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Rolling contact fatigue (RCF) cracks online detection using eddy current testing (ECT) is in urgent demand. However, RCF cracks detection and evaluation in this way under moving conditions remains challenging due to the velocity effect in ECT. This paper aims to study the response of a transmitter-receiver eddy current probe to cracks under moving conditions and evaluate the depth and inclination angle of RCF cracks. In this paper, a high-speed eddy current testing system is developed to experimentally investigate the influence of coil gap, detection speed, and the lift-off on the eddy current probe's response under moving conditions. In addition, the temporal and amplitude features of the eddy current signal are extracted to characterise the depth and inclination angle of the RCF cracks. The experimental results indicate the eddy current probe's response can be improved by increasing the coil gap (coil centre distance) suitably, which can be done to compensate for the attenuation of the eddy current signal caused by detection speed and lift-off. The probe's response hardly changes with an increase in detection speed when the driver and pick-up coils of the eddy current probe completely overlap. The crack depth and inclination angle can be evaluated under moving conditions. [ABSTRACT FROM AUTHOR]

Here a comprehensive 3-D wheel-rail rolling contact finite element model was developed to investigate the influence of wheel-rail material properties on the dynamic wheel-rail interaction. The wheel-rail contact responses induced by two typical wheel tread defects (i.e. wheel flat and tread spalling) were discussed and compared to the responses of perfect wheel in this work. The dynamic wheel-rail responses of three typical material models involved (i.e. rigid, elastic, and elastic-plastic models) were investigated in terms of contact force, contact pressure/stress, and wheel/rail degradation. Furthermore, the strain rate levels during wheel-rail rolling contact at various train speeds were estimated with emphasis on the elastic-plastic model, which was helpful for determining strain rate range of wheel/rail material property tests. Finally, the influences of strain rate effect and initial fatigue damage of wheel/rail materials on dynamic wheel-rail responses were discussed. These results illustrated that the rigid model grossly overestimated wheel-rail impact force caused by wheel tread defects, while the elastic model overestimated wheel-rail contact pressure/stress, damage index, and frictional work. The strain rate effect could significantly increase Mises stress and inhibit plastic deformation of wheel-rail system, while the initial fatigue damage would reduce Mises stress and aggravate plastic deformation. [ABSTRACT FROM AUTHOR]

Surface initiated rolling contact fatigue (RCF) of gears and rolling bearings can lead to premature failure in mechanical transmission systems. The orientation of surface crack initiation closely depends on surface tractive force, while the mechanism leading to this dependence is still obscure. In this work, a numerical model is proposed to investigate the effect of friction on surface crack initiation in RCF. An inhomogeneous contact solver based on semi‐analytical method (SAM) is developed by combining the damaged‐coupled constitutive relation and the Eshelby's equivalent inclusion method. The damage accumulation and concurrent degradation of material properties are modeled with continuum damage mechanics (CDM). The effect of surface tractive force on the orientation of surface crack initiation is simulated and analyzed. Results indicate that the progressive evolution of damage under the cyclic loading is accompanied by the stress variation around the damaged area. The damaged area increases, and the crack initiation life decreases with an increasing coefficient of friction. Highlights: A new damage evolution model incorporating an inhomogeneous contact solver is developed.The possible causes of the orientation of surface crack affected by friction traction are analyzed.The simulated results of damage evolution and stress variation agree well with open literatures. [ABSTRACT FROM AUTHOR]

Aiming at early detection of subsurface cracks induced by contact fatigue in rotating machinery, the knowledge-based data analysis algorithm is proposed for health condition monitoring through the analysis of acoustic emission (AE) time series. A robust fault detector is proposed, and its effectiveness was demonstrated for the long-term durability test of a roller made of case-hardened steel. The reliability of subsurface crack detection was proven using independent ultrasonic inspections carried out periodically during the test. Subsurface cracks as small as 0.5 mm were identified, and their steady growth was tracked by the proposed AE technique. Challenges and perspectives of the proposed methodology are unveiled and discussed. [ABSTRACT FROM AUTHOR]

• Typical factors affecting the wheel-rail dynamic response are comprehensively considered to reveal the short-term load characteristics of frog under complex conditions. Meanwhile, based on the ellipsoidal Gaussian extrapolation theory, long-term wheel-rail load characteristics are obtained. • Combined with the strain energy fatigue parameter theory, a prediction method for the fatigue life of nose rail under complex conditions based on load spectrum preparation was proposed. • The location between the 30 mm and 35 mm sections of the nose rail has the shortest rolling contact fatigue crack initiation life and should be focused on during damage inspection. The rail profile irregular variation and wheel-rail relationships complexity at the gap in the frog enhance the wheel-rail impact forces, frequently resulting in rolling fatigue damage under complex cyclic loads. Therefore, it is particularly important to study the transient rolling contact load characteristics of fixed frog and to compile load spectrum considering various complex conditions for analyzing the fatigue damage of frog and extending the service life of structure. In this paper, a 3D transient FE model of wheelset and fixed frog is established, revealing rolling contact behavior of wheel-rail in frog area from both macro and mesoscopic perspectives. In order to investigate long-term load characteristics of wheel and rail under complex service conditions, the changes of line conditions, load conditions, rail wear state and other influencing factors in actual operation are taken into account. Based on elliptic Gaussian kernel density theory, a method for compiling the load spectrum of fixed frog nose rail is proposed, and the FP method is used for fatigue crack initiation life prediction of nose rail in fixed frog. The results show that the force state is more severe when passing through the frog in straight reverse direction, and resonance is prone to occur in the third and fifth vertical bending modes of the frog, as well as the fourth and sixth vertical bending modes of the stock rail. The increase in factors such as speed, axle load, wheelset lateral displacement towards frog side and wing rail wear will exacerbate wheel-rail impact and shorten frog service life. Different factors have an impact on the overall amplitude, oscillation center range, and maximum frequency load of the wheel-rail force in the long-term load spectrum of the frog. The fatigue crack initiation area of nose rail is mainly in 30 mm–35 mm sections, which is consistent with the on-site flaw detection results, verifying the accuracy of the calculation method developed in this paper. [ABSTRACT FROM AUTHOR]

While large accumulated plastic deformations occur in the rail surface layer where rolling contact fatigue cracks initiate, many available Low Cycle Fatigue (LCF) crack initiation criteria focus on small plastic strains. Accordingly, this paper evaluates available fatigue crack initiation criteria for highly shear-deformed R260 steels, reflecting the conditions in the surface layer of rails. Furthermore, modified crack initiation criteria are suggested. The evaluation is based on three different experiments: Large shear strain increments under varying axial loading (predeformation), strain-controlled LCF tests after some predeformation, and axial High Cycle Fatigue (HCF) experiments. For the predeformation, Finite Element (FE) simulations, with a large-strain plasticity model for cyclic and distortional hardening, provide predictions of the local stress and strain histories. A cross-validation procedure is used to assess the accuracy and reliability of both established and modified fatigue crack initiation criteria. The proposed modifications to one of the criteria show an improved fit to the experimental data. However, there is a tendency to overfitting, which can be improved by including more experimental data. • Three crack initiation criteria in rolling contacts of rail steel are evaluated. • Large strain cyclic axial-torsion experiments are used. • Finite element simulations are done using a large strain plasticity model accounting for anisotropy. • A cross-validation approach is used to assess the criteria. • Improved crack initiation criteria are proposed, leading to enhancements in fitting without worsening the prediction. [ABSTRACT FROM AUTHOR]

