Rolling Contact Fatigue Cracks Research Articles

Development of autonomous ACFM rail inspection techniques

Rail tracks are subjected to high contact stresses leading to wear and fatigue. With increased axle weight and faster trains, there is a need for routine inspection of the rails. In order to improve punctuality and reliability of track rail inspection, intelligent systems and robotics can be employed. In this research, supported through the FP7 INTERAIL project, a robotic system is being developed for the detection and characterisation of rolling contact fatigue (RCF) defects on in-service track rails using alternating current field measurement (ACFM) techniques. The autonomous inspection system consists of a mechanised trolley, an FS02N Kawasaki robot arm and a commercially available AMIGO ACFM system, supplied by TSC Inspection Systems. The ACFM probe is used to both detect (as the trolley moves at a controlled speed with the sensor in a fixed position) and characterise (whilst the trolley is stationary using the robot arm to survey the rail head) RCF cracks. Results show that the ACFM sensor can detect RCF defects even at significant (approx 4 mm) lift-off; however, if lift-off exceeds 4 mm (for example due to rail-head wear), the data acquired may not be easily exploited to characterise the defects. This paper presents the results obtained based on the work carried out to date.

OS0509 放射光μCTを用いた高強度鋼の転動疲労下のき裂検出(OS5-2 き裂進展・計測,OS-5 材料の疲労挙動と損傷評価1)

In the present study, synchrotron radiation micro computed tomography (SR μCT) imaging of SPring-8 was applied to the observation of delamination defects in high-strength steels under rolling contact fatigue. The rolling contact fatigue crack initiation occurs under surface and these defects propagate to the surface. The conventional technique, such as optical microscope and scanning microscope, cannot give us the crack initiation and growth behavior under the surface. Therefore, the shape of rolling contact fatigue defects was observed by SR μCT imaging. The specimen with an artificial defect, which simulates an inclusion, was used to the contact rolling fatigue tests. The broken and non-broken specimens were observed respectively by SR μCT. The delamination defects and cracks can be detected from the artificial defetct. It is found from these results that the defect initiation and propagation behavior in the rolling contact fatigue process can be discussed from the measurements by SR μCT imaging. The number of crack, location and size of crack obtained by the CT imaging coincide with those by polishing and SEM measurements. It is found from these results that the defect initiation and propagation behavior in the rolling contact fatigue process can be discussed from the measurements by SR μCT imaging.

Effect of Wheel/Rail Friction on Rolling Contact Fatigue

A numerical model is presented to analyze effect of wheel/rail friction on rolling contact fatigue. A three-dimension finite element model of rail is built up to investigate the rail stresses and strain around wheel/rail patch. Then, based on the critical plane concept, a new model was proposed to predict the rolling contact fatigue (RCF) crack initiation life under different wheel/rail frictions by using stress and strain on the critical plane as fatigue parameter. The numerical results obtained show that the wheel/rail friction coefficient has a great impact on the fatigue crack initiation life.and the curve of fatigue crack initiation life under different wheel/rail friction coefficient is roughly "S" type. The results are very useful in the wheel/rail friction management and determining grinding interval and grinding removal.

The role of the environment in the rolling contact fatigue cracking of rails

▶ Environmental effect proposed in rolling contact fatigue cracking in rails. ▶ Previous studies showed environmental effect in cracking of bearing steels. ▶ Effect of water on RCF cracking proposed to extend beyond mechanical effects. The significance of an environmental contribution to the RCF cracking process in rails is presented and critically reviewed in the current work. Based on both field trials and laboratory observations, recent models of the RCF cracking process may be missing an important factor. The practical significance of an explicit environmental effect on the cracking process is also considered, as well as experimental approaches to quantify the effect of the environment.

Very early stage rolling contact fatigue crack growth in pearlitic rail steels

Abstract This paper reports on metallurgical characterisation of small cracks in pearlitic steels, using observations from both twin disc tests and rails removed from service, plus methods to determine three-dimensional (3D) aspects of such cracks. Tests to varying fractions of rolling contact fatigue (RCF) life have been assessed, based on twin disc tests where “average 100% RCF life” has been defined by eddy current detection of cracks between 200 and 800 μm in depth. Contact surface features and microstructural alterations have been mapped against near-surface strain and compared with worn rail examinations. Initial crack growth along favourably aligned and strained, prior-austenite (PA) grain boundaries, particularly if the boundary contains pro-eutectoid (PE) ferrite, was found to be restricted or halted at the PA grain limit or where the boundary altered to be less-favourably aligned. For samples with up to 55% RCF life, initial crack propagation length was still less than the average PA grain size. 3D aspects of this early growth and the deformed PA grain shapes were found to be linked to ratcheting flakes and flake cracks on test disc and rail contact surfaces. The results have shown how initial RCF crack propagation is very microstructurally influenced.

