Rolling Contact Fatigue Cracks Research Articles

Early stages of rail squat formation and the role of a white etching layer

A squat is a rolling contact fatigue crack below the surface of a rail that leads to a depression in the surface. Squats were thought to initiate by ratcheting (cyclic plastic shear deformation of the surface layer). However, the rail is often found to have a martensitic hard layer which refuses to etch (a white etching layer) on the surface, indicating a thermal crack initiation process. Grassie et al. [1] has suggested these cracks should be given a different name “studs” to distinguish their thermal origin. This paper describes detailed observations of cracks at a site where no white etching layer was evident. Hence these cracks can confidently be labelled as squats. Three stages of squat formation have been identified. Firstly, there is an early stage of crack initiation due to a white etching layer or ratcheting, and then growth of a subsurface crack leading to formation of a squat, caused by entrapped water inside the crack, and finally spalling off of a piece of rail surface. Rails with squat defects in different stages were removed from the site, investigated and inspected visually, ultrasonically, optically and by using electron microscopy to find the main cause of crack initiation and squat formation Microscopic observations revealed primary, secondary and tertiary cracks in the transverse section of rails, and differences in crack surface appearance. Primary cracks initiated at an angle of 10–15° with respect to the rail surface and then turned down at an angle of 50° with respect to the rail surface. Secondary cracks initiated at an angle of 160° with respect to the rail surface and further bifurcated in tertiary cracks. Microhardness of the damaged rail surface was higher than that of the adjacent rail matrix but was not comparable with that of a white etching layer or martensitic structure.

Measurement and modelling of ACFM response to multiple RCF cracks in rail and wheels

This paper presents the experimental and model results of the response of an alternating current field measurement (ACFM) sensor to clusters of rolling contact fatigue (RCF) cracks typical of those found in rails and rail wheels. Both artificial and real cracks occurring in rails taken from service are considered. Currently, commercially available ACFM software is capable of producing an estimate of crack pocket length for isolated cracks, assuming they are regularly shaped. The results presented are part of continuing work to link the ACFM signal to the whole range of complex shaped RCF cracks that appear in rail and rail wheels, including those appearing in clusters. The challenges in accurately sizing clustered RCF cracks using the ACFM technique are discussed.

Experience of premium grade rail steels to resist rolling contact fatigue (RCF) on GB network

Network Rail is involved in a number of initiatives to reduce rolling contact fatigue (RCF) crack initiation on wheels and rails on the GB network. One of these is the trial of premium grade rail materials to determine whether they provide enhanced resistance to RCF crack initiation. The sites where trials have been undertaken have been selected to comprise a range of track and traffic conditions, and were regularly monitored. This paper describes the results from one of these trial sites, on a tight radius curve, where it was found that the risk of RCF actually increased with the use of premium grade rail. It is shown that, although this was initially considered surprising, it is consistent with the current understanding of the mechanisms of RCF initiation. This demonstrates that premium grade rail steels may not always be the best solution to prevent track damage and that careful consideration of the track geometry, operating conditions and traffic should be taken when considering the most appropriate rail grade to select for use at a given location.

Rail grade selection and friction management: a combined approach for optimising rail–wheel contact

To investigate the mechanisms behind rolling contact fatigue (RCF) and wear, a collaborative test program was conducted by voestalpine Schienen GmbH (Leoben, Austria) and Kelsan Technologies Corp. (a part of LB Foster Friction Management) on the full scale test rig of voestalpine. A reference grade was compared to a series of pearlitic rail grades under dry and friction modifier (FM) contact conditions. Rail and wheel profiles were collected at predefined intervals, allowing the calculation of wear rates. The initiation and development of RCF cracks were monitored using optical means and magnetic particle inspection. Metallographic sectioning of the samples was conducted to characterise crack growth and crack orientation as well as the evolution of surface and subsurface material flow. Additionally, the test rig results were compared to the results obtained from the extensive track test network of voestalpine and Kelsan to outline similarities and differences between test rig conditions and the conditions in track. The combination of rail grade, FM application and an appropriate maintenance strategy is seen to have a significant effect on the development and propagation of wear and RCF. This offers a huge potential for reducing the track LCC costs for the infrastructure owners.

Subsurface microstructural analysis and mechanical properties of pearlitic rail steels in service

Ductile materials commonly exhibit plastic deformation at and near the contact surface and their flow behaviour at large strain has a clear effect on wear resistance. These materials almost always fail while under high levels of compression, but behaviour under these conditions cannot be investigated by standard materials tests at atmospheric pressure. In this work, the characteristics of the near-surface region of one new and four used rails are examined using samples from rails which have been in service in the UK. The results are presented as stress–strain response curves from the materials under high hydrostatic compression conditions, which are typical of rail–wheel contacts. In addition, shear strain and hardness variation with depth below the worn rail surface are presented, along with micrographs of the sites examined. A thin white surface layer, frequently called white etching layer (WEL), with high hardness (up to 1040HV) was observed on the surface of one of the used rails. These results have application in the modelling of rail failure by wear and in modelling rolling contact fatigue crack initiation, and can therefore be used to improve rail maintenance planning and risk assessments.

