Rolling Contact Fatigue Cracks Research Articles

Influence of original microstructure on rolling contact fatigue property of D2 wheel steel

In this work, the influence of different original microstructure on the rolling contact fatigue (RCF) property of D2 wheel steel under oil lubrication condition is thoroughly studied by the GPM-30 rolling fatigue tester. Before pre-wear, the RCF life of the tempered sorbite (TS) sample and the lamellar pearlite + proeutectoid ferrite (P + F) sample are 7.2 × 105cycles and 5.4 × 105 cycle, respectively. After pre-wear, the RCF life of the TS sample is 4 times higher than that of the original TS sample. However, the RCF life of the P + F sample is 4 times lower than that of the original P + F sample. After pre-wear, the formation of fatigue wear cracks at the P + F sample surface causes the flaking of hardening layer and acts as a crack initiation site to accelerate the RCF failure. While the hardening layer of the TS sample is not flaked. The formation of hardening layer and the decrease of surface roughness of the TS sample improve the RCF life. During the RCF crack propagation process, the ferrite grains are refined obviously at the edge of cracks due to huge internal stress on the inner wall of the RCF cracks. The thickness of refined ferrite grains of the TS sample at the edge of RCF cracks is higher than that of the P + F sample.

Effects of axle load transfer on wheel rolling contact fatigue of high-power AC locomotives with oblique traction rods

Severe wheel rolling contact fatigue (RCF) has been observed on two types of high-power AC locomotives with CO-CO axle arrangement and running in complicated environments in China. This RCF occurs as continuous lateral surface cracks in the zone approximately centered at the nominal rolling circle of wheels and can propagate downward up to 14.5 mm deep. To figure out the causes, field observations, monitoring tests and numerical simulations have been conducted in the past years together with systematic comparisons between two types of locomotives. Statistics have shown an interesting phenomenon, among others, that wheels on axles 3 and 4 are most susceptible to RCF cracks on Type B locomotives, but not so on Type A. It has been found that complicated running environments such as sharp curves and large slopes are the root causes of RCF. The axle load transfer among axles, which is aggravated by oblique traction rods, and inappropriate electrical compensation, and the curving behavior determine the axle preference of RCF on Type B locomotives. This explains the phenomenon that similar RCF damage has not been reported on Type B locomotives running in mild environments. At the end, the electrical compensation is optimized numerically with a purpose of easing RCF damage of Type B locomotives.

Evaluation of material models describing the evolution of plastic anisotropy in pearlitic steel

Rolling contact fatigue cracks often initiate in the highly deformed surface layer of railway rails. However, the behavior of the material in this region is not well known. In an earlier study by the author the behavior of a pearlitic rail steel subjected to large shear deformations was analyzed experimentally. The purpose of the present study is to evaluate the ability of four different material models, based on [Meyer et al. INT J SOLIDS STRUCT, pp. 122–132, vol 130–131, 2018], [Shi et al. INT J PLASTICITY, pp. 170–182, vol 63, 2014], [Qin et al. INT J PLASTICITY, pp. 156–169, vol 101, 2018] and crystal plasticity, to simulate these experiments. The second and third models are formulated into the finite strain framework used in the first model, and advanced kinematic hardening laws are incorporated. To enable such an evaluation, a simulation methodology of the experimental procedure is developed and presented. It is found that the model by Shi et al. could fit the experimental data most accurately. None of the models were able to predict all features observed in the experiments. Further development of constitutive models for evolving anisotropy in pearlitic steels subjected to large shear strains is therefore needed.

Influence of laminar plasma quenching on rolling contact fatigue behaviour of high-speed railway wheel steel

This study aims at determining the influence of laminar plasma quenching (LPQ) on the rolling contact fatigue (RCF) behaviour under water condition of high-speed railway wheel steel. The wheel and rail test rollers were treated by LPQ with different scanning speeds (700, 900, 1100, 1300 mm/min), which generated different depth of heat affected zones (HAZs) and residual compressive stresses in the surface layer. To obtain an insight of the microstructures and their corresponding thermal treatment applied, the temperature evolution was simulated during the LPQ by Finite Element Modelling and the micro-structures of cross-sections below the roller surfaces were characterized. The results showed martensite, retained austenite and undissolved cementite in the HAZ as a consequence of the ultrafast heating and cooling rates. The produced residual compressive stresses resulted in an increase of ~30% of the RCF life indicating that LPQ could effectively improve the RCF resistance. Further investigations showed that the high density dislocation martensitic structure of the HAZ of reduced the plastic deformation, which could delay the RCF crack initiation and decrease the depth of RCF crack growth. During the RCF test, some retained austenite of the LPQ treated wheel roller transformed into twin martensite under the effect of strain induced martensitic transformation. The plastic deformation in the untreated wheel roller resulted in a refined microstructure showing lots of sub-grains generated in both pearlite and ferrite regions. Furthermore, we have observed that the lattice structures of the martensite in the LPQ treated sample and sub-grains of the plastically deformed untreated sample were similar to the BCC structure of the ferrite at the substrate in the high-speed railway wheel steel. We conclude from the obtained results that the LPQ processing enhances the RCF life and that this improvement is influenced by the obtained microstructure in the HAZ.