Probability of subsurface fatigue initiation in rolling contact

Subsurface initiated rolling contact fatigue cracks initiate in the vicinity of material defects. As these exist randomly in the material, fatigue will appear randomly under otherwise constant conditions. By combining statistical methods with a contact mechanics and fatigue analysis, probabilities of fatigue initiation are evaluated. The developed model takes as input contact geometry, contact load and the statistical properties of the material defects. It is shown that under some well-defined presumptions the probability of rolling contact fatigue initiation in a rolling wheel can be expressed by a simple formula. Based on this, the parametric influence is assessed and it is found that the mean material defect size has a large influence. The benefit of material cleanliness testing procedures and the influence of the allowed defect size on the risk of subsurface initiated rolling contact fatigue in railway wheels are then quantified with the aid of numerical simulations.

An experimental study of EMAT ultrasonic surface waves modes in railhead

A lab-based laser-EMAT (Electromagnetic acoustic transducers) system has been developed to observe the ultrasonic surface wave propagation and interaction with surface breaking defects on the sample rail head surface. The frequency- wavenumber dispersion curves of the dominant surface wave modes were determined using 2D-FFT method based on the measurements of the out-of-plane displacement of the surface wave. A complete picture of the ultrasonic surface wave on the sample surface over time was reconstructed with exceptionally high spatial and temporal resolution. Despite the curvature of the rail head, the ultrasonic surface guided wave propagating down the rail is found to have similar properties to Rayleigh waves by direct comparison to those observed on flat samples using the same technique. Rail breaks caused by rolling contact fatigue (RCF) cracks are of growing concern to the modern high speed railway industry (1). The fatigue crack initiates on or very close to the rail rolling surface, which is not related to any material defect. Its occurrence is increasing on high speed passenger lines, mixed and heavy haul railways and can lead to expensive rail grinding in the attempt to remove it, premature removal of the rails and complete rail failure. The rolling contact fatigue damage on rails can be divided in headchecks, squats and spalling. They are caused by a combination of high normal and tangential stresses between rail and wheel, which cause sever shearing of the rolling surface layer of rail head and either fatigue or exhaustion of ductility of material. The microscopic cracks produced propagations through the heavily deformed surface layer of railhead at a shallow angle to the rail running face (about 10 ◦ ∼15 ◦ ) until it reaches a depth where the steel retains its original isotropic properties (2,3). At this stage the crack is a few millimeters deep into the rail head, and usually the crack may simply lead to spalling of material from the rail surface. However, for reasons still not very clearly understood, isolated oblique crack can down into the rail, and if not to detect by ultrasonic testing car, it may cause the rail to break. RCF damages are much more severe from the point of view of the structure integrity for high speed railway track.

Probabilistic prediction model for initiation of RCF cracks in heavy-haul railway

A comprehensive approach was developed for studying the probability of fatigue crack initiation under random wheel/rail force and random material properties. Firstly, a load spectrum was established based on the distribution of wheel/rail forces. Secondly, the discreteness of rail fatigue properties was analyzed by using Monte Carlo simulation and parametric assumed test method. Then three-dimensional finite element models were created to predict rail surface and subsurface stresses under different amplitudes of wheel/rail forces. Based on the critical plane concept, a new probabilistic prediction model was proposed to account for the effects of random wheel/rail forces on the rolling contact fatigue (RCF) crack initiation life. The results show that the prediction result from the model is in good consistence with the observations in field.

Ductile inclusions in rail, subject to compressive rolling–sliding contact

Abstract Driven by a requirement for increased performance and cost-effectiveness, the basic pearlitic microstructures of rail steels are being progressively refined and novel microstructures are being developed. With such progress, increased attention must be paid to potential ‘weak spots’ in the microstructure such as non-metallic inclusions. For the past few decades, rail steels have become progressively cleaner with regard to harmful brittle inclusions, however the effect of rolling–sliding contact forces on nominally benign, ductile inclusions requires further consideration. This work examines the characteristics of ductile, Mn-based inclusions in used rail and rail test discs. Where the structure near-surface has been subject to a high component of transverse creepage, ductile, axially aligned inclusion ‘stringers’ become strain-flattened to form planes of weakness along which micro-cracks initiate and propagate, thus facilitating wear and rolling contact fatigue (RCF) cracking. The progression of shape change of these ductile inclusions approaching the contact surface has been examined by optical and electron microscopy including focussed ion beam (FIB) analysis. As-manufactured ductile inclusions shapes and distributions have also been surveyed with respect to the (geographical) source of the rail steel. Comparative data for these different steel sources are given.