For premium grades (and rail surface treatments such as 'laser cladding') there is little plastic damage expected during typical operating conditions, and smooth surface contact models predict a long life. However, damage still develops, including rolling contact fatigue, affecting premium steels from multiple suppliers. A model is described predicting stress intensity factors and indicative crack growth rates for cracks driven by surface roughness stresses. Accompanying field data quantify the asperity separation and tip radii for rails from metro, mixed traffic and a freight line in ground or unground conditions. The results predict that in rough surface contact small surface cracks (50–100 μm, exceeding microstructural dimensions) can grow faster than crack removal by wear, even when smooth surface models predict no growth. The model has application in identifying how rail grinding marks or wheel roughness may be an important factor in determining rail life through raised stresses at the rail surface (leading to wear), and increased stress intensities and growth rates for incipient cracks. • Prediction of rough surface rolling contact fatigue crack stress intensity factors. • Relevant to harder steels where roughness is long lasting with little plastic flow. • Newly collected field data on rail surface roughness. • Cracks of 50–100 μm predicted to have growth faster than their removal by wear. • Crack growth predicted that would not be expected from smooth surface models. [ABSTRACT FROM AUTHOR]

• First S-N curve of surface initiated damage under rolling contact fatigue. • Uniquely conservative mechanical fatigue failure definition used for surface damage. • RCF and torsional fatigue S-N curves have similar fatigue strength exponent. • Study unveils that torsion tests can predict RCF surface damage results 10x faster. • Different operating regimes identified based on RCF surface damage severity. This paper presents fundamental findings on equivalence of rolling contact fatigue (RCF) driven surface damage and torsional fatigue modes of failure. Fatigue experiments were conducted for both modes of failure using AISI 4130 case carburized steel specimens. A Thrust Bearing Surface Pitting Rig (TBSPR) was designed and developed to perform RCF tests on flat test specimens using thrust ball bearings. The experiments were conducted in boundary lubrication regime, simulating the contact conditions in tribological machine components. The current investigation presents a novel approach where the effect of contact pressure and number of fatigue cycles on the initiation of surface damage was characterized by surface profilometry of the wear track after each test to generate a stress-life (S-N) map. The S-N map provides unique insights to early detection of RCF driven fatigue damage. The S-N map was utilized to generate the surface damage S-N curve. Torsional fatigue experiments were conducted on the same material, and the S-N curves of both failure modes were compared. The comparison shows key similarities between the two S-N curves, as they differ by a simple scaling factor. This investigation demonstrates that RCF driven surface damage characteristics can be predicted by conducting much simpler, and faster, torsion fatigue tests. This paper also presents a novel methodology to determine contact pressures to avoid incipient micropitting/microdenting damage. [Display omitted] [ABSTRACT FROM AUTHOR]

• A crystal plasticity damage model proposed for RCF of GGS. • FEM implementation to predict crack initiation and spalling lifetime. • Effect of the residual stress on RCF behavior is investigated. A dislocation density-based crystal plasticity damage model is presented for a numerical study on the spalling behavior of gradient grained structures under rolling contact fatigue loading. The topology of the gradient grained structure is constructed using Voronoi tessellations. The fatigue damage is coupled into a dislocation density-based crystal plasticity model where the evolutions of the dislocation density and fatigue damage are considered. The process of spalling is characterized by the evolution of fatigue damage in the polycrystalline structures. The evolutions of contact pressure, stresses, dislocation density, plastic strain, and fatigue damage of gradient grained structures with regard to rolling cycles are investigated. Moreover, the effect of the gradient form and residual stress on the rolling contact fatigue behavior is studied. The simulation results reveal that the gradient grained structures and the residual stress influence the rolling contact fatigue resistance. [ABSTRACT FROM AUTHOR]

[Display omitted] Material microstructure plays a key role in crack initiation under rolling contact fatigue. When studying microstructure with crystal plasticity finite element method (CPFE), mesh sensitivity study is of great importance, as the surface-near region is under high uniaxial stresses. In this paper, a new structured mesh strategy is purposed and compared with the classical unstructured mesh strategy. Modelling tests on a bi-grain and a polycrystal model show the calculation of geometrically necessary dislocation (GND) density, recently proposed as a suitable fatigue damage indicator, is highly dependent on mesh morphology, when GND hotspots tend to appear near distorted elements even in homogeneous materials. With uniform mesh size and shape, structured mesh elements can provide physically more acceptable GND calculations, which is particularly important in loading scenarios with complex stresses, such as rolling contact fatigue. Computational efficiency is also improved compared to unstructured models because a smaller number of elements are required in a structured mesh model and pre-processing of the mesh is not required. [ABSTRACT FROM AUTHOR]

The effect of contact forces on rail uneven longitudinal wear was investigated using three series of rolling tests, including tests with the same tangential force and varying normal force, tests with the same normal force and varying tangential force, and tests with varying tangential and normal forces. Results indicated that the effect of tangential force on rail uneven longitudinal wear was greater than that of normal force. When the tangential force was the same, rail uneven longitudinal wear occurred under the small normal force condition. As the normal force increased, the wear rate decreased. With the same normal force, rail uneven longitudinal wear occurred under the large tangential force condition. The main damage of the unevenly worn rail was oxidative wear and fatigue wear. The oxidative wear and ploughing were more severe at the crest. Plastic deformation, the degree of grain refinement and fatigue cracks were more severe at the trough. More severe fatigue cracks at the trough lead to more severe wear and then the aggravation of rail uneven longitudinal wear. • The influence of normal force and tangential force on rail uneven longitudinal wear was studied. • The effect of contact forces on rail uneven longitudinal wear was quantified. • The wear and rolling contact fatigue behaviors of rail were investigated in detail. • The damage mechanism of rail uneven longitudinal wear was proposed. [ABSTRACT FROM AUTHOR]

Rolling contact fatigue damage of railway rails in the form of squats, characterised by local depressions and cracks located at the rail surface, has been linked to the occurrence of local rail surface irregularities. This study concerns rolling contact fatigue cracks in the vicinity of fairly smooth surface irregularities, here denoted dimples. The influence of factors such as dimple geometry, cluster effects, and crack size is evaluated. To this end, dynamic vehicle–track simulations featuring realistic wheel and rail profiles are employed to characterise the dynamic impact during a wheel passage. The contact load in the vicinity of the dimples is then mapped onto a 3D finite element model of a rail section containing a crack in the rail head. The crack loading is finally quantified by multimodal stress intensity factors. The analyses establish that also shallow dimples might have a significant impact on the crack loading. This effect is increased for larger or multiple irregularities but decreases as the crack grows. [ABSTRACT FROM AUTHOR]

