Numerical simulation platform for slab track systems subjected to a moving vehicle

Drying‐induced cracks are an important issue for bi‐block ballastless track system consisting of foundation, precast sleepers, and cast‐in‐place track slab, which not only significantly affects the comfortableness and safety of rapid transit railway but also reduces the service life of ballastless track. In order to explore its damage mechanism, this work presents an evolution model of relative humidity (RH) in the CRTS I bi‐block ballastless track system by considering the actual construction sequence and environmental conditions to simulate the crack propagation induced by nonuniform RH field. Firstly, based on the node coupling technique, a three‐step transfer process of RH is designed to separately investigate the influence of the construction sequence on the early humidity field in the foundation, sleepers, and cast‐in‐place track slab, and then the nonuniform distribution of early humidity field in the ballastless track system is determined. Subsequently, the formation mechanism of shrinkage crack in the system is analyzed, and the crack propagation path is predicted by using the mixed‐mode fracture criterion. The results show that the maximum relative humidity gradient (RHG) appears at the interface between the track slab and the sleeper after concreting the cast‐in‐place track slab, which causes the maximum principal stress due to the drying shrinkage property of concrete materials. When the maximum principal stress exceeds the tensile strength of the interface, an interface crack will be generated and converted to a splayed crack with an initial angle of about 45° at the sleeper corner, which will be further propagated under the action of drying shrinkage deformation and finally forms a transverse through‐wall crack in the track slab. The simulated crack propagation path agrees with the observed one at the site well, and thus the results are beneficial to understand the formation mechanism of through‐wall crack in the track slab and further guide the construction design of the bi‐block ballastless track system.

Numerical analysis of interface damage in ballastless track on simply supported bridge due to thermal and vehicle dynamic load

Thermal-flow-elastic coupling (TFEC) numerical simulation platform has been an essential platform for designing turbo engines with high performance and efficiency. Generally, TFEC numerical simulation was achieved by predicting thermal and stress fields with finite element methods, while flow fields with finite difference methods, but such calculation was not popular in engineering design, because of too much size of data exchange and lower computing efficiency. However these shortcomings will not exist by using finite difference methods for all of the fields. To establish a three dimensional multifunction numerical simulation platform for turbines for all of the fields, the key technique was studied firstly. The technique included analysis on physical models, establishing of mathematical model equation, usage of curvilinear coordinate platform, construction of high accuracy difference scheme and selection of boundary conditions for multi-field coupling simulation. Then the algorithm including domain decomposition one and parallel one were studied to accelerate the coupling simulation. The purpose was to develop a completed TFEC numerical simulation platform by using of finite difference method and to apply the platform for numerical simulation in turbines. Firstly codes for predicting flow field in passage, thermal and stress fields in solid body were developed. Then a simple TFEC numerical simulation platform for turbines was obtained. The single code for predicting flow field was verified with experimental data, and the other two codes were validated with thermal and elastic analytic solutions respectively. And satisfying results were obtained. Then the code for thermal-flow was validated with experimental data of Mark II blade, and the code for thermal-elastic coupling simulations was validated with a cylinder by an analytic solutions. All of these are good basics for completing TFEC numerical simulation platform using finite difference methods for all of the fields and computing TFEC numerical simulation in a turbo engine.

Based on higher-order shear deformation theory (HSDT), the bending of China Railway Track System type II (CRTS-II) track laminated plates on elastic foundations and interlayer shearing performances are analyzed by meshfree radial point interpolation method (RPIM), which has the properties of the Kronecker function and discretizes the governing equations derived from HSDT. To verify the applicability of the model, the geometric configuration and key parameters of the CRTS-II slab ballastless track structure are analyzed, and the interlaminar slip model of the track structure is derived as a benchmark solution when calculating the track structure using HSDT-RPIM. The accuracy of the derivation is verified by comparing the HSDT-RPIM model with a double-layer slab model considering bidirectional interlayer slip. Then, the effect of resistant slip coefficient on the bending deformation of the track structure was analyzed using the bidirectional interlaminar slip model. The effect of elastic foundation coefficient on the bending of the track structure and the interlayer slip of the track structure (track slab – base slab) was analyzed. Finally, the effects of boundary conditions and elastic foundation coefficients on the interlaminar stresses of the track structure are investigated using HSDT-RPIM, and some interlaminar damages of the track structure are explained.