An EBSD Investigation on the Evolution of the Surface Microstructure of D2 Wheel Steel During Rolling Contact Fatigue

In this work, the relationship between rolling contact fatigue (RCF) failure and the microstructure of D2 wheel steel was studied using a GPM-30 fatigue tester under oil lubrication conditions. The microstructural evolution during the RCF process can be divided into three stages: In the first stage, the misorientation of the proeutectoid ferrite is 2°–10°, the ferrite phase in pearlite is less than 2°, and the dislocation density is low. In the second stage, with the increase in cycles, the misorientation of the proeutectoid ferrite increases to more than 10°, and the ferrite phase in pearlite increases to 2°–10°. In the third stage, the misorientation of the ferrite phase in pearlite increases to more than 10°, the ferrite phase is divided into fine grains, and the dislocation density is high. RCF cracks are formed in the third stage. Crack initiation is ascribed to the refinement of the surface ferrite phase and proeutectoid ferrite and the increase in dislocation density. RCF cracks are initiated and propagate primarily at the interface of pearlite/proeutectoid ferrite and in proeutectoid ferrite.

The crack propagation path for a system of surface and subsurface cracks and their interactions due to rolling contact fatigue

In this paper, the rolling contact fatigue crack growth in the presence of multiple cracks and their interactions is studied. The proposed formulation is based on linear elastic fracture mechanics and singular integral equations. The body under the rolling contact is modeled by a half-plane weakened by a set of surface, subsurface and surface–subsurface cracks. Rolling contact is simulated by translational motion of an elliptically distributed force along the half-plane boundary. Several parameters, such as the distance between cracks, the value of initial crack lengths, the value of the friction coefficient, and the initial angle between cracks and the boundary of the half-plane are studied. Results obtained from this investigation are in good agreement in a special case with those reported in the literature. It is observed that in the system of two parallel surface and subsurface cracks with equal lengths, changing the distance between the cracks changes the growth paths, and when this distance increases to a critical value, the cracks grow independently. In addition, in the case of two parallel surface cracks when the left crack is shorter, the cracks have a stronger tendency to join together, which leads to pitting phenomena on the contact surface. Furthermore, in the system of two parallel subsurface cracks, it is seen that fast fracture occurs sooner when the initial angle of the cracks increases. In the system of parallel surface and subsurface cracks, the dominant failure mode is spalling.

Investigation on rolling contact fatigue of railway wheel steel with surface defect

In this study, the effect of surface defect on the rolling contact fatigue (RCF) of railway wheel steel was investigated. Artificial defects with different dimensions were produced on unprerolled and prerolled wheel discs to simulate defects appearing in actual wheels. Subsequently, twin-disc tests were conducted under the typical contact stress and slip ratio. Finally, a novel method was used to investigate the RCF crack initiation of the wheel disc. Results show that the prerolling promotes RCF crack initiation for wheel steels with surface defects. The critical defect dimension resulting in the RCF crack initiation of the prerolled wheel disc is 100–200 μm. Considering the scaling factor between the actual wheel and test wheel disc, the predicted critical defect dimension is 2–4 mm for the actual wheel, which agrees with field observations. For the prerolled wheel steel, RCF cracks initiate from the region near the middle edge of the defect, where the most significant stress concentration appears. An increase in defect dimension increases the stress concentration, thereby reducing the RCF life. The Fatemi–Socie multiaxial fatigue parameter in conjunction with the theory of critical distances can predict the effect of defect dimension on variation in RCF life.

