Effects of sliding on rolling contact fatigue of railway wheels

This 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.     

Life prediction for rolling contact fatigue crack initiation of kiln wheels

This paper investigates the rolling contact fatigue life of kiln wheels with respect to the axis line deflection related with the applied supporting loads on wheels. Fatigue crack initiation criterions for elastic shakedown, plastic shakedown, and ratcheting material responses are applied to assess wheels responses with two sets of axial line deflection, one is measured in field and the other is optimal adjustment for the measured axial line deflection. The finite element simulations are performed by using the Bilinear material mode for nonlinear and kinematic hardening within ANSYS 11.0. By comparing life prediction from different criterions, it is showned that the low-cycle fatigue is the predominated failure. Results from different axial line deflection indicate that the optimum adjustment can greatly enhance the whole life of the supporting structure, that is useful for kiln adjustment and maintenance.     

Fatigue design of railway wheels: a probabilistic approach

This paper deals with a method to evaluate and optimize the design of railway wheels subjected to multiparameter variable fatigue loading. The fatigue loads are statistically evaluated from in‐service measurements. Representative realistic loading paths are built from the knowledge of the influence of various factors (such as train speed and track curvature). Using these paths, the method combines finite element computations and the fatigue equivalence method for damage evaluation in the structure. An extension of the Dang Van fatigue criterion in the high‐cycle fatigue finite life domain associated with a damage accumulation law is adopted. The probability of failure of the structure is directly obtained from the interference between a local fatigue equivalent stress and fatigue strength distributions (based on the stress–strength interference approach). The result is useful for the optimization during the design stage or the validation of the fatigue strength of structures.     

Interaction between cracks and microstructure in three dimensions for rolling contact fatigue in railway rails

The rail–wheel contact generates plastic deformation and cracks in the top layer of a rail. Rolling contact fatigue (RCF) cracks in rail samples from track and from a full scale test rig were examined. Due to the shear forces arising in the wheel rail contact, the microstructure close to the surface becomes aligned in the shear direction. Thereby, the pearlite becomes anisotropic, and resistance to cracks is lower in certain directions. RCF cracks follow the weakest direction of the microstructure, which in pearlitic railway rails is the aligned pearlite structure or singular weaknesses such as pro‐eutectoid ferrites or slags. The deformation of the microstructure is different depending on loading situation and original microstructure (rail grade). Once the plastic deformation is present, the cracks follow the path of the weakest crack resistance. Cracks close to each other can interact or shield each other; it is unclear, however, to what extent. In this paper, a new method is described that allows the presentation of RCF cracks in three dimensions.     

Analysis of subsurface crack propagation under rolling contact loading in railroad wheels using FEM

A general subsurface crack propagation analysis methodology for the wheel/rail rolling contact fatigue problem is developed in this paper. A three-dimensional elasto-plastic finite element model is used to calculate stress intensity factors in wheels, in which a sub-modeling technique is used to achieve both computational efficiency and accuracy. Then the fatigue damage in the wheel is calculated using a previously developed mixed-mode fatigue crack propagation model. The advantages of the proposed methodology are that it can accurately represent the contact stress of complex mechanical components and can consider the effect of loading non-proportionality. The effects of wheel diameter, vertical loading amplitude, initial crack size, location and orientation on stress intensity factor range are investigated using the proposed model. The prediction results of the proposed methodology are compared with in-field observations.     

Size effect in contact fatigue

The results of special experiments on the size effect in contact fatigue are presented. It is established that under constant contact loading conditions the durability is higher, the larger is the diameter of a tested cylindrical element. The methods for estimation of contact fatigue resistance of gear wheels, which is based on the statistical model for a deformable solid body having a critical volume, are proposed. The limiting stresses of a gear wheel are estimated using a regulated base for this machine parts. __________ Translated from Problemy Prochnosti, No. 1, pp. 113–120, January–February, 2009.     

