As a critical factor in the degradation of rails and wheels, wheel-rail contact heat has been investigated with various analytical and numerical methods. However, the predicted temperature distributions and thermal loads have not been directly validated through measurements due to the challenges associated with accurate measurements. This study employs an infrared camera to measure the temperature variation at the wheel-rail contact under various slip ratio conditions in an in-house wheel-rail dynamic contact test rig. Wheel braking is replicated, and a wheel flat is generated. The temperature field of the contact interface is measured and analyzed, revealing the heating and cooling processes before and after the formation of the wheel flat. The results demonstrate that the contact temperature between the wheel and rail progressively increases with increasing slip ratio until a flat is formed. Notably, at a slip ratio of 8.3%, the observed contact temperature reaches 337.2 ℃ and then rises to 432.8 ℃ at a higher slip ratio of 15.9%. When the wheel flat is generated at a slip ratio of 20.4%, the observed contact temperature between the wheel and rail reaches 652.4 ℃. After the formation of the flat, the contact temperature initially decreases due to more wheel material of lower temperature entering into the contact and rises again with the increase of slip ratio. These measurement findings are valuable for calibrating and validating simulation models and investigating thermal damage related to wheel-rail interactions.@en

This study experimentally and numerically investigated wheel–rail rolling contact fatigue (RCF), focusing on the initiation mechanisms of head check (HC). The experimental study was conducted using V-Track, a scaled test rig developed at TU Delft that is able to simulate real-life wheel–rail contact with controllable contact geometries and loading conditions. Ratcheting and HCs were generated on the V-Track rails with wheel–rail frictional rolling contact loading for up to 60,000 cycles. Rail samples with HCs were then examined with a microscopic analysis focusing on the R260MN steel grade. The boundary element method (BEM) and finite element method (FEM) were then applied to calculate wheel–rail contact-induced stress states in and below the rail surface under the same contact conditions as the experiment. The rail surface shear stresses calculated with BEM exhibited a strong correlation to the ratcheting observed within the rail running band in the microscopic analysis. Moreover, the plastic flows and cracks outside the running band identified by the microscopic analysis were correlated to the rail surface stresses, especially outside the contact patch, and subsurface stresses calculated with FEM: the results suggested that the accumulation of residual stresses could also contribute to plastic flow and the consequent initiation of cracks outside the running band.@en

Friction/adhesion management in railway networks is a challenge for infrastructure managers and railway operators. Friction/adhesion at the wheel–rail interface influences the braking and traction performance of railway vehicles and the formation of wheel and rail defects. A minimum level of friction/adhesion must be guaranteed to ensure appropriate braking and traction of vehicles, whereas high friction/adhesion increases wear and rolling contact fatigue (RCF) of wheels and rails, noise emissions and carbon footprint (transportation energy consumption). A crucial part of friction/adhesion management is to reliably measure the wheel–rail friction levels and creepage. A train-borne tribometer is desired because the wheelrail friction level depends on, among others, the normal contact load and speed.

A light vehicle will thus experience adhesion different from a heavy train, and the accuracy of hand-pushed tribometers is adversely affected by scaling and low speed. Aiming to contribute to the development of a train-borne tribometer for friction/adhesion management, this study conducted a comprehensive lab test in the V-Track in the Railway lab of TU Delft. The V-Track is a downscaled wheel-rail contact test rig consisting of a 4-meterdiameter ring track and 1~4 wheel assemblies running over it with well-controlled and measurable normal load and friction forces. The coefficient of friction (COF) was measured with two schemes: 1. Increase the angle of attack (AoA) to get friction saturation in the lateral direction and 2. Increase the traction/braking torque of the wheel to get friction saturation in the longitudinal direction. The wheel-rail contact forces in the three directions, AoA, wheel rolling speed and rotational/circumferential speed, and traction/braking torques were measured and analysed to obtain the COF of the V-Track.@en

