Search

In this study, finite element method (FEM) is applied to investigate the effect of material defect orientation on subsurface crack initiation in rolling bearings. First, the history of stress intensity factors (SIFs) of a subsurface cubic material defect is calculated. SIFs calculation is conducted when it is parallel to the rolling surface of a bearing ring ( φ = 0 ∘ ) under a ball rolling on the ring. Then, the effect of defect orientation, varying from φ = 0 ∘ to φ = 30 , 45 and 60 ∘ , on the history of SIFs is investigated. It is deduced that changing the defect orientation results in a threefold increase in the equivalent SIF range compared to parallel orientation. [ABSTRACT FROM AUTHOR]

In the circumstances of high-speed railways, the wheel-rail vibration is significantly aggravated by polygonal wheel wear and rail corrugation, which subsequently leads to the wheel-rail interaction at higher frequencies and potential failure of the rail fastening. In this paper, a ω-type clip of the fastening in the CRH high-speed rail was used to investigate the failure mechanism. First, a dynamic wheel-rail coupling model and a finite element analysis of the rail clip were developed, from which the rail vibration frequency and modal frequencies of the clip with different installation torques were obtained. The experimental tests and modal simulation results were mutually verified. In addition, the real-time vibration measurement and the wheel-rail wear monitoring were carried out at a CRH high-speed railway site. It was found that the resonant frequencies of the ω-type clip in the installation condition coincided with the excitation frequencies of the wheel-rail interaction induced by wheel-rail wear. The high-frequency dynamic failure mechanism of a typical ω-type clip, W300-1, is put forward for the first time. Moreover, a high-frequency rail clip fatigue test system was designed and developed specifically for this study. The loading excitation frequency of the clip test used was set as 590 Hz, and the loading amplitude was 0.05 mm. After 125-minute operation of the test system, the clip was broken at the expected location predicted by the FEA model. The high-frequency fatigue test result further verified that the failure mechanism of the ω-type clip was due to the resonance of the clip with its excitation force from the wheel-rail interaction. Finally, the clip was then structurally improved taking into account the stiffness and mass, which led to its resonant frequencies shifting away from the high-frequency excitation range, hence avoiding resonance failure of the subject clip. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Spin in frictional rolling contact can cause significant stress, which is the key to understanding and predicting the wear and fatigue behavior of contact components, such as wheels, rails, and rolling bearings. The lateral creep force arising from spin influences the kinematics of a wheelset and thus of vehicles. The solution that is currently employed in the field of elasticity and continuum statics was developed by Kalker and uses a boundary element method (BEM). In this paper, a new approach based on Lagrangian explicit finite element (FE) analysis is employed. This approach is able to consider arbitrary geometric profiles of rails and wheels, complex material behavior and dynamic effects, and some other factors. The new approach is demonstrated using a three-dimensional (3D) model of a wheel with a coned profile rolling along a quarter cylinder and can be easily adapted to apply to wheels and rails of arbitrary profiles. The 3D FE model is configured with elastic material properties and is used to obtain both normal and tangential solutions. The results are compared with those of the Hertz theory and the Kalker's model. The 3D FE model is then configured with elastoplastic material properties to study the spin-rolling contact with plasticity. The continuum dynamics phenomenon is captured by the FE model, which enhances the ability of the model to mimic reality. This improvement considerably extends the applicability of the FE model. The model can be applied to fatigue and wear analyses at gauge corners or rails as well as to deep groove bearings, where a large geometrical spin is present and plastic deformation may be of importance. [ABSTRACT FROM AUTHOR]

• A Voronoi finite element model for crack initiation life of RCF is built. • The coupling damage model considers the microstructure of material. • The impact mechanism of grain characteristics and material defects is revealed. The material microstructure of plays an important role in crack initiation of rolling contact fatigue. To address crack initiation of tapered roller bearings, the inhomogeneous characteristic of material is considered. The random distribution of material grains is simulated with Voronoi graph, and a Voronoi finite element model of rolling contact fatigue is given. Numerical analysis of damage-life constitutive relation is carried out with finite element analysis. The effect of material microstructure inhomogeneity on initiation life of fatigue crack is studied. Researches show that the smaller the grain size, the more difficult the fatigue crack appearing. The larger the difference of elasticity modulus between non-metallic inclusions and matrix material, the greater the impact on fatigue life of crack initiation. The angular and sharp non-metallic inclusions reduce fatigue life more than rounded shape inclusions. Initiation life of fatigue crack has a minimum value on a critical depth of nonmetallic inclusion. The longer distance between two laterally arranged inclusions, the greater it affect the initiation life of fatigue crack. The effect of distance between two vertically arranged inclusions on fatigue crack initiate depends on the depth of inclusions. [ABSTRACT FROM AUTHOR]

