Elena Kabo

Material defects in rolling contact fatigue — influence of overloads and defect clusters

Abstract The fatigue impact of material defects under rolling contact loading, with emphasis on wheel/rail contact, is studied. In particular, the response to overloads and the effect of clusters of defects are investigated. The study is carried out using numerical finite element (FE) simulations. Stresses and strains in the vicinity of the defects are analysed and the fatigue impact is quantified by means of a multiaxial fatigue criterion. It is found that overloads are detrimental, causing a permanent extension of the impacted zone surrounding a defect. Further, clustering of defects leads to a highly stressed zone between adjacent defects, through which a fatigue crack is likely to propagate. A combination of defect clusters and occasional overloads is found to be a plausible explanation to the initiation of deep subsurface fatigue cracks.

Fatigue initiation in railway wheels—a numerical study of the influence of defects

Stresses around a material defect in the rim of a railway wheel are studied during sequences of over-rolling contacts using elasto-plastic finite element analysis. Results in the form of animations show a complex evolution of the stress field. Based on multi-parametric studies, conclusions of practical interest to railway engineers are drawn. Insights into the physical process have been gained.

Prediction of Dynamic Train-Track Interaction and Subsequent Material Deterioration in the Presence of Insulated Rail Joints

Numerical analysis of high-frequency dynamic train–track interaction is combined with the analysis of material deterioration in terms of rolling contact fatigue (RCF) and plastic deformations to analyze the influence of insulated rail joints. These joints form local rail irregularities and lead to a local change of dynamic track stiffness. Dynamic responses at wheel passes are evaluated. Further, related plastic deformations at the joint and increased RCF impact along a stretch of the track adjacent to the joint are predicted.

A Numerical Study of the Lateral Ballast Resistance in Railway Tracks

Abstract Three-dimensional elastoplastic finite-element (FE) simulations of ballast deformation in a track with focus on lateral resistance have been conducted. Ballast geometry, vertical and lateral loading, and friction between ballast and sleeper are being varied in a parametric study. Sleeper displacements are studied under different conditions in order to find out which lateral resistance the ballast will provide and how this resistance depends on the various parameters. In addition to extending the state of the art, the knowledge gained in the study will be essential in the design of a structural element to represent the ballast in a model of the full track that is to be used for analysing lateral track stability [1, 2].

Wheel Tread Damage: A Numerical Study of Railway Wheel Tread Plasticity Under Thermomechanical Loading

A numerical study is presented where the impact of simultaneous thermal and mechanical loading on a railway wheel tread, as imposed by braking and rolling contact, is reported. A comparison is made of two-dimensional (2D) and 3D finite-element simulations of the thermomechanical problem featuring a material model that accounts for thermal expansion and plastic deformations. It is found that 2D simulations give unrealistic predictions of plastic deformations. The 3D simulations demonstrate a significant influence of the thermal loading also in cases of rather moderate temperature increases. In particular, the combination of thermal loading and high traction is found to be very detrimental.

A numerical study of the influence of lateral geometry irregularities on mechanical deterioration of freight tracks

Optimisation of railway track maintenance requires knowledge of how a deteriorated track geometry will affect subsequent loading and damage of the track. This is the scope of the current study where, in particular, the influence on track shift forces and rolling contact fatigue is investigated through numerical simulations. To this end, track geometries are obtained from field measurements. Lateral irregularities are extracted and scaled to represent different levels of geometry deterioration. Multibody simulations of dynamic train–track interaction featuring two freight wagon types are performed under different operational conditions. Track shift forces and rolling contact fatigue damage are further evaluated from simulation results. It is found that track shift forces tend to follow a normal distribution for moderate levels of lateral track geometry irregularities, and that an approximate linear relationship between standard deviations of lateral irregularities and track shift forces can be established. The relation between lateral track irregularity magnitude and rolling contact fatigue is more complex. Increasing levels of lateral irregularities will decrease the fraction of curve length affected by rolling contact fatigue for sharp curves, whereas for shallow curves it increases. As detailed in the article, this is caused by the lateral movement of the contact point as imposed by the track irregularities. Furthermore, the influence of wheel/rail friction and wear is investigated.

Rolling Contact Fatigue Prediction for Rails and Comparisons With Test Rig Results

Tests in a full-scale roller rig, a full-scale linear test rig, and a twin-disc machine are numerically evaluated and compared in terms of rolling contact fatigue loading. From previously evaluated contact patch sizes, a Wöhler curve relationship is established and matched towards experimentally established fatigue lives. In addition, non-linear finite-element simulations are carried out. Key data needed for the numerical evaluations, as well as difficulties in translating experimental data to numerical models are highlighted. A key parameter here is the interfacial wheel—rail friction. Additional simulations were carried out to establish the latter. However, the conformal contact in the linear test rig makes such simulations very uncertain.

Influence of plastic deformations on growth of subsurface rolling contact fatigue cracks in railway wheels

Abstract A subsurface rolling contact fatigue crack in a railway wheel is studied. Stress and strain fields are evaluated by elasto-plastic finite-element simulations. The robustness and validity of the numerical model are investigated. A suitable measure to characterize crack propagation is then sought. Numerical results show that the effect of crack tip plasticity is small for cases studied, indicating that linear elastic fracture mechanics approaches are applicable. Further, the magnitudes of the stress intensity factor in mode I, KI, are negative implying crack closure. Motivated by these findings, the stress intensity factor range in mode II, ΔKII, is employed as a measure of crack propagation. Finally, studies are carried out to quantify the effects of altered contact load conditions and crack face friction.

An Efficient Approach to the Analysis of Rail Surface Irregularities Accounting for Dynamic Train–Track Interaction and Inelastic Deformations

A two-dimensional computational model for assessment of rolling contact fatigue induced by discrete rail surface irregularities, especially in the context of so-called squats, is presented. Dynamic excitation in a wide frequency range is considered in computationally efficient time-domain simulations of high-frequency dynamic vehicle–track interaction accounting for transient non-Hertzian wheel–rail contact. Results from dynamic simulations are mapped onto a finite element model to resolve the cyclic, elastoplastic stress response in the rail. Ratcheting under multiple wheel passages is quantified. In addition, low cycle fatigue impact is quantified using the Jiang–Sehitoglu fatigue parameter. The functionality of the model is demonstrated by numerical examples.

Identifying the root causes of damage on the wheels of heavy haul locomotives and its mitigation

The paper illustrates how damage patterns in the form of rolling contact fatigue (RCF) on wheels, can be employed to identify and improve underlying operational conditions. The focus is on RCF of locomotive wheels operating on the Iron Ore Line in northern Sweden and Norway. Seasonal changes and damage patterns are charted. Potential root causes for observed damage patterns are identified and investigated. Mitigating actions are proposed and the efficiency of implemented actions is quantified.