Rolling Contact Fatigue Cracks Research Articles

Effect of wear on rolling contact fatigue crack growth in rails

The purpose of this paper is to analyze the influence of the load direction and the phenomenon of wear on rolling contact fatigue crack growth. For this purpose, a UIC60 rail with accurate geometry using FEM is studied. In this regard, a three-dimensional finite element model is provided. The maximum stress caused by wheel/rail contact for a new/worn wheel profiles was obtained using such model. Then, using Franc 3D software, during stress analysis for different situations of wheel, different values of stress intensity factors are obtained. The behavior of crack under fatigue loading was studied using a three-dimensional modeling.

Prediction of RCF and wear evolution of iron-ore locomotive wheels

Locomotives for the iron ore line in northern Sweden and Norway have a short wheel life. The average running distance between two consecutive wheel turnings is around 40,000km which makes the total life of a wheel around 400,000km. The main reason of the short wheel life is the severe rolling contact fatigue (RCF). The train operator (LKAB) has decided to change the wheel profiles to get a better match with the rail shapes in order to decrease the creep forces leading to RCF. Two wheel profiles optimised via a genetic algorithm were proposed. They have, however, not been analysed for long term wear development. There is a risk that the optimised profiles might wear in an unfavourable way and after a while cause even higher RCF or wear than the original one. This study predicts wheel profile evolution using the uniform wear prediction tool based on Archard’s wear law. RCF evolution on the surface of the wheel profiles is also investigated. The impact of wear on polishing the wheel surface and avoiding the RCF cracks to propagate is considered via introducing a correction factor to the calculated RCF index. Traction and braking are also considered in the dynamic simulation model, where a PID control system keeps the speed of the vehicle constant by applying a torque on the loco wheels. The locomotives are also equipped with a flange lubrication system, therefore the calculations are performed both for lubricated and non-lubricated wheels. The simulation results for the wheel profiles currently in use, which are performed to validate the model and the simulation procedure, show a good agreement with the measurements. It is also concluded that the lubrication system partly does not perform as expected. Comparison between the proposed optimised profiles for their long term behaviour suggests that one of them produces less RCF and wear compared to the other one.

Short rolling contact fatigue and thermal cracks under frictional rolling – A comparison through simulations

The relative severity of radial (thermal) and inclined rolling contact fatigue surface cracks of equal depth in a railway wheel is investigated by three-dimensional elastoplastic finite element analyses of a cracked wheel sector subjected to contact loading. Response is quantified by relative displacements of nodes close to crack tip and at the crack mouth. Highly inclined cracks give the highest magnitudes of crack tip shear displacements, which is the dominant deformation mode. Braking conditions are found to open the crack mouth. Initially higher temperatures on thermal cracks cause increased crack tip deformation and opening of the crack, whereafter subsequent mechanical load cycles impose crack closure.

Numerical study on fatigue crack growth in railway wheels under the influence of residual stresses

Accurate prediction of fatigue crack growth on railway wheels and the influence of residual stresses by finite element method (FEM) modeling can affect the maintenance planning. Therefore, investigation of rolling contact fatigue and its effect on rolling members life seem necessary. The objective of this paper is to provide a prediction of rolling contact fatigue crack growth in the rail wheel under the influence of stress field from mechanical loads and heat treatment process of a railway wheel. A 3D nonlinear stress analysis model has been applied to estimate stress fields of the railway mono-block wheel in heat treatment process. Finite element analysis model is presented applying the elastic–plastic finite element analysis for the rail wheel under variable thermal loads. The stress history is then used to calculate stress intensity factors (SIFs) and fatigue life of railway wheel. The effect of several parameters, vertical loads, initial crack length and friction coefficient between the wheel and rail, on the fatigue life in railway wheels is investigated using the suggested 3-D finite element model. Three-dimensional finite element analysis results obtained show good agreement with those achieved in field measurements.

Influence of grain boundary cementite induced by gas nitriding on the rolling contact fatigue of alloyed steels for gears

It is well-known that rolling contact fatigue life can be increased by modifying gear surfaces. For instance, gas nitriding is a thermochemical treatment enhancing superficial hardness, and introducing compressive residual stresses but nitriding alloys can lead to the precipitation of cementite at grain boundaries. In this study, experiments were performed on a twin-disc machine to investigate the influence of the microstructure of nitrided layers on rolling contact fatigue failure modes. Nitrided layers with small grains, thin and short grain boundary cementite filaments showed better resistance to surface originated pitting compared with nitrided layers with coarse precipitates, for similar mechanical properties in terms of hardness and residual stresses. Moreover, examinations carried out on cross sections throughout crack networks revealed the influence of these precipitates on the propagation of rolling contact fatigue cracks, which is at the origin of the differences in performance between thin and coarse microstructures.

