Influence of wheel tread damage on wheelset and track loading – Field tests and numerical simulations

Wheel tread damage leading to high magnitudes of vertical wheel–rail contact forces is a major cause of train delays in the Swedish railway network, in particular during the coldest months of the year. According to regulations, vehicles generating contact forces exceeding the limit value for allowed wheel–rail impact loads must be taken out of service for wheel maintenance. This may lead to severe traffic disruptions and higher costs. Increased wheel‒rail impact loads also cause elevated stress levels in wheels, axles and bearings and may shorten the life of track components, resulting in higher costs for vehicle and track maintenance. Wheel tread irregularities also lead to increased levels of rolling noise, impact noise and ground-borne vibration.The aim of the thesis is to enhance the understanding of wheel tread damage and its consequences and to identify better means of addressing them. To achieve this aim, the ability for numerical simulations to investigate different operational scenarios is crucial. A versatile and cost-efficient method to simulate the vertical dynamic interaction between a wheelset and a railway track, accounting for generic distributions and shapes of wheel tread damage, has therefore been extended and improved. The wheelset (comprising two wheels, axle and any attached equipment for braking and power transmission) and track with two discretely supported rails are described by three-dimensional finite element (FE) models. The dynamic coupling between the two wheel‒rail contacts (one on each wheel) via the wheelset axle and via the sleepers and ballast is considered. The simulation of dynamic vehicle–track interaction is carried out in the time domain using a convolution integral approach, while the non-linear wheel–rail normal contact is solved using Kalker’s variational method. Non-symmetric wheelset and track designs, as well as non-symmetric distributions of wheel tread damage or rail irregularities can be studied. Based on Green’s functions, a post-processing step has been developed to compute time-variant stresses at locations in the wheelset axle which are prone to fatigue. In an extensive parameter study, wheel–rail impact loads and axle stresses have been computed for different shapes and sizes of wheel tread damage.The simulations need to be calibrated and validated by tests. To this end, field tests with two different Swedish passenger trains with severe wheel tread damage have been carried out. Time histories of numerically evaluated axle stresses have been compared to measured data from an instrumented wheelset. Simulations have been used to demonstrate that variations in rail roughness level, and the angular position of a strain gauge with respect to that of a discrete wheel tread defect, may lead to a significant influence on predicted axle stresses. Developed numerical routines to predict stresses at critical locations in the wheelset from condition monitoring data will improve understanding and possibilities to handle wheel tread deteriorations. A discussion on future applications in terms of improved wheelset maintenance procedures is initiated

Influence of railway wheel tread damage and track properties on wheelset durability – Field tests and numerical simulations

Wheel tread damage leading to high magnitudes of vertical wheel–rail contact forces is a major cause of train delays in the Swedish railway network, particularly during the coldest months of the year. According to regulations, vehicles generating wheel–rail impact loads exceeding the limit values must be taken out of service for wheel maintenance. This may lead to severe traffic disruptions and associated high costs. On the other hand, increased wheel‒rail impact loads cause elevated stress levels in wheels, axles and bearings and may shorten the life of track components, resulting in higher costs for vehicle and track maintenance. Thus, alarm limits should provide a balance between preventing operational failures and minimising the number of stopped trains. The aim of this thesis is to enhance the understanding of the consequences of wheel tread damage and to identify better means of addressing them. To achieve this aim, the ability of numerical simulations to investigate different operational scenarios is crucial. A versatile and cost-efficient method to simulate the vertical dynamic interaction between a wheelset and a railway track, accounting for generic distributions and shapes of wheel tread damage, has therefore been extended and improved. The dynamic coupling between the two contact points (one on each wheel) via the wheelset axle and via the rails and sleepers is accounted for. Post-processing steps to evaluate fatigue impact at critical positions in the wheelset have been developed. The applied simulation models have been calibrated and verified by extensive field tests. Measurement campaigns with two different Swedish passenger trains have been carried out. In the first field test, impact loads generated by a wheelset with severe tread damage were measured. Measurements and simulations have been used to illustrate how wheel–rail loads and fatigue impact depend on the three-dimensional shape of the tread damage. The effects of speed and travelling direction of the vehicle, position in the sleeper bay where the defect strikes the rail, lateral position of the wheelset, and track stiffness on wheel–rail contact forces and wheelset durability have been investigated.In the second long-term field test, axle stresses have been monitored using an instrumented wheelset on a passenger train in revenue traffic. By post-processing of test results, statistical models of stress spectra for different stretches of the Swedish rail network were obtained. Moreover, the parameters describing such models have been related to track characteristics in terms of the presence of curves, switches & crossings and irregularities in track geometry. This allowed to develop numerical routines to evaluate wheelset durability depending on operational parameters. These studies are used to initiate a discussion on improved wheelset maintenance procedures

