-Disc Testing Parameters

Twin-disc testing is crucial for understanding wheel–rail interactions in railway systems, but the vast array of testing parameters and conditions makes data interpretation challenging. This review presents a comprehensive analysis of the twin-disc literature experimental data, focusing on how various parameters influence friction and wear characteristics under stationary contaminant conditions. We systematically collected and analyzed data from numerous studies, considering factors such as contact pressure, speed, material hardness, sliding speeds, adhesion, and a range of contaminants. This research showed inconsistent data reporting across different studies and statistical analyses revealed significant correlations between testing parameters and wear rates. For sand-contaminated tests, a correlation between particle size and flow rate was also highlighted. Based on these findings, we developed a simple predictive model for forecasting wear rates under varying conditions. This model achieved an adjusted R2 of 0.650, demonstrating its potential for optimizing railway component design and maintenance strategies. Our study provides a valuable resource for researchers and practitioners in railway engineering, offering insights into the complex tribological interactions in wheel–rail systems and a tool for predicting wear behavior.

Development of a Cost-Effective Twin-Disc Test Rig for Railway Wear Simulation

Maintenance due to the replacement of damaged wheels and rails due to rolling contact fatigue (RCF) and wear has been found to be the major problem to rail operating companies. This problem tends to lead to unavailability of railway networks. To solve this problem, costly wear simulators are developed to predict the wear behaviour of the rails and wheels to improve the preventive maintenance in pursuit of operational efficiency. Therefore, more studies that simulate a combination of rolling and sliding wear, together with RCF, are required, specifically for the Southern African, where good and cost-effective rail wear simulators are not readily available. The problem with wear and RCF simulators is high production costs, so this work aims to solve this problem by developing a cost-effective wear test rig that is capable of simulating RCF, sliding and rolling wear as experienced by the train wheel while moving along railway tracks. For this work, it was decided that twin-disc concept would be used, since literature clearly shown that the method was able to simulate the three damage mechanisms mentioned. The developed twin-disc wear simulator was able to simulate both rolling and sliding wear and parameters including contact load and slip ratio could be changed with ease so to simulate the actual contact conditions between the wheel and rail. Outputs such as coefficient of friction and wheel disc temperature were obtained. The results showed that the severity of wear is heavily dependent on slip ratio i.e., increased with slip ratio, with both coefficient of friction and wear rate increasing with slip ratio.

The correlation between material deformed microstructure and rolling contact fatigue crack propagation of U71Mn rail when matching with CL60 wheel

Deformed microstructure and rolling contact fatigue (RCF) cracks are the two predominant damage characteristics of the field wheel and rail materials from the cross-sectional view. The proportion of two damage characteristics varied under different operating conditions. Thus, this study aims to investigate the correlation between material deformed microstructure and RCF crack propagation of U71Mn rail when matching with CL60 wheel using a twin-disc testing machine. An initial deformed microstructure layer on U71Mn rail material was first obtained by testing under dry condition, and then the RCF crack propagation tests were conducted under water condition. Results indicated that when the crack depth was smaller than the initial deformed microstructure layer depth, the deformed orientation dominated the crack propagation direction; when the depth of crack propagation towards the matrix exceeded the depth of newly formed deformed microstructure region, crack propagation direction dominated the microstructure deformed orientation. The increase in the initial deformed microstructure layer depth would intensify both the crack length and depth, and also would result in thicker newly formed deformed microstructure region. The Pearson correlation coefficient between initial deformed microstructure layer depth and crack depth was 0.998. The influence of initial deformed microstructure layer depth on wear rate was slightly greater than crack depth.

An Experimental Investigation Replicating the Surface Behavior of Ground Steel Gears in Mixed Lubrication Using Twin-Disk Testing Part 2: Micropitting

Tooth surfaces in heavily loaded power-transmission gearing can often suffer from surface fatigue on the surface roughness scale, known as micropitting. In this paper, the influence of three variables (pressure, slide-roll ratio, and entrainment velocity) on micropitting is investigated using a full-factorial experimental program using a twin-disk test rig. The results showed that pressure was most influential on micropitting growth during the latter phases of the test, with slide-roll ratio being a more important factor during micropit initiation. The full-factorial nature of these tests allowed two- and three-factor effects to be investigated, demonstrating that micropitting is a complex phenomenon influenced by a network of interconnected effects.