Abstract: Current work presents Finite Elements study of hydrogen diffusion into a metal substrate loaded by rolling contact. The main goal of the presented study is to investigate stress effect on hydrogen transport in rolling contact conditions. In this work, rolling contact is defined by the Hertzian pressure, moving along the bearing ring. Although, hydrogen diffusion with respect to bearing is studied here, very similar approach can be used for other components of machinery subjected to rolling contact. Governing equations are given in the current study in dimensionless form, and parametric study, analyzing influence of different dimensionless parameters on subsurface concentration of hydrogen, is presented. [Copyright &y& Elsevier]

A method is described which is an extension of rolling contact models with respect to plasticity. This new method, which is an extension of the STRIPES semi-Hertzian (SH) model, has been implemented in a multi-body-system (MBS) package and does not result in a longer execution time than the STRIPES SH model [J.B. Ayasse and H. Chollet, Determination of the wheel–rail contact patch in semi-Hertzian conditions, Veh. Syst. Dyn. 43(3) (2005), pp. 161–172]. High speed of computation is obtained by some hypotheses about the plastic law, the shape of stresses, the locus of the maximum stress and the slip. Plasticity does not change the vehicle behaviour but there is a need for an extension of rolling contact models with respect to plasticity as far as fatigue analysis of rail is concerned: rolling contact fatigue may be addressed via the finite element method (FEM) including material non-linearities, where loads are the contact stresses provided by the post-processing of MBS results [K. Dang Van, M.H. Maitournam, Z. Moumni, and F. Roger, A comprehensive approach for modeling fatigue and fracture of rails, Eng. Fract. Mech. 76 (2009), pp. 2626–2636]. In STRIPES, like in other MBS models, contact stresses may exceed the plastic yield criterion, leading to wrong results in the subsequent FEM analysis. With the proposed method, contact stresses are kept consistent with a perfect plastic law, avoiding these problems. The method is benchmarked versus non-linear FEM in Hertzian geometries. As a consequence of taking plasticity into account, contact patch area is bigger than the elastic one. In accordance with FEM results, a different ellipse aspect ratio than the one predicted by Hertz theory was also found and finally pressure does not exceed the threshold prescribed by the plastic law. The method also provides more exact results with non-Hertzian geometries. The new approach is finally compared with non-linear FEM in a tangent case with a unidirectional load and a complete slip: when plasticity is taken into account, and for large adhesion values, friction forces have an influence on the size of the contact patch. The proposed approach enables also to assess extensively the level of plasticity along a track through an indicator associated with a given yield stress. [ABSTRACT FROM AUTHOR]

The development of contact theories and numerical solutions for various applications is a field which expands rapidly. Although railway system is a transportation system with a long tradition, still a large number of unsolved problems exist. One of these problems is the assessment of the stress and strain status of the rail and the wheel under rolling contact conditions. The assessment requires the investigation of the mechanically originated damage of wheel and rail, as cracklike damage, prediction of wear and fatigue as well as change of material properties. Since the mechanical problem is of high complexity because of the contact condition and the material behavior, only a numerical approach seems to be appropriate. This paper presents a method which has the advantage that the structural analyst is allowed to solve both the contact problem and material nonlinearity using an unique simulation model [ABSTRACT FROM AUTHOR]

