Dynamic calibration of slab track models for railway applications using full-scale testing
Introduction
The health and long-time performance of the infrastructure is critical in any rail system, not only due to safety aspects but also owing to the high maintenance costs involved. Besides, it is extremely important to minimize any disturbance in the railway service due to the social and economic repercussions. Despite its importance, the performance and maintenance management of the track is, scientifically, one of the least understood and least predictable elements of railway systems. This fundamental understanding is central to reduce the Life-Cycle Costs (LCC) and to increase the safety, capacity and reliability of the rail networks.
During train operation, tracks are subjected to high impact and fatigue loads that can originate rapid degradation and unexpected, unpredictable failure. The conventional approach used by the infrastructure managers consists of performing regular preventive maintenance, fixed lifetime replacement or applying corrective measures when an incident occurs. This approach is neither effective nor economical so health/performance assessment and predictive maintenance has a great potential for innovation due to the high costs associated. Any efficiency improvement on this would represent a great advantage to the rail industry.
Currently, both ballasted and concrete slab tracks are being used for railways worldwide and it is recognised that both forms have advantages and disadvantages. When compared with ballast tracks, slab systems have advantages from a structural point of view, such as higher lateral and longitudinal stability, no rail buckling and lower sensitivity to differential settlements [1], [2], [3]. They have also operational advantages such as lower maintenance needs, lower structure height and not presenting the problems that are associated to ballast shaking and flight [4], [5], [6], [7]. On the other hand, slab tracks have some disadvantages associated to the higher installation costs and to the lower noise and vibration absorption provided [4], [8]. Due to the overall poor performance of ballast for increased train speeds, the use of concrete slab became more popular and various slab track forms have been produced and tested in recent years [2], [5], [9].
The analysis of the track structure conditions and its dynamic response has devoted the attention of many researchers aiming to support the rail industry in their developments [10], [11], [12], [13], [14], [15], [16]. For this purpose, numerical models have been developed mainly using Multibody (MB) and Finite Element (FE) methodologies. The initial models were 2D based on an elastic foundation formulation [17]. Later, Zhai et al. [18] proposed a 2D numerical model which reproduces the dynamic interaction between a lumped mass vehicle and a discretely supported continuous rail track. This type of model has also been used by other researchers [1], [19]. Cai et al [20] proposed a 3D model where the track is considered as a periodic elastically coupled beam system resting on a Winkler foundation and uses spring and dashpot elements to simulate the rail pads and the sleepers. A more recent 3D model was proposed by Poveda et al. [21] to analyse the fatigue life of concrete slabs, which was calibrated with lab tests [22]. Other authors have also proposed co-simulation methodologies between MB and FE formulations in order to study the track structure under realistic trainset loads [23], [24]. These developments open the possibility of integrating more detailed wheel-rail contact models [25], [26], [27], [28], [29], [30], [31], to consider track irregularities [32], [33] and other track singularities [7], [13], [34], [35], [36] in the studies aiming to assess the track performance and degradation evolution [11], [37], [38], [39], [40] in realistic operation conditions.
Due to the multidisciplinary areas of knowledge involved, all issues involving the complete characterization of the railway infrastructure are complex. Therefore, the use of reliable computational tools that are able to reproduce the dynamic response of the track when subjected to the loads induced by the railway traffic is essential. The numerical models have to represent all the structural layers of the track and the elements that are used to fix and support the rails, namely the fastening system. The main difficulty of building such models is the uncertainty associated to the properties of the layers and components that compose them. In order to overcome this uncertainty, field measurements and laboratory tests can be used to validate the numerical models. Nevertheless, many of the track measurements that are performed by the infrastructure managers are unavailable for scientific use due to industry restrictions/confidentiality. The work proposed here is a contribution in this field by proposing a detailed track model with properties that are calibrated with experimental results obtained in a full-scale test facility.
The material layers have non-linear properties that vary with load conditions, frequency, etc. The fastening system is the component that presents higher non-linearities and this is the reason why dedicated lab experiments on it are performed in this work. It should also be noted that the slab technology has a much more predictable behaviour than the ballast tracks, which have a non-linear performance as they are composed of granular material. In the literature there are authors that use linear elastic material models to describe the constitutive relations of the slab track components [41]. Poveda et al [21] use FE solid elements modelled as linear elastic materials to perform studies on the fatigue life design of concrete slab tracks. Zhu et al [42] show that the bilinear cohesive zone model can be employed to capture the mechanical behaviour of the concrete interface of slab tracks. Ren et al [43] show that when debonding occurs the slab track stops exhibiting linear elastic mechanical response. Zhang et al [44] use viscoelastic parameters in the FE models to better predict the initiation of interlayer debonding of track structure. El-Ghandour et al [45] use the modal frequencies extracted from the FE model, instead of the nodal degrees of freedom, and uses the floating frame of reference formulation to obtain the elastic response of the track system. Using a non-linear FE formulation in this work would represent a heavy burden to the code and would make it almost impossible to use the validated models for vehicle-track interaction studies.
