Railway Track Structure Research Articles

Ultimate Impact Resistance and Residual Toughness of Pre-Stressed Concrete Railway Sleepers

The pre-stressed concrete sleepers (or railroad ties), which are installed in railway track systems as the crosstie beam support, are designed to carry and transfer the wheel loads from the rails to the ground. It is well known that railway tracks are subject to impact loading conditions, which are attributable to the train operations with either wheel or rail abnormalities such as flat wheels, dipped rails, etc. These loads are of very high magnitude but short duration. In addition, there exists the potential of repeated load experience during the design life of pre-stressed concrete sleepers. Pre-stressed concrete has played a significant role in maintaining the high endurance of sleepers subjected to low to moderate repeated impact loads. In spite of the common use of pre-stressed concrete sleepers in railway tracks, their impact response and behaviour under repetitions of severe impact loads are not deeply appreciated, nor taken into consideration in design. This experimental investigation was aimed at understanding the residual capacity of pre-stressed concrete sleepers in railway track structures under ultimate impact loading, in order to develop state of the art limit states design concepts for pre-stressed concrete sleepers. A high-capacity drop weight impact testing machine was constructed at the University of Wollongong to achieve this purpose. A series of severe impact tests on in-situ pre-stressed concrete sleepers was carried out, ranging from low to high impact magnitudes. The impact energy was evaluated in relation to the drop heights. The impact-damaged sleepers were re-tested under static conditions in order to evaluate the residual fracture toughness in accordance with the Australian Standard. It was found that a concrete sleeper damaged by an impact load could possess significant reserve capacity sufficient for resisting about 1.05 to 1.10 times the design axle loads. The impact behaviour and residual fracture toughness under different magnitudes of impacts are highlighted in this paper. The effects of track environment, including soft and hard tracks, are also presented together with a discussion related to the ultimate limit states design.

Lateral Hunting Stability of Railway Vehicles Running on Elastic Track Structures

Considering the viscoelastic property of railway track structure, it is suggested in this paper to include the influence of track dynamics in calculation of lateral stability of railway vehicles running on tracks. A direct numerical method based on the stability of time histories of vehicle components is proposed to ascertain the nonlinear critical hunting speed of railway vehicles. The time histories of the vehicle system are calculated by use of the three-dimensional vehicle-track coupled models, which are capable of taking into account the influence of dynamic properties of track structures on vehicle dynamics. The effect of track system properties such as the rail fastener stiffness and the rail profile on the lateral hunting stability of the vehicle is investigated. Differences of the critical hunting speeds of vehicles on rigid track model and on elastic track model are found out. Generally, the rigid track model overestimates the critical hunting speed by 5–10% for different types of vehicles, which implies that the classical vehicle dynamics predicts higher lateral stability of a vehicle than the vehicle-track coupled dynamics does. This conclusion is of significance to the safety design of railway vehicles and should be taken notice in the design stage. The reason that causes the differences is discussed. A full-scale field experiment was carried out to investigate the hunting behavior of a freight car with three-piece bogies on a straight track. Very obvious hunting motion was observed in the experiment when the unloaded vehicle ran at the speed of 75 km/h, which verified the theoretical calculation results.

Determination of the Transverse Resistance Characteristics in Railway Track

The paper deals with transverse resistance problems occurring in railway ballast bed viewed from the standpoint of construction, maintenance, and safety of the railway track in use. Various ways of an analytical approach to the phenomenon tested using illustrative examples of mathematical models found in literature have been analyzed. A survey has been made into the current state of experimental works related to transverse resistances, and different methods of conducting such experiments in railway track during operation have been discussed. Furthermore, the idea of undertaking research, by a group of scientists from the Gdansk University of Technology, on the application of the tamping machine, is considered. Attention is focused on the process of preparing and carrying out the experimental works on transverse resistances, inclusive of an appropriate measuring system. The results of the experiments were analyzed and are included in the paper. The topic on how to deal with the time signals obtained from measurements and how to interpret them is discussed. The basic part of the paper is related to the theory of modeling the railway track structure paying special attention to the presented experimental work. An explanation has been provided to the mathematical description of the adopted model. The presented algorithm makes allowance for running simulation of the phenomenon on computer. The results obtained from simulation and experiments were aimed at convergence. Finally the results of such a simulation to enable a numerical description of the transverse resistance to the rail track ballast bed are tested.