Deflection of a railway reinforced concrete slab track: Comparing the theoretical results with experimental measurements

As the scale of development continues to expand, carbon emissions of urban rail transit (URT) in China have gradually increased each year, posing certain pressure on the carbon emission reduction efforts in the URT industry. To evaluate the current status of low-carbon development in the URT industry, this paper first constructs an annual carbon emission accounting model for URT that considers both the construction and operation processes. Based on this model, the annual carbon emissions of URT in China from 2015 to 2023 are calculated. Furthermore, based on decoupling theory, the relationship between carbon emissions and industry development in URT is explored from both the construction and operation aspects. The results show that: (1) From 2015 to 2023, the total carbon emissions of URT in China increased significantly from 14.425 million tons to 24.7072 million tons. The proportion of carbon emissions from construction is higher during the study period, but the growth rate of carbon emissions from operation is faster. (2) The order of carbon emissions from URT in different regions from largest to smallest is: East > West > Central > Northeast. (3) During the study period, the carbon emissions from URT construction in China were weakly decoupled from the operational lines. (4) There was an expansive coupling relationship between carbon emissions from URT operation and passenger transport turnover in China. These findings can provide some guidance for the sustainable development of the URT industry.

To study the impact of upwarp deformation in the ballastless track on the jumping behavior of the high-speed vehicle, utilizing UM and ANSYS joint simulation, a vertical vibration model of high-speed vehicle on CRTS II slab ballastless track was developed based on the non-Hertz wheel–rail contact model of virtual penetration theory. By using the single-wave cosine curve simulating the characteristics of upwarp deformation in the track slab, we calculated the whole process of wheel jumping. This allowed us to analyze how the amplitude and wavelength of the track slab upward deformation influence the vibration response of the vehicle–track system. Our findings indicate that when a wheel passes through the arch section of the track slab, the entire wheel jumping process consists of distinct stages: “wheel–rail bonding, wheel–rail separation, wheel–rail impact (one or more times), and wheel–rail bonding.” As the amplitude of upwarp deformation increases and the wavelength decreases, significant changes occur in several parameters, including the vertical force between the wheel and rail, wheel unloading rate, wheel jump height, frequency, duration, and vertical displacement of the rail. Additionally, when the wavelength is between 2 and 6[Formula: see text]m and the amplitude is 8[Formula: see text]mm, the vertical force between the wheel and rail becomes zero, the wheel load reduction rate is one, and the wheel jumps. When the wavelength is less than 3[Formula: see text]m, the wheel jump height exceeds the flange height, increasing the risk of derailment. Meanwhile, during the first wheel–rail impact, the wheel–rail vertical force and the rail vertical displacement reach their maximum, potentially impacting rail service performance negatively. Finally, compared to the amplitude of the track slab camber deformation, its wavelength has a greater impact on the entire process of wheel jumping. It is recommended that attention be paid to the change in the wavelength of the track slab camber during the maintenance and repair of the ballastless track.

A significant part of railway track maintenance management is the periodic and systematic evaluation of track conditions and representation of these conditions by quantitative parameters. In spite of a significant global increase in the construction of railway slab tracks, there is no practical method for evaluating and quantifying slab-track quality conditions. This research is aimed at developing a slab-track condition evaluation algorithm and a methodology for predicting the required repair and maintenance strategy. This is done using a large set of data from a comprehensive field investigation, implementing experts' evaluations and applying statistical techniques. Two methods for slab-track condition inspections are introduced. Track indices are developed, expressing structural and geometric conditions in numerical forms. An algorithm for prioritising required repair activities is proposed by establishing correlations between structural conditions and geometry indices. The research results are evaluated by applying the proposed methods and algorithm in a light rail transit line.