The Coefficient of Friction (CoF) is an important parameter affecting acceleration and braking behavior of trains, and consequently the inter-train distance and utilization of track. To optimize operation schedules, maximize railway capacity, and realize automatic train operations, reliable measurements of the CoFs experienced by in-service trains are desirable. In this study, a train-borne measurement approach is proposed based on a torque modulation concept. It involves superimposing a small-amplitude sinusoidal signal on the motor torque. Because the wheel-rail friction force acts as variable damping, it causes a phase difference between the angular velocity response of the wheelset and the input modulated torque signal. This phase difference can be used to determine the creep coefficient, i.e., the slope of the creep curve, and then to estimate the CoF in combination with the measured Coefficient of Adhesion (CoA), i.e., the ratio between the wheel-rail friction force and normal load. Simulations of torque modulation with VI-Rail are conducted. Variation of phase difference with the increase of the modulated torque is derived theoretically and compared with numerically obtained results using the VI-Rail multibody dynamics model under different CoF conditions. The good agreement between the results indicates the effectiveness of the proposed measurement concept.@en

Various models for simulating rail ratcheting behaviour were developed to study rolling contact fatigue (RCF) damage in rails. However, limitations remain in terms of the accuracy of wheel–rail contact modelling and computational efficiency of the cyclic loading simulation. This study developed an efficient 3D finite element (FE) procedure to simulate ratcheting in rails subjected to numerous load cycles. The procedure simulates a wheel rolling repeatedly over a rail section with updated stress–strain states, enabling automatically executed cyclic loading simulation given a predefined number of cycles. To ensure the accuracy of the contact modelling, the effect of meshing schemes on subsurface stress distribution was examined. In addition, the FE contact model with the selected meshing scheme, which balances accuracy and computational efficiency, was verified against the widely accepted CONTACT program. Subsequently, a non-linear kinematic hardening (NLKH) steel material was used in the FE model for ratcheting simulations with up to 100 wheel-loading cycles. The rail surface and subsurface stress states were replicated under partial-slip wheel–rail rolling contact conditions with traction coefficients of 0.10, 0.20 and 0.35, respectively. The ratcheting behaviour was extensively analysed in terms of plastic deformation, contact patch evolution, and ratcheting rates. The simulated plastic deformation was found to alter the contact geometry and thus contact stresses, which in turn affect further accumulation of plastic deformation and subsequent ratcheting strains. These findings highlighted the importance of considering the interplay between the rail ratcheting behaviour of the rail and evolving contact conditions for predicting ratcheting and RCF damage in rails.

@en

A review of prediction methods for wheel-rail rolling contact was first prcsented. It is found that thc 3-D transient rolling contact model dcveloped using the explicit finite elemcnt method seems to be the most suitable approach for analyses of transient wheel-rail rolling-sliding-jumping contact at 400 km/h, or at higher speeds, and in the presence of short/medium-wave irregularities. Therefore, a transient rolling contact model was developed to simulate the dynamic curving of a wheelset on a typical curved track in the time domain. Harmonie or ideal rail corrugation was taken as the typical short/medium-wave irregularity to study its influences on transient rolling-sliding-jumping contact, with a wave-length ränge of 30-210 mm considered on the basis of field observations of high-speed wheels and rails in Operation. Be-sides corrugation geometry, corrugation oecurring on the high and/or the low rails was also taken into aecount properly, with the speed up to 500 km/h. Considering dynamic unloading caused by dynamics of low and medium frequency, the critical sizes of short/medium-wave irregularities were determined, at which the wheel-rail contact loss just oecurred. Fi-nally, proposals for management of wheel-rail short/medium-wave irregularities were discussed from the aspects of impor-tance, rationality and application limitations, and compared with the corrugation limit recommended for rail grinding Operations in Management Measures of High-speed Railway Rail Grinding.