Rolling contact fatigue cracks and thermally induced defects are common problems in the railway industry especially as demands for increasing loads, speeds, and safety continue to rise. Often, the two types of defects are found together in the field, however, whether one causes the other to occur is not completely agreed upon. The effect of thermal damage, in the form of a martensite spot on pearlitic steel test bars, on the fatigue life in uniaxial low cycle fatigue experiments was investigated by the authors. However, the focus of the current work was to characterize the damage evolution from the low cycle fatigue (LCF) tests and correlate the crack initiation and propagation with the initial thermal damage. Residual stress measurements, digital image correlation, and X‐ray tomography were used to characterize the effects of the thermal damage before, during, and after fatigue testing, respectively. It was found that the thermal damage causes strain accumulation and crack initiation at the interface between the two materials. The strain evolution was visualized using digital image correlation (DIC), clearly showing the strain concentrations at the top and bottom of the white etching layers (WEL), where the residual stresses are also most tensile. X‐ray tomography confirmed the planar crack growth from the martensite spot. [ABSTRACT FROM AUTHOR]

Rail rolling contact fatigue (RCF) crack and other rail damage have great impact on the normal operation of the railway system. The detection and evaluation of RCF crack can effectively ensure the running safety of the train. Compared with the traditional non-destructive testing methods, the characteristics of Magnetic Barkhausen Noise (MBN) detection technology such as easier implementation, higher efficiency and faster inspection process etc., make it an ideal choice for non-destructive detection technology of rail RCF crack. Therefore, this paper studied the influence of plastic deformation layer and crack of rail roller on MBN signal. The detection equipment based on MBN signal was built and used to detect and characterize the rail PDL and cracks. The wheel-rail rolling-sliding tests were conducted on a twin-disc tribometer to prefabricate the rail RCF crack and PDL. Experimental results proved that MBN signal decreased with the increase in plastic deformation layer (PDL) thickness. Crack initiation would release the residual compressive stress (RCS) of material surface, resulting in the abnormal increase in MBN signal under the general trend of gradually weakening. The influence mechanism of cracks on MBN signals was analyzed and discussed. • With the increase in the PDL thickness, the MBN eigenvalues decrease. • A linear relationship between PDL thickness and MBN signal is established. • Crack initiation releases the residual compressive stress of the roller surface, causing the increase in MBN signal. • The propagation behavior of the crack attenuates the MBN signal intensity. [ABSTRACT FROM AUTHOR]

• A first study of the effect of full creepage on RCF crack growth behaviour. • Predicted crack growth directions show a good agreement with field observation. • Lateral and spin creepages can significantly affect K II and K III evolutions. • Increase in spin creepage is more detrimental to RCF crack driving force. In heavy-haul railway systems, the experience of high traction force of rails in curved tracks significantly accelerates the rolling contact fatigue crack growth. The overall level of traction force and its detailed distribution within a contact patch can be determined by the extent of wheel slips in different directions, also referred to as creepage. However, few studies have focused on quantifying the influence of creepage on rolling contact fatigue crack growth behaviour. In this paper, a numerical method is proposed to investigate the non-proportional mixed-mode rolling contact fatigue crack growth behaviour in the presence of severe longitudinal, lateral and spin creepages. To partially validate the proposed numerical method, the predicted crack growth directions on the rail surface are compared with the cracking patterns obtained from field observations. A parametric study is also conducted to further quantify the influence of different creepage combinations on the evolution of stress intensity factors at the crack front. Results show that both lateral and spin creepages can significantly affect the phase and maximum magnitude of K II and K III during one complete loading cycle. Moreover, the increase of spin creepage is found to have a more detrimental effect on overall RCF crack driving force than the increase of the other two creepage components. [ABSTRACT FROM AUTHOR]

The rolling contact fatigue behavior of a newly developed bearing steel with carbide-free bainite, modified by a combination of tungsten inert gas surface melting and subsequent isothermal transformation was investigated. The rolling sliding tests were conducted to determine the vibration response and rolling contact fatigue life. The rolling contact fatigue life of the bainitic layer is 6.623 × 106 cycles, significantly higher than that of the commercial GCr15 steel sample (4.496 × 106 cycles). Focused ion beam technology and transmission Kikuchi diffraction studies on fatigue-tested samples showed that the toughness of the bainitic layer could be improved by the bainitic ferrite and retained austenite, which recrystallizes and forms nanocrystals during the rolling contact fatigue process. Furthermore, high-resolution transmission electron microscope study elucidated that the bainitic layer has better dislocation slip ability and is more prone to recrystallization on the rolling contact fatigue surface, preventing the rapid expansion of fatigue cracks. Moreover, the effect of dissolution of the Fe 3 C in GCr15 steel on the structure of distorted body-centered cubic crystals and pitting defects was also comparatively investigated. [Display omitted] • The bainitic surface layer has high hardness and compressive stress. • More than 35% of the fatigue life can be obtained by TIG surface bainitic treatment. • Recrystallization in the RCF layer prevents the fatigue crack from further propagation. • RCF surface in the carbide-free bainitic layer generates the nanocrystal leading to a better dislocation slip ability. [ABSTRACT FROM AUTHOR]

Additive manufacturing is steadily gaining acceptance in certain industry segments as a process for the manufacturing of dense metallic components. The Ni-based superalloys belonging to the Inconel family have for many years been in focus for AM research and AM produced components are now becoming commercially available. However, it is still unclear how the microstructural anisotropy, inherent to most AM materials, affects the material performance in a given application. The anisotropy may e.g. influence the mechanical properties and the performance in certain tribological situations, such as when subjected to rolling contact fatigue. Like most AM methods, the powder bed fusion – laser beam process gives the produced components a relatively rough surface. To perform well in demanding tribological situations, the components are commonly machined to a smooth finish. In this work, Inconel 625 produced using PBF-LB is evaluated in a rolling contact fatigue test. Test cylinders (Ø10 mm) have been produced using different build directions and scan strategies, resulting in varying microstructures and textures. In the rolling contact fatigue test, a cylindrical sample is mounted between two Ø 140 mm metal rollers, pulled together via a spring. After testing, the contact tracks are studied using SEM and EBSD to reveal cracks. Cracks were analysed with respect to the microstructure and anisotropies. It was found that the anisotropy influences both the nucleation and growth of cracks. The AM produced specimens were also found to be more prone to transgranular cracking than conventional Inconel 625, which predominantly displayed intergranular cracks. • AM IN625 tend to crack transgranularly, conventional IN625 intergranularly. • In AM rolling components, texture may dictate specific sites for crack nucleation. • In AM rolling components built with axis along build direction small damage dominate. • When built perpendicular to build direction the bottom part is most prone to fatigue. • AM rolling components built with contour will have fatigue crack along contour lines. [ABSTRACT FROM AUTHOR]

The combined theoretical-experimental technique to estimate high-tempered 65Γ steel resistance to rolling contact fatigue cracking is used. As a result of complex studies, the fatigue life data of high-tempered 65Γ steel is obtained. The dimensions of spall formed by pitting mechanism for deferent contact pressure values is determined. It is shown that the significant role in rolling contact fatigue estimation of similar steels has an initiation shear crack length. The influence of tempering temperature after quenching on rolling contact fatigue resistance of 65Γ steel is evaluated. [ABSTRACT FROM AUTHOR]

