Development of bainitic rail steels with potential resistance to rolling contact fatigue

Rails are a major capital and maintenance cost for railways in North America. While manufacturers produce clean steels with high quality, most rails made today retain the basic carbon–manganese chemistry of traditional pearlitic rails. This paper describes the development of a bainitic rail steel with potential additional resistance to rolling contact fatigue damage. It is shown that rails can be produced in bainitic steel without the need for complex heat treatments after rolling, and that bainitic rails can have higher hardness and fracture toughness than pearlitic rails. Although small‐ and full‐scale tests indicate that the wear performance of bainitic steel depends considerably on test conditions, the indication is that bainitic steel rails can have significantly better rolling contact fatigue performance compared to pearlitic rails. Reasons for the superior fatigue performance are not fully understood, although a number of hypotheses exist. A conclusion is that continued research would be useful to understand quantitatively the physics and metallurgy of wheel/rail contact.     

On propagation of short rolling contact fatigue cracks

Recent accidents involving railway rails have aroused demand for improved and more efficient rail maintenance strategies to reduce the risk of unexpected rail fracture. Numerical tools can aid in generating maintenance strategies: this investigation deals with the numerical modelling and analysis of short crack growth in rails. Factors that influence the fatigue propagation of short surface‐breaking cracks (head checks) in rails are assessed. A proposed numerical procedure incorporates finite element (FE) calculations to predict short crack growth conditions for rolling contact fatigue (RCF) loading. A parameterised FE model for the rolling‐sliding contact of a cylinder on a semi‐infinite half space, with a short surface breaking crack, presented here, is used in linear‐elastic and elastic–plastic FE calculations of short crack propagation, together with fracture mechanics theory. The crack length and orientation, crack face friction, and coefficient of surface friction near the contact load are varied. The FE model is verified for five examples in the literature. Comparison of results from linear‐elastic and elastic–plastic FE calculations, shows that the former cannot describe short RCF crack behaviour properly, in particular 0.1–0.2 mm long (head check) cracks with a shallow angle; elastic–plastic analysis is required instead.     

A rolling contact fatigue crack driven by squeeze fluid film

ABSTRACT A new model of surface breaking rolling contact fatigue (RCF) crack driven by a coupled action of a squeeze oil film built up in the crack interior and a pressure exerted at the external contact interface was developed. The model can be applied to the ‘nominally dry’ contact couples with an occasional presence of liquid in the crack interior (wheel/rail contact) as well as to the elastohydrodynamic lubrication (EHL) conditions. In the first case, the contact load is a result of solid/solid interaction and can be determined by solving the FE contact problem, but the liquid contained in the crack interior forms a thin film between the crack faces changing their interaction into the type of liquid/solid. This liquid is being periodically squeezed under contact load and acts as a ‘squeeze film’ known from the lubrication theory. In the second case, the liquid (lubricating oil) is permanently present in the contact area and consequently in the vicinity of the crack mouth. This creates conditions for filling the crack with oil. Similarly as in the first case, the ‘squeeze oil film’ is built between the crack faces. The contact load in this case results from a liquid/solid interaction and can be approximated by the pressure and traction distributions obtained from the numerical solution of the elastohydrodynamic contact problem. In both cases the model can be used to determine the Linear Elastic Fracture Mechanics (LEFM) crack tip stress intensity histories during cyclic loading and consequently to predict the crack growth rate and direction. An example of applying the model to the EHL case is given to explain the mechanisms and phenomena leading to the crack front loading. The cycle of rolling a roller over the crack was numerically simulated to obtain the mixed mode (I and II) SIF histories. In the analysis, the EHD pressure and traction were determined through the full solution of the EHD line contact problem accounting for the presence of a crack, whilst the pressure in the crack was found with the use of the wedge shaped squeeze oil film (SOF) model. Possible effects of the mode I and mode II stress intensity cycles on crack growth rate and direction are discussed. The solution indicates high pressure in the neighbourhood of the crack tip, exerted on the crack faces by the squeeze oil film. This leads to the ranges of the mode I and mode II SIF variations, which are larger than for the ‘dry’ and ‘fluid entrapment’ models, and can be an explanation for the crack growth rate observed in practice     

The effect of contact load reduction on the fatigue life of pearlitic rail steel in lubricated rolling–sliding contact

Twin-disc contact simulation tests were carried out to investigate the influence of contact pressure variation on rail steel fatigue life. Both a colloidal suspension of molybdenum disulphide in an oil carrier fluid (similar to a commercial flange lubrication product) and water were used as lubricants. It was found that the reduction from 1500 to 900 MPa of the maximum Hertzian contact pressure (at which a molybdenum–disulphide-lubricated and previously worn rail sample was tested) extended the fatigue life of the rail steel by over five times. For water lubrication a similar reduction in contact pressure produced only a marginal increase in fatigue life. The results were found to be in qualitative agreement with the predictions of the newly developed Three Mechanism (TM) model of rolling contact fatigue, which is introduced here. This model combines the mechanisms of ratcheting and the fracture mechanics-based mechanisms of both shear stress- and tensile stress-driven, fluid-assisted, crack growth.     