Dynamic Crack Propagations in Prestressed Concrete Sleepers in Railway Track Systems Subjected to Severe Impact Loads

Prestressed concrete sleepers (or railroad ties) are the crosstie beam support in railway track systems. They are designed and constructed under flexural constraints in order to carry and transfer the dynamic wheel loads from the rails to the ground. Under perfect wheel and rail conditions, the dynamic loading on railway tracks could be treated as a quasi-static load using a dynamic impact factor. The current design method for the prestressed concrete sleepers taking into account the quasi-static effect is based on allowable stress where crack initiation is not permitted. In reality, the impact events are often detected due to the uncertainties of wheel or rail abnormalities such as flat wheels, dipped rails, etc. These loads are of very high magnitude but short duration. Over the design life span of the prestressed concrete sleepers, there exists the feasibility of extreme and repeated impact loading events. These have led to two proposed limit states for the consideration of structural engineers: ultimate limit states and fatigue limit states. Prestressing techniques have been long used to maintain the high endurance of the sleepers under repeated impact cycles. In spite of the most common use of the prestressed concrete sleepers, their impact behavior and capacity under the repetitions of severe impact loads are unclear. This paper presents the experimental investigation aimed at understanding the dynamic crack propagations in prestressed concrete sleepers in railway track structures under repeated impact loading. The impact forces have been correlated against the probabilistic track force distribution obtained from an Australian heavy haul rail network. The effects of track environment including soft and hard tracks are highlighted in this paper.

Isolated Railway Track Structures

Isolated Railway Track Structures

Progressive failure of prestressed concrete sleepers under multiple high-intensity impact loads

Prestressed concrete sleepers (or railroad ties) are among the major and the most common structural components of railway track structures. Their main duty is to carry and transfer the wheel loads from the rails to the ground. In general, railway tracks suffer with the extreme loading conditions, which are attributable to the train operations with either wheel or rail abnormalities such as flat wheels, dipped rails, etc. These loads are of very high magnitude but short duration, as well as they are of low-possibility occurrence during the design life of the prestressed concrete sleepers. In spite of the most common use of the prestressed concrete sleepers in railway tracks, their impact responses and behaviours are not deeply appreciated nor taken into the design consideration. This experimental investigation was aimed at understanding the progressive dynamic behaviours of prestressed concrete sleepers in railway track structures under repeated impact loading, in order to form the state of the art of limit states design concept for prestressed concrete sleepers. A high-capacity drop weight impact testing machine was constructed at the University of Wollongong as to achieve the purpose. Series of repeated impact tests for the in-situ prestressed concrete sleepers were carried out, ranging from a low drop height to the ultimate drop height where the ultimate failure occurred. The accumulative impact damage and crack propagation are highlighted in this paper. The effects of track environment including soft and hard tracks together with the relationship between the resultant bending moment of prestressed concrete sleepers and the applied impact force as a design guideline were also presented.

Analysis of dynamic behaviour of low vibration track under wheel load drop by a finite element method algorithm

According to finite-element method (FEM) analytical solution (based on penalty algorithm) and the control equations of wheel—rail contact (developed from point-to-surface contact elements), the vibration responses were simulated for the two alternative railway track structures impacted by a falling wheel-set. The dynamic behaviour of low vibration track (LVT) is investigated based on this numerical method, which is verified by the field experiments. The results show that, when compared with the conventional short sleeper buried track (SSBT), the LVT is more beneficial for the reduction of track structure vibration levels — thanks to its properly matched stiffness under rail and block. Furthermore, it is demonstrated that the proposed wheel-load-drop FEM numerical procedure and field experiment can effectively and economically study the dynamic properties of track structures.