Transient wheel-rail rolling contact theories

This paper provides an overview of different theories to analyse unsteady rolling contact phenomena between wheel and rail: the exact formulation by Kalker, the simplified model based on the Winkler approximation, and the recent two-regime model. The classic solution to the transient problem derived by Kalker using the complete theory of elasticity is first recalled. The more involved situation of combined creepages and spin is analysed using Kalker’s simplified model. Analytical solutions are reported in integral form concerning the time-varying and constant creepages. Qualitative results are additionally provided for the case of a time-varying contact patch. Finally, a novel theory, which describes the transient evolution of the force-creepage characteristics using a system of ordinary differential equations (ODEs), is introduced

Relating the influence of track properties to axle load spectra through onboard measurements

This work aims at investigating how variations in measured stresses are affected by track conditions and, if possible, extract information on track conditions from onboard measurements. Axle bending strains measured in extensive field tests are employed to evaluate axle stress spectra. Correlation between stress spectra and parameters describing track design and condition for three sections of the Swedish mainline “V\ue4stra stambanan” have been investigated. The study shows how switches and other discontinuities in the track running surface increase the scatter of the stress spectra and increase the number of overloads. Circular and transition curves mainly increase stress amplitudes with magnitudes close to quasistatic conditions load. A decrease in track quality leads to both a shift of stress spectra towards higher values and a higher number of overloads. The influence of bridges/tunnels and decreased track stiffness were found to be difficult to distinguish from effects of curves, switches and crossings and track quality. If effects exist, they are likely to be small. Results from the study aids future track condition monitoring, maintenance planning of track and running gear, and the estimation of stress spectra for track stretches with known characteristics

Railway wheelset fatigue life estimation based on field tests

Field tests using an instrumented powered wheelset were performed to investigate fatigue damage accumulation in railway axles. Axle bending strains were measured and post-processed to obtain axle stress spectra. Statistical analyses were used to investigate the variations of axle stress spectra due to changes in railway operation parameters. The study indicates that measured axle stress spectra can be modeled using truncated normal distributions, where the large majority of measured stress amplitudes are lower than 50 MPa. Stress cycles at higher amplitudes are affected by operation parameters such as track design, number of switches and crossings, and whether the wheelset is in a leading or trailing position. Variations in the obtained statistical distributions of axle stresses have been used as input for fatigue life analyses. It was concluded that fatigue damage can potentially initiate on axles suffering from corrosion or small surface cracks/scratches

Wheel–rail impact loads and axle bending stress simulated for generic distributions and shapes of discrete wheel tread damage

Wheel–rail impact loads generated by discrete wheel tread irregularities may result in high dynamic bending stresses in the wheelset axle, leading to a decrease in component life and an elevated risk for fatigue failure. In this paper, a versatile and cost-efficient method to simulate the vertical dynamic interaction between a wheelset and railway track, accounting for generic distributions and shapes of wheel tread damage, is presented. The wheelset (comprising two wheels, axle and any attached equipment for braking and power transmission) and track with two discretely supported rails are described by three-dimensional finite element (FE) models. The coupling between the two wheel‒rail contacts (one on each wheel) via the wheelset axle and via the sleepers is considered. The simulation of dynamic vehicle–track interaction is carried out in the time domain using a convolution integral approach, while the non-linear wheel–rail normal contact is solved using Kalker’s variational method. Wheelset designs that are non-symmetric with respect to the centre of the axle, track support conditions that are non-symmetric with respect to the centre of the track, as well as non-symmetric distributions of tread damage on the two wheels (or irregularities on the two rails) can be studied. Time-variant stresses are computed for the locations in the wheelset axle which are prone to fatigue. Based on Green’s functions for stress established using the wheelset FE model, this is achieved in a post-processing step. An extensive parametric study has been performed where wheel–rail impact loads and axle stresses have been computed for different distributions and sizes of tread damage as well as for different train speeds

Digitalisation of condition monitoring data as input for fatigue evaluation of wheelsets