This work aimed to evaluate the effects of contact stress and slip ratio on wear and rolling contact fatigue of ER9 wheel steel and R260 rail steel couple under unlubricated conditions using a dual disk test. It was demonstrated that as either contact stress or slip ratio increased, so did the wear rate and surface damage of both ER9 wheels and R260 rail rollers. The wear rate of the ER9 wheel steel containing proeutectoid ferrite and the smaller structure size of pearlite increased significantly when compared to the rail steel containing fully pearlite in each pair. After testing according to the conditions used in this work, the surface hardness of the pearlitic rail steel was higher than that of the ferrite-pearlite wheel steel. The average hardening ratio of ER9 wheel steel and R260 rail steel was 0.7 and 1.28, respectively. This was consistent with the subsurface deformation angle. With full pearlite in the microstructure of rail steel, the main wear mechanism was predominantly caused by fatigue cracking and peeling, with some adhesive wear. Whereas the most prominent mechanism of the ferrite-pearlite wheel steel was adhesive wear, which was followed by spalling and small fatigue cracks. • Wear behavior of the ER9 wheel and R260 rail couple under different conditions. • Hardening ratio of the ER9 wheel steel is lower than that of R260 rail steel. • Voids occur at proeutectoid ferrite/ferrite and broken cementite interfaces. • Relationship of wear rate, surface crack angle, and deformation angle. • Surface damage caused by microstructure in the ER9 wheels and R260 rail steel. [ABSTRACT FROM AUTHOR]

The surface crack of wheel/rail is a significant problem affecting the safety and maintenance of railway. This study presents the experimental results to explore the formation and damage mechanism of rolling contact fatigue (RCF) surface cracks of wheel/rail materials under dry condition. The results indicate that the surface cracks mouths on the rail roller are perpendicular to the resultant direction of shear force. Fatigue cracks grow along a larger angle to depth on the rail rollers than on the wheel rollers due to the difference of materials. Meanwhile, there are visible subsurface and secondary cracks as well as lamellar structure cracks. The vertical force changed in the later 1.5×10 5 cycles leads to a significant difference in the cracks propagation. [ABSTRACT FROM AUTHOR]

Local rolling contact fatigue (LRCF) has occasionally been observed on wheels of high-speed electrical multiple units (EMUs) in recent years in China. It typically propagates to 2.5–8.5 mm deep along shallow angles to the tread surface, leading to large material removal during lathe turning. To reveal the initiation and propagation mechanisms of LRCF, a thorough investigation has been conducted by means of field observations, statistical analyses, and numerical simulations. It is found that 69% of LRCF occurred on the leading axles of leading coaches, and its occurrence rate on the 380 km/h class EMU was 2.87 times that on the 250 km/h class. Records of a 380 km/h class EMU train have shown that LRCF initiated with an occurrence rate of 14.3% from indentations of 2.3–3.0 mm deep and 4 mm in diameter, and the crack depth increased by 0.011–0.031 mm every 1000 km. All these phenomena, among others, suggest that indentations are the main causes of LRCF. The cracks, according to 3D transient and 2D static simulations of wheel–rail rolling contact, probably initiate at the bottom of deep indentations with a characteristic dimension longer than 2–4 mm. Preventive measures against LRCF are recommended at the end. [ABSTRACT FROM AUTHOR]

• Propagation rates of rolling contact fatigue cracks are measured experimentally. • FE model of the observed cracks is used to predict ΔK values during over-rolling. • Propagation rates of longer cracks follow Paris law of type dc dN ∝ Δ K eff 4 . • Mode II propagation (ΔK II) is the main driver of surface breaking RCF cracks. • These findings can aid predictions of remaining useful life of machine elements. Rolling contact fatigue (RCF) is a major life limiting factor for machine elements that employ non-conformal, rolling sliding, lubricated contacts such as rolling bearings and gears. This paper explores the application of linear elastic fracture mechanics (LEFM) principles, as commonly used in structural fatigue, for prediction of RCF crack propagation. A triple-disc contact fatigue machine is used to generate RCF cracks of varying lengths in AISI 52100 bearing steel roller specimens. Crack propagation rates across the surface are measured using optical inspection of test specimens and the final crack geometry is established through specimen sectioning. A numerical finite element model of surface breaking RCF cracks based on LEFM methods is devised to predict the evolution of stress intensity factors (SIFs) during over-rolling of the contact over the experimentally observed crack geometries. The model employs a suitable fracture mechanics mesh to resolve stresses at the crack tip and accounts for Hertzian contact stresses, contact friction and crack face friction. Potential effects of lubricant pressurisation within the crack are not modelled. The predicted SIFs are then related to the experimentally measured crack propagation rates to establish the applicability of the LEFM principles to RCF crack propagation. Results show that LEFM can be used to predict the growth of surface braking RCF cracks. For cracks longer than about 100 μm, a Paris law relationship with the stress intensity exponent of about 4 is derived. Mode II was seen to be the dominant mode of propagation for surface braking RCF cracks. Mode I SIFs are much smaller but can exhibit significant values when the contact is located just ahead of the crack mouth. Decreasing the crack face friction significantly increases mode II stress intensity suggesting that this is one important mechanism by which lubricant entry into the surface braking crack can accelerate its propagation. The findings can help in improving the reliability of mechanical systems by supporting the development of new tools for prediction of remaining useful life of machine components such as bearings and gears. [ABSTRACT FROM AUTHOR]

Surface defects in different railway lines experience different wheel-rail rolling contact actions owing to various railway line characteristics and vehicle operation parameters. This study focused on the evolution of defective wheel-rail rolling contact fatigue (RCF) behaviour under different operation conditions. Conical defects were created using an indentation apparatus. Wheel-rail RCF tests were performed using a twin-disc testing machine. Various operational conditions were simulated considering different slip ratios and wheel linear velocities on uphill and flat lines, as well as different vehicle axle loads and train configurations. The results indicate that surface damage, crack depths, and wear debris size increased only with decreasing wheel linear velocities, whereas the crack angles increased only with decreasing slip ratio. Meanwhile, both the wear rate and oxidation degree of wear debris were simultaneously affected by the slip ratio and wheel linear velocities. Under rolling conditions on flat lines, vehicle axle loads had a negligible influence on the wheel-rail RCF behaviour. • Operation conditions were simulated considering uphill and flat railway lines, also axle loads and train configurations. • Slip ratio cannot affect surface damage, crack depths and wear debris sizes, velocities cannot affect crack angles. • RCF behavior under constant and adjustable axle load during rolling contact tests was almost identical. • Crack angles, oxidation degrees of wear debris perform some differences on uphill railway lines. [ABSTRACT FROM AUTHOR]

