Influence of track conditions and wheel wear state on the loads imposed on the infrastructure by railway vehicles J. Pombo a,J. Ambrósio a, M. Pereira a, R. Verardi b, C. Ariaudo b, N. Kuka ba IDMEC/Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugalb Railway Dynamics, Experts, ALSTOM Ferroviaria, Savigliano (CN), Italy【Abstract】Nowadays, one of the most sensible issues in the railway industry is the damage on vehicles caused by the track conditions and the infrastructure deterioration due to the trains’ passage. Therefore, it is essential to acquire a better understanding on how the operation conditions influence the wear evolution of the railway wheels and the consequences of their changing profiles on vehicle–track interaction forces. In this work, a computational tool is used to simulate the dynamic performance of integrated railway systems and to predict the wear evolution of wheel profiles. The tool is applied to realistic operational scenarios with the purpose to evaluate the influence of the track conditions, defined by the track geometry and by its irregularities, on the wear progression of railway wheels. The loads imposed to the railway infrastructure by a trainset running at different velocities, on a track with and without irregularities,and equipped with wheelsets having new and worn profiles is also studied. The studies performed here show that the levels of track irregularities considered have a negligible influence on the wear progression.Furthermore, the loads imposed to the track during trainset operation are not affected by the wear state of the wheels. On the other hand, the track imperfections can affect significantly the vehicle–track interaction forces.【Keywords】:Railway dynamics，Vehicle–track interaction，Track irregularities，Wheel–rail contact，Wheel profiles wearArticle history:Received 9 November 2009Accepted 10 May 2011Available online 2 June 2011《Computers and Structures》1.IntroductionThe dynamic analysis of the loads imposed to the railway infrastructure by trainsets and, conversely, the damages on vehicles provoked by the track conditions has been attracting the attention of railway community in recent years. The raising interest on this subject has occurred mainly due to the development of new high-speed railway lines and to the common drive to upgrade the existing infrastructures. The increasing demands on railway transportation require improvements of the network capacity, which can be achieved either by increasing the speed of the traffic or by increasing the axle loads. However, both of these options place pressures on the existing infrastructures and the effects of these changes have to be carefully considered.In general, the increasing demand for safer, more efficient and better cost-effective railway transport is satisfied by two alternative approaches. In the first one, dedicated high-speed lines are built to carry passenger-only traffic (e.g. France and Japan) or mixed passenger-freight traffic (e.g. Germany, Italy and UK). This approach significantly increases the lines capacity as the traditional tracks can be freed of the intercity passenger trains, leaving more room for freight and regional passenger traffic. Nevertheless, this solution is also very costly as it requires building completely new high-quality lines with the accompanying infrastructure.A second alternative approach consists of upgrading the existing infrastructures to allow for faster passenger trains and heavier freight trains. This approach was adopted, for instance, in Sweden (Stockholm–Malmo line) and in the UK (West Coast Main Line from London to Glasgow) where tilting passenger trains operate on modernised tracks at speeds up to 200 km/h. Despite this solution is much less efficient in terms of increasing the line capacity, it is used when the efficiency improvements are required within a shorter time-frame or when the demand does not justify high expenses on building new lines.The latter approach is currently being implemented in several European countries (e.g. Portugal and Ireland) where the railway networks are undergoing modernisation to cope with the ever increasing demand on railway transportation. However, a great part of these infrastructures date back to the second half of the 19th century and to the first half of the 20th. Clearly, they were designed to carry loads different from those they are subjected to today. Furthermore, the dynamic effects were not fully recognized and appreciated at the time the structures were designed and built. This raises concerns regarding the dynamic response of these tracks caused by the current and planned enhancements of the network capacity. In particular, the dynamic effects caused by the increase in the speed and axle loads of trains and by the introduction of new types of the rolling stock are under question. These dynamic effects include excessive vibrations, risk of resonance, stability of the track and running safety of rolling stock, among others.The improvement of the modelling capabilities and the understanding of the dynamic response of vehicle–track interaction can provide refined methods of analysis and assessment of existing railway systems. These, will help to address the ageing problems of rolling stock and of infrastructures and will allow the development of more cost-efficient maintenance and modernisation procedures. In addition, enhancements to the capacity of railway networks can be achieved if the dynamic effects, resulting from increased speeds and loads of trains, are accurately predicted and evaluated.During trainset operation, the wheels of railway vehicles are subjected to wear. When