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.     

Rolling contact fatigue cracking in rails subjected to in‐service loading

Rolling contact fatigue (RCF) is one of the most important failure mechanisms in rails with significant cost‐ and safety‐related implications on the operation of railway systems. In this work, a metallurgical analysis of RCF crack initiation and propagation, including geometrical characteristics of RCF cracks – length, depth from surface, angle of propagation and spacing between cracks, is presented. The role of proeutectoid ferrite in crack initiation has been studied. Analysis of the fracture surface of an RCF crack revealed a ductile initiation zone followed by a quasi‐cleavage crack propagation. Iron oxide formed in the interior of all cracks in rails exposed to stagnant water with implications to crack propagation rate because of crack closure effects. Sequential sectioning parallel to the rolling surface revealed that RCF cracks possess convoluted surfaces. The crack trace expands with depth from the rolling surface. Subsurface crack initiation has also been documented.     

The development of a crack propagation model for railway wheels and rails

Rolling contact fatigue (RCF) and wear of railway wheels and rails are the main phenomena that affect their maintenance costs. When crack propagation and wear rates can be predicted, maintenance planning can be optimised, and cost‐effective measures can be developed. Several RCF models exist, but none which can be used in combination with vehicle dynamics simulations and can predict the actual crack depth. This study shows the development of a crack propagation model that can be applied for both railway wheels and rails. Two unknown material parameters in the model were calibrated against crack measurements in a curve on the Dutch railways over a period of 5 years. Two different RCF models were used to calculate the stress magnitudes for the propagation model. The propagation model can be used in combination with vehicle‐track dynamics simulations and shows promise in predicting the actual crack depth and/or surface length. Further research is needed to determine the model's validity for other operational conditions.     

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

Rolling 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.     

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.     

Vertical dynamic responses of a simply supported bridge subjected to a moving train with two‐wheelset vehicles using modal analysis method

The vertical dynamic responses of a simply supported bridge subjected to a moving train are investigated by means of the modal analysis method. Each vehicle of train is modelled as a four‐degree‐of‐freedom mass–spring–damper multi‐rigid body system with a car body and two wheelsets. The bridge, together with track, is modelled as a simply supported Bernoulli–Euler beam. The deflection of the beam is described by superimposing modes. The train and the beam are regarded as an entire dynamic system, in which the contact forces between wheelset and beam are considered as internal forces. The equations of vertical motion in matrix form with time‐dependent coefficients for this system are directly derived from the Hamilton's principle. The equations of motion are solved by Wilson‐θ method to obtain the dynamic responses for both the support beam and the moving train. Compared with the results previous reported, good agreement between the proposed method and the finite element method is obtained. Finally, the effects of beam mode number, vehicle number, beam top surface, and train velocity on the dynamic responses of the entire train and bridge coupling system are studied, and the dynamic responses of beam are given under the train moving with resonant velocity. Copyright © 2005 John Wiley & Sons, Ltd.     

Taguchi sensitivity analysis of damage parameters for predicting the damage Mechanism of 9Cr steel under low‐cycle fatigue test

Numerical investigations of low‐cycle fatigue damage parameters of a 9Cr steel have been studied and compared with the previous results in order to understand the effect of the damage parameters on predicting the damage development of the material. Using the nonlinear kinematic softening criterion, the Chaboche constitutive equation is combined with the hysteresis total stress–strain energy concept to implement damage initiation and evolution; the remaining life of the specimen can be predicted. In this paper, the cyclic softening model in conjunction with the progressive damage evolution model successfully predicted the failure times of the experimental tests. By using a novel sensitivity analysis of the damage parameters c₁, c₂, c₃ and c₄ based on the Taguchi method, the highest parameter effect has been determined.