Abstract: Rolling contact fatigue cracks in railway track called squats are studied in this paper. In the first part, the effects of an elastic foundation (sleepers and the ballast) on stress intensity factors obtained at a crack tip are studied. A simplified finite element model (FEM) and an extended finite element model (XFEM) were created to investigate these effects, the XFEM model being limited in geometrical size, but more able to model crack growth. Both FEM and XFEM confirmed that an elastic foundation leads to an additional bending stress which increases the crack growth rate significantly. Field results also authenticate that squat form cracks appear on timber sleepers more commonly than on concrete ones. These results indicate that considering these bending stresses in a FE model, is important to achieve a more realistic model of squat development. In the second part, a short crack of 250μm length is simulated to investigate how variations of traction ratio (TR), friction coefficient between the crack faces (FC) and the crack angle affect SIFs when the rail is mounted on an elastic foundation. Simulations show that a crack on a rail laid on elastic foundation (clips, sleepers and ballast) can lead to significantly higher SIFs in many conditions and consequently raises crack growth rate. This indicates that foundation stiffness is as important as water entrapment and friction coefficient between the crack faces. [Copyright &y& Elsevier]

Highly localized stresses generated at the gauge corner of the outer rail cause fatigue cracking. The longitudinal bending stresses in the rail underhead radius position are of special interest, as tension spikes have been identified at this location during in-track measurements under high axle load conditions. The tension spike is a result of vertical and lateral head bending on the web. This effect is highly localized and is additional to the stresses generated due to vertical and lateral bending of the whole rail profile (the so-called global bending). This study examined contact and bending stresses by modelling the rail on a discrete foundation using finite elements and by considering the loading to be a fully slipping elliptical Hertzian contact patch. The analysis revealed that the tensile longitudinal stress was highly dependent on several service conditions: the contact patch offset from the rail centreline, the ratio of lateral (L) to vertical (V) loads, the direction of lateral traction, foundation stiffness and seasonal temperature variations. The tension spike increases, and the depth below the contact surface at which the stresses become tensile reduces, as the contact patch offset and/or the L/V ratio increases. Both these enhance the tendency for rolling contact fatigue cracks to turn downwards and become transverse defects. This is because an increase in tensile bending stresses together with both residual and thermally induced stresses can facilitate cracks to turn perpendicular to the tensile stresses once they reach a critical length. Shear traction towards the gauge corner was found to be the most damaging phenomenon. [ABSTRACT FROM AUTHOR]

Abstract: In this investigation an elastic–plastic Voronoi finite element (EPVFE) model for rolling contact fatigue of Hertzian line contact was developed. The EPVFE model coupled with damage mechanics approach is used to investigate rolling contact fatigue (RCF) of tribo-components. Mises based plasticity model with kinematic hardening was employed to incorporate the effects of material plasticity. The model considers both; stress and accumulated plastic strain based damage laws in the constitutive damage modeling. The results indicate that once a fatigue crack initiates within the domain, the fatigue damage induced due to the accumulated plastic strains around the crack tip drives the majority of the crack propagation stage. The results from this investigation reveal that depending on the contact pressure the crack propagation stage can constitute 15–40% of the total life. The spall shape, fatigue lives and Weibull slopes obtained from the EPVFE model correlate well with the experimental results. The developed model was also used to investigate the effects of initial material imperfections such as inclusions and elastic modulus inhomogeneity on the fatigue lives. The results indicate that material flaws within the domain lead to a significant decrease in fatigue lives and increase in life scatter. [Copyright &y& Elsevier]

Abstract: This paper discusses the relationship between ACFM signal and RCF defects in railway rails and wheels using an FEM model developed with COMSOL Multiphysics that has been verified for single and multiple RCF type cracks. The response to semi-elliptical defects of surface length 2mm to 40mm, and elliptical ratio 1:1 to 1.75:1, has been simulated and it is shown that the ACFM signal is most sensitive to, and therefore able to accurately size, RCF defects <20mm surface length (classified by Network Rail on rails as light-moderate). The sensitivity to sensor lift off and sensor angle relative to crack angle, which is of importance for high-speed and fixed sensor systems, is presented for a semi-elliptical crack. For a complex defect extending into the rail beyond the surface breaking component (a morphology that can be seen for moderate, heavy or severe RCF cracks in rail), the effect of using a sizing algorithm based on semi-elliptical shapes is quantified; it is shown that the defect would be undersized by 34.4% using this approach. Finally, the ACFM response to clusters of 4 semi-elliptical defects with inter-crack spacings of between 1mm and 19mm is presented. It is shown that closely spaced cracks give a large signal that could be interpreted as one large defect; this effect is quantified. [Copyright &y& Elsevier]