the worn state of the profiles reaches a limit value defined by international standards [1], the wheels have to be re-profiled. Furthermore, the railway wheels only can be re-profiled 3 or 4 times and the wheelset substitution is a very expensive maintenance procedure. Therefore, it is also important to acquire a better understanding on how the track conditions influence the wear evolution and the vehicle-track interaction forces. Such evaluation is an important contribute to reduce the operation and maintenance costs, by increasing the life cycle of both vehicles and tracks.In the literature, several approaches to estimate wheel and rail ear using dynamic simulations are available [2–6]. However, less emphasis has been placed on the consequences of that wear on the performance of the railway vehicles and on the loads imposed to the infrastructure. The work reported by Nielsen et al. [7] focus on the train-track interaction and mechanisms of irregular wear. These authors discuss the causes, consequences and suggest solutions to minimize the problems for several types of wheel and rail wear. Hur et al. [8] use a 1/5 scaled roller rig bogie prototype to analyze the influence of the wheel profile wear on the running stability (critical speed) of railway vehicles. Froling [9] addresses the asymmetric wheel profile wear and analyses the consequential damages to the wheels, rails, turnouts and bogie components. Wu [10] discusses the effects of wheel and rail profiles on vehicle curving and lateral stability through the evaluation of the North American freight railways wheel profile. Fergusson et al. [11] tackle the wheel wear problem in another perspective. They present a methodology to minimize the wheel wear by optimising the primary suspension stiffness and the centre plate friction of a self-steering three-piece bogie without compromising the vehicle stability.In the work presented here, a computational tool, based on a multibody formulation, is used to simulate the dynamic performance of integrated railway systems that include the vehicle, the track and their interaction. The tool is also able to predict the wear evolution of the wheel profiles as a function of the distance run. In this work, the tool is first applied to realistic operational scenarios in order to study how the wheel wear progression is affected by the track conditions, defined by its geometry and irregularities. Then, special attention is given to the comparison of the vehicle-track interaction forces obtained during the operation of a trainset, running atdifferent velocities, on tracks with and without irregularities. These simulations are performed with the railway vehicle assembled with wheels having new and worn profiles in order to evaluate how the wear state influences the loads transmitted to the railway infrastructure.2.Description of the computational toolThe computational tool used here is composed of two parts. The first one is the railway dynamics block that uses the commercial software V AMPIRE [12,13] to study the dynamic behaviour of railway vehicles. This software uses a multibody formulation [14–19] to simulate the dynamic performance of integrated railway systems that include the vehicle, the track and the wheel–rail contact interaction. The code allows simulating accurately the vehicle, including the masses and inertias of the structural elements, and the characteristics of suspensions. It is also possible to represent accurately the track geometry, which is defined by its layout (macro-geometry) and by the track irregularities. This allows including, in the railway dynamic studies, the perturbations arising from the track imperfections. The vehicle–track interaction is studied through an appropriate wheel–rail contact formulation [20–26]that is used to compute the normal and tangential forces that develop in the contact area. Here, in particular, the fully non-linear creep law from V AMPIRE tool [12,13], with non-linear contact forces and non-linear contact geometry, is used.The second block of the computational tool is a purpose-built code [27–31] that is used to manage the pre and post-processing data of the first block in order to compute the wheel profiles wear or a given railway system. The strategy consists of providing an initial profile to all wheels of the trainset and running a simulation, for a pre-defined travel distance using the commercial multibody software. Then, the wear prediction block collects the necessary data from the dynamic analysis results and calculates the wear, i.e. the amount of material to be removed from the wheel surfaces. The resulting updated profiles are then used as input for a new railway dynamic analysis. This methodology, represented in Fig. 1, is repeated as many times as necessary to reach the distance required for the wear study.Fig. 1. Outline of the computational tool.The core of the wear prediction block is the wear computation rocedure thatcalculates the amount of worn material to be removed from the wheel surfaces. This block is composed of the contact model and of the wear function. The contact model processes the dynamic analysis results to obtain the wheel-rail contact parameters. The wear function uses these contact parameters as input to compute the quantity of worn material to be removed from the wheel surfaces.3.Influence of track conditions on wheels wearIn the following, the computational tool described here is used to evaluate the influence that the track conditions, defined by its geometry and irregularities, have on the wear evolution of railway wheels.3.1．