@en

Polygonal wear is a type of damage commonly observed on the railway wheel tread. It induces wheel-rail impacts and consequent train/track components failure. This study presents a finite element (FE) thermomechanical wheel-rail contact model, which is able to cope with the three possible generation and development mechanisms of polygonal wear: initial defects, thermal effect, and structural dynamics. The polygonal wear-induced impact contact and further development of wear are simulated. The simulated elastic contact solutions are verified against the program CONTACT. Different material properties (elastic, elasto-plastic and elasto-plastic-thermo, i.e. with thermal softening) and initial polygonal profiles are then applied to the FE model to investigate the influence of wheel/rail material and wear amplitude on wheel-rail contact stress and wear development. The simulations indicate that the wheel-rail impact-induced temperature may reach up to 362 ℃ at the contact interface, and the high temperature at the contact area influences wheel-rail contact stress and wear depth.

@en

A time-domain finite element model is developed to study the transient rolling contact of a driving wheelset over a curved track with Low Adhesion Zones (LAZs) shorter than 1.0 m. LAZs on one rail, i.e., unilateral LAZs occurring more likely, is treated for a speed up to 500 km/h. Structural vibrations of wheelset are analyzed to explain the transient contact forces, creepages and the resulting irregular wear. LAZs on high rails are found more detrimental than those on low rails. The results explain the occurrence of flats and rolling contact fatigue in bad weather, although significant wheel idling is absent.

@en

Polygonal wear is a common type of damage on the railway wheel tread, which could induce wheel-rail impacts and further components failure. This study presents a finite element (FE) thermomechanical model to investigate the causes of wheel polygonal wear. The FE model is able to cope with three possible causes of polygonal wear: thermal effect, initial defects, and structural dynamics. To analyse the influences of the three causes on wheel-rail contact stress and wear depth, different material properties (i.e., elastic, elasto-plastic, thermo-elasto-plastic with thermal softening), and wheel profiles (i.e., round and polygonal) were used in the FE model. The simulation indicates that a high temperature up to 264.20 ℃ could be induced by full-slip wheel-rail rolling contact when the polygonal profile is used. The thermal effect, similar to that induced by tread brake, may then have a significant influence on wheel-rail contact stress and wear depth. In addition, the involvement of initial defects, i.e., polygonal profile, causes wheel-rail impact contact and remarkably increases the contact stress and wear. By reliably considering all the three possible causes, the proposed FE model is believed promising for further explaining the generation mechanisms of wheel polygonal wear.

@en

A better understanding of wheel-rail dynamic interaction is necessary for the capacity increase of railway transportation. This chapter briefly introduces the consequences, modeling and detection methods of wheel-rail dynamic interaction, and maintenance of the weak track spots where wheel-rail dynamic interaction often occurs. Wheel-rail dynamic interaction causes high-amplitude vibration and noise and accelerate structural degradation. Owing to the capabilities of handling nonlinear material properties and arbitrary contact geometries and considering dynamic effects, the explicit finite element approach is proposed to model wheel-rail dynamic interaction. Three numerical examples are presented to simulate wheel-rail impacts, frictional instability, and flange contact, respectively; and Rayleigh waves are observed in the simulation results. Wheel-rail dynamic interaction-related problems can be detected with the measurements of wheel-rail contact force, axle box acceleration, and track vibration. The weak track spots can be maintained by stiffness, geometric controls, and friction management.

@en

By modifying friction to the desired level, the application of friction modifiers (FMs) has been considered as a promising emerging tool in the railway engineering for increasing braking/traction force in poor adhesion conditions and mitigating wheel/rail interface deterioration, energy consumption, vibration and noise. Understanding the effectiveness of FMs in wheel–rail dynamic interactions is crucial to their proper applications in practice, which has, however, not been well explained. This study experimentally investigates the effects of two types of top-of-rail FM, i.e. FM-A and FM-B, and their application dosages on wheel–rail dynamic interactions with a range of angles of attack (AoAs) using an innovative well-controlled V-track test rig. The tested FMs have been used to provide intermediate friction for wear and noise reduction. The effectiveness of the FMs is assessed in terms of the wheel–rail adhesion characteristics and friction rolling induced axle box acceleration (ABA). This study provides the following new insights into the study of FM: the applications of the tested FMs can both reduce the wheel–rail adhesion level and change the negative friction characteristic to positive; stick–slip can be generated in the V-Track and eliminated by FM-A but intensified by FM-B, depending on the dosage of the FMs applied; the negative friction characteristic is not a must for stick–slip; the increase in ABA with AoA is insignificant until stick–slip occurs and the ABA can thus be influenced by the applications of FM.