Dynamic Wheel–Rail Interaction Over Rail Squat Defects

Dynamic interaction between wheel and rail generates vibration and acoustic radiations transmitted through other media such as structural track systems, air, and rolling stock. Noteworthy, both wheel and rail irregularities often induce additional impact vibration and noise along the rail corridor. Rail squats and studs are typically classified as the development of any cracks that have grown longitudinally and laterally through the subsurface and become a laminated crack resulting in a depression of the rail surface. The rail defects are commonly referred to as ‘squats’ when they are initiated from rolling contact fatigue cracks, and as ‘studs’ when they are associated with a white etching layer due to wheel slips or excessive tractive effort. Interestingly, recent research shows that rail squats can be found almost everywhere, and every type of track structures, gradients and geometries in either light passenger or heavy freight traffics and for low, medium or high speed trains all over the world. A more pronounced issue appears when the crack grows and finally flakes off the rail. This paper highlights the monitoring of wheel–rail interaction over rail squat defects. Using the axle box acceleration data obtained from the calibrated track recording vehicle, the frequency spectra can be analysed. The results demonstrate an apparent influence of squats on the wheel–rail interaction over the defective sections and increased acoustic radiation at the high frequency range. The practical understanding will also enhance the development of acoustic- and vibration-based monitoring systems of rail surface defects.

Observing early stage rail axle bearing damage

Rail axle-bearing failure is a serious issue that can affect passenger safety and generate large costs for operators from penalties, recovery, reduced train availability and repairs. Southeastern have fitted bearing sensors to their Electrostar fleet; these sensors identify rail axle-bearing degradation. In this paper (presented at ICEFA VI) we introduce an on-going study where ex-situ rail axle bearings with recorded vibration histories are being forensically examined. The ultimate aims of this work are (1) to link vibration signatures to levels of physical damage and (2) to identify any common factors that are causing bearing failures. The current work presents results from one removed bearing; this work highlights the methods and techniques being used to measure and quantify the physical damage, which include profilometry, X-ray tomography and metallography. The bearing described herein presented large subsurface white etched layers, rolling contact fatigue crack propagation and evidence of electrical arcing damage.

Evaluation of rolling contact fatigue crack path in high strength steel with artificial defects

The objectives of the present paper are to clarify the rolling contact fatigue (RCF) crack path in high strength steel with artificial defects using synchrotron radiation micro-computed tomography (SR micro CT) imaging and to discuss the mechanism of RCF crack propagation by finite element (FE) analysis. Two crack types, a vertical crack and a horizontal crack, were observed around the artificial defect by SR micro CT. The stress intensity factor (SIF) of horizontal cracks, calculated by FE analysis, increased by the existence of vertical cracks. The interaction between these two crack types dominates RCF crack propagation originating from artificial defects.

A novel laboratory test rig for probing the sensitivity of rail steels to RCF and wear – first experimental results

Increased traffic and intensified operating conditions cause more and more damage on rails manifesting themselves in rolling contact fatigue cracks and wear. Rolling contact fatigue, e.g. head checking, is classified safety critical as these cracks may lead to the derailment of a vehicle. For that reason research and development is mainly focused on designing novel, more damage resistant rail steels whereas part of the research is concentrated on testing their sensitivity to head checking. In this study a new type of laboratory test rig is presented which is capable of generating head check like rolling contact fatigue cracks under realistic contact and loading conditions. The main idea behind the design is that the contact patch size is significantly increased in a convex–concave contact configuration, realized in this special case by a shaft running inside a ring.

2D fatigue crack propagation in rails taking into account actual plastic stresses

Due to increase axle loads, repeated passages of the wheels, rolling contact fatigue cracks initiate in the surface or subsurface of the rails. These defects can propagate and lead to the rail failure. A two-scale frictional contact fatigue crack model developed within the X-FEM framework is used to address the cracked rail problem, calculate the stress intensity factors, perform crack propagation and fatigue life prediction. Actual plastic stresses, quantified through a dedicated software developed by SNCF within the consortium IDR2, are taken into account in the propagation simulation via projection of the asymptotic mechanical fields. The effects of those actual plastic stresses are investigated on the crack growth path and rate. Interactions between multiple cracks of a network are also analyzed.