Rolling contact fatigue behaviour of rails: Wedge model predictions in T-Gamma world

In this study, T-Gamma and Wedge models have been compared with each other for the prediction of surface-initiated rolling contact fatigue cracks on rail surfaces. Both models are able to account for different observed rolling contact fatigue-wear regimes in tracks, but with very different physical backgrounds. The T-Gamma model uses empirically determined damage functions by introducing a relationship between the wear number (T-Gamma) and the rolling contact fatigue damage increment. Different rolling contact fatigue-wear regimes are considered in this empirical approach based on the idea that initiated cracks get partially or fully removed by the wear mechanism, not accounting for the full complexity of the occurring tribological phenomena. The Wedge model represents a physical approach, where contact stresses and its impact on plastic deformations and related material anisotropy are considered. Thus, the prediction of different rolling contact fatigue-wear regimes is based on these physical relationships, where plastic shear deformations in the near-surface layer play a key role. For comparison, the wheel–rail contact data from stochastic multibody dynamics simulations of a metro vehicle with conventional bogie technology running in three curve radii have been used. While the T-Gamma model always predicts the same rolling contact fatigue damage increment for a given T-Gamma value, the Wedge model shows a scattering of the predicted rolling contact fatigue damage increments when plotting them over T-Gamma because of the explicit consideration of contact stresses. Thus, each scenario consisting, for example, of certain vehicles, curve radius, wheel–rail profile combination, friction conditions, rail material, etc. needs its own damage function in the T-Gamma world. This should be kept in mind when applying the standard T-Gamma model to scenarios which differ significantly from the scenario it has been parameterised for.

Numerical assessment of the loading of rolling contact fatigue cracks close to rail surface irregularities

AbstractRolling contact fatigue damage of railway rails in the form of squats, characterised by local depressions and cracks located at the rail surface, has been linked to the occurrence of local rail surface irregularities. This study concerns rolling contact fatigue cracks in the vicinity of fairly smooth surface irregularities, here denoted dimples. The influence of factors such as dimple geometry, cluster effects, and crack size is evaluated. To this end, dynamic vehicle–track simulations featuring realistic wheel and rail profiles are employed to characterise the dynamic impact during a wheel passage. The contact load in the vicinity of the dimples is then mapped onto a 3D finite element model of a rail section containing a crack in the rail head. The crack loading is finally quantified by multimodal stress intensity factors. The analyses establish that also shallow dimples might have a significant impact on the crack loading. This effect is increased for larger or multiple irregularities but decreases as the crack grows.

An energetic approach for the prognosis of thermally induced white etching layers in bearing steel 100CrMn6

Abstract Thermally induced white etching layers (WEL) are known to increase the risk of rolling contact fatigue (RCF) cracks. This study focuses on the potential of an energetic approach for the prognosis of thermally induced WEL in oil-lubricated rolling/sliding contacts. For the determination of power and energy density thresholds a laser surface treatment was selected. The thresholds were determined using non-lubricated flat specimens made of 100CrMn6 bearing steel. The applicability of the energetic approach for the prognosis of WEL appearance in oil-lubricated rolling/sliding contacts was shown using a two-disc test rig. The formation of WEL was achieved by an acceleration process under varying contact parameters such as speed and load. The appearance of WEL was confirmed through metallographic investigations.

Improvement in the wear resistance of a hypereutectoid rail via heat treatment

The rail, as the most important component in railways, needs to be routinely checked to maintain proper service conditions to prevent catastrophic accidents by any premature failure of the rail. The severe plastic deformation of rails due to cyclic wheel/rail contact forces forms a superficial deformed layer, referred to as the white etching layer, because of cementite decomposition and the formation of the carbon-supersaturated nanostructured ferrite phase. This supersaturated superficial layer is highly prone to the initiation and propagation of rolling contact fatigue cracks. However, it has been observed that transition carbides and cementite can precipitate when the first and third stages of tempering starts at approximately 150 and 400 °C, respectively, due to carbon ejection from the body-centered tetragonal structure. A controlled laboratory heat treatment was conducted on a used hypereutectoid rail at 200 and 500 °C. Then, the wear resistances of the heat-treated samples were compared with the as-received rail sample using pin-on-disc testing. A significant increase in the wear resistance of the sample treated at 200 °C confirmed the improvement in the wear resistance of used pearlitic rails, which is a great interest for railway industries.