OS1504 放射光μCT及び小型転動疲労試験機を用いた転動疲労き裂進展過程の観察

In rolling contact fatigue (RCF), cracks are usually initiated from inclusions under the surface and propagated to the surface. In the present study, the micro computed tomography (μCT) imaging and laminography using synchrotron radiation in SPring-8 is applied for observation of RCF cracks. The compact RCF testing machine is developed to observe RCF crack behavior m-situ. In this testing machine, the steel ball rolls on the specimen, linearly and reciprocally, and a specimen can be attached and removed easily. Since the size of the sample for μCT imaging has to be limited by penetrative power, the sample for μCT imaging has 500μm×500μm cross-section. On the other hand, laminography can be applied to a sheeted material. The sample, where an artificial defect is introduced, is used for observation of crack initiation and propagation. It is found from this experiment that cracks initiate and propagate in the depth direction along the artificial hole with increasing the number of cycles. Flaking and RCF crack in sheeted sample are observed by laminography. RCF cracks which initiate and propagate under contact surface can be detected. The shape and location observed by laminography coincide with that by SEM.

Rolling contact fatigue (RCF) defects of rails in Japanese railways and its mitigation strategies

Rail is the one of the most important materials to support and guide railway vehicles safely and smoothly. Since rail sufferes from various interacting forces and environmental atmosphere, wear and fatigue pose large problems with wheel and rail. Hence, wear and fatigue of wheel and rail have been studied so far to keep running safety and some level of riding comfort of vehicle taking into account track maintenance cost. In this review, rolling contact fatigue (RCF) of rail which is one of typical fatigue phenomenon for steel wheelon- rail system is focused on and the history of RCF defects and the maintenance experience of their mitigation measures in Japanese railways are described. The concept of mitigation strategy is balance between wear and RCF. Controlling wear amount is a key word to mitigate RCF defects based on selecting rail material suitable for vehicle/track interaction together with grinding and lubrication. Furthermore, the purpose of Japanese bainitic steel rail is to obtain the suitable amount of wear to prevent the initiation of RCF crack.

Effects of sliding on rolling contact fatigue of railway wheels

ABSTRACTThis paper proposes a numerical approach based on a steady‐state algorithm to predict the rolling contact fatigue crack initiation in railway wheels in practical conditions. This work suggests taking into account the cyclic hardening of the wheel's material and one of its originality is to conduct a complete numerical approach whatever the loading level. The main stages are the characterization and modelling of the material behaviour, the determination of the stress–strain fields using a numerical steady‐state method and the application of a high cycle fatigue criterion. Computations were made with the Abaqus FE commercial software. Three cases are studied: rolling with or without sliding and skating. The numerical results give several types of mechanical responses: elastic or plastic shakedown. Otherwise, the results show that the location where the shear stress is maximal is not the same as where the risk of crack is the highest.

Detection and evaluation of rail surface defects using alternating current field measurement techniques

Reliable and cost‐effective inspection of rail tracks is of paramount importance to ensure the safety of rolling stock operations. In this paper alternating current field measurement (ACFM) sensors are used to carry out experiments on artificially induced rail surface defects at various speeds using testing configurations that simulate actual inspection conditions found in the field. From the obtained results it can be clearly seen that the ACFM sensors can detect the artificially induced rail surface defects even when relatively significant lift-off is involved, i.e. ∼5 mm and that the effect of increasing speed on the amplitude of the Bx signal, which is directly related to the depth of the crack, is negligible. However, clustered defects cannot be easily resolved and the overall amplitude is related to the spacing of the defects within a cluster. The order of clustered defects also significantly influences the maximum amplitude of the recorded Bx signal. The validity of the results obtained from the tests on artificially induced defects was verified by conducting further ACFM tests on a rail sample removed from service that contained mild rolling contact fatigue cracks.

Micro-mechanism of rolling contact fatigue in Hadfield steel crossing

The formation of the fatigue cracks was due to massive vacancy clusters in the subsurface layer of Hadfield steel crossing, which are induced by the accumulated plastic deformation under the conditions of impact and contact stresses from train wheels. The high concentration layers of vacancy clusters were formed parallel to the working surface of the crossing, which caused the initial rolling contact fatigue cracks to be parallel to the working surface with a laminar distribution in the depth direction. It can be predicted that metals containing elements with larger atomic diameter should have better rolling contact fatigue and wear performances.