Several authors have devoted their studies to highlight the importance of the fastenings to the rails’ dynamic response. Wei et al. [46] concluded that the properties of the rail pads have a non-linear behaviour and are dependent on the frequency and amplitude of the applied loads. Fenander [47] concluded that the rail pad stiffness increases with the preload and with the frequency, although the effect of the frequency is less relevant. Kaewunruen et al. [48] concluded that the level of preloading have a substantial influence on the natural frequencies and dynamic properties of rubber pads. Wei et al. [49] also showed the influence of the frequency in the pads stiffness especially for the vibrations produced by subways in tunnels. Zhu et al. [50] appreciated an increase in the dynamic stiffness and damping of the rail pads with the increment of the excitation frequency. Carrascal et al. [51] studied the deterioration in the rail pads produced by its normal working conditions and defined a test methodology to determine the dynamic behaviour of railway fastening setting pads in working conditions [52], [53].
Other authors have dedicated their research activities to investigate the performance of various parts of the railway track structure and have used experimental tests for the purpose. For example, full-scale model tests with simulated train moving loads hace been developed to explore the dynamic performance and long-term behaviour of concrete slab tracks [4], [22], [54]. In the case of ballasted track, a two-layer railway track model was developed and tested [55]. Pita et al. [56] and Colaço et al. [57] observed that a low track stiffness value can result in a flexible track with poor load distribution, whereas a high track stiffness can cause greater dynamic overloads on the rail, which induces increased vehicle-track interaction forces that lead to rail defects such as corrugation.
The main goal of this work is to develop and calibrate computational models of slab tracks that can be used for the consistent assessment of the performance and dynamic response of the railway infrastructure when subjected to realistic loads imposed by the rolling stock. The properties of the fastening elements have a noticeable uncertainty as they depend on the material, load, preload produced by the fastening system (toe load), frequency and on the degradation degree of the components. Furthermore, there are also uncertainties associated to the properties of the material layers that compose the slab track structure, such as the subgrade and the different concrete types. The main contribution of this work is that the uncertainties associated to the physical parameters of the material layers and of the components that compose the track system are overcome by performing full-scale tests of the slab track structure and of the fastening system. Such experimental data is used to calibrate the numerical models such that they present a frequency response similar to the real physical models.
It is foreseen that the calibrated slab track model proposed here can be used together with detailed vehicle models, in a co-simulation environment, to study the long-term behaviour of the rail infrastructure. This approach can then be used together with suitable track degradation models to develop decision support tools to promote the implementation of science-based maintenance strategies.
Section snippets
GRAFT II: Description of the test apparatus
A slab track system is generally composed by a track superstructure and a substructure. The superstructure includes the rail, fastening system, slab track and a concrete supporting layer. The track substructure comprises the roadbed, subgrade and subsoil [4]. The dynamic response of the slab track depends on the properties of all the different layers [58], [59]. In this work full-scale laboratory tests of a slab track system are performed comprising the layers represented in Fig. 1, namely
Description of the test apparatus
The rail fastening is the system used to fix the rails to the sleepers that, not only prevents the rails from rotating, but also provide elasticity to the track and damp the transmission of noise and vibrations to the infrastructure resulting from the train operation. The fastening system used in this work is the Vossloh system 300, represented in Fig. 5, which is a highly elastic solution for slab track with applications for both conventional and high-speed rail.
As shown in Fig. 5(b), the
General description
The purpose of this work is to develop a detailed and reliable FE slab track model that includes all components of the rail infrastructure. The characteristics of the track layers and of the rail supporting elements are identified by performing full-scale tests of the slab track and of the fastening system as previously described. The FE model is built and studied using the commercial software ANSYS and considering the dimensions shown in Table 1. The dimensions of the fastening system model
Computational model calibration
The calibration of the slab track numerical model is performed by comparing the computational results with the ones obtained in the laboratory experiments. To this end, the reference values for the properties of the track material layers are adjusted to get the better possible correspondence between the numerical and experimental results. The calibrated model considered here is the one with the material properties detailed in Tables 8 and 9 and the mesh size defined in Table 10.
The comparison
Conclusions and future developments
The aim of this work is to boost the overall performance of rail transport infrastructure by developing reliable numerical models that enable to predict the track behaviour, contributing to reduce the LCC of the infrastructure and to minimize the traffic disruptions, which are mainly caused by unpredicted failures or events. For this purpose, fastening characterization tests and full-scale slab track experiments are performed in realistic operation conditions. Then, a detailed 3D slab track
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for funding this work under Grant Number EP/NOO9215/1. Tarmac, Tensar and Max-Bögl are also acknowledged for their support with regards to the experimental tests. This work was supported by FCT, through IDMEC, under LAETA, project UID/EMS/50022/2019.
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