Surface crack growth of silicon nitride bearings under rolling contact fatigue

Surface crack growth of silicone nitride ceramic bearings under rolling contact fatigue has been investigated from the viewpoints of contact stresses (ring crack model) and fluid pressure (wedge effect model). The mechanisms of these two models have been investigated independently; however, it was impossible to separate the effects of contact stresses and fluid pressure on surface crack growth. In this paper the effects of contact stresses (ring crack model) on surface crack growth are investigated. In the ring crack model the crack growth is caused by contact stresses around the circumference of the contact circle. The growth of surface cracks located inside and outside the contact track was observed in order to obtain data from which we could reexamine the ring crack model. The outside cracks under rolling contact fatigue were propagated by contact stresses alone and also the inside cracks grew as slowly as the outside cracks. We concluded that the cracks are propagated by the single effect of contact stresses. Preliminary observations of surface crack growth showed that the cracks were unaffected by wear and residual stresses.     

On the Ekberg, Kabo and Andersson calculation of the Dang Van high cycle fatigue limit for rolling contact fatigue

Recently, various methods have been proposed to assess the risk of rolling contact fatigue failure by Ekberg, Kabo and Andersson, and in particular, the Dang Van multiaxial fatigue criterion has been suggested in a simple approximate formulation. In this note, it is found that the approximation implied can be very significant; the calculation is improved and corrected, and focused on the study of plane problems but for a complete range of possible friction coefficients. It is found that predicted fatigue limit could be much higher than that under standard uniaxial tension/compression for ‘hard materials’ than for ‘ductile materials.’ This is in qualitative agreement, for example, with gears' design standards, but in quantitative terms, particularly for frictionless condition, the predicted limit seems possibly too high, indicating the need for careful comparison with experimental results. Some comments are devoted to the interplay of shakedown and fatigue.     

Investigation of rolling contact crack initiation in bainitic and pearlitic rail steels

The stress–strain history and the crack initiation lives of bainitic and head‐hardened pearlitic rail steels were determined under rolling contact loading by implementing the semi‐analytical Jiang–Sehitoglu rolling contact model that incorporates both ratchetting and multiaxial fatigue damage. The calculations revealed that the bainitic steel withstands higher loads than the pearlitic steel at low shear tractions, however; both materials behave in an increasingly similar manner as the shear tractions increase. Furthermore, maximum damage occurs in both steels when ratchetting and fatigue damage coincide on the surface. In addition to shedding light on the rolling contact fatigue (RCF) performance of bainitic and pearlitic rail steels, the current work also establishes a methodology for the realistic prediction of crack initiation under RCF.     

Effects of sliding on rolling contact fatigue of railway wheels

This paper proposes a numerical approach based on a steady‐state algorithm to predict the rolling contact fatigue crack initiation in railway wheels in practical conditions. This work suggests taking into account the cyclic hardening of the wheel's material and one of its originality is to conduct a complete numerical approach whatever the loading level. The main stages are the characterization and modelling of the material behaviour, the determination of the stress–strain fields using a numerical steady‐state method and the application of a high cycle fatigue criterion. Computations were made with the Abaqus FE commercial software. Three cases are studied: rolling with or without sliding and skating. The numerical results give several types of mechanical responses: elastic or plastic shakedown. Otherwise, the results show that the location where the shear stress is maximal is not the same as where the risk of crack is the highest.     

A numerical approach to subsurface crack propagation assessment in rolling contact

Subsurface crack mode II propagation parallel to the contact surface is a damage mechanism leading to dramatic failure in many components subjected to cyclic loading. A weight function (WF) was elaborated for calculating the applied mode II stress intensity factor (SIF) of a crack in a two‐dimensional half‐space in plane strain condition, for crack completely closed and frictionless contact between the crack faces. With respect to other methods, the WF allows faster SIF calculation, thus being suitable for simulation of many repeated load cycles and fatigue crack propagation. The WF was applied for simulating a case of rolling contact experiments found in the literature, and good agreement between experimental and numerical results was obtained, showing the effectiveness of the WF method in damage tolerant design.     