Railway vehicleThe 3D model of the railway vehicle is build using a multibody approach [14–19]. This methodology allows representing accurately the mass and inertia properties of the structural elements that compose the vehicle. It also includes the kinematic joints, which control the relative motion between the bodies, and the force elements, that represent suspension components of vehicle.The trainset considered for the studies conducted here is a nonarticulated conventional trainset composed by seven vehicles interconnected by linking elements, as represented in Fig. 2.Fig. 2. Non-articulated conventional trainset.Due to the trainset configuration, it is assumed that, concerning the studies performed here, the dynamic behaviour of each vehicle has a non-significant influence on the others. According to this assumption, each vehicle of the trainset can be studied independently, as shown in Fig. 3. In this way, the vehicle model considered is composed only by one unit of the trainset. This composition is a motor vehicle that is assembled with two trailer wheelsets, represented in white in Fig. 3 and two motor wheelsets, represented in black. The vehicle is initially equipped with new wheels, having a diameter of 890 mm and a S1002 profile [1].Fig. 3. Motor vehicle of trainset3.2.Track conditionsThe dynamics behaviour of railway vehicles is dependent, on a great extent, of the track conditions. Also the loads transmitted to the infrastructure by the trainsets operation depend on the track geometry. The accurate description of the track is, therefore, essential for the dynamic analysis of railway systems.The computational tool used here allows creating realistic track models to run the railway dynamic studies. The track geometry is defined by the track layout, representing its macro-geometry, and by the track irregularities, representing its imperfections.3.2.1.Track layoutThe railway tracks are, in general, composed of straight (or tangent) sections, transition curves and circular curves. In the computational tool used here, the track layout is defined by the following design parameters [12,13]: (a) plan view curvature;(b) vertical offset; and (c) cross level offset. The plan view curvature c represents the curvature of the track in the horizontal plane, being defined as:c=1R(1) where R is the radius of the curve, as represented in Fig. 4a).When travelling in horizontal curves, railway vehicles are influenced by centrifugal forces, which act in a direction away from the center of the curve and tend to overturn the vehicles. In order to counteract this force, the outer rail in a curve is raised. This difference of heights between the two rails is called cross level offset h (or cant). The vertical offset zO represents the height of the track centerline in a curve. These two parameters are represented in Fig. 4(b).Fig. 4. Parameters to define the track layout: (a) plan view curvature; (b) vertical offset and cross level offset (cant).The track considered for the wear studies conducted here is from the Italian railway network, between the cities of Cuneo and Ventimiglia. This track has about 96 km length and it is particularly curved, with 61% of its curves having radii with less than 450 m, as represented in Fig. 5.Fig. 5. Curve radii distribution of the Cuneo–Ventimiglia track.The track model is assembled with UIC60 rails [32], with 1/20 cant, and has a track gauge of 1435 mm. The track flexibility is modelled here differently in the vertical and lateral directions. Laterally the rails can move independently of the sleepers, as represented in Fig. 6. The lateral model thus has rail to sleeper lateral flexibility as well as sleeper to ground flexibility. Vertically the rails are constrained to the sleepers and so the track model just has a vertical flexibility to ground. The track flexibility is represented here by stiffness and damping parameters with the following characteristics [12,13]:●Track stiffness:❍Sleeper-ground lateral stiffness: ky = 37.0 * 106 N/m.❍Rail-sleeper lateral stiffness: kr-y = 43.0 * 106 N/m.❍Sleeper-ground vertical stiffness: kz = 50.0 * 106 N/m.●Track damping:❍Sleeper-ground lateral damping: cy = 0.24 * 106 N s/m.❍Rail-sleeper lateral damping: cr-y = 0.24 * 106 N s/m.❍Sleeper-ground vertical damping: cz = 0.1 * 106 N s/m.Fig. 6. Track stiffness and damping.3.2.2.Track irregularitiesThe realistic description of a track requires not only the definition of its layout, as previously referred, but also the description of the irregularities. This data represents the deviations of the track from its design geometry and result mainly from construction imperfections, usage operations and change on the foundations. In the railway industry, the track irregularities are measured experimentally using special testing vehicles.In the studies carried out in this work, the track imperfections are characterized by several parameters [12,13]: (a) cross level (cant) irregularities; (b) curvature irregularities; (c) lateral irregularities; (d) vertical irregularities; and (e) gauge variation. The curvature irregularities contain the long wavelength lateral data,while the lateral irregularities define the short wavelength lateral displacements [12,13]. During the dynamic analysis, the railway vehicle follows the curvature data but not the lateral irregularity. The gauge variation gives the variation of the gauge about the nominal value and a positive value indicates increasing gauge.In Fig. 7 the absolute measured values of the irregularities parameters are presented for the first 2 km of the track. These values are representative of the irregularities that exist along the 96 km of track length. With the inclusion of irregularities