Investigation of the crack initiation of subsurface rolling contact fatigue in railway wheels

To investigate the crack initiation behaviour of the subsurface rolling contact fatigue (RCF) of railway wheels, a novel experimental procedure was developed to determine the actual shapes and sizes of macroscopic defects in the wheel rim. The experimental results show that the macroscopic defect can be approximated as an ellipse with an axis ratio of 2.5–7. Most of the macroscopic defects in the wheel rim are composed of different oxides, which is in accordance with observations for scrapped wheels subjected to subsurface RCF. The mode II threshold stress intensity factor range ΔKII,th of the wheel steel was measured using a double cantilever specimen. Finally, the initiation behaviour of subsurface RCF crack was evaluated based on a fracture mechanics approach, considering the morphologies of the macroscopic defects. The evaluation results show that the subsurface RCF is unlikely to occur in railway wheels that satisfy the requirements of relevant standards under the straight condition, while it will likely occur under the limited condition. For a given defect size, the initiation of subsurface RCF crack is the least likely when the axis ratio of the macroscopic defect is controlled to be a/b = 2.

Three-dimensional study of rolling contact fatigue using crystal plasticity and cohesive zone method

A three-dimensional (3D) finite element model that includes the microstructure of grains and grain boundaries is developed for studying the rolling contact fatigue (RCF) damage caused by a rolling steel wheel on a steel railway. The 3D model uses a crystal plasticity-based micromechanical framework for simulation of stress-strain behavior inside the grains, and cohesive elements with traction-separation law for modeling the intergranular fatigue damages. A combination of moving normal Hertzian load and tangential loads are employed to mimic the rolling contact conditions. The effect of various parameters such as friction coefficient, traction, partial slip contact and wheel load on RCF is studied. The results reveal that RCF cracks initiate slightly below the surface for large wheel loads and low tractions. The cracks migrate toward the surface as the wheel-rail traction increases. For high tractions, the cracks initiate at the rolling surface. Under partial slip contact condition, there is a large change in rail life for small normalized tractions. With a slight increase in the latter, the rail life significantly decreases. For moderate to large tractions, rail life remains relatively unchanged or slightly increases. The results of the study appear to agree with anecdotal observations by the U.S. railroads from revenue service operation.

Fatigue Crack Growth under Non-Proportional Mixed Mode Loading in Rail and Wheel Steel Part 2: Sequential Mode I and Mode III Loading

Rolling contact fatigue cracks in rail and wheel undergo non-proportional mixed mode I/II/III loading. Fatigue tests were performed to determine the coplanar and branch crack growth rates on these materials. Sequential and overlapping mode I and III loading cycles were applied to single cracks in round bar specimens. Experiments in which this is done have been rarely performed. The fracture surface observations and the finite element analysis results suggested that the growth of long (does not branch but grown stably and straight) coplanar cracks was driven mainly by mode III loading. The cracks tended to branch when increasing the material strength and/or the degree of overlap between the mode I and III loading cycles. The equivalent stress intensity factor range that can consider the crack face contact and successfully regressed the crack growth rate data is proposed for the branch crack. Based on the results obtained in this study, the mechanism of long coplanar shear-mode crack growth turned out to be the same regardless of whether the main driving force is in-plane shear or out-of-plane shear.

Visualization of the Rolling Contact Fatigue Cracks in Rail Tracks with a Magnetooptical Sensor

The rolling contact fatigue cracks (RCF), produced at the surface of the rail tracks, can be detected using a magnetooptical (MO) sensor. Rail tracks are carbon steels with pearlite microstructure. This microstructure has a lamellar texture composed of alternating layers of ferrite and cementite. Both phases are soft ferromagnetic materials at room temperature. If an external magnetic field is applied on the surface of a rail track, the reduced magnetic permeability causes a magnetic leakage field above the cracks. When the external magnetic field is removed, in most cases, a residual stray magnetic field remains above the cracks. When a MO sensor is placed on the surface of the rail track, the sudden change of the stray remanent magnetic field near a crack, yields a significant rotation of the polarization plane of the reflected light, resulting in high MO contrast, exactly above the cracks. Using a polished surface and a cross-section from the head of the rail track, we succeeded in visualizing the RCF cracks in the laboratory. The RCF cracks can also be detected on the surface of the rail track, in field measurements, using a portable commercial polarized light microscope equipped with a MO sensor. Finally, we use computer vision methods, to automatically detect the RCF cracks, using video recorded by displacing the portable microscopy with the MO sensor, on the surface of the rail tracks. We tested an unsupervised automatic crack detection algorithm, which exploits the tubular contrast of the RCF cracks to pinpoint the pixels that correspond to them.

2D and 3D characterization of rolling contact fatigue cracks in manganese steel wing rails from a crossing

Rail wheel contact at switches and crossings (S&Cs) induces impact stresses along with rolling contact stresses, resulting in plastic deformation and eventually crack formation. Damaged and deformed wing rails of a manganese steel crossing are studied and the microstructure, hardness and 3D crack network within the steel are characterized. It is found that the surface of the rail receives the maximum deformation resulting in a hardened top layer. The deformation is manifested by a high density of twins and dislocation boundaries in the microstructure. A complex crack network is revealed in high resolution by X-ray tomography.