Mechanism and improvement of the rolling contact fatigue of the surface layer with heterogeneous microstructures of the rail steel treated by laminar plasma jet

Surface layer with heterogeneous microstructures (SLHM), characterized by hardened spots discretely embedded in a soft substrate, can be prepared by laminar plasma quenching (LPQ) to improve the wear resistance of the railway rail. However, severe rolling contact fatigue (RCF) cracks and spalling would occur in the hardened spots that only consist of quenched microstructure. This study analyzed the RCF damage mechanism of the LPQ prepared hardened spots by contact simulation. To improve the RCF resistance while maintaining the high wear resistance of the treated rail, a novel SLHM, whose discrete hardened spots are surrounded by transitional zone with the hardness lower than that of the quenched zone but higher than that of the substrate, was proposed and prepared by laminar plasma quenching-tempering (LPQT). Then, twin-disc tests and contact simulations were conducted to investigate the wear and RCF resistances of the LPQT treated rail steel. Results showed that the cause of the severe RCF cracks and spalling at the entry side of the LPQ prepared hardened spots was that the cyclic tensile stress at the entry side was always higher than that at the exit side under the cyclic rolling contact loading. The high tensile stress at the entry side of the LPQ prepared hardened spots resulted from the elastic deformation induced by tensile plastic deformation of the neighboring substrate. When the thickness of the transitional zone was about 0.1 mm, the generation of severe RCF cracks and spalling was significantly inhibited in the LPQT prepared hardened spots because the transitional zone decreases the cyclic tensile stress at the entry side of the quenched zone. Besides, the wear loss of such LPQT treated rail steel was only increased by 18.1 % compared with that of the LPQ treated rail steel, indicating that the wear resistance of the LPQT treated rail steel was not significantly reduced. These findings are promising for designing the microstructure of the surface layer to obtain wear-resistant rail with high RCF resistance.

Novel in-situ real-time line scan optical monitoring of wear and surface damage initiation in a laboratory twin disc test

A new optical monitoring system has been developed to photograph in-situ in real time the initiation of damage on the running surface of a rail steel twin-disc sample undergoing wear testing. The line-scan camera system has been demonstrated on the Sheffield University ROlling Sliding 2 (SUROS2) twin-disc machine. The results show the system can continuously track the development of wear flakes, with wear flake initiation and stabilisation of wear flake size observed without test interruption for the first time. Image analysis to quantify the total wear flake shadow pixel count showed a good correlation with the mass loss results, indicating the potential for the optical data to quantify rail steel wear without interruption to testing. Furthermore, in a water-lubricated test the new system enables observation of rolling contact fatigue (RCF) crack initiation through a water layer present on the specimens, without requiring test interruption. The improving knowledge of the wear and RCF performance of rail steels available from the new observation method can help improve understanding of steel performance and support to the selection of rail steel grades according to their performance.

Influence of HAZ microstructure on RCF under twin-disc test of a flash-butt welded rail

A new twin-disc test was performed using discs containing all microstructures of the heat-affected zone (HAZ) of flash-butt welded rails. It was observed that the lower hardness regions (spheroidized cementite in a ferritic matrix) at the HAZ boundary underwent a local deformation, generating valleys and reproducing a common defect type observed in rail welds: double squat-like defect. Longer crack lengths or crack clusters were observed in the HAZ boundary and central weld regions. The cross-section metallography showed that these cracks are directly related to the microstructure of spheroidized cementite at the HAZ edges or pro-eutectoid ferrite at the central region. The decrease in surface roughness before the cracks, the presence of oxide on the bottom crack surface, and superior spheroidized cementite microstructure in the upper crack region suggest that these larger/cluster cracks are predominated by extrusion of the softer material above the harder material (pearlitic/base metal).