• Surface undergoes a cyclic process of in-situ NL generation-stripping-generation. • The NL effectively hinder the fatigue crack propagation to the deep. • The decrease of elastic modulus indicates that bainitic steel begins to damage. • The strengthening of surface deformation layer is accompanied by damage. The formation process of nanocrystalline layer (NL) on rolling contact surface of bainitic steel are studied. The results show that the surface repeatedly goes through a cycle of NL generation, spalling and production. The decrease of elastic modulus of the steel surface indicates that fatigue properties have begun to damage. [ABSTRACT FROM AUTHOR]

Most existing wheel/rail hardness matching studies have been carried out using pearlite steels with different hardnesses; no relevant wheel–rail hardness matching research has been carried out for other microstructure types such as bainite and other tempered materials. In this paper, a CL60 wheel (pearlite) was quenched and tempered to obtain wheel materials with a certain hardness gradient, and matching tests between different wheel materials and a quenched U75V rail were carried out to explore the influence of wheel microstructures on wheel–rail damage. The results showed that with increasing tempering temperature, the wheel microstructure changes from tempered troostite to tempered sorbite, with ferrite changing from a needle shape to a multilateral shape; additionally, the cementite particles grow obviously larger, resulting in a gradual decrease in wheel hardness. With the increase in the wheel/rail hardness ratio, the wear rate of the wheel and wheel–rail system first increases and then decreases; the wear rate of tempered sorbite is the highest. Wear surface analyses indicate that typical peeling appears on the wear surfaces of the CL60 wheel as well as the coupled rail. Tempered sorbite shows slight peeling; however, severe peeling and delimitation occur on the wear surface of its coupled rail. Moreover, severe peeling and serious material delamination are present on the wear surface of tempered troostite, and typical peeling appears on its coupled rail surfaces. The fatigue crack initiation and propagation behaviors of wheel and rail materials are closely related to their microstructures. The cracks in pearlite always initiate on the surface and propagate along the ferrite line. Short and shallow fatigue cracks appear on the tempered sorbite with a high wear rate, and the cracks in the tempered troostite easily expand into the centre of the samples, indicating poor rolling contact fatigue (RCF) resistance. Therefore, the pearlite structure shows better comprehensive mechanical properties for wheel service; moreover, more attention should be given to wheel microstructure when conducting research on wheel–rail matching. • The CL60 wheel were heat treated and wheel materials with a certain hardness gradient were obtained. • Matching tests between quenched U75V rail and different wheels were carried out. • Wear and rolling contact fatigue behaviours were proposed and studied. • Ratcheting behaviours and propagating mechanism of RCF cracks were analyzed and revealed. [ABSTRACT FROM AUTHOR]

• A two-stage simulation strategy was proposed to investigate the mechanisms of RCF cracks on the rail, consisting of a dynamic FEM for wheel and rail interaction and a coupled static FEM/BEM for crack growth. • The critical wheel positions for crack propagation during wheel approaching to and departing from the crack were determined and correlated with the different cracking mechanisms. • The orientation of the crack in rail played an essential role in crack growth behaviors. Rolling contact fatigue (RCF) crack is one of dominating damages on the rail surface due to repeated wheel and rail contact. RCF defects, including squats and head checks, can potentially cause rail fracture failure if not treated in time. It is critical to accurately predict the pertinent damage for different operational conditions. A two-stage simulation strategy was proposed to investigate the mechanisms of RCF-induced cracks on the rail, consisting of a dynamic FEM for wheel and rail interaction and a coupled static FEM/BEM for crack growth. It was found that there are two critical wheel positions for crack propagation during wheel approaching to and departing from the pre-existed crack. The different cracking mechanisms at these two critical wheel positions are correlated with the local physical phenomena, i.e. , the traction at the wheel/rail interface, the relative distance between the contact patch and crack, and the sliding at crack faces. The twisting of the cracks from the transverse plane to the longitudinal plane at the railhead significantly reduces crack growth in mode I, enhances crack propagation in modes II and III, and shifts the dominant crack growth from the crack front center to the crack corners. The increased incline of the crack in the rolling direction promotes the crack to grow downward in opening mode I. [ABSTRACT FROM AUTHOR]

From lubricated rolling contact tests on silicon nitride rolling elements, genuine subsurface fatigue process from the Si3N4 microstructure was demonstrated and visualised for the first time by testing two different microstructure quality levels. Subsurface fatigue cracks initiating at microstructure features were found to be associated with local orthogonal shear stresses from the Hertzian stress field. All fatigue cracks displayed evidence of mode II and III crack type propagation with an angle orientation that conformed to the shear stress direction. [ABSTRACT FROM AUTHOR]

• FASTSIM algorithm implementation in Abaqus to consider slipping and adhesion zones of the wheel/rail contact patch. • XFEM rolling contact fatigue (RCF) analysis of a 60E1 rail profile considering creepages. • Validation of the developed rail model with data available in the literature. • Sensitivity analysis to determine the influence of contact conditions on the fatigue life of rails. In this work the influence of considering wheel-rail contact creepages on fatigue crack growth rates due to Rolling Contact Fatigue (RCF) is studied. For this purpose, the FASTSIM algorithm, which considers the moving complex pressure distribution with slipping and adhesion zones of the wheel-rail contact patch, has been implemented in Abaqus using FORTRAN code subroutines. The developed methodology has been validated with a 60E1 rail profile model which uses XFEM, by comparing the obtained Stress Intensity Factors (SIFs) and sub-surface shear stresses with numerical results available in the literature. Finally, the RCF crack propagation analysis of a 60E1 rail profile with different contact conditions has been performed using the XFEM. The obtained results justify the necessity of considering contact creepages on contact shear stresses for crack growth analysis. [ABSTRACT FROM AUTHOR]

Rolling contact fatigue (RCF) and wear are competing phenomena in railway wheels, but they are typically studied as independent events. In this work, preliminary tests are carried out with a twin-disc bench to explore the relationship between RCF and wear. First, the collected tests are reported on the shakedown maps to assess the cyclic response and calculate the fatigue index parameter used to predict the surface-initiated fatigue damage. Second, the fatigue index and material cyclic yield strength are related to the experimental wear rate. This work proposes maps to predict how the wear rate and cyclic response change by varying the contact conditions (clean or contaminated with water or sand), slip ratio and extraction position of specimens. Furthermore, the estimated wear rate is superimposed to the shakedown map to investigate a possible relationship between RCF and wear. The proposed approach allows to understand how a railway wheel steel behaves as a function of the working conditions and whether some working parameters can be changed without compromising the material performance. • Predictive maps for bi-disc tests are proposed. • Wear rate is superimposed to shakedown maps. • A higher fatigue index results in a higher wear rate. • Water contaminant drastically reduces the wear rate. • Reprofile operations increase the fatigue index and wear in clean contact. [ABSTRACT FROM AUTHOR]