Comparative assessment of fatigue and fracture behaviour of cast and forged railway wheels

A comparative evaluation of fatigue and fracture behaviour of commercially produced cast and forged rail wheels has been made using specimens extracted from various locations of the wheel quadrant. A systematic investigation in the web and rim regions of the wheel quadrant with various notch orientations showed that the forged material exhibited a better intrinsic resistance to fatigue crack growth than the cast material. Since linear elastic fracture mechanics (LEFM) based fracture toughness could not be validated for both the cast and forged wheel material, elastic plastic fracture mechanics (EPFM) based characteristic fracture toughness was used. Results showed that fracture resistance of the forged material is superior to that of the cast material. Cast wheel specimens exhibited unstable crack extension in comparison to substantial stable tearing in forged specimens. Microstructural and fractographic analyses showed that the cast wheel material contained large amounts of inclusions. The poor fracture resistance of cast wheel material is therefore attributed to the inferior material quality.     

On the Ekberg, Kabo and Andersson calculation of the Dang Van high cycle fatigue limit for rolling contact fatigue

Recently, various methods have been proposed to assess the risk of rolling contact fatigue failure by Ekberg, Kabo and Andersson, and in particular, the Dang Van multiaxial fatigue criterion has been suggested in a simple approximate formulation. In this note, it is found that the approximation implied can be very significant; the calculation is improved and corrected, and focused on the study of plane problems but for a complete range of possible friction coefficients. It is found that predicted fatigue limit could be much higher than that under standard uniaxial tension/compression for ‘hard materials’ than for ‘ductile materials.’ This is in qualitative agreement, for example, with gears' design standards, but in quantitative terms, particularly for frictionless condition, the predicted limit seems possibly too high, indicating the need for careful comparison with experimental results. Some comments are devoted to the interplay of shakedown and fatigue.     

Roles of microstructure,inclusion, and surface roughness on rolling contact fatigue of a wind turbine gear

Contact fatigue is a key feature limiting the service lives and reliabilities of gears. The gear contact fatigue failure mechanism has not been understood fundamentally due to the complexities of structural factors, material properties, and operating conditions. In this work, an integrated finite element model of a megawatt level wind turbine gear is established considering the real gear geometry, material microstructure heterogeneity, existence of nonmetallic inclusion, and the tooth surface roughness. The gear steel material properties are defined based on the crystal elasticity anisotropy framework. The modified Dang Van multiaxial criterion is utilized to estimate the material fatigue failure probability during gear engagement. With the developed model, the roles of microstructure, inclusion, and surface roughness on the gear contact fatigue behaviour are comparatively investigated. Additionally, the influences of different inclusion size and surface roughness profile on gear failure risk are investigated and discussed in detail.     

Fatigue analysis of railway wheel using a multiaxial strain‐based critical‐plane index

A fatigue damage model to assess the development of subsurface fatigue cracks in railway wheels is presented in this paper. A 3‐dimensional finite element model (FEM) is constructed to simulate repeated cycles of contact loading between a railway wheel and a rail. The computational approach includes a hard‐contact over‐closure relationship and an elastoplastic material model with isotropic and kinematic hardening. Results from the simulation are used in a multiaxial critical‐plane fatigue damage analysis. The employed strain‐based critical‐plane fatigue damage approach is based on Fatemi‐Socie fatigue index that takes into account the non‐proportional and out‐of‐phase nature of the multiaxial state of stress occurs when a railway wheel rolls on a rail. It predicts fatigue‐induced micro‐crack nucleation at a depth of about 3.7 mm beneath the wheel tread, as well as the crack plane growth orientation which indicates the possible failure pattern. Additionally, the influence of various factors such as contribution of normal stresses, higher wheel load, and material model have been investigated.     

