Initiated Rolling Contact Fatigue Research Articles

Effect of primary carbides on rolling contact fatigue behaviors of M50 bearing steel

It is perceived that the fatigue performance of high-strength steel is preponderantly affected by internal defects. Inclusions may be efficaciously controlled and primary carbides have been proved to be the principal type of defects in M50 bearing steels. Primary carbides in service-status M50 were systematically characterized and the rolling contact fatigue (RCF) properties were evaluated under different friction conditions in this work. An innovative method of shakedown analysis was proposed for the first time, extending the shakedown analysis of materials as an integral based on traditional shakedown theory to that of weaker internal defects. By the response of primary carbides, the RCF process was separated into a plastic instability period and an elastic shakedown period. The accumulation of plastic deformation around carbides before shakedown was proposed as the RCF damage parameter, and the influence of primary carbides was studied based on the damage criterion. The competition between surface and internal fatigue initiation was revealed via different friction coefficients. A predictive model for RCF initiation was established, which could predict the critical sizes for disparate types of primary carbides not deleterious to RCF performance, and the critical sizes under typical industrial service condition were reckoned.

Influence of railway wheel tread damage on wheel–rail impact loads and the durability of wheelsets

Dynamic wheel–rail contact forces induced by a severe form of wheel tread damage have been measured by a wheel impact load detector during full-scale field tests at different vehicle speeds. Based on laser scanning, the measured three-dimensional damage geometry is employed in simulations of dynamic vehicle–track interaction to calibrate and verify a simulation model. The relation between the magnitude of the impact load and various operational parameters, such as vehicle speed, lateral position of wheel–rail contact, track stiffness and position of impact within a sleeper bay, is investigated. The calibrated model is later employed in simulations featuring other forms of tread damage; their effects on impact load and subsequent fatigue impact on bearings, wheel webs and subsurface initiated rolling contact fatigue of the wheel tread are assessed. The results quantify the effects of wheel tread defects and are valuable in a shift towards condition-based maintenance of running gear, and for general assessment of the severity of different types of railway wheel tread damage.

The effect of friction on surface crack initiation in rolling contact fatigue considering damage accumulation

AbstractSurface 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.

Numerical study on ratcheting performance of heavy haul rail flash-butt welds in curved tracks

Ratcheting at the rail heads due to cyclic rolling contact stresses is often observed in heavy haul railways, especially when there are large steering forces developed in low-radius curves. Additionally, ratcheting behaviour can be exacerbated when associated with material strength loss in the heat affected zones of rail welds. However, the numerical evaluation of this failure mode requires a sophisticated representation of elastic–plastic contact pressures and traction distributions, which, to the best of the authors’ knowledge, cannot be directly measured by any existing tools. In this paper, in order to accurately evaluate the ratcheting performance at rail welds in curved tracks, attempts were conducted to modify the FaStrip algorithm to analytically estimate the traction distributions based on contact pressures and creepages obtained from static finite element analysis and multi-body dynamic simulations, respectively. Cyclic rolling contact was then simulated by repeatedly applying contact pressure and traction on the rail surface consisting of both the rail weld and the parent rail regions. The weld region was divided into 23 sub-zones in the rolling direction to represent the material inhomogeneity within the heat affected zones. Cases on a 2000 m radius curved track and a tangent track were studied for comparison. It was found that the ratcheting strain rate in curved tracks can be significantly elevated due to the higher magnitude of traction, and the location of the elements with the highest ratcheting strain rate was dependent on the traction distribution patterns. The proposed method provides a general tool for predicting rolling contact fatigue initiation life and location, which will benefit the maintenance efficiency of heavy haul rail welds in curved track.

Effect of superimposed compressive stresses on rolling contact fatigue initiation at hard and soft inclusions

A semi-analytical framework is introduced for the evaluation of favorable residual stresses for delaying rolling contact fatigue initiation. Subsurface stress histories are applied to a micro-scale model accounting for isolated inclusions of different types and geometries. Micro-scale von Mises stresses are calculated based on Eshelby’s method and considered as an estimator of crack initiation due to plastic deformation. The most critical cases in terms of micro-scale plasticity are identified, and the effect of macro-scale compressive residual stresses is considered. Finally, optimized residual stresses are determined that minimize the maximum attained micro-scale von Mises stress at different depths.