Abstract: Spalling is rolling contact fatigue (RCF) damages of railway wheels, as a result of unexpected wheel slides on a rail. The major characteristic in spalling is that fatigue cracks initiate from white layers. In this study, in order to evaluate the influence of the white layer geometries on spalling properties, RCF tests and elasto-plastic FEM analyses were carried out. Moreover, the Dang Van model, which is the multiaxial fatigue strength evaluation methods, was applied to predict the crack initiations in RCF tests. There are three main results from this study. Firstly, RCF tests yielded that the crack initiations were less in the smaller white layers than in the larger ones. In addition, the crack initiations of the matrices were less than those of the white layers. Secondly, FEM analysis results showed that the maximum stresses were lower in the smaller white layers than in the larger ones. This result corresponded to the crack initiation behaviours of RCF tests. Finally, the fatigue strength prediction results by the Dang Van model were reasonably equivalent to the RCF test results. Therefore, it was concluded that the crack initiations in spalling could be reduced by the smaller white layer. [Copyright &y& Elsevier]

The continuum theory of elasticity and/or homogeneously discretized finite element models have been commonly used to investigate and analyze subsurface stresses in Hertzian contacts. These approaches, however, do not effectively capture the influence of the random microstructure topology on subsurface stress distributions in Hertzian contacts. In this paper, a finite element model for analyzing subsurface stresses in an elastic half-space subjected to a general Hertzian contact load with explicit consideration of the material microstructure topology is presented. The random internal geometry of polycrystalline microstructures is modeled using a 3D Voronoi tessellation, where each Voronoi cell represents a distinct material grain. The grains are then meshed using finite elements, and an algorithm was developed to eliminate poorly shaped elements resulting from "near degeneracy" in the Voronoi tessellations. Hertzian point and line contacts loads are applied as distributed surface loads, and the model's response is evaluated with commercial finite element software ABAQUS. Internal stress results obtained from the current model compare well with analytical solutions from theory of elasticity. The influence of the internal microstructure topology on the subsurface stresses is demonstrated by analyzing the model's response to an over rolling element using a critical plane approach. [ABSTRACT FROM AUTHOR]

Abstract: Uncertainty analysis and parametric studies are presented for estimating the fatigue failure probability of surface cracks in silicon nitride ball bearings subjected to rolling contact fatigue. Uncertainty quantification of input parameters are presented first based on experimental data, inspection capability, and geometric reasoning. Surrogate models for equivalent stress intensity factors are then used for uncertainty propagation, which are built upon high fidelity finite element modeling with half-penny-shaped surface cracks. Instead of black-box type surrogate modeling, physical observations are employed to decompose the high dimensional surrogate model into multiple one-dimensional models. The cross-validation technique is used to find the best surrogate that has the smallest prediction variance. The probability of failure is estimated using Monte Carlo simulation and surrogate models. The parametric studies show that reducing the maximum crack size (by limiting inspection threshold) and increasing the fatigue threshold (by improving fracture toughness of a material) are the most effective ways of reducing the probability of failure. For example, decreasing the maximum crack size by 4.4% and increasing the lowest fracture threshold by 2.8% results in the reduction of probability of failure by 40%. Ball survivability increases with decreasing ball diameter, for a given peak Hertzian stress. In order to apply the current study to hybrid ball bearing design, the survivability results are generalized through non-dimensionalization. [Copyright &y& Elsevier]

Abstract: Fatigue life investigations have been made for cylindrical hollow rollers in pure rolling contact. In addition to normal loading, the rollers have been subjected to tangential loading of 1/3rd the normal load value. Sufficient coefficient of friction has been used to ensure no slipping occurs. Two main models were built with different hollowness percentages to investigate the hollowness percentage that gives the longest fatigue life. The first model consists of two cylindrical rollers of same size, while the second model consists of two rollers of different sizes. Two cases have been studied, when both rollers are hollow and when only one roller is hollow. The stress distribution in the roller body and the resulting deformation has been investigated using the finite element package, ABAQUS. Then the Ioannides–Harris (IH) theory was used to predict the fatigue life of the hollow rollers in pure rolling contact. Investigations have been made for five different materials, CVD 52100, Carburized steel, VIMVAR M50, M50NiL and Induction-hardened steel. It has been found that the optimum hollowness percentage with the longest fatigue life ranges is around 50%. Many factors affect the optimum hollowness percentage, like the kind of the material used for the cylindrical roller, whether the rollers in contact are of the same size or different size and whether the hollow roller is in contact with another hollow roller or in contact with solid roller. [Copyright &y& Elsevier]

