To mitigate and alleviate low wheel–rail adhesion, a train-borne system is utilised to deposit sand particles into the wheel–rail interface via a jet of compressed air in a process called rail-sanding. Britain Rail Safety and Standards Board introduced guidelines on the sand particles’ shape, size, and uniformity which needs to be adhered to for rail-sanding. To further investigate these guidelines and help improve them, this research presents a parametric study on the particle characteristics that affect the rail-sanding process including density, size and size distribution, coefficient of uniformity, and shape, utilising a coupled computational fluid dynamics–discrete element method (CFD–DEM) model. The efficiency of rail-sanding is estimated for each case study and compared to the benchmark to optimise the sand characteristics for rail-sanding. It is concluded that particle size distribution (within the accepted range) has an insignificant effect on the efficiency while increasing particle size or the coefficient of uniformity decreases the efficiency. Particle shape is shown to highly affect the efficiency for flat, compact and elongated particles compared to the spherical shape. The current numerical model is capable of accurately predicting the trends in the efficiency compared to the actual values obtained from full-scale experiments.

The presented surface indentation model is one step towards building a DEM model for wheel–rail sanding. In railways, so-called low-adhesion conditions can cause problems in traction and braking, and sanding is used to overcome this problem. Sand grains are blasted towards wheel–rail contact, fracture repeatedly as they enter the nip and are drawn into the contact and then increase adhesion. Research on this topic has mostly been experimental, but focussed on adhesion enhancement measurement. Thus, physical mechanisms increasing the adhesion are not well understood. Previous works involved experiments and DEM modelling of single sand grain crushing tests under realistic wheel–rail contact pressures of 900 MPa, focusing on sand fragment spread and formation of clusters of solidified fragments. In the experiments, indents in the compressing steel plates were also observed, which are also observed on wheel and rail surfaces in railway operation. These are now modelled by adapting an existing surface indentation model from literature to the case of surface indentations caused by granular materials. Two test cases are studied, and experimental spherical indentation tests for model parametrisation are presented. In a proof of concept, the mentioned single sand grain crushing tests under 900 MPa pressure are simulated including the surface indentation model. This work contributes to DEM modelling of wheel–rail sanding, which is believed to be a good approach to deepen the understanding of adhesion increasing mechanisms under sanded conditions.

In railways, low adhesion conditions lead to problems in braking and traction. Sanding increases adhesion by blasting sand grains towards the wheel-rail contact. Which physical mechanisms increase adhesion during sanding is poorly understood, as research in this field has been almost exclusively experimental. This work is the first step in developing a DEM model describing the sanding process. Previous experimental studies involve single grain crushing tests focusing on initial breakage and repeated breakage under realistic wheel-rail contact pressures of 900 MPa. Tests on two types of rail sand under dry and wet contact conditions, showed different spread of fragments as well as the formation of clusters of solidified fragments at high stresses. The developed DEM model captures the observed behaviour for both types of sand, and both contact conditions. Combining experimental research and DEM modelling aims to improve the understanding of adhesion increasing mechanisms in sanded wheel-rail contacts.

Sand in wheel-rail contacts is successfully used for a long time to increase adhesion. However, the physical effects responsible for this adhesion increase remain unclear. To improve the understanding of these effects an approach combining experiments with Discrete Element Method (DEM) modeling is followed. Results from initial breakage and high loading experiments on single sand grains under purely normal loading conditions show that the spreading behavior and the forming of clusters of solidified sand fragments depend on the type of sand and the contact conditions, namely dry and wet. The experimental data were used to develop, calibrate and validate DEM models including particle breakage. These DEM models can describe the observed effects. Finally, a systematic classification is presented where tangential relative motion due to slip can occur in a sanded wheel-rail contact. This is seen as an important basis for future experimental and modelling work.

Herein the authors report a detailed insight into the tribological and chemical mechanisms that take place when leaf matter is present at the wheel/rail interface. The approach used enables further insight into the formation and subsequent effects on friction from leaf derived layers. Results shed insight on the process that facilitates the formation of leaf derived layers, their structure and chemical make-up. Our analysis indicates that polyphenols, a chemical family that include tannins, have an important role in layer formation and hypothesis on the layer's stability, when formed under high pressures. The data indicate that the high pressure found at the wheel/rail interface facilitates conversion of biomass into a tenacious, thin film. This is shown by the increase in the amount of phenolic compounds present. Phenolic compounds are typically rich in oxygen functional groups that have the ability to bind to metal ions. This insight into the composition of the film is expected to enable the development of novel remediation strategies. It highlights the potential for cleaning agents to be used as tools for restoring friction to safe values. This should lead to improvements in the operational performance and safety of rail transport for passengers and train operators.