Scaling-up laser cladding of rails

With rail travel getting more popular more demands are being put on rail infrastructure. The wheel/rail contact is the one of the main parameters that is key to maintaining a high performing network. With increasing demands for higher loads and higher speeds, wear and rolling contact fatigue (RCF) are the main two problems.New maintenance techniques can overcome some problems, but new solutions that reduce the damage are needed. Laser cladding is one of the best methods for improving wear and RCF of rail steels by adding layers of more durable materials to the rail.In this article full-scale tests of clad layers have been carried out at realistic contact conditions to verify outcomes seen in twin-disc testing to see if the results can be scaled to predict field performance of clad layers. Martensitic Stainless Steel (MSS) and Stellite 6 were used for the clad layers and R260 was used as the base material. Un-clad R260 was used as reference material.Results of full-scale tests in terms of wear rates, sub-surface deformation, hardness and roughness evolution were comparable. This showed that the clad layers deposited on actual rail performed very well compared to un-clad rail and that the tests run at the small-scale were appropriate for predicting full-scale behaviour.

Study of the Initial Cycles Number Influence on the Retentivity of a Commercial Friction Modifier Using the Twin-disk Test

The rails and wheels are one of the most critical and expensive components of railway transport, and any problem could generate numerous consequences. The ability to manage friction at the wheel-rail interface currently represents one of the greatest challenges and one of the most powerful tools in railway engineering. The application of friction modifiers, lubricants, and friction enhancers to friction control have been the focus of extensive research of wheel-rail systems. The friction modifier (FM) provides a targeted coefficient of friction (COF) of 0.3–0.4. Currently, the laboratorial retentivity tests conducted by manufacturers do not describe the standards used, neither the surface integrity before the FM application. This work studies the influence in the retentivity of the different surface integrity states before application of FM using a twin-disk test. It will be possible to define the surface condition necessary for the application of FMs in retentivity tests, using a twin-disk tribometer. A commercial friction modifier was applied before the test (zero cycles) and after 500; 1,000; 1,500; 2,000; 3,000; 4,000; and 5,000 dry cycles, ending the tests when the coefficient of traction (COT) reached 0.4. To compare the retentivity of different FMs, a complete standardization of the twin-disk test is essential, applying the FM in a controlled amount, with over 2,000 dry cycles, and using a disk with controlled initial roughness.

Tribological and rheological performance of friction modifiers with different viscosities

Friction modifier (FM) in the rail/wheel applications is a special lubrication material that could be applied between the top of rail and the tread of wheel. The application of FM brings many benefits to the wheel-rail interface such as reducing noise, alleviating wheel-rail wear and mitigating corrugation. Viscosity is an important parameter to reflect fluid properties of FM materials. In this study, FM samples with different viscosities were prepared by changing the content of sodium carboxymethyl cellulose (CMC). The tribological and rheological performance of these FM samples were explored using the twin-disc testing machine and rotational rheometer. Furthermore, the relationship between the tribological and rheological results were tried to establish. The results showed that the increase in the content of CMC could increase the viscosity of FM and enhance the solid-like characteristic of FM, which could help FM reduce coefficient of adhesion (COA) level and wheel-rail damage. For the FM material developed in this study, the rational applying amount and the range of viscosity were about 15 μL/time and 200 Pa s to 400 Pa s, respectively, considering the factors of wheel-rail adhesion, wear, potential waste and pumping process in the practical application.

A new scaled testing methodology for the analysis of rolling contact fatigue in railway steels.