On propagation of short rolling contact fatigue cracks

Recent accidents involving railway rails have aroused demand for improved and more efficient rail maintenance strategies to reduce the risk of unexpected rail fracture. Numerical tools can aid in generating maintenance strategies: this investigation deals with the numerical modelling and analysis of short crack growth in rails. Factors that influence the fatigue propagation of short surface‐breaking cracks (head checks) in rails are assessed. A proposed numerical procedure incorporates finite element (FE) calculations to predict short crack growth conditions for rolling contact fatigue (RCF) loading. A parameterised FE model for the rolling‐sliding contact of a cylinder on a semi‐infinite half space, with a short surface breaking crack, presented here, is used in linear‐elastic and elastic–plastic FE calculations of short crack propagation, together with fracture mechanics theory. The crack length and orientation, crack face friction, and coefficient of surface friction near the contact load are varied. The FE model is verified for five examples in the literature. Comparison of results from linear‐elastic and elastic–plastic FE calculations, shows that the former cannot describe short RCF crack behaviour properly, in particular 0.1–0.2 mm long (head check) cracks with a shallow angle; elastic–plastic analysis is required instead.     

Fracture toughness of nanostructured railway wheels

This paper describes nanostructured railway wheels made of Si–Mn–Mo–V low-carbon steel through an advanced metallurgy process and fabrication technology. The microstructure of the wheels, particularly in the rim portion, is composed of carbide-free bainite that consists of bainitic ferrite laths and retained austenite films along the lath boundaries. The thickness of the laths and films is in nanometer scale. For comparison, traditional pearlite–ferrite wheel steels are also investigated. Test results show that carbide-free bainite steel is superior to pearlite–ferrite steel not only in yield strength but also in fracture toughness. Theoretical explanation of these phenomena is also elucidated.     

An investigation on rotatory bending fretting fatigue damage of railway axles

Fretting damage failure analysis of a Chinese carbon railway axle RD₂ was carried out. The wheel hub was in situ cut to expose the damaged surface of the wheel seat to avoid additional damage. A small‐scale axle test rig was developed, and simulation tests were performed at different rotator speeds of 1800 and 2100 rpm. The wear mechanism of fretting damage areas was a combination of abrasive wear, oxidative wear and delamination. The fracture surfaces exhibited characterization of multisource and step‐profile. The fretting fatigue crack initiated at the subsurface and propagated along an inclined angle at the first stage. The fretting damage at the higher speed was more severe compared with the lower speed, which lead to a relatively shorter fatigue life. The damage morphologies of the axle in the simulation tests were in good agreement with that observed in the failure analysis on real axle.     

Accounting for microstructure sensitivity and plasticity in life prediction of heavily loaded contacts under rolling contact fatigue

Probabilistic models employed for life prediction in the bearing industry (Lundberg‐Palmgren [LP], Ioannides‐Harris [IH], and Zatersky models) utilize stresses calculated based on Hertzian assumptions that ignore plastic deformation occurring under high radial loads. However, it is known that subsurface plasticity produces significant deviations from elastic calculations in contact dimensions, pressure, and stresses. In this work, we show that accounting for plastic deformation leads to significantly more accurate life predictions than the general practice of utilizing elastic stress fields. Conventional probabilistic models are also restricted to utilizing stress measures at the macroscopic scale, while the onset of damage under rolling contact fatigue (RCF) manifests on a much smaller scale. By utilizing Dang Van criterion (DVC)—a multiscale high cycle fatigue criterion—in a probabilistic framework to account for stresses at multiple length scales, we propose a formulation that is second in accuracy only to the Zaretsky model, thus outperforming LP and IH models. Thus, we improve the accuracy of conventional probabilistic models by accounting for plastic deformation, while also presenting a new microstructure‐sensitive probabilistic formulation that betters their predictions.     

A356‐T6 wheels: Influence of casting defects on fatigue design

Rotating bending fatigue tests were executed on A356‐T6 wheels. The tests were performed both on components containing only the casting defects and on wheels in which artificial holes were machined. Results show the detrimental effect of sub‐superficial porosities on fatigue endurance. A predictive model was proposed. It resulted in a useful tool for the designer because knowing the applied stress, it is possible to estimate the number of cycles for crack nucleation considering the size of the defect, whereas knowing the endurance target requested it is possible to find the maximum defect size acceptable considering the applied stress.     