In this paper an approach for generating predicted full-scale creep force – creep curves from small-scale tribological tests has been outlined and examples produced using rail sands and low adhesion contaminants. A high pressure torsion rig was used to measure the change in shear stress over increasing displacement for British and Austrian rail sands in dry, wet, and leaf contaminated conditions under different normal pressures. In addition, tests with just sycamore leaf powder and graphite were also conducted. This data was then used to parameterise extended creep force models for each contact condition, from which predictions of full-scale behaviour were then made and validated using full-scale data from literature.

Particulate materials are utilised in many applications to manipulate the friction between surfaces. This dataset provides the characteristics of particulates used as rail sand in the train's wheel/rail interface (via an on-board system) to facilitate the train's acceleration and deceleration. Seven materials are studied including Austrian rail sand, standard Great British rail sand, waste glass beads, recycled crushed glass, non-coated alumina, coated alumina, and dolomite. The main objective of this research is to provide a physical and mechanical characterisation of these granular materials in terms of their density, bulk behaviour, particle size, particle shape, hardness, reduced modulus, and mineralogical properties. In particular, three-dimensional raw and post-processed micro-computed tomography images of more than 1200 particles are shared. The results provide a detailed dataset which can be used in ongoing and future experimental and numerical investigations studying the role of particulates in the wheel/rail interface.

This paper presents an assessment of newly-developed conductive adhesion materials (Products A-E) in comparison to standard rail sand used in Britain. Current rail sand is an insulating material which can affect track circuits; newly-developed conductive materials could reduce the risk of this and allow for more material to be applied to further mitigate against low adhesion. The particles were characterised to determine their densities, and size and shape distributions. Bulk behaviour was assessed through three characteristics: angle of repose, bulk shear strength, and particle breakage index. Materials were then assessed using a high pressure torsion approach to measure their effects on adhesion and electrical resistance in dry, wet, and leaf contaminated conditions. It was found that all products produced better or equivalent conductivity compared to the currently used GB rail sand and that Product D and Product E should be considered for future field testing.

In the UK Network Rail Environmental Sustainability Strategy 2020–2050, minimal waste and the sustainable use of materials are highlighted as core priorities. The ambition is to reuse, repurpose or redeploy all resources. In low adhesion conditions, sand particles are used to enhance traction throughout the network. However, sand is in danger of becoming scarce as many applications demand it. In this study, an alternative adhesion enhancing particle system made of recycled crushed glass is examined in terms of density, size, shape distribution, mineralogy, mechanical properties, and bulk behaviour to better understand their characteristics in comparison with the typical Great British rail sand currently in use and reported in the literature. Their effects on tribological behaviour and surface damage are also investigated using the High-Pressure Torsion test in dry, wet, and leaf-contaminated conditions. Both particle characterisation and tribological testing show promising results. Recycled glass particles provide an acceptable level of traction with a similar level of rail damage as typical rail sand. It is suggested to perform full-scale laboratory and field tests to further confirm the suitability of this material.

Sufficient adhesion in a wheel-rail contact is one of the key requirements for safe and efficient railway operations.Low adhesion conditions significantly increase the risk of braking issues leading to extended braking distances, passing signals at danger, reduced acceleration rate, and damage to the wheels and rails.Sanding remains one of the most common methods to overcome low adhesion conditions.However, over application of electrically insulating sand may interfere with railway track circuits of some signalling systems leading to loss of train detection and therefore limits of application have been imposed.The development of alternative, novel adhesion enhancing materials with higher electrically conductivity may mitigate the risk of electrical insulation and allow for larger amounts of material to be applied to improve wheel/rail adhesion.As well as not interfering with track circuits, these new materials must demonstrate good deposition efficiency using conventional sanders as well as providing a significant increase in friction levels under low adhesion conditions.This work describes the testing of proprietary coatings which can be applied to sand and other particles to improve conductivity and deposition efficiency.Laboratory scale testing of the deposition efficiency, adhesion enhancing and electrical characteristics of these materials were carried out at the University of British Columbia and LB Foster facilities in Canada and field testing of the influence on track circuits was

The process of applying sand particles to increase the traction between wheel and rail is reported to be less than 20% efficient. To better understand entrainment efficiency, the process is simulated using the Discrete Element Method. The simulation results are validated against full-scale experimental observations in terms of entrainment efficiency and particle velocity for ten case studies with different positioning of the sand nozzle. The numerical simulations confirm the experimental observations wherein the highest efficiency can be achieved when the sander is aimed at the wheel/rail nip. When aiming the sander at the wheel, the values of entrainment efficiency from simulations and experiments show some discrepancy which can be related to the numerical assumptions. Calculating coefficients of traction between the rail and wheel from the simulation data for the four cases of an un-sanded contact, and with the sander aimed at the rail, the nip, and the wheel (all with the same angle) show an increase in the coefficient of traction for all sanded cases compared to the un-sanded case.

