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

The mechanism of rail short pitch corrugation has remained elusive in the past. The damage mechanisms of the corrugation are reported to be differential wear or plastic deformation. The former has been extensively studied, while the plastic deformation, especially under multiple wheel passages has been seldom studied. To uncover the facts behind it, an integrated dynamic vehicle-track model with the rail material treated in elasto-plasticity is developed. Further, a novel method which can simulate the material deformation under cyclical axle loads is proposed. This method is used to study the rail material response at corrugation. Our research found that for the cases studied, the rail material undergoes cyclic plastic deformation at corrugation peaks only for a limited number of cycles (2–4 cycles) before reaching the elastic shakedown limit. After that, no further residual stresses and strains accumulate. The plastic deformation at corrugation peaks weakens the corrugation amplitude, serving as an early corrugation attenuation mechanism. Conversely, work-hardening at corrugation peaks increases wear resistance at those peaks, promoting corrugation in the long term. The explanation of the corrugation development process under the interplays of the plastic deformation and wear has been validated by field corrugation data. Additionally, we propose a wear coefficient in the wear model to account for the work-hardening and change in the wear resistance. Experimental results of the hardness distribution show the similar characteristics to the numerical results.

@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

Rolling contact fatigue (RCF) is an important form of damage in wheels and rails that typically has surface and subsurface cracks. Squats are one of the major RCF defects that occur in the running band of rails and can create high dynamic forces and cause rail fracture if they are not detected and treated in time. In the current research, three advanced methods are developed in order to obtain a better understanding of the formation mechanism of RCF defects and, especially, squats in rails: 1) A new thermomechanical tool for numerically modelling the wheel–rail contact, 2) A new experimental setup for physically simulating the wheel–rail interaction and 3) A new computed tomography (CT) procedure for characterizing the wheel–rail defects. The first part presents a coupled thermomechanical modelling procedure for the wheel–rail contact problem and computes the flash–temperature and stress–strain responses when thermal effects are included. The contact temperature and thermal stresses could be driving factors for squats initiation. A three–dimensional (3D) elasto–plastic finite element model is built considering the wheel–track interaction. When the wheel is running on the rail, frictional energy is generated in the contact interface. The model is able to convert this energy into heat by using a coupled thermomechanical approach. The numerical models calculate the flash–temperature and thermomechanical stresses in the wheel and rail. In the second part, a new downscale test setup is designed and built for investigating the interaction between wheel and rail, especially under impact–like loading conditions, which are supposed to be often associated with rail squats. The test rig is intended to remedy the lack of dynamic similarity between the actual railway and the existing laboratory testing capability, by considering the factors that contribute to high–frequency dynamics of the wheel–track system. This part of the thesis further presents the results of some experiments carried out using the newly–built setup to verify the ideas behind its development. The third part presents the development of a computed tomographic (CT) scanning technique to reconstruct the 3D geometry of the RCF cracks in the railhead. Squat defects are associated with complex crack networks at the subsurface. Sample rails having squats of different severities are taken from the Dutch railway network. Various specimens of different sizes are prepared and investigated with the CT scanner. A detailed procedure of the CT experiment and post–processing is described. The proposed 3D visualization method, together with the necessary geometric definitions, is then used for enabling effective measurement and characterization of the squat cracks. Based on this research, the main new insights into the formation of rail squats are as follows: i) the WEL formation via martensitic phase transformation turns out to be possible; this is confirmed through the thermomechanical wheel–rail contact modelling; ii) the impact–like loading conditions and high–frequency dynamic characteristics of the wheel–track system appear to be essential for the squat formation; this is confirmed through the vehicle–track testing using the new test rig; and iii) the occurrence of different crack orientations followed by the primary and secondary V–shaped cracks turns out to be important in the squat formation; this is confirmed through the CT scanning and metallographic observations. @en