Flaking failures originating from microholes of bearings under rolling contact fatigue

Rolling contact fatigue tests were carried out using plates with microholes (diameter was about 100 μm and depth was about 140 μm) under three different loads (maximum values of Hertzian stress were about 3250, 3550 and 3840 MPa, respectively) and the surface cracks initiating from those holes were observed. It was found that there is a threshold value of maximum Hertzian stress whether surface cracks originate from microholes or not, and its value is between 3250 and 3550 MPa. However, flaking failures occurred even when the stress values were lower than the threshold value. In order to investigate the relation between the flaking failures and the cracks, sectional observations of the subsurface cracks were made before and after the surface layer separations. From these observations, it was found that the subsurface cracks caused the flaking failures even when the maximum value of Hertzian stress was lower than the threshold value of surface crack initiation.     

Numerical simulation of fatigue crack growth

A numerical simulation of fatigue crack growth which uses currently available crack tip stress and strain fields is described. The essential features of the numerical model are the concepts of damage accumulation cycle by cycle and repeated re-initiation at the tip of the growing crack. The failure criteria employed are a combination of a failure condition and a critical distance over which this condition must be achieved. This critical distance, the material size parameter, has a magnitude which depends on the failure mechanism.

The use of the model to illustrate the effects of stress ratio and environmental effects is described and the ability of the model to predict the onset of bursts of crack growth due to static failure mechanisms is demonstrated. The phenomenon of self-arresting cracks is also displayed.

Material characteristics are included in the model and comparisons with experimental data are presented for a C-Mn steel used in the fabrication of offshore structures.     

Experimental and numerical rolling contact fatigue study on the 32CrMoV13 steel

The aim of this work is to study pure rolling contact fatigue in 32CrMoV13 quenching and tempering steel. The study involves both experimental and numerical work. The influence of the roughness and the residual stresses on the mechanisms and zones of cracking were studied. The results show a rapid reduction in roughness during the first minute of test but even so there will be specimen deterioration. The residual stress profile after rolling contact tests have high compression values in the surface and at a depth of approximately 240 μm, which is related with the Hertzian maximal shear stress. The numerical simulation of the Hertzian loading was used both to determine the elastic shakedown of the material and to apply a high‐cycle multiaxial fatigue criterion. The three‐dimensional finite element analysis used in the numerical calculation includes elastic‐linear kinematic hardening plastic material and allows the introduction of an initial residual stress state. Taking into account the elastoplastic load induced by the Hertz pressure, low‐cycle fatigue tests were used to characterize the mechanical properties of the material. In order to validate the numerical simulation, the results of the calculation after elastic shakedown were compared with the values measured by X‐ray diffraction after rolling contact tests. The results showed a reasonable agreement between calculated and measured stresses. The Dang Van high‐cycle multiaxial fatigue criterion showed a good relationship with the experimental findings.     

Investigation of rolling contact fatigue cracks in a grade 900A rail steel of a metro track

Rolling contact fatigue (RCF) has been of increasing concern in the recent years in respect of the safe operation of high‐speed railway track with high traffic intensity. The present paper summarizes the results of the first investigation of RCF damage encountered in the Athens Metro. The objective of the investigation was to determine the initiation and propagation of RCF cracks and to determine their geometrical characteristics. A thorough metallographic investigation of track regarding shelling and spalling defects showed the development of a subsurface network of cracks. An analysis of the Hertzian stress field was used to determine the conditions for first yield and shakedown limits as a function of loading.     

Crack initiation from micro surface holes in bearings under rolling contact fatigue

Investigations concerning surface crack growth are necessary for understanding the mechanism of rolling contact fatigue (RCF) of bearings because the surface defects cause flaking failures. In the present work, micro holes were artificially made prior to the RCF tests and the initiation of the surface cracks from the micro holes was observed in order to find the key factors for understanding their features. Crack initiation directions were compared to the stress intensity factors calculated by a simple method based on the theory. The extent to which ‘contact pressure (wedge effect)’ and ‘contact stresses’ are applicable for understanding the correlations between the crack initiation directions and stress intensity factors is discussed. The crack initiation directions are strongly correlated to the stress intensity factors caused by the contact stresses alone. We concluded that the crack growth and initiation are dominated by stress intensity factors caused by contact stresses rather than the wedge effect.     