Impact of wheel profile evolution on wheel-rail dynamic interaction and surface initiated rolling contact fatigue in turnouts

Hollow wear is a common type of wheel profile evolution in high-speed railway, causing special contact characteristics in turnouts. Taking into account the flexibility of wheelsets and rails in turnout as well as complex constraint conditions, the vehicle-turnout coupling model is built and wheel-rail dynamic interaction in turnouts is analyzed. After calculating the wheel-rail non-Hertz contact behavior with wheel profile evolution, the accumulated surface initiated rolling contact fatigue (RCF) of turnouts are evaluated based on the shakedown theory. Results indicate that the wheel transition position is shifted backwards with wheel profile evolution. The vertical wheel-rail force is contributed by wheelset bending modes in the switch panel, and its maximum value decreases by 22% due to the 87% reduction in vertical contact irregularity in the crossing panel. The severe contact between the false flange and wing rail leads to the aggravation of lateral axle box acceleration (ABA) while the positions of its characteristic frequencies are changed from the point rail to wing rail. When the degree of hollowing becomes medium, wheel profile evolution improves the conformal contact that decreases the accumulated surface initiated RCF in the switch panel. As the degree of hollowing becomes severe, the false flange's contact with the wing rail is prone to RCF in the crossing panel.

Assessment of non-Hertzian wheel-rail contact models for numerical simulation of rail damages in switch panel of railway turnout

Rail damages in railway turnout switch panels are closely related to the complicated wheel-rail contact conditions in the turnouts. In order to assess the applicability of different non-Hertzian contact models for simulating rail damages in railway turnout switch panel, three engineering approaches, i.e. Kik-Piotrowski, Ayasse-Chollet and Sichani, are adopted to simulate rail wear and surface initiated rolling contact fatigue (RCF) damages by studying a Chinese CN60-1100-1:18 high-speed turnout switch panel. A comparison is then carried out taking into account the results calculated by Kalker’s three-dimensional (3D) theory and an elastic-plastic FEM (Finite Element Method) model. The analyzed results show that the computational efficiencies of the three engineering approaches are all quite high for simulating rail damages in switch panel. Under elastic conditions, the calculation accuracy of the Sichani approach is found to be the highest, meaning that this would appear to be the preferred method for simulating rail damages in railway turnout switch panel. The research findings can be taken as theoretical guidance for selecting wheel-rail contact models and analyzing these types of damages in railway turnout switch panels.

Model based detection and diagnosis of bearing defects

In this paper we present the Amplitude Spectral Density of Spectrogram [1] (ASDS) and how its output is post processed for determining the size of defects in bearings using acceleration signals. Fatigue modeling of the bearing is then used to define the moment to stop the operation and replace the bearing. Initially, small defects in the order of a 0.1 to 1 millimeter may arise in the running surface of the bearing. This is the moment when vibrations start to be excited by the rolling contact forces and can be observed by accelerometers on the structure of the machine. At this stage most machines are still operating well. The rolling bearing modeling work shows that characteristic bandwidth of the force impulse for defects smaller than the Hertzian length approximates the speed rolling divided by the sum of the length of the defect and length of the Hertzian contact. This simple result has been proven experimentally by running a bearing with a small defect and varying the running speed, and by varying the size of the Hertzian contact by changing the load [1]. When the small defect grows, the amplitude of the acceleration is growing with defect size but it’s bandwidth is hardly changing until the Hertzian contact is completely disrupted. At that point the force impulse’s bandwidth starts going down as the defect length becomes the dominant factor. This property is used to calculate the actual size of the defect in order to avoid operational disruption and the decision to stop is based on fatigue modeling. Eventually when the defect is about 20 percent of the roller spacing in the bearing, the bandwidth starts to become similar to the repetition frequency and the ASDS transform will no longer be able to estimate it. Experimental work with artificial defects proves the size quantification. The diagnosis with defect size quantification makes future prognosis possible. For small defects, we have proof that the defect’s rate of growth is slow [2], [3]. When the defect passes a certain size stage which depends on the load, the erosion process will exponentially increase the area damaged in the bearing . Using rolling contact fatigue modeling, under the condition of defects in a rolling bearings, it is possible to predict the likely defect growth rate [2],[4]. By fitting the estimated defect size to the exponential growth rate curve, the decision to replace the defective bearing can be deferred to the right moment without much risk of immediate failure. [1] Mol, H.A., van Nijen, G.C., “Method for Analysing Regularly Recurring Mechanical Vibrations”, Patent US5698788. Published December 16, 1997. [2] Morales-Espejel G.E., Gabelli A., “The progression of surface rolling contact fatigue damage of rolling bearings with artificial dents”. Tribology Trans. (2015); 58: pp. 418–31. [3] Rycerz et al: Propagation of surface initiated rolling contact fatigue cracks in bearing steel, International Journal of Fatigue 97 (2017) 29-38. [4] Gabelli A., Morales-Espejel G.E., “Improved Fatigue Life Analysis of Pre-Dented Raceways Used in Bearing Material Testing”, Bearing Steel Technologies 11th Volume, “Progress in Steel Technologies and Bearing Steel Quality Assurance”, ASTM STP1600, 2017, pp. 167–191 [5] A.V. Oppenheim and R.W. Schafer, "Digital Signal Processing", Englewood Cliffs, NJ: Prentice Hall, 1975, pp 358 ff. [6] D.A. Rice, V. Venkatachalam, M.J. Wegmann: "A simple envelope detector". IEEE transactions on instrumentation and measurement, vol 37, no. 2, June 1988, pp 223-226.