This study investigates the angles θ₁, θ₂, and θ₃ that squat crack faces form with respect to three orthogonal planes: the rail top, the longitudinal-vertical cross-section and the lateral-vertical cross-section. Rail samples with squats of various severities are obtained from the field. Their three-dimensional crack networks are reconstructed using CT (computed tomography) scanning and serial cutting. A 3D visualization method, together with the necessary geometric definitions, is developed for enabling effective measurement and characterization of the squat cracks. It is found that the cracks can be characterized by four orientations (T1 – T4). The variation ranges of the crack angles are determined for each orientation that satisfies 132° ≤θ₁ ≤ 150°, 6° ≤θ₂ ≤ 36° and 67° ≤θ₃ ≤ 81°. By investigating the occurrence frequency of the orientations, it is found that T4 and T1 together form the primary V-shaped cracks of the squats, and T2 and T3 together form the secondary V-shaped cracks. A finite element modelling of the wheel-track system, in combination with contact mechanics and multi-axial fatigue analysis, successfully relates the stress state to the RCF cracks.

@en

This research presents a coupled thermomechanical modelling procedure for the wheel-rail contact problem and computes the flash-temperature and stress-strain responses when thermal effects are present. A three-dimensional elasto-plastic finite element model was built considering the wheel-track interaction. When the wheel is running on rail, frictional energy is generated and converted into heat. To evaluate the contribution of thermal effects and plasticity, five different material models were studied among them TEPS was nonlinear and temperature-dependent including thermal softening. Discussions were made on the effect of solution type and material type. The rail temperature, calculated for a critical creepage case, confirmed the potential of martensitic phase transformation. Thermal effects were also important at lower creepages, where a synchronization effect causes earlier damage.

@en

In this paper, a decision support approach is proposed for condition-based maintenance of rails relying on expert-based systems. The methodology takes into account both the actual conditions of the rails (using axle box acceleration measurements and rail video images) and the prior knowledge of the railway track. The approach provides an integrated estimation of the rail health conditions to support the maintenance decisions for a given time period. An expert-based system is defined to analyse interdependency between the prior knowledge of the track (defined by influential factors) and the surface defect measurements over the rail. When the rail health conditions is computed, the different track segments are prioritized, in order to facilitate grinding planning of those segments of rail that are prone to critical conditions. In this paper, real-life rail conditions measurements from the track Amersfoort-Weert in the Dutch railway network are used to show the benefits of the proposed methodology. The results support infrastructure managers to analyse the problems in their rail infrastructure and to efficiently perform a condition-based maintenance decision making.@en

In this paper, we present a condition-based maintenance decision method using knowledge-based approach for rail surface defects. A railway track may contain a considerable number of surface defects which influence track maintenance decisions. The proposed method is based on two sets of maintenance decision factors i.e. (1) defect detection data and (2) prior knowledge of the track. A defect detection model is proposed to monitor surface defects of the track including squats. The detection model relies on track images and Axle Box Acceleration (ABA) signals to give both positions of severity and defects. To acquire the prior knowledge, a set of track monitoring data is selected. A fuzzy inference model is proposed relying on the maintenance factors to give the track health condition in a case study of the Dutch railway network. The proposed condition-based maintenance model enables infrastructure manager to prioritize critical pieces of the track based on the health condition.@en

A new downscale test rig is developed for investigating the contact between the wheel and rail under impact-like loading conditions. This paper presents the development process of the setup, including review and synthesis of the potential experimental techniques, followed by scalability, mechanical and operational analysis of the new setup. The new test rig intends to remedy the lack of dynamic similarity between the actual railway and the existing laboratory testing capability, by taking into account the factors that contribute to high-frequency dynamics of the wheel-track system. The paper first reviews the functionalities of the existing test techniques in the literature. Based on this survey, the category of the scaled wheel on the rail track ring is chosen. Afterwards, three potential alternatives are identified under the chosen category and the optimum mechanism is achieved through finite element modelling and analysis of the structures. A downscale test rig, consisting of multiple wheel components running over a horizontal rail track ring, effectively fulfilled the requirements needed for analogical testing of the wheel-rail contact behaviour. The new test rig is a unique experimental setup due to the involvement of high-frequency dynamic vibrations in the wheel-track system and analogy of the incorporated elements and loading to those of the real-life system. This paper further presents the results of some real experiments carried out using the newly-built setup to support substantial ideas behind its development.@en