A new test to study the cyclic hardening behaviour of a range of high strength rail materials

Plastic ratcheting plays a key role in wear and rolling contact fatigue crack formation at the wheel–rail interface. Tests to examine the wear and rolling contact fatigue behaviour of rail materials over a wide range of service conditions are expensive and can be impractical. A physical simulation of the deformation behaviour associated with ratcheting is an attractive replacement for such tests. In this work, the Plane Stress Local Torsion (PSLT) test is proposed as a novel mechanical testing method to physically simulate near-surface deformation in rails and to characterize the cyclic deformation behaviour of rail materials. Contrary to the orthodox mechanical tests, the proposed method is capable of producing a nonlinear strain gradient in test samples which resembles the real gradient in the rail–wheel system. The PSLT testing was performed on specimens of the nominated rail steels in a strain-controlled fashion to simulate the unidirectional as well as the fully reversing strain cycles. The test was used to examine the cyclic hardening behaviour and ratcheting characteristics of a range of high strength rail materials under cyclic loading at room temperature. The effect of the cyclic strain amplitude under symmetrical strain cycling on the cyclic hardening behaviour was investigated. Experimental torque–twist data were used to compare the plastic flow behaviour of commonly used rail materials in heavy haul applications under cyclic loading. The cyclic and ratcheting strain accumulation behaviour in the test samples was characterized based on the torque–twist data to allow a comparative study of the mechanical properties. Optical microscopy of the tested samples was also performed to compare the microstructures at the flow localization zone for different materials subjected to cyclic strain accumulation.

OS2121 高輝度放射光ラミノグラフィを用いた高強度鋼中の転動疲労き裂の観察

In rolling contact fatigue (RCF), cracks are usually initiated from inclusions under the surface and propagated to the contact surface. In the present study, RCF cracks was observed using synchrotron radiation computed laminography (SRCL) in SPring-8. For observation of crack propagation, ex situ rolling fatigue test and SRCL were applied to same sample. Laminography is suitable for the observation of sheeted material and sheeted sample is useful for operation of rolling fatigue tests. The specimen with flaking damage was measured by SRCL. It was found that two types of cracks, vertical crack and horizontal crack under the contact surface, which is typically rolling fatigue crack, can be measured by SRCL. For observation of RCF crack propagation, vertical crack which initiate from inclusion and propagated in the direction perpendicular to the contact surface, could be detected. SRCL and fatigue tests were conducted three times. As results, crack propagation could be observed between first and third measurements, while the different part of crack was detected in second measurement. Residual compression stress is considered to affect the observation of RCF crack. SRCL is concluded to be effective for evaluation of initiation and propagation behavior of crack in the rolling fatigue.

Observation of Rolling Contact Fatigue Cracks by Laminography Using Ultra-bright Synchrotron Radiation

Micro computed laminography imaging using ultra-bright synchrotron radiation (SRCL) was applied to the observation of flaking defects under rolling contact fatigue (RCF) of a high strength steel. Specially fabricated inclusion rich steel plate specimens were employed for the experiments. RCF fatigue tests were carried out near the measurement hatch of SRCL for successive observation of crack initiation and growth behaviors. Specimens before and after the initiation of flaking were observed by SRCL, and the flaking defects and cracks under the surface were successfully detected. The shape and location of RCF crack obtained by SRCL imaging were almost coincident with those obtained by SEM, and detail process of crack initiation and flaking in RCF process can be discussed.

Analysis of wear debris in rolling contact fatigue cracks of pearlitic railway wheels

In the current study a severe subsurface crack network in a railway wheel has been studied and compared to typical rolling contact fatigue cracks found in the wheel tread surface. Microstructural characteristics, chemical composition and microhardness within the cracks and around crack faces were examined. While the two damage types are principally different, both showed similar crack characteristics, with short cracks branching along the main crack paths and a discontinuous sheared layer of wear debris and metallic flakes within them. Analyses of the wear debris showed that it does not originate from external contamination or being the result of corrosion primarily. Instead it has most likely been produced by shear deformation and wear mechanisms within crack faces caused by mixed-mode crack growth. Although microstructural appearance at lower magnification seemed to differ from the bulk material, at high magnification a lamellar structure was observed consisting of layers of deformed metallic flakes and particles of the base metal. Auger electron spectroscopy was used to analyze these sheared layers; higher concentration of oxygen was measured in between flakes, indicating the presence of oxides and flakes being of similar chemical composition as the base material. A possible explanation is that these layers are created due to high shear forces and friction between crack faces in the service of the wheel. With continued rolling the material being sheared by the cyclic relative motion of the crack faces disintegrates into smaller wear debris particles with concurrent oxidation.