Twin disc testing is a relatively cost effective and practical method of investigating the damage resistive properties of rail steels in a scaled environment. The loading conditions and dimensions used in testing are scaled in such a way that the Rolling Contact Fatigue (RCF) generated on a twin disc rig are representative of the conditions experienced at the full-scale. Typical methodologies to generate RCF on a twin disc rig involve the application of water to the contact patch throughout the test, with perhaps a few thousand dry cycles being run beforehand. This research explores the use of weather data to reflect the real-world contact conditions more accurately between a wheel and rail. Met Office weather data collected from 1981 to 2010 shows the average annual precipitation in the UK is around 133 days of rain or snow, which is nearly 36 % of the year. In this study, weather data was used to determine the ratio of wet to dry cycles in the testing methodology.  50k dry cycles are run in intervals of 5k and then transitioned to 25k of wet cycles, which is 30% of the total cycles. Intervals were kept at 5k so attaining 36%. If RCF is not initiated after the first 75k cycles, which ends with 25k wet cycles, then the same dry and wet cycles are repeated. RCF data generated using this methodology demonstrated expected differences in the RCF resistive properties of two rail steel grades, R260 and R350HT. In test 1 the point of initiation for each grade was not captured, though cracks were visible around the expected cycles to initiation, which was verified using optical microscopy. The cracks viewed on the softer R260 steel samples, ‘’260’’ referring to the Brinell hardness value of the steel, were more developed at 85k cycles and showed signs of spalling, whilst the cracks on the harder R350HT sample were much smaller. In test 2 the point of initiation was found for each grade. Optical microscopy was also used to show the depth and direction of head check cracks to ensure the damage viewed from the head was RCF and not plastic flow or cyclic wear. This paper shows the necessity for considering the number of wet cycles applied during twin disc testing and demonstrates it is possible to run a high number of wet cycles which are more representative of real-world conditions when generating RCF on twin disc rigs.  

Peridynamic modeling of rail wear during sliding contact considering thermal effects

Numerical simulations of rail wear during sliding contact pose significant challenges owing to the complex coupling between thermomechanical contact loads, material properties, and profile changes. This study proposes a modeling approach for rail wear during sliding contact, using nonlocal peridynamic (PD) thermomechanics and a bond damage criterion based on ratchet failure. To facilitate test verification, a two-dimensional PD model for twin-disc testing with elastoplastic materials was developed, and numerical simulations and corresponding twin-disc testing were performed during wheel idling under different loads, idling speeds, and idling times. The PD model accurately predicted rail wear under all operating conditions, exhibiting excellent agreement with the test results, and thus validating the proposed method and supporting the ratchet failure mechanism. Subsequently, thermomechanical modeling of sliding contacts with different creepages was performed. The PD model predicted continuous wear along the contact surface, with an increase in wear depth corresponding to higher creepage values. As the contact temperature increased with creepage, thermal stress and thermal softening caused slight and sharp increases in rail plastic strain, respectively. Therefore, thermal softening is the main factor responsible for the significant increase in wear rate with temperature. Finally, wear results of the PD model and classical Archard’s wear model were compared. The PD models can capture variations in wear rates that are difficult to address using the Archard model.

Investigation of friction and scuffing during loss of lubrication on a high velocity twin-disc test rig

Increasing demands on gear components such as high-pressure and high speeds are aggravated by weight saving strategies eliminating an optional secondary lubrication system during negative g operations or windmilling, which result in loss of lubrication conditions (LOL) and scuffing. In order to understand gear design options able to withstand LOL at high velocities, systematic, fundamental studies on model test-rigs, e.g. twin-disc rigs, are required. For this purpose, a twin-disc rig was designed which is able to perform test at high entrainment velocities of up to 80 m/s and high lubricant injection temperatures. Systematic studies have been carried out in continuous lubrication and LOL condition and evaluated in terms of friction and time-of failure (TOF). A mixed EHL model complemented the experimental matrix for wider range of temperatures and additionally compared the influence of the grinding direction on the lubricant gap.Entrainment velocity showed to have the most prominent influence on friction in continuous mode, mostly for the increase from 8 m/s to 23 m/s, followed by minor changes upon further increase to 30 m/s. The influence of injection temperature was mostly prominent at high entrainment velocities. Contact pressure had the strongest influence on TOF. Comparisons with superfinished disc surfaces exhibited a high increase in TOF and significantly less scuffing damage fractures on the runway.

Development of a heating system and the effect of temperature in the twin-disc test

Development of a heating system and the effect of temperature in the twin-disc test

Surface treatment of rail to enhance rolling contact fatigue and wear resistance: Combined spot laminar plasma quenching and tempering method

The railway industry faces the challenge of improving the fatigue and wear resistance of rails simultaneously. Laminar plasma quenching (LPQ) can enhance wear resistance but may result in severe rolling contact fatigue (RCF) damage. This study proposes a combined LPQ and tempering method to address this issue and evaluates its effect on rail RCF damage and wear resistance using twin-disc tests. The findings demonstrate that tempering diminishes the grain density and mitigates the lattice distortion in the quenched zone of LPQ rails, thereby enhancing the RCF resistance in that zone. This treatment leads to a substantial reduction in surface fatigue cracks and spalling of the rails and a wear resistance improvement of over one-fold compared to ordinary rails. Thus, this rail surface treatment approach effectively improves both the fatigue and wear resistance of rails simultaneously.