Fatigue crack propagation in rocker and roller–rocker bearings of railway steel bridges

In this work, a method is proposed for rolling contact fatigue crack propagation analysis using contact and fracture theories in conjunction with fatigue laws. The proposed method is used in the fatigue analysis of rocker and roller–rocker bearings of a railway open web girder bridge which is instrumented with strain gages. Using a contact algorithm based on the minimum energy principle for bodies in rolling contact with dry friction, the normal and tangential pressure distribution are computed. It is seen that the most critical location of a crack in bearings is at a point very close to the contact region, as expected.

Effect of Lubricant on Surface Rolling Contact Fatigue Cracks

This study performed the finite element analysis of the cycle of stress intensity factors at the surface initiated rolling contact fatigue crack tip under Hertzian contact stress including an accurate model of friction between the faces of the crack and the effect of fluid inside the crack. A two-dimensional model of a rolling contact fatigue crack has been developed with FRANC-2D software. The model includes the effect of Coulomb friction between the faces of the crack. The fluid in the crack was assumed not only to lubricate the crack faces and reduce the crack face friction coefficient but also to generate a pressure.

Visualization and modelling to understand rail rolling contact fatigue cracks in three dimensions

Visualization and modelling to understand rail rolling contact fatigue cracks in three dimensions

Planning Rail Grinding Using Crack Growth Predictions

Rail grinding is a widely used rail industry technique for maintaining the quality and safety of railway track. Rail grinding can be used to restore the profile of the rails, remove surface plastic damage and remove or reduce in size very early stage surface breaking rolling contact fatigue cracks. This paper focuses on the use of rail grinding to extend the safe life of a rail that would otherwise be limited by rolling contact fatigue (RCF) defects. The optimum grinding strategy for a variety of conditions has been investigated using a model that predicts the growth of surface breaking cracks in rails loaded by rail vehicle wheel contacts, taking into account natural wear of the rail and rail grinding. It was found that a grinding strategy which removes incipient cracks in a specific period of time without being over-conservative and producing excessive material loss must take into account all traffic using the line, not just the traffic perceived to be most damaging. A mixed traffic case is modelled, revealing the potential for interaction between different vehicle types in determining maintenance requirements.

Rolling Contact Fatigue of White Etching Layer on Pearlite Steel Rail

White etching layer(WEL) is a phenomenon that occurs on the surface of rail due to wheel/rail interactions such as excessive braking and acceleration . Rolling contact fatigue(RCF) cracks on the surface of rail have been found to be associated with the WEL. In this study, we have investigated RCF damages of white etching layer in the laboratory using twin disc testing. These tests consist of wheel flat tests and rolling contact fatigue tests. The WEL has been simulated by wheel flat test. It has been founded that the WEL with a bright featureless contrast is formed on the surface of specimen by etching. Rolling contact fatigue test was conducted by using flat specimens with the WEL generated by the wheel flat test. It has been observed that two types of cracks occur within the specimen, the first initiated at the interface between the WEL and the undeformed area, the second initiated at the center of the WEL.

Numerical analysis for predicting the rolling contact fatigue crack initiation in a railway wheel steel

Abstract This paper concerns the modelling of the rolling contact fatigue of a railway wheel steel, which is simulated with moving Hertzian contact pressure. Parametric studies are carried out with a two-dimensional elastic–plastic finite element model of a part of a wheel containing defects. Several parameters, namely the size and shape of material defects, the load magnitude and the friction coefficient are varied to investigate their effect on the railway wheel fatigue damage. Defects or small friction coefficient are a plausible explanation to the initiation of deep subsurface fatigue cracks.

Modelling rolling contact fatigue cracks in the hydrodynamic lubrication regime: A coupled approach

In this paper we present a coupled method for modelling fluid-solid interactions within a crack generated by rolling contact fatigue (RCF) in the presence of a lubricant. The technique describes the fluid flow in the contact area and within a surface-breaking crack and explores how this affects the elastic deformation of the solid while the moving load traverses the surface crack. The approach sheds light on the transient response of the system by explicitly modelling the localised fluid flow and support within the crack, providing a physically-accurate description of the phenomenon under investigation.