Rolling contact fatigue of three crossing nose materials—Multiscale FE approach

In this work finite element models at different length scales are applied to predict the performance of three different crossing materials (Manganese steel, Hardox and Marage 300) in view of the development of rolling contact fatigue (RCF) cracks. A model of the whole crossing (crossing model) is used for the calculation of the dynamic forces and movements of wheel and crossing. For the prediction of RCF repeated loadings have to be calculated, but only a reduced model permits a sufficiently fine mesh and reasonable computing times. Therefore, a simplified model of the wheel and the crossing nose (impact model) is developed, which uses the dynamic movements of the crossing model as boundary conditions. The accumulation of plastic strains in the crossing, the build-up of residual stresses and the geometric adaption of the crossing to the loads is studied for 81 load cycles. The contact pressures, shear stresses and residual stresses of the impact model with the adapted geometries of the 81st cycle are applied to a two-dimensional model with a surface crack (crack model). Using data from measured crack growth curves, the three materials can be compared in terms of crack development and growth.

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

ABSTRACTThe 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.

Modeling damage mechanics of railway tracks to evolve control strategies for derailment prevention

Purpose – With an eye to prevent derailment of high-speed trains, vis-à-vis unwarranted loss of lives and property, this paper aims to develop a formalism of designing a suitable control system with embedded decision support system. Design/methodology/approach – A model of rolling contact fatigue (RCF) crack propagation in railway tracks is designed, simulating the alarming stress intensity factor around the advancing fatigue cracks. COMSOL multi-physics software is employed to design the RCF crack monitoring system with acoustic emission (AE) count signals, describing the damage threshold of railway tracks. Findings – Simulation experiment on stress intensity factor for cracks in real life rail sections has enabled to describe the maximum working stress; it has been noticed that the threshold value of stress intensity factor (∼ 41 MPa m1/2) for the onset of unstable crack propagation is reached at a fatigue crack length of 11.5 mm. It is further noticed that the observed AE count at a particular instant of time in a specific location of railway track is a true indication of the vulnerability of rail failures. Originality/value – The proposed model, a completely new of its kind, bears a high socio-technological value as it entails the design of an intelligent control system to prevent train accidents.

Prediction of Crack Initiation of Rail Rolling Contact Fatigue

A plain-strain finite element (FE) model of the rail is developed by ABAQUS. The 1070 rail steel is considered in the model by Jiang and Sehitoglu's cyclic plastic constitutive equation. The wheel-rail rolling contact is represented by a moving load applied on the contact surface. Based on the FE results the initiation of rolling contact fatigue (RCF) crack is further evaluated by Jiang's fatigue model. Effects of the rail material inner defect and the friction coefficient are also investigated. The results show that the stress-strain state of the rail surface material becomes stable after about thirty passages. The maximum residual stress and strain are located in the subsurface. The life of a defected rail can be as low as 1/27~1/17 of a qualified rail, and the most probable location for crack initiation is highly dependent on the inner defect. The fatigue life is found to considerably decrease with the friction coefficient, while the location of crack initiation and its direction of propagation are less affected.

An integrated robotic system for automatic detection and characterisation of rolling contact fatigue cracks in rails using an alternating current field measurement sensor

This paper considers the development of an integrated robotic system which may be used to automatically locate and then systematically scan rolling contact fatigue cracks on rails to provide more detailed information on the cracks’ sub-surface size. The system consists of a mechanised trolley, a robot, a commercially available AMIGO alternating current field measurement (ACFM) system and a laser distance sensor. The dynamic results, when the trolley moves at a controlled speed to detect defects, and the static results, when the defect location has been determined and the robot arm is used to characterise (size) the defect, are both considered. During field trials it was found that in the detection test most of the artificial cracks were detected automatically by employing a novel combined threshold and signature match (CTSM) method. The results of the characterisation test also suggest that the system can automatically take reliable constant lift-off ACFM measurements over the cracks at close to optimum range for characterisation where the relative angle offset between the ACFM sensor and the crack is bounded to be within 5°.

Micromechanics behavior of fatigue cracks in Hadfield steel railway crossing

In order to predict the mechanism of the formation and propagation of rolling contact fatigue cracks in Hadfield steel railway crossings (HSRC), the residual strain, microstructure, crystal orientation, and microhardness surrounding fatigue cracks were investigated. Results show that the formation and propagation of fatigue cracks in HSRC have no correlation with the crystal orientation and boundaries of grains. The hardness and residual strain in the field around the fatigue crack are lower than those in other regions. The compressive strain around the fatigue crack is released after crack propagates, which reduces the hardness around the fatigue crack. Deformation twins and dislocations play a key role in the work hardening of HSRC.