Metallurgical and physical understanding of rail squat initiation and propagation

This paper describes the initiation and propagation behaviour of squat-type cracks in rail steel using mechanical and metallurgical analysis simultaneously. Rails with squat defects were removed from the site and inspected visually, ultrasonically, optically and by using electron microscopy. The contact stresses on a rail from a wheel are critically evaluated and rolling contact fatigue crack direction, initiation and propagation have been evaluated using stress theory and validated with metallurgical observations. The hardness of the rail surface was much higher than that of the adjacent rail matrix and this was comparable with that of a white etching layer. Squats were observed near the white etching layer. The squats formed an angle of 20–30° with the horizontal surface. Ultrasonic testing predicted the actual shape of the squat under the surface. Scanning electron microscopic observation revealed differences in the rolling contact behaviour between the direction of train travel and the opposite direction. Based on metallurgical research, a mechanism for squat initiation and propagation has been proposed in this paper.

Modelling a squat form crack on a rail laid on an elastic foundation

Rolling contact fatigue cracks in railway track called squats are studied in this paper. In the first part, the effects of an elastic foundation (sleepers and the ballast) on stress intensity factors obtained at a crack tip are studied. A simplified finite element model (FEM) and an extended finite element model (XFEM) were created to investigate these effects, the XFEM model being limited in geometrical size, but more able to model crack growth. Both FEM and XFEM confirmed that an elastic foundation leads to an additional bending stress which increases the crack growth rate significantly. Field results also authenticate that squat form cracks appear on timber sleepers more commonly than on concrete ones. These results indicate that considering these bending stresses in a FE model, is important to achieve a more realistic model of squat development. In the second part, a short crack of 250μm length is simulated to investigate how variations of traction ratio (TR), friction coefficient between the crack faces (FC) and the crack angle affect SIFs when the rail is mounted on an elastic foundation. Simulations show that a crack on a rail laid on elastic foundation (clips, sleepers and ballast) can lead to significantly higher SIFs in many conditions and consequently raises crack growth rate. This indicates that foundation stiffness is as important as water entrapment and friction coefficient between the crack faces.

Modeling Stress Intensity Factor of Rail Steel under Situation of Growing Fatigue Crack—A Novel Technique

Simulation of stress intensity factor as function of rolling contact fatigue cracks of railway tracks and the vehicle load is made with the help of COMSOL Multiphysics software. It is found that the critical stress intensity factor i.e. 41.6 MPa. m1/2 is reached at a stress level of 32 MPa and at the crack size 11.5 × 10-2 m.Noting the power law variation of acoustic emission count with increase in crack size (analogous to Paris Law), the simulation was further carried out to model the dependence of measured AE count with the stress intensity factor ahead of a growing RCF crack tip. It is demonstrated that AE measurement can be effective to trigger a control loop for avoidance of fatigue failure of railway track. In view of potential difference in the intensity of back scattered light from surface irregularities, a model is developed to find out the threshold intensity of scattered light that insures safety in the railway system against fatigue failure.

Wheel surface damage: relating the position and angle of forces to the observed damage patterns

A new method of presenting simulated wheel/rail forces and relating these to the observed wheel damage has been developed. This indicates a good correlation between the predicted forces and the regions of the wheel where damage is observed in practice. There is also a good correlation between the angle of the predicted forces and the observed cracks. The angle evidence suggests that the dominant rolling contact fatigue cracks on the field side of the wheel tread are initiated by the occasional high forces when the opposite wheel is running in flange contact on sharp curves. Cracks may then be propagated by more frequent lower forces on moderate curves.

Using bogie-mounted sensors to measure wheel rolling and sliding on railway tracks

The goal of this paper is to present both the development of an intelligent sensor that can be used to obtain information about the existence of rolling contact fatigue cracks created by wheel–rail interactions and also the data obtained in a real-world study performed for its validation. The design of a bogie-mounted instrumentation system to detect and measure wheel rolling in railway units is introduced, and experimental results are presented. The system is designed to analyse and compare the angular velocity of the wheels of a railway unit. It uses photodiodes to acquire information about rolling conditions and a radio frequency link to send that information to a data logger, which acquires and saves the data for post-processing and analysis.

Finite element modelling of the rail gauge corner and underhead radius stresses under heavy axle load conditions

Highly localized stresses generated at the gauge corner of the outer rail cause fatigue cracking. The longitudinal bending stresses in the rail underhead radius position are of special interest, as tension spikes have been identified at this location during in-track measurements under high axle load conditions. The tension spike is a result of vertical and lateral head bending on the web. This effect is highly localized and is additional to the stresses generated due to vertical and lateral bending of the whole rail profile (the so-called global bending). This study examined contact and bending stresses by modelling the rail on a discrete foundation using finite elements and by considering the loading to be a fully slipping elliptical Hertzian contact patch. The analysis revealed that the tensile longitudinal stress was highly dependent on several service conditions: the contact patch offset from the rail centreline, the ratio of lateral ( L) to vertical ( V) loads, the direction of lateral traction, foundation stiffness and seasonal temperature variations. The tension spike increases, and the depth below the contact surface at which the stresses become tensile reduces, as the contact patch offset and/or the L/ V ratio increases. Both these enhance the tendency for rolling contact fatigue cracks to turn downwards and become transverse defects. This is because an increase in tensile bending stresses together with both residual and thermally induced stresses can facilitate cracks to turn perpendicular to the tensile stresses once they reach a critical length. Shear traction towards the gauge corner was found to be the most damaging phenomenon.