Parameters Studies on Surface Initiated Rolling Contact Fatigue of Turnout Rails by Three-Level Unreplicated Saturated Factorial Design

Surface initiated rolling contact fatigue (RCF), mainly characterized by cracks and material stripping, is a common type of damage to turnout rails, which can not only shorten service life of turnout but also lead to poor running safety of vehicle. The rail surface initiated RCF of turnouts is caused by a long-term accumulation, the size and distribution of which are related to the dynamic parameters of the complicated vehicle-turnout system. In order to simulate the accumulation of rail damage, some random samples of dynamic parameters significantly influencing it should be input. Based on the three-level unreplicated saturated factorial design, according to the evaluation methods of H, P and B statistic values, six dynamic parameters that influence the rail surface initiated RCF in turnouts, namely running speed of vehicle, axle load, wheel-rail profiles, integral vertical track stiffness and wheel-rail friction coefficient, are obtained by selecting 13 dynamic parameters significantly influencing the dynamic vehicle-turnout interaction as the analysis factors, considering four dynamic response results, i.e., the normal wheel-rail contact force, longitudinal creep force, lateral creep force and wheel-rail contact patch area as the observed parameters. In addition, the rail surface initiated RCF behavior in turnouts under different wheel-rail creep conditions is analyzed, considering the relative motion of stock/switch rails. The results show that the rail surface initiated RCF is mainly caused by the tangential stress being high under small creep conditions, the normal and tangential stresses being high under large creep conditions, and the normal stress being high under pure spin creep conditions.

Long-term wear and rolling contact fatigue behaviour of a conformal wheel profile designed for large radius curves

ABSTRACTThere are many reasons to optimise the wheel–rail interface through redesign or maintenance. Minimising wear and rolling contact fatigue (RCF) initiation on wheels and/or rails is often at the forefront of such considerations. This paper covers the design of a conformal wheel profile and its long-term wear and RCF performance to optimise the wheel–rail interface and subsequently reduce the occurrence of surface-initiated RCF on South Africa’s iron ore export line. A comparative study is performed using multibody dynamics simulation together with numerical wheel wear and RCF predictions. The advantages of a conformal wheel profile design are illustrated by evaluating the worn shape and resulting contact conditions of the conformal design. The conformal design has a steadier equivalent conicity progression and a smaller conicity range compared with the current wheel profile design over the wheel’s wear life. The combination of a conformal wheel profile design with 2 mm hollow wear and inadequate adherence to grinding tolerances often result in two-point contact, thereby increasing the probability of RCF initiation. The conformal wheel profile design proved to have wear and potential RCF benefits compared with the current wheel profile design. However, implementation of such a conformal wheel profile must be accompanied by improved rail grinding practices to ensure rail profile compliance.

The effect of rolling contact fatigue mitigation measures on wheel wear and rail fatigue

The maintenance costs associated with heavy haul operations are mainly driven by wheel and rail damage in the form of wear and rolling contact fatigue (RCF). RCF initiated on the surface of the rail is the dominant damage mode on South Africa's iron ore export line. Two potential rail RCF mitigation measures were adapted from service tests and those published in literature and studied. The mitigation measures involved changes in suspension stiffness in an attempt to spread wheel wear across the tread and changes in rail profile design. These mitigation measures were evaluated by means of multi-body dynamics simulations including wheel wear predictions. Changes in suspension stiffness and rail profile design caused concentrated hollow wear on the wheels. These worn shapes of the wheels are conducive to RCF initiation with the worst performance coming from the application of a rail profile with gauge corner relief. Contact between the gauge side false flange of the wheel and the relief section of the rail profile were shown to increase the probability of RCF initiation.