A numerical approach to subsurface crack propagation assessment in rolling contact

Subsurface crack mode II propagation parallel to the contact surface is a damage mechanism leading to dramatic failure in many components subjected to cyclic loading. A weight function (WF) was elaborated for calculating the applied mode II stress intensity factor (SIF) of a crack in a two‐dimensional half‐space in plane strain condition, for crack completely closed and frictionless contact between the crack faces. With respect to other methods, the WF allows faster SIF calculation, thus being suitable for simulation of many repeated load cycles and fatigue crack propagation. The WF was applied for simulating a case of rolling contact experiments found in the literature, and good agreement between experimental and numerical results was obtained, showing the effectiveness of the WF method in damage tolerant design.     

Evaluation of fatigue life of aluminium alloy wheels under bending loads

This paper presents the details of S–N curve for aluminium alloy (Al) A356.2‐T6 and fatigue life of alloy wheels under bending load of cornering fatigue test (CFT). Development of S–N curve has been carried out by conducting rotary bending fatigue test at different stress levels as per Standards IS 5075. The rotary bending fatigue test has been performed under constant amplitude fatigue loading. The CFT of the wheel in normal driving mode has been carried out as per the procedure given in Japanese Industrial Standard Disc Wheels (JIS D_4103). It has been observed from the test that the cracks are initiated at the spoke and hub joining area closer to spanner hole on the front face of the wheel. Fatigue life of the alloy wheel has been predicted by finite element analysis (FEA), simulating the realistic test conditions. From finite element analysis, it has been observed that the maximum stress occurs at the mounting face of the wheel. Further, it has been observed that there is significant difference between the computed fatigue life and experimental value. Parametric study has been carried out for reliable fatigue life estimation and proposed an appropriate safety factor for fatigue life estimation under rotary bending test.     

A refined CLNA model in fretting fatigue using asymptotic characterization of the contact stress fields

Using the Atzori–Lazzarin criterion, the first author has recently proposed a unified model for fretting fatigue (FF), called the crack‐like notch analogue (CLNA) model. Two possible types of behaviour were suggested: either ‘crack‐like’ or ‘large blunt notch,’ and these are immediately studied in the typical condition of constant normal load and in phase oscillating tangential and bulk loads. The former condition (‘crack‐like’) was treated by approximating the geometry to the perfectly flat one of the crack analogue (CA), improved in some details, reducing all possible geometries to a single contact problem. The latter (‘large blunt notch’), with a simple peak stress condition i.e. a simple notch analogue model. In the present paper, the calculation of the ‘crack‐like’ behaviour is improved using the recent asymptotic characterisation developed by Dini, Hills and Sackfield, which extracts the asymptotic singular stress field of the fretting contact. A significant difference is found in the ‘equivalent’ geometric factor obtained for the Hertzian geometry, particularly near full sliding, where the new criterion is more conservative, but still not large enough to permit to find, for example in Nowell's FF experimental data, if the refinement is an improvement of predictive capabilities. In flatter geometries, the difference is expected to be even smaller than in the case of the Hertzian geometry, and in this case, the original CLNA model, for its simplicity, remains a very convenient simple closed form criterion.     

Retained austenite stability on rolling contact fatigue performance of 8620 case-carburized steel

Varying levels of retained austenite (RA) were achieved through varying undercooling severity in uniformly treated case carburized 8620 steel. Specimens were characterized via XRD and EBSD techniques to determine RA volume fraction and material characteristics prior to rolling contact fatigue (RCF). Higher RA volume fractions did not lead to improvement in RCF lives. XRD measurements after RCF testing indicated that little RA decomposition had occurred during RCF. A continuum damage mechanics (CDM) finite element model (FEM) was then developed to investigate the effects of RA stability on RCF. The results obtained from the CDM FEM captured similar behavior observed in the experimental results. Utilizing the CDM FEM, a parametric study was undertaken to examine the effects of RA quantity, RA stability, and applied pressure on RCF performance. The study demonstrates that the energy requirements to transform the RA phase are critical to RCF performance.