Berechnungsmethode zur Beurteilung von Verschleiß bei Stahl-Bronze-Wälzpaarungen hinsichtlich Schneckengetrieben

AbstractWorm gears are usually used in industrial applications with the material pairing of hard worm and softer worm wheel, where the failure mode wear often limits the service life. Up to now, extensive, costly and time-consuming component wear tests need to be carried out for each tribological pairing in order to calculate the amount of wear on worm wheels. Therefore, no practical and efficient experimental method is currently available to optimize material-lubricant pairings with respect to wear capacity for worm gear drives. For this reason, a model contact wear test was developed applying a twin-disk test rig. Based on this, the paper covers a new wear calculation method to characterize the wear behavior of steel-bronze rolling-sliding contacts regarding worm wheels. Therefore, the basics for the new calculation method for a model contact will be presented. Furthermore, the validation of the newly developed wear calculation method on the basis of test results of experimental investigations using the twin-disk test rig is addressed. Subsequently, results of the new wear calculation method are compared to results of the wear calculation method for worm wheels for individual tribosystems. Special focus will be given to similar trends in the calculated wear between the worm gear contact and the model contact. This new calculation method enables, in combination with wear tests on the twin-disk machine, efficient information on the wear behavior of worm gears and the development of optimized material-lubricant pairings concerning wear performance for worm gear drives.

Determination of the critical depths of indentations for local rolling contact fatigue initiation

Scaled twin-disc tests are conducted to study the initiation of local rolling contact fatigue (LRCF) from indentations on discs of U71MnG rail indented by sharp tap tips. Critical depths over which LRCF initiates are found for indentations of different geometries under oil lubrication. Simulating the indenting process with a static finite element modelling approach, a critical plastic strain of 170% corresponding to the critical depths is determined. Assuming this critical strain is transferrable, size-dependent critical depths are further derived for indentations on actual rails using the same modelling approach. After verification by LRCF damages in practice, a linearized relationship between the critical depth and the characteristic length is proposed for actual indentations, being 2.3–3.2 mm deep for ones with a characteristic length of 4.0–10.0 mm.

Nano and micro-indentation driven characterisation of asperity and bulk plasticity at the surface of modern premium rail steels

The plastic deformation behaviour of rail steels due to cyclic rail-wheel contacts is important to understand due to its connection to wear and rolling contact fatigue (RCF) damage initiation in service. Simulation models such as the ‘Layer’ and ‘Brick’ model have previously been developed to estimate the accumulation of plastic damage in a rail steel; however, the data available to drive these models is currently sparse, with limited applicability to modern rail steel grades. This paper presents the research examining the shear stress-strain curve relationships of rail steels derived from plastic shear strain and shear yield stress data collected from twin-disc test samples. A combination of microhardness and nanohardness testing was used to derive the shear yield stress data, whereas the plastic shear strain was acquired from optical microscopy. Six different conditions were investigated for this research for the purpose of examining how shear stress-strain curve relationships compared between the standard R260 and the premium HP335 and R350HT rail steels and how this compares to wear damage data. The influence of the maximum Hertzian contact pressure on the shear stress-strain curve relationships of R260 between 600 and 1500 MPa contact pressure was also investigated. The wear rate results derived from the mass loss in interrupted twin-disc tests showed HP335 wearing the least, followed by R350HT and then R260 for 1500 MPa, dry contact conditions. However, the highest shear yield stress achieved was for R350HT, then HP335, and R260. The results show that the shear stress-strain curve relationships by themselves are insufficient to determine rail steel wear performance in a laboratory environment. The shear stress-strain curve relationships for R260 collected under different contact pressures showed the results are near independent of the contact pressure within the range explored.