Contact fatigue initiation and tensile surface stresses at a point asperity which passes an elastohydrodynamic contact

Abstract Contact mechanics and tribology was combined with fundamental fatigue and fracture mechanics to form a new mechanism for surface initiated rolling contact fatigue. Following, fatigue was investigated numerically for single asperities and craters in lubricated rolling contact surfaces. The hypothesis suggests that asperity point contacts can create sufficiently large tensile stresses for fatigue. The investigated case corresponded to a heavily loaded truck gear with ground surfaces. Reynolds equation resolved the elastohydrodynamic effect of the asperity in the transient three dimensional contacts. The Findley critical plane criterion was used for multiaxial and non-proportional fatigue evaluation. The simulations confirmed the new mechanism for rolling contact fatigue and showed how asperities can create contact fatigue in the lubricated contacts even without slip.

Propagation of surface initiated rolling contact fatigue cracks in bearing steel

Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Most research on rolling contact fatigue considers total life to pitting. Instead, this work studies the growth of rolling contact fatigue cracks before they develop into surface pits in an attempt to better understand crack propagation mechanisms. A triple-contact disc machine was used to perform pitting experiments on bearing steel samples under closely controlled contact conditions in mixed lubrication regime. Crack growth across the specimen surface was monitored and crack propagation rates extracted. The morphology of the generated cracks was observed by preparing sections of cracked specimens at the end of the test. It was found that crack initiation occurred very early in total life, which was attributed to high asperity stresses due to mixed lubrication regime. Total life to pitting was dominated by crack propagation. Results provide direct evidence of two distinct stages of crack growth in rolling contact fatigue: stage 1, within which cracks grow at a slow and relatively steady rate, consumed most of the total life; and stage 2, reached at a critical crack length, within which the propagation rate rapidly increases. Contact pressure and crack size were shown to be the main parameters controlling the propagation rate. Results show that crack propagation under rolling contact fatigue follows similar trends to those known to occur in classical fatigue. A log-log plot of measured crack growth rates against the product of maximum contact pressure and the square root of crack length, a parameter describing the applied stress intensity, produces a straight line for stage 2 propagation. This provides the first evidence that growth of hereby-identified stage 2 rolling contact fatigue cracks can be described by a Paris-type power law, where the rate of crack growth across the surface is proportional to the contact pressure raised to a power of approximately 7.5.

Switch panel design based on simulation of accumulated rail damage in a railway turnout

A methodology for numerical prediction of accumulated rail damage in railway turnouts is presented. Based on simulation of dynamic vehicle-track interaction followed by a discretisation of the conditions in each wheel-rail contact, distributions of rail wear are calculated by the Archard model of sliding wear, while surface initiated rolling contact fatigue (RCF) damage is evaluated by the Palmgren–Miner rule and an index building on shakedown theory. Partial slip in the wheel-rail contacts and variable amplitude loading are considered. For freight traffic in the diverging route, the influence of rail inclination and switch rail elevation on damage in the switch panel is investigated in a demonstration example. Two-point contact situations with one contact on the switch rail and one on top of the stock rail induce relative motion and slip between wheel and rail leading to high energy dissipation. In agreement with field observations, it is concluded that wear is the dominating damage mechanism on the gauge side of the switch rail while the risk for RCF is higher on the crowns of the switch and stock rails. For accurate prediction of rail life for given combinations of wheel/rail materials and traffic conditions, the methodology needs to be calibrated by field measurements.

Stress gradient effects in surface initiated rolling contact fatigue of rails and wheels

The paper investigates gradient effects, which relate to how highly stressed regions should be dealt with in fatigue design analyses. In particular stress gradients in rolling contact are investigated with a focus on differences in response between full and partial slip conditions. To this end the multiaxial state of stress beneath a wheel–rail contact featuring full or partial slip is quantified using a multiaxial equivalent stress criterion. A comparative study shows that the significant differences in peak interfacial shear stress magnitudes between full and partial slip conditions are significantly reduced when translated to equivalent stress magnitudes. An innovative procedure to quantify the gradient effects by comparing the multiaxial contact stress field to uniaxial conditions is developed and employed. For the studied cases stress gradients beneath the frictional contact were found to be similar to stress gradients outside a uniaxially loaded large plate featuring a small hole with a radius in the order of 0.5–0.7mm. The study concludes that the use of local magnitudes of interfacial shear stress in the analysis of surface initiated rolling contact fatigue under partial slip conditions is conservative. The analysis framework established in the current study can be used to estimate the level of conservativeness.