in the whole track model, the complete characterization of the track Fig. 2. Non-articulated conventional trainset. Fig. 3. Motor vehicle of trainset.conditions is achieved. This allows performing railway dynamic studies considering the track perturbations.Fig. 7. Track irregularities parameters for the first 2000 m of track: (a) cross level (cant) irregularities; (b) curvature irregularities; (c) lateral irregularities; (d) vertical irregularities; (e) gauge variation.3.3.Wear study on the wheels of railway vehiclesIn order to evaluate the influence of the track imperfections on the wear evolution, two wear studies are carried out. In the first one, an ideal track model is considered, having a perfect design geometry, defined in Fig. 5, with no irregularities. In the second wear study, a realistic track model is adopted having the same layout of the previous one, but considering the measured track irregularities represented in Fig. 7. In both cases all remaining input data, used to study the railway dynamic problem, and analysis parameters, required for the wear computations, are equal.The comparative wear studies are carried out by performing several journeys of railway vehicle on both tracks until reaching the total distance of 5000 km. The vehicle velocity is defined according to the service conditions, varying between 80 and 95 km/h along the track length.The new and the worn profiles obtained in the two tracks, with and without irregularities, are presented in Fig. 8. These results correspond to the left and right wheels of leading wheelset. From these plots, no differences are perceptible among results obtained with the two tracks. In order to assess the differences with more detail,the wear depth results on the profiles of both wheels are also analysed.The comparison between the wear depth values obtained when travelling on the two tracks is shown in Fig. 9. The results are presented as a percentage of the maximum wear depth value obtained on both wheels. In this comparative study, no significant differences are observed between the wear depth results obtained with the tracks with and without irregularities. This means that the levels of imperfections considered in this track, represented in Fig. 7, do not affect the wear progression on the trainset wheels when operating this line. In other work by the same authors [28], a comparative wear study between two tracks with different layouts is performed. In that work it is demonstrated that the severed curved track, considered here and having the design characteristics represented in Fig. 5, originates levels of wear 20–40% higher than the ones obtained in a track with a mainly straight geometry.From Fig. 9 it is also interesting to note that the wear distribution along the profiles is slightly wider when travelling on the track with irregularities. This can be explained by the fact that the track imperfections originate more lateral oscillations of the wheelsets during trainset operation. Consequently, the lateral position of the wheel–rail contact points along the profiles has more amplitude. The wear results also show that, after 5000 km of trainset operation, the highest wear on the left wheel occurs on both tread and flange zones, while, on the right wheel, it occurs on flange.Fig. 8. Influence of track irregularities on wear: (a) Left wheel; (b) right wheel.4.Influence of wheel wear and of track irregularities on track loadsAn important and very sensitive issue in railway industry is the impact of train operations on the infrastructure and, conversely, the damages on vehicles provoked by the track conditions. This topic has a significant economical impact on the vehicles maintenance but also affects the life cycle costs of tracks. As a consequence, there is a growing tendency to define the track access charges, i.e. the prices billed by the infrastructure managers to the railway operators, according to the damage that the trainsets operation is supposed to cause to the tracks.In the following, the dynamic behaviour of a railway vehicle is studied in several service conditions. The purpose is to assess how the wear state of its wheels and the track imperfections influence the loads imposed to the infrastructure. These studies are performed considering different velocities for the trainset operation.Fig. 9. Wear depth results on: (a) left wheel; (b) right wheel.4.1.Railway vehicleThe railway vehicle used in the studies conducted here is represented in Fig. 3. It is the same as the one considered in the wear evolution studies described in the previous section.4.2.Track conditions4.2.1.Track layoutA section of the track described previously, and represented in Fig. 5, is considered here to study the influence of the wheel wear state and of track irregularities on track loads. The design geometry of this track section is depicted in Fig. 10. It corresponds to the first 2000 m between Cuneo and Ventimiglia, which is representative of the whole railway network between these two cities. The track layout is composed of a straight segment L1 followed by two circular curves L3 and L5, with radii R1 and R2, respectively, and finalized by a tangent segment L7.When trains are operated at normal speeds, a circular curve with cant cannot be followed directly by a tangent track, and vice-versa [20,21]. A transition between these two types of segments, designated by transition curve, is required in order to guarantee the curvature continuity and to minimize the change of lateral accelerations of the vehicles. Usually, the radius of a transition curve is changed continuously, decreasing from an infinite radius, at the tangent