Abstract: The fundamental knowledge of fatigue damage mechanism is necessary for understanding manufacturing process effects. The traditional method of artificially created surface defects may accelerate crack propagation and fatigue. However, the artificial defects will change the surface integrity and therefore alter the nature of fatigue damage, and thus, the fatigue damage in the presence of artificial defects in literature may not reflect the true mechanism of real-life fatigue processes. This paper studies the fatigue damage resulting from real-life rolling contact tests and finite element analysis of AISI 52100 steel and identifies the possible mechanisms for fatigue failure in the presence of process induced surface integrity. Rolling contact fatigue tests were then performed until surface spalling had occurred. Surface and subsurface fatigue damage of the samples was then characterized using optical and scanning electron microscopy (SEM) and surface profiling. Two types of subsurface cracks were observed: main cracks that propagate parallel to the surface due to subsurface shear stress induced fracture/debonding of inclusions or second phase particles. Shear stress induced surface cracks propagate at shallow angles (≈35°) from the surface. Branching cracks eventually form and connect the main crack to surface. The crack SEM images show that the formation sequence of fatigue cracks is different for the turned and ground surfaces. Material loss (spalling) occurs as a combined effect of the main, surface, and branching cracks creating small discontinuous sections of material at the surface which are eventually lost after continued rolling. [Copyright &y& Elsevier]

Two-dimensional elastic-plastic finite element (FE) simulations of a rolling contact problem are presented. The objective was to evaluate three elastic-plastic material models, all of them able to simulate decay in ratchetting rate, by studying the cyclic ratchetting material response and failure of a pearlitic rail steel. The FE simulation, which were performed for different magnitudes of contact pressure and traction forces showed a large accumulation of shear deformation of the surface at the rolling contact. The critical plane (crack plane) for crack initiation on the crack planes was calculated using this model, as well as an empirical model for failure by ratchetting, for comparison. The results from the evaluation of the three material models show that the material model which could best mimic the stress-strain cycles and ratchetting rate, from cyclic uniaxial experiments, was also able to resolve the stress and strain field correctly in the FE simulations and to give reliable results. Low-cycle fatigue governed fatigue crack initiations, and the angles calculated for the crack plane showed good agreement with observations from experiments. [ABSTRACT FROM AUTHOR]

Rolling contact fatigue failure is one of the very common causes for the replacement of heavy haul car wheels. There are two crack initiation modes in rolling contact fatigue failures, which are surface and subsurface crack initiation modes. Internal defects such as voids and non–metallic inclusions are known to be the possible origins of subsurface cracks. In this study, torsion and axial fatigue tests are conducted on railway wheel steel with artificial defects, and finite element analyses of heavy haul car wheels are performed to study the subsurface crack initiation due to cyclic rolling contact. The fatigue limits of the wheel steel decrease with an increase in the artificial defect size. The shear and axial fatigue limits with different defect sizes agree well the El–Haddad formula. Based on the finite element analysis considering the multiaxial stress state, the critical defect size for subsurface crack initiation is estimated to be 1.1 mm. This indicates that subsurface cracks might be initiated in service conditions as the critical size is less than 1.6 mm, which is the maximum permissible size of an internal defect in the wheel rim as per Association of American Railroads specifications. Therefore, higher–cleanliness wheels would be required to prevent subsurface cracks. • Critical defect size for subsurface crack initiations of railway wheel was studied. • Fatigue limits of wheel steel decreased with an increase of defect size. • Predicted critical defect size was 1.1 mm in actual wheels from multi-axial fatigue evaluation. • Higher cleanliness wheels have superior resistance to subsurface crack initiations. [ABSTRACT FROM AUTHOR]