Temperature-dependent evolution of the cyclic yield stress of railway wheel steels

The evolution of the cyclic yield stress for a railway wheel steel (UIC ER7T) during cyclic plastic straining has been characterized at different temperatures from −60 to 600°C. Different constant strain amplitude levels were examined and for temperatures above 200°C, hold periods were included to study stress relaxation during constant compressive strain. The results are of use in predicting material deformation and damage. This is demonstrated by the application to improve a criterion for surface initiated rolling contact fatigue damage.

“Brown etching layer”: A possible new insight into the crack initiation of rolling contact fatigue in rail steels?

Abstract A field sample of rail steel was metallurgically examined to characterize its rolling contact fatigue (RCF) damage. In addition to the well-known white etching layer (WEL), a possible different type of surface modification layer was identified in parallel. The layer has some similar features as the WEL but exhibits a significantly different etching response to 3 vol% Nital etchant. After etching, the new layer exhibits a brown color under the same light reflection. This layer was named as “brown etching layer” (BEL) to distinguish it from the WEL. Similar to the WEL, cracks are observed to be closely related to the BEL. The cracks are found to penetrate deeper than those initiated by the WEL reported in existing publications. Further, they are found to propagate downwards without branching, which may eventually cause rail fracture. Although its formation mechanism is not yet clear, WEL has been considered by some authors in the literature as a possible RCF initiation source. It is therefore of critical importance to understand the characteristics of the BEL and its formation mechanism. This may also lead to better understanding of the formation mechanism of the WEL. To this end, microstructural features of the BEL were studied using micro-hardness tests, optical microscopy and scanning electron microscopy. The BEL was found to be distinctly softer than the WEL and lamella-type features are found within the BEL. The microstructural features of the BEL were compared with the WEL reported in the literature. Finally, the formation mechanism of the fatigue damage was discussed based on the comparison, observations and material characterization.

Influence of wheel and rail profile shape on the initiation of rolling contact fatigue cracks at high axle loads

ABSTRACTThe influence of wheel and rail profile shape features on the initiation of rolling contact fatigue (RCF) cracks is evaluated based on the results of multi-body vehicle dynamics simulations. The damage index and surface fatigue index are used as two damage parameters to assess the influence of the different features. The damage parameters showed good agreement to one another and to in-field observations. The wheel and rail profile shape features showed a correlation to the predicted RCF damage. The RCF damage proved to be most sensitive to the position of hollow wear and thus bogie tracking. RCF initiation and crack growth can be reduced by eliminating unwanted shape features through maintenance and design and by improving bogie tracking.

Rolling contact fatigue and wear properties of 0.1C–3Cr–2W–V nitrided steel

Rolling contact fatigue (RCF) characteristics of 0.1C–3Cr–2W–V nitrided and untreated steel have been investigated at ambient temperature. The results indicate that the friction coefficients of the nitrided specimens decreased from 6% to 29%, and the RCF life of the nitrided specimens have been significantly prolonged. The failure modes of the nitrided surface could be classified as spalling and delamination within the layer, and the inner defects generated by the plasma nitriding play an important role in the surface and subsurface initiated RCF. Meanwhile, decreased surface roughness and better wear-resistance are demonstrated to be useful to improving the RCF performance.

Numerical investigation of the spall opening angle of surface initiated rolling contact fatigue

The spall opening angle was studied for surface initiated rolling contact fatigue with the purpose to allow assessment of the volume of detached material. The influence of friction and the crack inclination angle on the damage spread in the contact surface was investigated with the asperity point load mechanism and a simplified three-dimensional rolling contact fatigue load. Crack arrest due to crack closure was proposed as explaining mechanism for the spall opening angle of the typical v-shaped or arrowhead crack configurations. A new three-dimensional crack geometry was presented allowing the study of the spalling surface morphology in a gear application. Stress intensity factors along the crack front were computed using the eXtended Finite Element Method (XFEM) implemented in Abaqus (6.12). Both low crack inclination angles and increased friction resulted in larger spall opening angles. For cracks with small inclination angles the effects of increased friction on the spall opening angle appeared however very little. The findings increase understanding of the surface morphology and the damage process and further motivate the asperity point load mechanism as an important source for surface initiated RCF damage.