@en

With dynamic behaviour different from that of traditional discretely supported tracks, continuously supported embedded rail systems (ERSs) have been increasingly used in railway bridges, level crossings, trams, and high-speed lines. However, studies on ERSs have been limited, and none of them have addressed the wheel-rail impact-induced dynamic response, although wheel-rail impact is a main cause of ERS degradation. This paper studies, numerically and experimentally, the wheel-rail impact at an insulated rail joint (IRJ) used in the ERS. As a weak spot of the track, the IRJ results in discontinuities in the track support stiffness and wheel-rail contact geometry. This study first develops an explicit finite element model to simulate the vibration responses of the IRJ in the ERS when excited by a hammer and passing wheel loads. The simulated dynamic behaviours (represented by the hammer-excitation frequency response function) at a frequency up to 5 kHz and a wheel-rail impact vibration frequency up to 10 kHz are then validated with a field hammer test and a train pass-by measurement, respectively. Both the experimental study and numerical modelling reveals that the major frequencies of the impact vibration at the IRJ in the ERS depend mainly on geometric irregularities in the IRJ region and the train speed, rather than on the resonances of the track structure, as in the case of the discretely supported IRJ. This finding is meaningful to the engineering practice because it indicates a continuously supported IRJ in the ERS is more impact resistant, especially when the IRJ geometry is adequately maintained, e.g. by timely grinding.@en

The modeling of dynamic frictional rolling contact is crucial for accurately predicting behavior and deterioration of structures under dynamic interactions such as wheel/rail, tire/road, bearings and gears. However, reliable modeling of dynamic frictional rolling contact is challenging, because it requires a careful treatment of friction and a proper consideration of the dynamic effects of the structures on the contact. This study takes the wheel-rail dynamic interaction as an example to systematically explore the core algorithms for the modeling of dynamic frictional rolling contact by way of explicit finite element analyses. The study also theoretically demonstrates that the explicit finite element method handles nonlinearities in friction, material properties, arbitrary contact geometries and boundary conditions, and fully couples the calculation of frictional rolling contact with the calculation of high-frequency structural dynamics. An indirect validation method for dynamic contact solutions is proposed. To promote the broad use of the method, this paper proposes a detailed procedure for establishing robust wheel-rail dynamic interact tion models and obtaining dynamic contact responses. The proposed procedure can also be applied to the modeling of dynamic interactions occurring to tire-road, bearings and gears.

@en

Complex frictional rolling contact and high-frequency wheel dynamic behavior make modeling squeal greatly challenging. The falling-friction effect and wheel mode-coupling behavior are believed to be the two main mechanisms that generate unstable wheel vibration and the resulting squeal noise. To rigorously consider both mechanisms in one model, we propose an explicit finite element (FE) model to simulate wheel-rail dynamic frictional rolling. Wheel-rail squeal-exciting contact is investigated with considerations of dynamic effects, unsteady lateral creepage and velocity-dependent friction. With the inclusion of the dynamic effects in the contact solution, large-creepage-induced waves, which share features with Rayleigh waves, are discovered. The solutions of the dynamic contact calculated using the proposed model indicate that the explicit FE method is able to reproduce the falling-friction effect. The transient analyses of wheel-rail frictional rolling with wheel lateral creepage show the coupling of the axial and radial dynamics of the rolling wheel model, suggesting that the explicit FE method can also reproduce the mode-coupling behavior. This study improves the understanding and modeling of squeal-exciting contact from the perspective of wheel-rail dynamic interaction.