A review of loading conditions for railway track structures due to train and track vertical interaction

Train and track interactions during services normally generate substantial forces on railway tracks. Such forces are transient by nature and of relatively large magnitude and are referred to as impact loading. There has been no comprehensive review of the typical characteristics of the loading conditions for railway track structures, in particular, impact loads due to the wheel/rail interaction, published in the literature. This paper presents a review of basic design concepts for railway tracks, abnormalities on tracks, and a variety of typical dynamic impact loadings imparted by wheel/rail interaction and irregularities. The characteristics of typical impact loads due to wheel and rail irregularities, e.g. rail corrugation, wheel flats and shells, worn wheel and rail profiles, bad welds or joints, and track imperfections, are presented with particular emphasis on the typical shapes of the impact load waveforms generally found on railway tracks. Copyright © 2007 John Wiley & Sons, Ltd.

Field trials for dynamic characteristics of railway track and its components using impact excitation technique

Assessment of condition of railway track is crucial for track design, repair, and effective maintenance operations. In-field dynamic testing in combination with track modelling represents an efficient strategy for identification of the current condition of railway track structure and its components. Field investigations for the dynamic characteristics of a railway track and its components were carried out and are presented in this paper. A non-destructive technique using impact excitation, so-called ‘ modal testing’, was utilized in these trials. Integrated approach combining field measurements, experimental modal analysis, and finite element modelling to evaluate the dynamic parameters of the in situ railway track components are appended. A ballasted railway track site in Central Queensland managed by Queensland Rail (QR) was selected to perform the field tests. Six sleeper-fastening-rail assemblies were selected for dynamic testing. The frequency response functions (FRFs) were recorded by using Bruel & Kjaer PULSE vibration analyser in a frequency domain between 0 and 1600 Hz. The data obtained were best fitted using the least-square technique to determine the dynamic stiffness and damping constants of the tested track components. In addition, the experimentally determined resonance frequencies along with the dynamic properties of the track components can provide an important input for determining the maximum speed and axle load for the future track upgrades. This paper also points out on how to judge the dynamic responses (e.g. FRFs) together with the visual inspection of existing conditions from the field experience. Examples of testing results representing the deficient integrity are additionally highlighted. Based on the results, the impact excitation technique is an efficient method susceptible to the structural integrity of railway track structures.

Sensitivity analysis of free vibration characteristics of an in situ railway concrete sleeper to variations of rail pad parameters

The vibration of in situ concrete sleepers in a railway track structure is a major factor causing cracking of prestressed concrete sleepers and excessive railway track maintenance cost. Not only does the ballast interact with the sleepers, but the rail pads also take part in affecting their free vibration characteristics. This paper presents a sensitivity analysis of free vibration behaviors of an in situ railway concrete sleeper (standard gauge sleeper), incorporating sleeper/ballast interaction, subjected to the variations of rail pad properties. Through finite element analysis, Timoshenko-beam and spring elements were used in the in situ railway concrete sleeper modeling. This model highlights the influence of rail pad parameters on the free vibration characteristics of in situ sleepers. In addition, information on the first five flexural vibration modes indicates the dynamic performance of railway track when using different types of rail pads, as it plays a vital role in the cracking deterioration of concrete sleepers.