Stress‐dependent fatigue mechanisms of CrC–NiCr coatings in rolling contact

The rolling contact fatigue (RCF) behaviour of the plasma‐sprayed CrC–NiCr cermet coatings under different tribological conditions of contact stress was investigated. Four sets of fatigue life data of coatings were characterized by Weibull distributions. The failure modes of the coatings were classified on the basis of worn surface observations of the failed coatings. Results showed that the failure mode of the coating was related to the magnitude of contact stress. The RCF life data of the coatings tested at high contact stresses exhibited high scattering, because the bimodal distribution of the fatigue life data was seen in the Weibull plot. Generally, when the contact stress was relatively low, the coatings were prone to fail in spalling and cohesive delamination. However, at high contact stress, the coatings often failed due to interfacial delamination. At different contact stress levels, the maximum shear stress amplitude was the main reason for the generation of spall and delamination.     

Rolling contact fatigue behavior of sintered and hardened steels

Powder-metal-processed bearings and gears are finding increasing application because of their economical and technical advantages. The residual pores from the sintering operatives act as lubricant pockets and dampen sound and vibration. However, porosity also decreases the mechanical strength and reduces the life of components fabricated by powder processing relative to similar wrought components. The rolling contact fatigue behavior of sintered and heat treated steel rollers was investigated using a fatigue test machine designed and fabricated for that purpose. The powder-metal-processed and the wrought steel rollers that were tested had similar composition and hardness and were mated against wrought steel rollers of high hardness. The contact stress versus number of cycles to failure data showed that the wrought steel had a very high endurance limit under rolling contact fatigue compared to the sintered steels investigated. Rolling contact fatigue behavior was found to depend on the porosity present in the material. Large surface peeling failures and pitting type fatigue failures were observed in the sintered and hardened steels, while only pitting type failures were observed in the wrought steels     

Analysis of subsurface crack propagation under rolling contact loading in railroad wheels using FEM

A general subsurface crack propagation analysis methodology for the wheel/rail rolling contact fatigue problem is developed in this paper. A three-dimensional elasto-plastic finite element model is used to calculate stress intensity factors in wheels, in which a sub-modeling technique is used to achieve both computational efficiency and accuracy. Then the fatigue damage in the wheel is calculated using a previously developed mixed-mode fatigue crack propagation model. The advantages of the proposed methodology are that it can accurately represent the contact stress of complex mechanical components and can consider the effect of loading non-proportionality. The effects of wheel diameter, vertical loading amplitude, initial crack size, location and orientation on stress intensity factor range are investigated using the proposed model. The prediction results of the proposed methodology are compared with in-field observations.     

Similarities of stress concentrations in contact at round punches and fatigue at notches: implications to fretting fatigue crack initiation

A linear elastic model of the stress concentration due to contact between a rounded flat punch and a homogeneous substrate is presented, with the aim of investigating fretting fatigue crack initiation in contacting parts of vibrating structures including turbine engines. The asymptotic forms for the stress fields in the vicinity of a rounded punch-on-flat substrate are derived for both normal and tangential loading, using both analytical and finite element methods. Under the action of the normal load, P , the ensuing contact is of width 2 b which includes an initial flat part of width 2 a . The asymptotic stress fields for the sharply rounded flat punch contact have certain similarities with the asymptotic stress fields around the tip of a blunt crack. The analysis showed that the maximum tensile stress, which occurs at the contact boundary due to tangential load Q , is proportional to a mode II stress intensity factor of a sharp punch divided by the square root of the additional contact length due to the roundness of the punch, Q /(√( b − a )√ π b ). The fretting fatigue crack initiation can then be investigated by relating the maximum tensile stress with the fatigue endurance stress. The result is analogous to that of Barsom and McNicol where the notched fatigue endurance stress was correlated with the stress intensity factor and the square root of the notch-tip radius. The proposed methodology establishes a 'notch analogue' by making a connection between fretting fatigue at a rounded punch/flat contact and crack initiation at a notch tip and uses fracture mechanics concepts. Conditions of validity of the present model are established both to avoid yielding and to account for the finite thickness of the substrate. The predictions of the model are compared with fretting fatigue experiments on Ti–6Al–4V and shown to be in good agreement.     

Influence of inclusion content on rolling contact fatigue in a gear steel: Experimental analysis and predictive modelling

Rolling contact fatigue tests were carried out on ring specimens made of quenched and tempered SAE 5135 gear steel with three different steel-production processes, through a bi-disc machine under pure rolling condition and water lubrication. Early formation of micro-pits then coalescing into macro-pits was observed on the rolling surface, while the final failure was caused by subsurface originated spalling phenomena. Microscope analysis of specimens section highlighted the complex surface and subsurface crack layout, and permitted to recognise sulphides as preferential sites for cracks initiation. The inclusion content was analysed throughout the extreme value statistics and the maximum expected inclusion in the Hertzian contact zone was introduced in a failure assessment diagram recently proposed, which resulted effective in predicting the specimen failures.