Rolling contact fatigue (RCF) is the critical damage from rolling bearings constraining the service, performance, and reliability, which is directly caused by fatigue crack propagation. The crack paths on both the contact surface and subsurface were observed after the bearing fatigue test. The three-dimensional geometry of the RCF crack was characterized and reconstructed based on experimental statistics. Finite element analysis was performed to obtain the mixed mode stress intensity factors (SIFs) along the crack fronts, and the crack propagation behavior was numerically studied. Results indicated that the propagation of cracks on the contact surface and subsurface causes the gradual formation of micropits and the sudden spalling of material respectively. Initial micro-cracks with an inclination angle of 15–30° have the highest growth rate, which should be prevented during the bearing raceway manufacturing process. The propagation paths of the RCF cracks can be more accurately predicted by considering the superposition of fracture mode I + II + III in three-dimensional, and mechanisms of crack propagation on contact surface and subsurface were revealed respectively. [Display omitted] • Surface cracks propagation results in the gradual formation of micropits, while the subsurface cracks cause the spall off. • RCF cracks on the surface tend to propagate perpendicular to the rolling direction, while subsurface cracks grow and deflect. • Micro-cracks with an initial inclination angle of 15–30° have the highest growth rate. • Fracture mode II dominates the propagation of subsurface cracks, while it plays the least important role in surface cracks. • By considering the combined influence of mixed mode fracture, the propagation paths can be more accurately predicted. [ABSTRACT FROM AUTHOR]

The present work proposes a novel efficient numerical approach for the simulation of three-dimensional propagation of non-planar frictional crack under Rolling Contact Fatigue (RCF). The developed model relies on a global–local strategy involving the Semi-Analytical Method (SAM), dedicated to the resolution of 3D contact problems, and the eXtended-Finite Element Method (X-FEM), dedicated to the solving of 3D crack problems. Similar to a submodeling technique, it consists in performing a local X-FEM analysis of the 3D RCF crack problem by means of boundary conditions extracted from a prior global SAM analysis which solves the contact problem between two semi-infinite bodies without considering the crack. A powerful procedure for the transfer of relevant mechanical quantities between the SAM and X-FEM models is developed. It allows the use of non-matching and non-conforming discretization scheme for the global SAM model and the local X-FEM model, particularly worthwhile for the pre-processing stage. An initial arbitrary stress profile can be considered to investigate the influence of residual stresses on crack behavior. The non-intrusive SAM/X-FEM coupling is integrated into a fully automatic crack propagation algorithm. This provides a powerful and user-friendly tool suitable for industrial applications, which easily handles 3D long non-planar frictional crack growth in the region of interest. After introducing some details concerning the implementation of the model, a validation of the innovative SAM/X-FEM coupling is performed by using results from the literature. Furthermore, a practical example demonstrates the great potential of this novel numerical technique to simulate in a fast, robust and accurate way the 3D complex behavior of fatigue crack under moving contact. • Simulation of the 3D frictional crack propagation under rolling contact conditions. • Non-intrusive global–local approach based on the coupling of SAM and X-FEM. • Field transfer procedure allowing independent meshes for the SAM and X-FEM models. • Fast, efficient and robust resolution coupled with a pre-processing time saving. • Influence of residual stresses on crack behavior highlighted. [ABSTRACT FROM AUTHOR]

Three types of differently shaped lasered microtextures (microdimples) were produced on the surface of a U75V hot-rolled rail roller with a TY-FM-20S laser marking machine. Wheel flange/rail gauge lubrication starvation tests of four kinds of rail samples running against CL60 wheel samples were carried out, and the friction properties of the wheel/rail steels were studied. The results indicate that the injection of lubricating grease dominates the friction coefficient reductions and that the lubricating effects are closely related to the shapes of the lasered microtextures; for instance, the rectangular and rhombic textures show a better lubricating effect. Furthermore, the wear rates depend on the wear mechanism of the wheel/rail steel; for example, delamination and spalling always result in severe wear while peeling results in mild wear. The materials from the rolling-sliding surface to the matrix region of the wheel/rail materials can be divided into three parts: the plastic deformation region, transition region and undeformed matrix region. The average crack length and angle values of the textured rail and coupled wheel steels are much smaller than those of the untextured rail and coupled wheel steels; thus, the textured rail and coupled wheel steels exhibit better rolling contact fatigue (RCF) resistance. The effects of the lasered microtextures shapes on the wear and fatigue damage are determined based on the fluid dynamic pressure on the microtexture. Notably, rhombic lasered microtextures can be adopted to improve the grease storage capacity, thereby reducing wear and fatigue damage. • Three types of differently shaped lasered microtextures were produced on the rail surface. • Lubrication starvation tests of four kinds of rail samples were carried out. • Lubricating effect, wear and rolling contact fatigue behaviours were proposed and studied. • The textured rail and coupled wheel steels demonstrated better RCF resistance. • The lubricating and damage mechanism of textured rail were analysed and revealed. [ABSTRACT FROM AUTHOR]

Twin-disc tests were carried out between two discs of rail steel (fully pearlitic and spheroidized microstructures) to investigate the effect of the flash-butt welded rail HAZ microstructure. The spheroidized cementite microstructure was obtained by heat-treating simulating the HAZ softer region. Ten tests were performed for each condition, considering maximum Hertz pressure of 1100 MPa, rotation speed of 400 rpm and slip of 0% and 1%. The results showed that the spheroidized cementite discs (with less hardness) presented more significant surface hardness due to deformation, more clearly for the tests carried out with 1% slip. This factor led to a cracking process for both heat-treated discs and counter-discs with a completely pearlitic microstructure. The number of cycles for the beginning of contact fatigue between the specimens compared to the untreated pairs proved to be statistically equal in the tests carried out with 1% slip. In tests without slip, however, this phenomenon did not occur, as the spheroidized discs showed a significant decrease in the number of cycles before the beginning of contact fatigue. Plastic deformation was observed around the fatigue cracks (even in the spheroidized microstructure's discs), and the presence of inclusions influenced the cracking process. • Twin-disc tests were run on pairs of different microstructures. • The number of cycles before the RCF had a 19% difference between test conditions. • The initial softer disc showed a harder surface after the tests when compared. • The initially harder counter discs presented a higher RCF cracks density. [ABSTRACT FROM AUTHOR]

The subsurface white etching microstructure (WEM) was an important reason to cause the rolling contact fatigue (RCF) failure of wheel materials. The WEM formation mechanism in the subsurface layers of failed pearlite steel wheel was analyzed by using a scanning electron microscope, transmission electron microscope, and nanoindenter. The results showed that RCF cracks were mostly at the interface between the WEM and pearlite matrix. A large amount of the WEM was found in areas with no RCF cracks. This indicates that the WEM was formed prior to the RCF cracks. The WEM was comprised of nanostructured ferrite grains and residual cementite particles, and its hardness (approximately 920 HV) was about 3 times as high as that of the pearlite matrix. A soft area model for WEM formation is proposed based on the microstructural characteristics of two types of ferrite grains and the dissolution of cementite in pearlite resulting from continuous plastic deformation. During the wheel–rail contact, continuous plastic deformation in the subsurface under high contact stress conditions causes the successive refinement of the proeutectoid ferrite (PF) with low strength and ferrite grains in pearlite, the dissolution of cementite, and the improvement in the corrosion resistance, which cause the WEM to look white under an optical microscope. The WEM formation processes induced by alternating fatigue load were similar to that of surface nanocrystallization through surface mechanical attrition treatment. The great difference between the WEM and the pearlite matrix in the strength results in the crack propagating along their interface, therefore the nonuniform distribution of the WEM along both edges of RCF cracks was found commonly. • A subsurface white etching microstructure (WEM) is found in wheel materials. • A soft area model is proposed to the formation mechanism of subsurface WEM. • The relationship between subsurface WEM and fatigue cracks is discussed. [ABSTRACT FROM AUTHOR]