X-Ray Investigations of the Natural and Artificial White Etching Layer

We analyzed X-Ray diffraction data on the White Etching Layer (WEL) excised from the surface of a railway track and WEL-like structures obtained by high pressure torsion of the same steel as the rail was made. Data analysis was performed by the whole-pattern fitting method. The railway WEL shows a clear asymmetry of the diffraction lines, which can be attributed to either tetragonal martensite structure or to the formation of a ferrite nanophase with a lattice parameters larger than that of the raw material. X-Ray patterns of the natural WEL were fitted with the two models. The quality of the fit clearly favors the model with two cubic phases over the model with a single tetragonal phase. Artificial WEL samples could be fitted with a single cubic phase model. Several alternative methods were simultaneously used to extract information on crystallite size and internal stress. The analysis of the natural WEL shows that the secondary cubic nanophase is formed of the crystallites of the average size of 8 nm essentially free of any strain. The lattice parameter 0.2881nm corresponds to the 0.26 wt.% concentration of the dissolved carbon. In the artificial WEL samples the crystallite size decreases down to 9 nm at the maximum shear stress of 430, while the internal strain gradually increases. There is however no change of the lattice parameter. This means that no carbon from cementite dissolves in the lattice of ferrite. It must therefore be located in the grain boundaries and dislocation cores. We conclude that the high pressure torsion produces a WEL with a different distribution of carbon than in the natural process of the WEL formation. It involves an immediate severe deformation the sample while natural process takes numerous subsequent cycles of modest deformation with simultaneous local heating of the deformation area. Such a process may lead to the gradual diffusion of the carbon atoms from the grain boundaries into the crystal lattice of ferrite.

A vehicle-track-bridge interaction element considering vehicle's pitching effect

A finite element method is applied to analyze the dynamic responses of railway track and bridge under a moving railway vehicle. The vehicle, track and bridge are regarded as an integrated system. The moving vehicle is modeled as a two-axle mass-spring-damper system having four degrees of freedom. The rails and the bridge deck are modeled as an elastic Bernoulli–Euler upper beam with finite length and a simply supported Bernoulli–Euler lower beam, respectively, while the elasticity and damping properties of the rail bed are represented by continuous springs and dampers. A vehicle-track-bridge interaction element considering vehicle's pitching effect is proposed for studying the interactions among moving vehicles, and its supporting railway track structure and bridge structure. The element consists of a vehicle modeled as a two-axle mass-spring-damper system having four degrees of freedom, two upper beam elements to model the rails, two lower beam elements to model the bridge deck, and continuous springs and dampers to model the rail bed between two upper beam elements and two lower beam elements. The equations of motion for the vehicle-track-bridge interaction element are directly derived by means of Hamilton principle. After by assembling the stiffness matrices, the damping matrices, the mass matrices and the vectors of nodal loads of all elements, the global equations of motion for the integrated system are obtained. These equations are solved by step-by-step integration method, to obtain simultaneously the dynamic responses of vehicle, track and bridge. Several illustration examples are presented.

Experimental and mathematical analysis of a multi-layer system of railway track

Based on experimental measurement of a model of a multi-layer non-linear system with inserted geosynthetic layers of the permanent way structure, a new design method will be designed for dimensioning the sleeper subgrade structure using classical and geosynthetic materials or sub-ballast vibration-damping matting. The results of measurements on a track structure model in a 1:1 scale performed in a testing box will be verified by means of a numerical model. A two-dimensional solution will be subsequently verified by means of a spatial model. Numerical modelling will serve for transferring the values measured on the experimental model into a current railway track structure.

Vibration of railway bridges under a moving train by using bridge-track-vehicle element

During the last two decades, much attention has been paid to various vibration problems associated with railways. They include the dynamic response of railway bridges and railway tracks at grade under the action of moving trains. However, studies on the role of track structures on the vibration of railway bridges are rather limited. In this paper, a new element called bridge-track-vehicle element is proposed for investigating the interactions among a moving train, and its supporting railway track structure and bridge structure. The moving train is modelled as a series of two-degree-of-freedom mass-spring-damper systems at the axle locations. A bridge-track-vehicle element consists of vehicles modelled as mass-spring-damper systems, an upper beam element to model the rails and a lower beam element to model the bridge deck. The two beam elements are interconnected by a series of springs and dampers to model the rail bed. The investigation shows that the effect of track structure on the dynamic response of bridge structure is insignificant. However, the effect of the bridge structure on the dynamic response of the track structure is considerable.