@en

Recent finite element (FE) simulations have revealed the generation and propagation of waves in rail surfaces induced by wheel-rail frictional rolling. These waves have rarely been addressed in the literature. This paper presents an in-depth analysis of these waves, aiming to give new insights into the contact mechanics, a research area in which waves have generally been ignored. The study first categorises the simulated contact-induced waves according to their generation mechanisms as impact-induced, creepage-induced and perturbation-induced waves. The link between the generation of perturbation-induced waves and the stick-slip contact mechanism is then explored. Next, by examining the rail surface nodal motion that forms the wave, the creepage-induced wave is demonstrated to be a Rayleigh wave; this result also shows that the explicit FE method can effectively simulate physical contact-induced waves and provide reliable dynamic contact solutions. Finally, FE modelling is presented to investigate the effects of surface cracks on the waves, which may contribute to wave-based crack detection.

@en

This paper presents an analysis of the transient contact solutions of wheel-rail frictional rolling impacts calculated by an explicit finite element model of the wheel-insulated rail joint (IRJ) dynamic interaction. The ability of the model to simulate the dynamic behavior of an IRJ has been validated against a comprehensive field measurement in a recent paper (Yang et al., 2018). In addition to the measured railhead geometry and bi-linear elastoplastic material model used in Yang et al. (2018), this study adopts a nominal railhead geometry and an elastic material model for the simulations to provide an overall understanding of the transient contact behavior of wheel-IRJ impacts. Each simulation calculates the evolution of the contact patch area, stress magnitude and direction, micro-slip distribution, and railhead nodal vibration velocity in the vicinity of the joint during the wheel-IRJ impacts. The simulations apply small computational and output time steps to capture the high-frequency dynamic effects at the wheel-IRJ impact contact. Regular wave patterns that indicate wave generation, propagation and reflection are produced by the simulations; this has rarely been reported in previous research. The simulated waves reflect continuum vibrations excited by wheel-rail frictional rolling and indicate that the simulated impact contact solutions are reliable.@en

This paper presents an analysis of the transient contact solutions of wheel-rail frictional rolling impacts calculated by an explicit finite element model of the wheel-insulated rail joint (IRJ) dynamic interaction. The ability of the model to simulate the dynamic behavior of an IRJ has been validated against a comprehensive field measurement in a recent paper (Yang et al., 2018). In addition to the measured railhead geometry and bi-linear elastoplastic material model used in Yang et al. (2018), this study adopts a nominal railhead geometry and an elastic material model for the simulations to provide an overall understanding of the transient contact behavior of wheel-IRJ impacts. Each simulation calculates the evolution of the contact patch area, stress magnitude and direction, micro-slip distribution, and railhead nodal vibration velocity in the vicinity of the joint during the wheel-IRJ impacts. The simulations apply small computational and output time steps to capture the high-frequency dynamic effects at the wheel-IRJ impact contact. Regular wave patterns that indicate wave generation, propagation and reflection are produced by the simulations; this has rarely been reported in previous research. The simulated waves reflect continuum vibrations excited by wheel-rail frictional rolling and indicate that the simulated impact contact solutions are reliable.@en

The modelling of wheel-rail dynamic interactions is crucial for accurately predicting wheel/track deterioration and dynamic behaviour. A reliable wheel-rail dynamic interaction model requires a careful treatment of wheel-rail frictional rolling contact and a proper consideration of dynamic effects related to the contact. Since the wheel-rail interaction due to the frictional rolling contact significantly influences the vehicle dynamics and stability, and the dynamic effects involved in wheel-rail interactions can be increased by wheel rail highspeed rolling, a systematic study of wheel-rail dynamic interactions is highly desired within the context of booming high-speed railways.@en

By considering real wheel and track geometry and structures, as well as their coupled interaction, an explicit finite element method is applied to simulate the transition of wheel–rail rolling from single-point tread contact to two-point tread-flange contact. The evolutions of the contact position, stress magnitude and direction, adhesion–slip distribution, and wheel–rail relative velocities in the two contact patches are considered to investigate the transient dynamic effects during the contact transition. Important findings include that the transition to two-point contact can result in friction saturation and excite waves in the contact, causing local intensification and relaxation of compression, as well as turbulence of the micro-slip; the local relative velocity in the contact patch is a good measure of the dynamic effects@en