This paper aims to propose a new and fast method to estimate the multiaxial non-proportional elastic–plastic stress and strain time histories developed in rails due to rolling contact and evaluate its accuracy for fatigue analysis. The method has the pseudo-elastic stress time history as input, which is obtained by a coupled analytical and numerical method. Therefore, a method originally proposed for notch-root elastic–plastic stress–strain analysis was extended to the Rolling Contact Problem (RCF). The problem is solved incrementally and is established considering a set of equations composed by constitutive and multiaxial notch correction equations. Additionally, a kinematic cyclic plasticity model was included. An equivalent model based on the Finite Element Method was used for comparison purposes. The comparison was made in terms of multiaxial fatigue damage parameters in critical planes, according to fatigue analysis models developed by Matake and Brown & Miller. The more relevant advantage of the method over the non-linear FEM analysis is the possibility to readily obtain long elastic–plastic stress and strain histories and to perform material parameter and load case studies. The method proposed agrees with the equivalent FEM and proved to be a powerful tool for fast wheel-rail failure analysis, especially in RCF problems. [ABSTRACT FROM AUTHOR]

• FASTSIM algorithm implementation in Abaqus to consider slipping and adhesion zones of the wheel/rail contact patch. • XFEM rolling contact fatigue (RCF) analysis of a 60E1 rail profile considering creepages. • Validation of the developed rail model with data available in the literature. • Sensitivity analysis to determine the influence of contact conditions on the fatigue life of rails. In this work the influence of considering wheel-rail contact creepages on fatigue crack growth rates due to Rolling Contact Fatigue (RCF) is studied. For this purpose, the FASTSIM algorithm, which considers the moving complex pressure distribution with slipping and adhesion zones of the wheel-rail contact patch, has been implemented in Abaqus using FORTRAN code subroutines. The developed methodology has been validated with a 60E1 rail profile model which uses XFEM, by comparing the obtained Stress Intensity Factors (SIFs) and sub-surface shear stresses with numerical results available in the literature. Finally, the RCF crack propagation analysis of a 60E1 rail profile with different contact conditions has been performed using the XFEM. The obtained results justify the necessity of considering contact creepages on contact shear stresses for crack growth analysis. [ABSTRACT FROM AUTHOR]

• The wear performance of the defected rail materials under various slip ratios and contact stresses was investigated. • The surface damage of the defected rail materials under various slip ratios and contact stresses was studied. • The effect of slip ratio and contact stress on crack propagation mechanisms near the dent was discussed. The defects on rail surface may have negative effects on the rolling contact fatigue (RCF) performance of the rail materials. This investigation researched on the RCF behaviors of the defected rail materials under various slip ratios and contact stresses. The results show that both the wear loss and the average crack length increase if the slip ratio or the contact stress increases. Moreover, the surface damage characteristics change from simple adhesion spots to significant fatigue crack grouping when slip ratio and contact stress rise. Finally, the experimental results also highlight that the slip ratio provides the deformation area for crack initiation whereas the contact stress accelerates the crack propagation by emphasizing the geometric effects. [ABSTRACT FROM AUTHOR]

A numerical model for rolling contact fatigue has been developed using finite element method and the Tanaka-Mura micromechanical model to calculate fatigue crack threshold. Two different approaches are compared: macroscopic modeling and mesoscopic modeling which considers a Voronoi polycrystalline geometry. Smooth and dented surface contacts are considered and different methods for computing shear stress are compared. It is shown that a mesoscopic length scale consideration offers benefits over a macroscopic approach in the case of dented surface meanwhile orthogonal shear stress computation on the macroscopic model can legitimately be used in the case of Hertzian contact. The article also highlights that mesoscopic modeling allows for life scatter on fatigue crack threshold. Initiation depths are consistent with literature experiments. • A polycrystalline model is developed for rolling contact fatigue. • Fatigue crack initiation is calculated using Tanaka-Mura approach. • Different shear stress computations are compared. [ABSTRACT FROM AUTHOR]

Wheel rail rolling contact fatigue is a very common form of damage, which can lead to uneven rail treads, railhead nuclear damage, etc. Therefore, ANSYS software was used to establish a three-dimensional wheel–rail contact model and analyze the effects of several main characteristics, such as the rail crack length and crack propagation angle, on the fatigue crack intensity factor during crack propagation. The main findings were as follows: (1) With the rail crack length increasing, the position where the crack propagated by mode I moved from the inner edge of the wheel–rail contact spot to the outer edge. When the crack propagated to 0.3–0.5 mm, it propagated to the rail surface, causing the rail material to peel or fall off and other damage. (2) When the crack propagation angle was less than 30°, the cracks were mainly mode II cracks. When the angle was between 30 and 70°, the cracks were mode I–II cracks. When the angle was more than 70°, the cracks were mainly mode I cracks. When the crack propagation angle was 60°, the equivalent stress intensity factor reached the maximum, and the rail cracks propagated the fastest. [ABSTRACT FROM AUTHOR]

• The state of residual stresses of on-line heat-treated rails was studied. • Rail manufacturing influences the stress state of rails after repeated wheel loads. • FP max differs with residual stresses applied or not. • The relationship between FP max and residual stresses is complicated. Rolling contact fatigue cracks shorten rail service life, and increase costs of rail maintenance and replacement significantly. The aim of the present study is to investigate the effect of residual stresses induced by manufacturing and repeated wheel-rail contact on fatigue crack initiation in rails. To this end, three-dimensional finite element models of rail on-line heat treatment, rail straightening, and wheel-rail contact were established to study the state of residual stresses of on-line heat-treated rails. The effect of residual stresses induced by rail manufacturing and repeated wheel-rail contact on fatigue crack initiation was analyzed based on a fatigue parameter FP max by Jiang–Sehitoglu low cycle fatigue criterion. Rail fatigue crack initiation mechanism under the combined action of wheel load and residual stresses was studied. The results show that residual stresses induced by rail manufacturing affect the stress state of on-line heat-treated rails after repeated wheel loads. The influence of residual stresses induced by rail manufacturing and repeated wheel-rail contact on the FP max of rails is significant. However, there is no positive or negative correlation between residual stresses and FP max of rails under different wheel loads and friction coefficients. [ABSTRACT FROM AUTHOR]

• Residual stress affects crack tip displacements of rail cracks. • CTSD amplitude increases with increasing wheel-rail friction coefficient. • Oblique crack propagates as the combined type of II/III dominated by type II. Fatigue crack is one of the main damages that reduce rail service life and raise rail maintenance costs. In this paper, a three-dimensional elastic–plastic finite element model of wheel-rail contact is established, in which a three-dimensional oblique crack is inserted in the rail. Taking crack tip displacements as fracture parameters, the effect of residual stress on fatigue crack propagation of rails is quantitatively analyzed, and the fatigue crack propagation mechanism of rails under the combined action of wheel load and residual stress are studied. Results show that crack tip displacements under the action of wheel load are quite different from that under the combined action of wheel load and residual stress. For the oblique crack with crack tip entering the residual tensile stress zone, crack tip sliding displacement amplitude increases with the increase of wheel-rail friction coefficient. Under the combined action of wheel load and residual stress, the oblique crack propagates as the combined type of II/III dominated by type II. [ABSTRACT FROM AUTHOR]