end, to a radius equal to that of the circular curve, at the other end. The transition curves are also used between circular curves with different radius. In Fig. 10 the track segments L2, L4 and L6, represented with dashed lines, are the transition curves of the track considered here.Also the cant is changed gradually over the transition length, leading to the so-called superelevation ramp. It represents the cant variation along the transition, ensuring a smooth cant evolution from a null value, at the straight track, to the nominal cant of the circular curve. The design characteristics of the track depicted in Fig. 10 are represented in Table 1.Fig. 10. Track layout.Table 1Design characteristics of the track.4.2.2.Track irregularitiesThe imperfections associated to the track layout depicted in Fig. 10 are also considered here. They are defined by the track irregularities parameters described before in this text and represented in Fig. 7.4.3.Influence of wheel wear state on track loadsThe purpose here is to analyse the dynamic behaviour of the railway vehicle in different operation conditions in order to assess how the wear state of its wheels influence the loads imposed to the infrastructure. For this purpose, the vehicle is assembled with wheelsets having the new and the worn wheel profiles, represented in Fig. 11, and comparative dynamic studies are carried out.The comparative studies with new and worn wheels are performed considering two different velocities for the trainset. The velocity of 95 km/h is adopted as it corresponds to the service conditions of the vehicle in this 2000 m section of the track.The velocity of 135 km/h is also considered since it represents the maximum velocity that a railway vehicle can operate a track having curves with the design characteristics presented in Table 1 [33].Fig. 11. New and worn wheel profiles: (a) left wheel; (b) right wheel.4.3.1.Track loads caused by the leading wheelsetThe first indicator considered here to assess the loads imposed to the track by the trainset operation is the wheelset ripage force FRipage. This force represents the total lateral force transmitted to the track by a wheelset, being obtained as the sum of the lateral contact forces exerted on the left and right rails. These forces are represented in Fig. 12 for the cases in which the wheelset is running on a straight track or negotiating a curve.The ripage force results, originated by the leading wheelset of the railway vehicle, are presented in Fig. 13. These results are obtained for running velocities of 95 and 135 km/h and with the vehicle assembled with new and worn wheels. According to the UIC 518 standard [33], these results should have two steps of signal post-processing. First, the lateral track forces data is processed with a sliding window 2 m running average filter. Then, a low pass filter with a cut-off frequency of 20 Hz and 2 poles is used to obtain the final results for the ripage force.The results from Fig. 13 show that, in curve, the lateral track forces obtained with the velocity of 135 km/h are about 150% higher than when running at 95 km/h. When comparing the results obtained with new and worn profiles, it is observed that the new wheels originate slightly higher lateral track forces on curve.It is also interesting to observe that in the tangent track after the curve, the vehicle assembled with new wheels and running at the velocity of 135 km/h exhibits a periodic lateral oscillation that originates low frequency lateral track forces. This phenomenon is known in the railway industry as vehicle lower sway and it only occurs when using new wheel profiles S1002 and new rail profiles UIC 60 with 1/20 cant. In such conditions, the equivalent conicity [20,34,35] of the wheelsets is very small and, consequently, after a perturbation the vehicle has more difficulty to center itself on the track. This phenomenon does not occur when using the worn wheel profiles since the equivalent conicity is higher and the wheelsets have a stronger tendency to center themselves on the track.Another indicator considered to evaluate the loads imposed to the track by the trainset operation is the vertical wheelset force on the left FL and on the right FR rails. These forces are represented in Fig. 12 for the cases in which the wheelset is running on straight and curved tracks.The vertical track forces results obtained on the left and right rails are presented in Figs. 14 and 15 , respectively. These results are defined in the wheelset axis and are filtered with a low pass filter with a cut-off frequency of 20 Hz and 2 poles, according to the UIC 518 standard [33]. When comparing the results obtained with new and worn profiles, no differences are perceptible among them. This means that the vertical track forces are not sensitive to the wear state of the wheels.The graphs of the vertical track forces also show that, in curve, the track forces on the inner (left) rail are lower when running with the velocity of 135 km/h. On the outer (right) rail, the opposite happens, i.e. the higher velocity originates higher vertical forces. These results show that the railway vehicle is running with cant deficiency, which is more pronounced at 135 km/h. This means that, for these velocities, the track cant is not sufficient to assure zero track plane acceleration and a resultant centrifugal force arises, pointing towards outside of the curve. Consequently, the passengers are pushed in that direction and the vertical contact forces are higher on the outer wheels.Fig. 12. Ripage (lateral) and vertical wheelset forces transmitted to the track: (a) tangent track; (b) curved track.。