Some comments on stress intensity factor calculation using different mechanisms and procedures for rolling contact fatigue cracks

Recently, attempts have been underway to simulate rolling contact fatigue (RCF) crack growth in the railhead, including also the effect of wear on maintaining the integrity of the rail and saving cost. At this juncture, it is essential to confirm whether the past analyses are adequate and what extent of differences exists when the different mechanisms or numerical procedures are applied to the same conditions in the RCF problem. Therefore, boundary-element analyses of stress intensity factors (SIFs) at the inclined surface crack tip under RCF conditions have been performed. Comparisons were made between SIFs calculated by the present analyses and those done by the numerical procedures of other researchers in the RCF problem. From this study, it was recognized that a special program should be developed to analyse the SIFs when the fluid pressure is taken into account. It was also found out that, for the analyses of SIFs, the iteration procedure with convergence calculation to specify the extent and location of locked, slipped, and separated regions on the crack faces should be used.

Rail rolling contact fatigue dependence on friction, predicted using fracture mechanics with a three-dimensional boundary element model

Rolling contact fatigue crack growth continues to affect many railways worldwide. It is most often controlled through rail grinding in a preventive maintenance strategy, but to plan the required frequency and depth of grinding, prediction of crack growth rates has a vital role. This paper presents crack growth rate results from a new three-dimensional rail model containing an inclined surface breaking rolling contact fatigue crack. The calculations are based on a shear mode of crack growth, driven by the Hertzian contact pressure on the railhead and moderated by friction between the crack faces (“crack face friction”). The results from the model show good correlation with those from the previously published work in the area, with particularly good agreement at higher levels of surface friction coefficient. Applying the new model to a range of surface and crack face friction coefficients predicted that crack growth rate will rise with reduced internal crack face friction at all crack sizes. For small cracks (2 and 5 mm radius) rates were predicted to rise with increased surface traction, but this trend was reversed at larger crack sizes (10 and 19 mm radius). Identical trends were found when the modelling was repeated using the previously developed half-space based “2.5d” model, indicating that although this older model cannot represent the rail geometry its high speed means it remains a useful tool for investigating the effects of contact parameters on rail rolling contact fatigue. The next study in this area could therefore consider if there is a uniform or crack size related ratio for mapping 2.5d results to three-dimensional rail geometry to produce closer agreement in crack growth rates as well as trends. For the three-dimensional model, consideration of alternative crack morphologies and movement of the contact running band away from the rail centreline would provide additional data on the effect of rail grinding and re-profiling on rolling contact fatigue.

10304 数値解析によるレール頭部におけるき裂発生寿命の予測(計算機シミュレーションとその応用(1),OS.14 計算機シミュレーションとその応用)

A methodology for fatigue life prediction of rolling contact fatigue (RCF) crack initiation on railhead was developed. It consists of finite element analyses (FEA), multi-axial fatigue crack initiation models with critical plane approach and fatigue damage accumulation. In simulations, both the global track response and the three-dimensional local elastic-plastic material response caused by the wheel-rail rolling-sliding contact were taking into consideration. It used real train traffic situations in curved sections of Shinkansen site where head checks initiate on the gauge corner of the railhead. The results from the numerical analyses were compared with observations made at the sections.

The role of deformed rail microstructure on rolling contact fatigue initiation

Metallurgical examinations are being used to validate progressive development of a mechanics model of contact damage in rail, with data from worn and fatigued rails removed from track and from SUROS twin-disc, lubricated tests, where standard grade rail steel discs were tested both to failure initiation (detected by eddy-current probe) and to proportions of fatigue life. This paper describes microstructural changes induced by compressive-shear, cyclic contact. Initial rail examinations indicated strain-partitioning between the pro-eutectoid ferrite phase at the prior austenite grain boundaries and the pearlite phase, with surface rolling contact fatigue (RCF) crack initiation primarily along the more highly strained, pro-eutectoid ferrite phase boundaries, therefore some rail discs were heat-treated prior to testing in order to alter the volume of pro-eutectoid ferrite. The SUROS results supported the rail examinations with regard to surface RCF crack initiation and the RCF life and the part-life results showed the development of strain-partitioning.