Investigations for inclination angle characterization of angular defects using eddy current pulsed thermography

As a typical branch of active thermography techniques, eddy current pulsed thermography (ECPT) has been used to detect surface and subsurface defects in many scenarios. Among different applications, localization and characterization of rolling contact fatigue cracking (a common angular defect) in rail tracks has gained more attentions. This paper investigates three features for the inclination angle characterization of artificial angular defects. Specifically, one pixel-level and two spatial-level features, i.e., max pixel response, skewness- and kurtosis-based spatial features, are proposed to build the relations to the inclination angle. Based on the 3D FEM, the simulation results show that the relation between the max pixel response and the inclination angle is non-monotonic under different heating pulses. In contrast, the skewness- and kurtosis-based spatial features can give overall monotonic relations to the inclination angle under the condition that a longer pocket length (≥0.38 mm) occurs and a longer heating pulse (≥100 ms) is used. Further, the above simulation results were experimentally verified.

Fatigue Crack Growth under Non-Proportional Mixed Mode Loading in Rail and Wheel Steel Part 1: Sequential Mode I and Mode II Loading

Fatigue tests were performed to estimate the coplanar and branch crack growth rates on rail and wheel steel under non-proportional mixed mode I/II loading cycles simulating the load on rolling contact fatigue cracks; sequential and overlapping mode I and II loadings were applied to single cracks in the specimens. Long coplanar cracks were produced under certain loading conditions. The fracture surfaces observed by scanning electron microscopy and the finite element analysis results suggested that the growth was driven mainly by in-plane shear mode (i.e., mode II) loading. Crack branching likely occurred when the degree of overlap between these mode cycles increased, indicating that such degree enhancement leads to a relative increase of the maximum tangential stress range, based on an elasto–plastic stress field along the branch direction, compared to the maximum shear stress. Moreover, the crack growth rate decreased when the material strength increased because this made the crack tip displacements smaller. The branch crack growth rates could not be represented by a single crack growth law since the plastic zone size ahead of the crack tip increased with the shear part of the loading due to the T-stress, resulting in higher growth rates.

Comparison Study of Different Features for Pocket Length Quantification of Angular Defects Using Eddy Current Pulsed Thermography

As one promising nondestructive testing technique, eddy current pulsed thermography has been used for detecting the defects in rail tracks, e.g., rolling contact fatigue (RCF) cracking. This paper introduces and compares three features for the pocket length quantification of artificial angular defects and natural RCF cracks. Specifically, the maximum thermal response and two principal component analysis (PCA)-based features (PCA-based thermal pattern and PCA-based first-order differential pattern) are utilized to quantify the pocket length by linear fitting. For evaluating and comparing the fitted relations between three introduced features and the pocket length, two criteria ( $R^{2}$ and 2-norm of the residual) are used. Through comparisons, three features’ strengths and limitations are summarized. Furthermore, a real RCF specimen is used to verify the comparison results.

Rolling contact fatigue crack propagation relative to anisotropies in additive manufactured Inconel 625

Additive manufacturing is steadily gaining acceptance in certain industry segments as a process for the manufacturing of dense metallic components. The Ni-based superalloys belonging to the Inconel family have for many years been in focus for AM research and AM produced components are now becoming commercially available. However, it is still unclear how the microstructural anisotropy, inherent to most AM materials, affects the material performance in a given application. The anisotropy may e.g. influence the mechanical properties and the performance in certain tribological situations, such as when subjected to rolling contact fatigue. Like most AM methods, the powder bed fusion – laser beam process gives the produced components a relatively rough surface. To perform well in demanding tribological situations, the components are commonly machined to a smooth finish.In this work, Inconel 625 produced using PBF-LB is evaluated in a rolling contact fatigue test. Test cylinders (Ø10 mm) have been produced using different build directions and scan strategies, resulting in varying microstructures and textures.In the rolling contact fatigue test, a cylindrical sample is mounted between two Ø 140 mm metal rollers, pulled together via a spring. After testing, the contact tracks are studied using SEM and EBSD to reveal cracks. Cracks were analysed with respect to the microstructure and anisotropies.It was found that the anisotropy influences both the nucleation and growth of cracks. The AM produced specimens were also found to be more prone to transgranular cracking than conventional Inconel 625, which predominantly displayed intergranular cracks.