A field test in which a train was run at different speeds over an impact load detector is described. One of the wheelsets in the train had severe wheel tread damage. The results are presented and the relation between the speed of the train and the magnitude of the impact loads registered for the two wheels is discussed. The defects on the wheel tread have been studied and scanned by means of 3D laser and their characteristics are described. An in-house software for the simulation of dynamic wheel–rail interaction has been improved by including the possibility to account for the cross-coupling of the two wheels within the same wheelset. The contact algorithm and a possible implementation of discrete defects in the in-house software are discussed. The in-house software gives, among other possible outputs, the maximum dynamic loads occurring at both wheels of the wheelset. To show an example of the utility of such information, fatigue analyses for the axle are performed for the different running conditions used during the field tests. The impact loads measured on the day of the tests are given as input to the fatigue analyses

Railway wheel tread damage and axle bending stress – Instrumented wheelset measurements and numerical simulations

A combination of instrumented wheelset measurements and numerical simulations of axle bending stresses is used to investigate the consequences of evolving rolling contact fatigue (RCF) damage on a passenger train wheelset. In a field test campaign, stresses have been monitored using a wheelset with four strain gauges mounted on the axle, while the evolution of wheel tread damage (out-of-roundness) has been measured on regular occasions. The strain signals are post-processed in real time and stress variations are computed. Based on a convolution integral approach, the measured wheel out-of-roundness has been used as input to numerical simulations of vertical dynamic wheelset–track interaction and axle stresses. Simulated and measured axle stresses are compared for cases involving combinations of low or high levels of rail roughness and the measured levels of RCF damage. The study enhances the understanding of how wheel tread damage and track quality influence axle stress amplitudes

Finite element analysis of thermal fields during repair welding of discrete rail defects

Discrete defects in a rail head may form due to aggressive wheel–rail contact in terms of thermal and/or mechanical loads, or due to indentations from foreign objects trapped in the contact. If large, such defects need to be repaired or the rail section removed. These are costly operations that cause operational disturbances. To decrease mitigation costs, discrete defect repair (DDR) procedures that include repair welding have been developed. These operations typically require high preheat temperature (350 \ub0C) and long working process times.This MSc-thesis work investigates a novel DDR rail welding procedure through numerical simulations. The new technique employs significantly lower preheat temperature (60–80 \ub0C) and equipment that can easily be carried to the working place. However, the low preheating temperature introduces high temperature differences between the molten filler material and the surrounding rail steel. This may lead to the formation of defects, welding related cracks or martensitic areas.The aim of the work is to simulate the DDR procedure and thereby be able to analyse the thermal history in the rail during the welding process. In this manner, cooling curves for critical locations in the rail head can be evaluated and the risk of weld related defects and metallurgical transformations to hard microstructures can be assessed. To achieve these ends, numerical models of a milled rail head were created in ABAQUS/CAE. The repair welding procedure was then simulated and the results compared to experimental data from the literature.The results show temperature trends that are in line with temperature measurements from trials carried out some years ago. The simulations show the sensitivity to parameters such as the temperature of the molten filler and cooling times. There is thus a high potential in simulating operational procedures and thereby be able to e.g. investigate effects of various process parameters. However, to this end more highquality test data are required. In particular the simulations show how sensitive a calibration is to the exact position of thermocouples. On the other hand, the simulations performed in the thesis have shown that small variations in the geometry of the numerical model of the repair process do not have a significant influence on the predicted cooling curves

Thermo-mechanical analyses of discrete defect repair process for rails

Discrete defects in a rail head may form due to aggressive wheel–rail contact in terms of thermal and/or mechanical loads, or due to indentations from foreign objects trapped in the contact. If large, such defects need to be repaired or the rail section removed. These are costly operations that cause operational disturbances. Discrete defect repair (DDR) procedures that include repair welding provide a cost-efficient alternative to rail replacements. Investigations of such repair methods have been conducted in the frame of the European project In2Rail and reported in Deliverable report D3.1, see Kallander (2017). The present work has been focused on investigating a novel DDR procedure through numerical simulations. The new technique employs significantly lower preheat temperature (80\ua0\ub0C in contrast to widely used 350\ua0\ub0C) and equipment that can be carried to the working place. However, the low preheating temperature introduces high temperature differences between the molten filler material and the surrounding rail steel. This may lead to the formation of defects and has to be assessed.The quality of the weld is highly influenced by the thermal field induced by the repair welding process. This topic was investigated in detail by thermal analyses in Maglio (2017). The study showed the possibility of numerical simulations and investigated in detail requirements and sensitivities of numerical models. In particular, it quantified the sensitivity related to both simulation parameters and to experimental measurements. The thermal analysis has been extended to a thermomechanical analysis that is able to investigate the residual state of stress after repair welding. Such analyses can be used to compare repair welding methods and also investigate the sensitivity to different operational parameters