The wear and rolling contact fatigue of wheel-rail materials were investigated through varying wheel/rail hardness ratios (H w / H r) and creepages. The results indicated that with the H w / H r increasing from 0.927 to 1.218, the wheel wear rate reduced significantly in the case of H w / H r = 1.218, the rail wear rate showed an increasing trend. Both the wheel and rail wear rates increased as the creepage enlarged. The synergistic results of H w / H r and creepage caused a transition of the wear and damage mechanisms on the wheel-rail steels. Besides, the fatigue damage of ER7 and CL60 wheel materials was dominated by slender multi-layer cracks, while the fatigue cracks were short and contained lots of interlayer broken materials on C-class wheel steel. • As H w / H r increases, the wheel wear rate declines while the rail wear rate increases. • With the slip ratio increasing, both of the wheel and rail wear rates increase. • The wear and fatigue damage mechanisms transform as the H w / H r and slip ratio increase. • The fatigue crack morphology is closely related to the microstructures of materials. [ABSTRACT FROM AUTHOR]

Although both wear and fatigue are inevitably associated with the wheel/rail contact, they are normally studied as different phenomena and both are commonly considered excluding. Wear of railway wheels and rails is usually studied by relating wear rate to the T γ or T γ / A parameters or by plotting the so called "wear maps", where different rolling parameters are related to the wear rate. On the other hand, fatigue in wheel/rail contact is mainly studied from the surface and subsurface stress or deformation fields and from the crack growth rate. However, one of the main causes of wear in rolling contact is the loss of material due to delamination processes that have their origin in the presence of surface fatigue cracks. So, if wear and fatigue are related, it should be possible to study both of them using the same method. In this paper, the fatigue index, based on the shakedown theory and used to predict the surface initiated fatigue of railway wheels, is proposed as a means to study the wear rate in rolling contact. The fatigue index is directly related to the apparition and growth of fatigue cracks at the surface and, thus, directly related to the peeling and spalling processes that constitute the severe and catastrophic wear mechanisms. The data collected from a series of twin-disc tests show that the wear rate correlates with the fatigue index following an exponential evolution independently of the normal load. Furthermore, a fatigue index around 0 marks the onset of severe wear. • The fatigue index is a suitable option to predict rolling contact wear. • Severe wear in takes place when the fatigue index is equal or greater than 0. • The relation between delaminative wear and the fatigue index is exponential. [ABSTRACT FROM AUTHOR]

Rail wheel contact at switches and crossings (S&Cs) induces impact stresses along with rolling contact stresses, resulting in plastic deformation and eventually crack formation. Damaged and deformed wing rails of a manganese steel crossing are studied and the microstructure, hardness and 3D crack network within the steel are characterized. It is found that the surface of the rail receives the maximum deformation resulting in a hardened top layer. The deformation is manifested by a high density of twins and dislocation boundaries in the microstructure. A complex crack network is revealed in high resolution by X-ray tomography. • For Manganese steel, damage in the wing rail is similar to that in the nose rail. • The depth of work hardening reaches a depth of 10 mm and 600Hv at the surface. • It can be assumed the impact from the wheel causes the crack formation. • 3D mapping of the crack network reveal presence of surface and sub-surface cracks. • The crack network appears similar to that of normal straight track pearlitic steel. [ABSTRACT FROM AUTHOR]

To investigate the relationship between surface microstructure and wear mechanism in D2/U71Mn wheel-rail material under different contact stress conditions, rolling wear tests using a GPM-40 wear machine to simulate the wheel-rail operation was performed. After wear tests, an optical microscope (OM), scanning electron microscope (SEM) and micro-hardness testers were used to characterize the microstructure and fatigue wear cracks. The results show that the thickness of the plastic deformation layer and surface hardness is increased with the increase of contact stress. Under high contact stress condition (1200 MPa), the severe plastic deformation layer led to the formation of fatigue wear of wheel-rail samples. Under a contact stress of 700 MPa, the wear mechanism of samples is adhesive wear and wear rate is low. With the increase of contact stress, the fatigue cracks are gradually severe. Under a contact stress of 1200 MPa, the wear mechanism of samples becomes fatigue wear and the fatigue wear cracks cause the increase of wear rate. The fatigue wear can accelerate the wear failure of wheel-rail samples. The fatigue wear cracks of wheel samples are severer than that of rail samples due to both the rate of plastic strain and the content of proeutectoid ferrite. [ABSTRACT FROM AUTHOR]

The rolling contact fatigue cracks (RCF), produced at the surface of the rail tracks, can be detected using a magnetooptical (MO) sensor. Rail tracks are carbon steels with pearlite microstructure. This microstructure has a lamellar texture composed of alternating layers of ferrite and cementite. Both phases are soft ferromagnetic materials at room temperature. If an external magnetic field is applied on the surface of a rail track, the reduced magnetic permeability causes a magnetic leakage field above the cracks. When the external magnetic field is removed, in most cases, a residual stray magnetic field remains above the cracks. When a MO sensor is placed on the surface of the rail track, the sudden change of the stray remanent magnetic field near a crack, yields a significant rotation of the polarization plane of the reflected light, resulting in high MO contrast, exactly above the cracks. Using a polished surface and a cross-section from the head of the rail track, we succeeded in visualizing the RCF cracks in the laboratory. The RCF cracks can also be detected on the surface of the rail track, in field measurements, using a portable commercial polarized light microscope equipped with a MO sensor. Finally, we use computer vision methods, to automatically detect the RCF cracks, using video recorded by displacing the portable microscopy with the MO sensor, on the surface of the rail tracks. We tested an unsupervised automatic crack detection algorithm, which exploits the tubular contrast of the RCF cracks to pinpoint the pixels that correspond to them. [ABSTRACT FROM AUTHOR]

Twin disc tests were carried out to evaluate the wear resistance and Rolling Contact Fatigue (RCF) of premium R400HT rail samples in contact with E8 wheel samples. The wear rate and friction coefficient were correlated with the frictional work expended at the contact interface (the Tgamma approach). Accelerated RCF tests were also carried out on the premium R400HT rail and the results were compared to those obtained for standard R260 rail. The wear rates of rail samples were consistently lower than those reported in the literature for other contacting pairs in which the rail material studied is softer than R400HT. Also, the energy needed for the transition from the moderate to severe wear regime significantly increased for the hardened rail. Fatigue cracks were shallower for R400HT when compared with standard rail material. Hardened rails also showed lower mean spacing between fatigue cracks. This new information can be used to improve wear simulations of wheels and rails by using more realistic wear equations. [ABSTRACT FROM AUTHOR]