Low adhesion in the wheel/rail contact is a problem for the rail industry in Great Britain as it causes significant scheduling and safety issues. Applying sand to the wheel/rail contact is used to mitigate against low adhesion however, there is not a consensus on what makes a “good” particle for restoring adhesion, especially with regards to when the particle has entered the wheel/rail contact. The aim of this work was to investigate what particle characteristics had the greatest effect on wheel/rail adhesion and surface conditions, using a process of particle characterisation, tribological testing and statistical modelling. Particle size, shape, and hardness were all found to affect tribological performance. This knowledge can help guide future changes to sanding operations.

In railway operation, the sanding process is used to overcome low adhesion conditions in the wheel–rail contact. In the literature, previously conducted research has been experimental, e.g., measuring adhesion coefficients (ACs) under different contact conditions (dry, wet, …) or applying different sands. Under dry conditions, sanding can reduce measured ACs, while under wet conditions different types of rail sand can leave ACs unchanged or increase adhesion. Despite active research, the physical mechanisms causing the change in ACs under sanded conditions are still poorly understood. A possible remedy is the development of advanced models of sanding including local effects. As a basis for such a model, this study presents experimental results concerning single grain crushing behaviour of two types of rail sand under dry and wet contact conditions. Firstly, initial breakage behaviour is investigated with focus on the particle fragments’ size and spread as only fragments within the running band are available to influence the AC during roll-over. Secondly, single grain crushing tests are conducted under realistic wheel–rail load showing the formation of solidified clusters of sand fragments, as well as their size and thickness. This information is important for understanding mechanisms and for future physics-based modelling of the sanding process in wheel–rail contacts.

Despite ongoing research, the parametrisation of a DEM model is a challenging task, as it depends strongly on the particle shape representation used, particle-particle contact law and the simulated applications: for railway ballast e.g. lab tests or track conditions. The authors previously modelled railway ballast with a DEM model using a simple particle shape. The DEM model was parametrised, by trial-and-error, to compression and direct shear test results. A good agreement between DEM model and experimental results was achieved only when the Conical Damage Model (CDM) was used as the contact law. Compared to the well-known linear-spring Cundall-Strack law or the Hertz-Mindlin law, this contact law takes into account additional physical effects (e.g. edge breakage) occurring in the experiment. Little is known on the influence of the CDM model parameters on the simulation results or on possible parameter ambiguities. This lack of knowledge hinders a reliable and efficient parametrisation of DEM models using different particle shapes. Both points are addressed in this work in detail by investigating a DEM model for railway ballast using one simple particle shape. Suggestions for a parametrisation strategy of reduced computational effort are formulated and tested using a second particle shape. In future works, the newly presented parametrisation strategy can help to calibrate different DEM models and to study the influence of particle shape.

Particle shape analysis is conducted, to compare two types of railway ballast: Calcite and Kieselkalk. Focus lies on the characterisation of particle angularity using 3D scanner data. In the literature, angularity is often characterised using 2D data, as these types of data are easier to collect. 3D scanner data contain a vast amount of information (e.g. curvatures) which can be used for shape analysis and angularity characterisation. Literature approaches that use 3D data are often not thoroughly tested, due to a lack of test cases. In this work, two new curvature-based angularity indices are introduced and compared to one from the literature. Analytical test bodies with shapes ranging from spherical towards cubic are used for a first plausibility test. Then, 3D scans of ballast stones are compared to artificially rounded meshes. Only one out of three evaluated angularity indices seem to be suited to characterise angularity correctly in all of the above tests: the newly introduced scaled Willmore energy. A complete shape analysis of the scanned ballast stones is conducted and no difference between the two types of ballast can be seen regarding form, angularity, roughness, sphericity or convexity index. These findings of shape analysis are set in the context of previous works, where experimental results and DEM simulations of uniaxial compression tests and direct shear tests were presented for the same ballast types.

Simulating the wheel/rail contact of a train is traditionally conducted using small-scale twin-disc testing. Here an emerging alternative method, the "high pressure torsion" (HPT) test is introduced. This has advantages over twin-disc testing as only one cycle is used and it is easier to control third body layers during testing. Shear stress data generated is useful on its own, but can also be used to parameterise analytical prediction tools such as the extended creep force model (ECF), which produces creep-force curves from HPT data. This can then be validated against field data to develop a full-scale predictive capability. A detailed account of the HPT methodology development and the procedures for running a typical test are presented along with case studies that illustrate the typical output of HPT testing.