Rolling Contact Fatigue Crack Growth Prediction by the Asperity Point Load Mechanism

The crack path and growth life of surface initiated rolling contact fatigue was investigated numerically based on the asperity point load mechanism. Data for the simulation was captured from a gear contact with surface initiated rolling contact fatigue. The evolvement of contact parameters was derived from an FE contact model where the gear contact had been transferred to an equivalent contact of a cylinder against a plane with an asperity. Crack propagation criteria were evaluated with practically identical crack path predictions. It was noted that the trajectory of largest principal stress in the uncracked material could be used for the path prediction. The mode I fracture mechanism was applicable to the investigated rolling contact fatigue cracks. The simulated path agreed with the spall profile both in the entry details as in the overall shape, which suggested that the point load mechanism was valid not only for initiation but also for rolling contact fatigue crack growth. Different equivalent stress intensity factor ranges were used to estimate the fatigue life, which agreed with the life of the investigated gear wheels.

The effect of slip ratio on the rolling contact fatigue property of railway wheel steel

Shelling is one of the typical rolling contact fatigue (RCF) failures of railway wheels. Creepage between wheels and rails due to rolling radius difference in curves is inevitable on railway tracks. Therefore, there are tangential stresses and slip phenomena on wheel treads. Eventually, initiation of RCF cracks being the origin of shelling is accelerated by creepage. The objective of the present paper is to evaluate RCF property of railway wheel steel. The effect of the slip ratio on the RCF strength of the railway wheel steel was evaluated. RCF tests were conducted using two cylindrical contact specimens under water lubrication at a slip ratio of 0.0–1.0%. The range of slip ratio is determined to assume a creepage between wheel and rail during curving. As a result, it was found that the traction coefficient increased with the increase in the slip ratio, and the fatigue strength decreased simultaneously. The results were evaluated by a shakedown map and Hirakawa’s RCF map. Experimental fatigue limits could be expressed more precisely by the criterion of Hirakawa’s RCF map than that expressed by the well-known shakedown map. Stress intensity factors (SIFs) of the cracks produced by the RCF test were calculated by finite element (FE) analysis where the effect of water pressure due to the penetration of water into the cracks is taken into account. Two peaks of the maximum tangential SIF occurred during one cycle of rolling contact. The direction of the crack propagation was estimated by the maximum tangential stress criteria. The results of the RCF test showed that the cracks were initiated at the surface, propagated obliquely in the depth direction and then branched into two directions. One was towards the surface and the other was towards the depth. These two crack directions were inspected experimentally, corresponding to the estimated crack directions from the two peaks of SIF by FE analysis. The effect of slip ratio on RCF crack propagation was discussed by using the SIFs of the RCF cracks where water penetration into the cracks was considered.

Rolling contact fatigue crack path prediction by the asperity point load mechanism

The crack path of surface initiated rolling contact fatigue was investigated numerically based on the asperity point load mechanism. Data for the simulation was captured from a gear contact with surface initiated rolling contact fatigue. The evolvement of contact parameters was derived from an FE contact model where the gear contact had been transferred to an equivalent contact of a cylinder against a plane with an asperity. Five crack propagation criteria were evaluated with practically identical crack path predictions. It was noted that the trajectory of largest principal stress in the uncracked material could be used for the path prediction. Different load types were investigated. The simplified versions added some understanding but the full description with cylinder and asperity pressures was required for accurate results. The mode I fracture mechanism was applicable to the investigated rolling contact fatigue cracks. The simulated path agreed with the spall profile both in the entry details as in the overall shape, which suggested that the point load mechanism was valid not only for initiation but also for rolling contact fatigue crack growth.

PEC thermography for imaging multiple cracks from rolling contact fatigue

With the development and operation of high speed trains, condition based maintenance becomes an important approach for the improvement of reliability and safety of rail transportation. This paper reports a feasibility study using pulsed eddy current thermography for imaging multiple cracks caused by rolling contact fatigue (RCF). After reviewing rail track inspection and RCF cracks, a PEC thermography system is introduced and applied to the imaging of multiple rolling contact fatigue cracks. Potential on-line inspection for rail track is also discussed.