GROUND VIBRATION GENERATED BY A HARMONIC LOAD ACTING ON A RAILWAY TRACK

A calculation method has been produced for the propagation of vibration in the ground from a stationary oscillating load applied via a railway track structure. The model includes the track as an infinite layered beam structure resting on a ground made up of infinite parallel homogeneous elastic layers. These layers may either be constrained at the lower interface or coupled to an elastic half-space. A similar model, based on wave propagating finite elements, has previously been shown to be useful in predicting the behaviour of real soils and railway tracks, but the applicability of that model was limited by long computation times. The present method is more efficient in calculating the responses at a large number of positions. The development of the theory allows analysis in terms of the amplitudes of different wave types propagating along, and normal to, the track. Example calculations are presented for a ground consisting of a layer on a half-space. By changing only the depth of the layer, two different wave propagation regimes are found, the first where propagation takes place via modes of the layer and the second, where propagation takes place via the bulk waves in the layer and the Rayleigh wave in the substratum. Both examples show the track structure to have a strong effect on the directivity and amplitude of the response of the ground surface.

Experience in Railway Track Testing for Validation of Theoretical Dynamic Analysis

The objective of this paper is to present some experimental approaches in dynamic testing of the railway track structure. The load - deflection track characteristics are particularly important in the design, in the evaluation of serviceability of concrete-sleepers tracks, and to improviding data for computer modelling of the track dynamic behaviour and the dynamic vehicle/track interaction analyses. Thus, results of theoretical dynamic analyses of the track structure must be verified and compared with obtained experimental results. The herein presented track testing for validation of dynamic analyses can give many useful indications concerning the evaluation of the superstructure behaviour.

Shear Behavior of Railway Ballast Based on Large-Scale Triaxial Tests

Quarried rock fragments (ballast) constitute one of the most commonly used construction materials in railway engineering practice. Ballast is subjected to high stress levels as well as being always exposed to environmental changes. Unsatisfactory performance of railway tracks is often associated with the loss of cross level, track profile and track alignment. The in situ condition and the engineering behavior of ballast are also important aspects governing the stability and performance of a given railway track structure. This paper presents the findings of a series of isotropically consolidated, triaxial compression tests on two modeled fractions of uniformly graded latite basalt, which is currently being used by the Railway Services Authority (RSA) of New South Wales, Australia, in the construction of new railway tracks. These tests form part of an extended research program sponsored by RSA to study the stress-strain relationships, strength properties and degradation characteristics of various types of railway ballast. The program is associated with the necessity of upgrading railway tracks for the looming Olympics in 2000. Large-scale triaxial equipment has been employed during the testing program, which is formulated to provide specific geotechnical information on the shear strength and the angle of internal friction of ballast as a function of the particle size distribution. The effect of maximum principal stress ratio on the deformation and degradation of ballast is also studied. Nonlinear relationships are developed to describe appropriately the variation of shear strength, angle of internal friction, dilation rate and degree of particle crushing at different confining pressures and principal stress ratios.

DYNAMIC ANALYSIS OF BEAMS ON AN ELASTIC FOUNDATION SUBJECTED TO MOVING LOADS

A simple procedure based on the finite element method has been developed for treating the dynamic analysis of beams on an elastic foundation subjected to moving point loads, where the foundation has been modelled by springs of variable stiffness. The effect of the speed of the moving load, the foundation stiffness and the length of the beam on the response of the beam have been studied and dynamic amplifications of deflections and stresses have been evaluated. The technique is extended to the analysis of railway track structures, where the effect of the spring stiffness of the moving load is also incorporated. The entire analysis has been programmed to run on a microcomputer and gives fast and accurate results. Several numerical examples are presented. The technique and the findings will be useful in railway track design.