This paper reviews the academic and industrial research conducted into the practical aspects of using abrasive particles in the wheel/rail contact, such as: wheel/rail isolation, surface damage, and the application of the particles into the contact. Abrasive particles are applied to the wheel/rail contact to restore traction when low adhesion situations exist on the rail head; this process is referred to as "sanding" because sand particles are the preferred particle type in the railway industry. This aspect of sanding was covered in another sanding review. Currently, particles are applied either by firing dry particles into the wheel/rail contact via an air stream or by suspending them in a gel which can be applied using train-borne or trackside methods. The papers looked at in this review were scrutinised using a gap analysis method which grades each paper based on seven criteria, these criteria assessed: whether the papers had been peer reviewed, whether the conclusions matched the results, the range of testing scales used, and the presence of fundamental modelling work. When the findings of the research in this review were analysed, it was apparent that the negative effects of sanding (damage and isolation) have not been researched in much depth compared to its positive effects (adhesion restoration and leaf layer removal). In addition, the academic research that has been conducted has not been taken forward by industry, and industry research has not been studied in more depth by academia, suggesting a communication gap between the two branches of research; this was also the case for research into application methods.

In recent years several high profile projects have questioned the repeatability and validity of scientific research in the fields of psychology and medicine. In general, these studies have shown or estimated that less than 50% of published research findings are true or replicable even when no breaches of ethics are made. This high percentage stems from widespread poor study design; either through the use of underpowered studies or designs that allow the introduction of bias into the results. In this work, we have aimed to assess, for the first time, the prevalence of good study design in the field of tribology. A set of simple criteria for factors such as randomisation, blinding, use of control and repeated tests has been made. These criteria have been used in a mass review of the output of five highly regarded tribology journals for the year 2017. In total 379 papers were reviewed by 26 reviewers, 28% of the total output of the journals selected for 2017. Our results show that the prevalence of these simple aspects of study design is poor. Out of 290 experimental studies, 2.2% used any form of blinding, 3.2% used randomisation of either the tests or the test samples, while none randomised both. 30% repeated experiments 3 or more times and 86% of those who repeated tests used single batches of test materials. 4.4% completed statistical tests on their data. Due to the low prevalence of repeated tests and statistical analysis it is impossible to give a realistic indication of the percentage of the published works that are likely to be false positives, however these results compare poorly to other more well studied fields. Finally, recommendations for improved study design for researchers and group design for research group leaders are given.

Low adhesion between a train's wheel and the rail can cause performance and safety issues, costing the UK rail industry similar to 345 pound m/annum. Sand is applied to the wheel/rail interface to increase traction when low adhesion conditions are present. In order to improve performance, an understanding of how particles are entrained into and act within the interface is needed. This paper outlines a particle characterisation framework and applies it to sands used in the rail industry: Leighton Buzzard (LB), Central European (CE), and Derbyshire Youlgreave (DY) sand. The largest difference found in this framework was between the sand's particle size, LB being largest, then CE, then DY. A high pressure torsion rig measured traction when the sands were applied to dry, wet, and leaf extract contaminated conditions, the latter two representing low adhesion conditions. All sands had no impact on wheel/rail adhesion in dry conditions; in low adhesion conditions DY had little influence, whereas LB and CE were found to increase traction. Particles in dry conditions had no effect on test specimen surface roughness, whereas roughness increased when sand was applied in low adhesion conditions. The developed characterisation framework provides a platform for assessing prospective adhesion enhancing particles.

This paper aims to provide a comprehensive review into academic and industrial research concerning the use of particles as a means of recovering adhesion when low adhesion conditions exist within the wheel/rail contact. The most common particle used is sand, generally possessing a high silica content, usually between 0.85 and 1.4 mm in size. Sand is currently being applied in low adhesion conditions in two ways: firing the sand into the wheel/rail contact by means of a train-borne sanding system, or by suspending it in a gel and applying it to the rail head with either a train-borne system or using a wayside applicator. Sanding has been used for a long time, but little research has been conducted on the sanding system, with research shortfalls surrounding particle properties and models of the mechanical behaviour of the particles' effect in the wheel/rail contact. This paper includes a gap analysis method whereby previous research has been categorised based on seven criteria, designed to help assess the papers. The research was then graded as either 'A', 'B' or 'C' with 'A' grade research representing peer-reviewed work conducted across a range of scales or with an aspect of modelling. Most academic research was of 'B' grade due to the lack of multiple scales or modelling, which was also lacking in industrial research, but due to the lack of peer review most industrial research received a 'C' grade. The review also found there was evidence to suggest a lack of linkage between academia and industry with regards to taking sanding research findings forward. Additionally, this review helps clarify what future work is needed to optimise the sanding system to best recover adhesion and remove lubricating layers in the wheel/rail contact.