Rolling contact fatigue (RCF) defects are associated with complex crack networks at the subsurface. A computed tomographic (CT) scanning technique has been developed to reconstruct the 3D geometry of the RCF cracks in the railhead. Sample rails having squats of different severities were taken from the Dutch railway network. Four specimens of different sizes were prepared and investigated with the CT scanner. The detailed procedures of the CT experiment and post-processing work were described. A sequence of high-quality X-ray images was collected during each scan. These 2D images were combined to construct the 3D visualization of the specimen. Various image processing tools were applied to extract and rebuild the internal crack geometries, thus allowing the crack networks to be differentiated from the bulk steel. For validation, the CT results were compared with metallographic observations of the rail surface for all the defects and the vertical section when needed. Discussions were made regarding the proper size of the rail samples and severity of the squats. According to the results, CT allows for a 3D visualization of RCF defects, providing high-quality data on the geometry of the internal cracks. By choosing the appropriate settings and specimen size, CT could accurately reconstruct the squat cracks at different growth stages. This research shows the potential of the CT technique as an intermediate detection and characterization tool among the methods for detecting macro cracks and those for characterizing micro/nano cracks. Finally, a practical specimen design and a detailed scanning procedure are proposed, based on the CT experiments performed in this research.

@en

Employing a coupled model for vehicle-track interaction, this paper presented the dynamic analysis of railway track system in the presence of weld irregularities with different geometric profiles in two parallel rails. A 3D model of a standard railway wagon running on a ballasted track system is used to examine the vibrations of the system under track irregularities. Four samples of measured rail profiles at rail welds are considered as the source of dynamic excitations, in which the profiles of parallel rails are unequal. Hence, the individual profiles of left/right rails are imported into the numerical model, taking into account the nonuniform conditions of rail welds in both sides of the railway track. The dynamic responses of the track system caused by irregularities at welds are studied by performing the time-domain vibration analysis. The time histories of the nodal responses under various weld scenarios (measured in an actual railway track site) are recorded. Finally, the peak responses of dynamic forces along the track are determined for left/right rails. The results of dynamic simulations in various weld scenarios are compared and the values of dynamic amplification factors (DAFs) are evaluated. Such findings provide an estimation for the levels of dynamic forces occurring under impact loading conditions of nonuniform rail welds in left/right rails.

@en

A three-dimensional elasto-plastic finite element (FE) tool has been developed to study the thermo-mechanical originations of squats in rails. The initiation process of rail squats is in principle related to the response of materials against cyclic loading. The thermo-mechanical issues in this process are not yet clear, due to the complicated wheel-rail interaction and the variability of the loads, parameters and material properties. This paper examines the role of thermo-mechanical-related causes on the formation mechanism of squats using numerical modelling. Assuming a range of material behaviour, thermal softening and temperature dependencies, the sliding wheel-rail contact problem is investigated using a three-dimensional FE tool. The temperature rise and thermal stress due to the frictional heat, generated at the contact surface of the wheel and rail, are taken into account. The magnitudes and distributions of thermo-mechanical stresses on rail surface are calculated for realistic loading conditions. Particular attentions are paid to the temperature rise, residual stress and plastic strain with thermal effects under high tractive forces. The outputs are used to scrutinise if the so-called white etching layer is formed as a result of the temperature rise, needed for structural phase change and to consider the role of plastic deformations.@en

White etching layer (WEL) is a frequently observed microstructural phenomenon in rail surface, formed during dynamic wheel/rail contact. It is considered as one of the main initiators for rolling contact fatigue cracks. There are several hypotheses for the formation mechanism of WEL. However, due to the complicated wheel/rail contact conditions, none is directly proven. Currently, the most popular hypotheses refer to either formation of martensitic WEL by phase transformations or formation of nanocrystalline ferritic WEL by severe plastic deformation. In this work, WEL formation by martensitic transformation in R260Mn grade pearlitic rail steel was simulated by fast heating and quenching experiments. Microstructural characteristics of the simulated WEL and WEL observed in a field rail specimen were characterized by microhardness, optical microscopy, scanning electron microscopy and electron backscatter diffraction. Microstructures of the two WELs were compared and similarities in morphology were identified. Numerical simulation shows the possible temperature rise up to austenitizing temperatures. Combining comparisons of experimental simulation with observation of WEL in the rail and the thermodynamic calculations, the hypothesis for WEL formation via martensitic transformation is supported.

@en