<p indent="0mm">In this paper, magnetic confinement fusion is introduced focusing on its fluid mechanics relevant to a liquid lithium divertor. The main research problem is the flow stability of a liquid metal film flow under the influence of a magnetic field. Therefore, a linear stability analysis numerical calculation platform and a direct numerical simulation platform are developed to understand the effect of magnetohydrodynamics on the flow stability of thin films. These platforms will be used to reveal the physical mechanism of a magnetic field on the film flow stability and to guide the design and implementation of a liquid lithium divertor.

Cable-stayed bridges carrying urban rail transit are often equipped with rail expansion devices (REDs) to avoid possible breaking or buckling of the rails. Such devices not only increase construction and maintenance budgets of the track system, but also endanger ride comfort and safety of the trains. Therefore, the maximum expansion length of cable-stayed bridges without the adoption of RED is investigated and presented in this paper. Finite element models of cable-stayed bridges with various lengths are developed based on design parameters of existing bridges with similar lengths. A friction-spring model is utilized to simulate the nonlinear resistance history of fasteners under various loading cases. Track–bridge interactions are then studied by varying the longitudinal restraint conditions for the cable-stayed bridges. The investigation reveals that the admissible overall rail stress allows a maximum expansion length of about 400[Formula: see text]m for urban rail transit double-tower cable-stayed bridge paved with the ballastless track and WJ-2 small resistance fasteners. This finding provides theoretical bases and practical guidelines for the saving of REDs for many large-span urban rail transit bridges in future design.

PurposeTemperature is an important load for a ballastless track. However, little research has been conducted on the dynamic responses when a train travels on a ballastless track under the temperature gradient. The dynamic responses under different temperature gradients of the slab are theoretically investigated in this work.Design/methodology/approachConsidering the moving train, the temperature gradient of the slab, and the gravity of the slab track, a dynamic model for a high-speed train that runs along the CRTS III slab track on subgrade is developed by a nonlinear coupled way in Abaqus.FindingsThe results are as follows: (1) The upward transmission of the periodic deformation of the slab causes periodic track irregularity. (2) Because of the geometric constraint of limiting structures, the maximum bending stresses of the slab occur near the end of the slab under positive temperature gradients, but in the middle of the slab under negative temperature gradients. (3) The periodic deformation of the slab can induce periodic changes in the interlayer stiffness and contact status, leading to a large vibration of the slab. Because of the vibration-reduction capacity of the fastener and the larger mass of the concrete base, the accelerations of both the slab and concrete base are far less than the acceleration of the rail.Originality/valueThis study reveals the influence mechanism of temperature gradient-induced periodic deformation in the dynamic responses of the train-track system, and it also provides a guide for the safe service of CRTS III slab track.

Ballastless track slab offers excellent stability and durability and has been well accepted in high-speed railways worldwide. Rails are typically laid on precast concrete slabs that are subjected to dynamic load transferred from the rails. Cracks can be induced by shrinkage and mechanical loading in concrete, which accelerates the degradation and affects the performance of the track slab. As tens of thousands of miles of ballastless track are constructed, effective and efficient maintenance for the concrete slabs has become an issue. In this paper, ultra-high performance concrete (UHPC) is proposed to fabricate ballastless track slab. UHPC is a superior fiber-reinforced, cementitioius mortar, which has greatly-improved mechanical strengths and durability. A recently-developed UHPC is evaluated in terms of the flowability, durability, shrinkage, and mechanical properties. A functionally-graded slab design is proposed with the consideration of initial material cost. The slab is cast with two layers: a layer of conventional concrete at the bottom, and a layer of UHPC on the top. A three-dimensional finite element model is developed for ballastless track slab whose flexural performance is investigated and compared with that of slab made with conventional concrete. Concrete damage plasticity model is incorporated to consider the post-cracking behavior. The results indicate that the proposed UHPC is promising for fabricating ballastless track slab with superior performance.

Rational planning for the underground space development in subway station areas, in coordinate with the urban rail transit development, has positive impacts on the sustainable development of cities. This paper induces the space syntax method for calculating the quantitative accessibility index of the urban rail transit and detects the coordination between urban underground space use intensity and rail transit system of the 7 selected subway lines in Tokyo. The results show that the station areas with higher accessibility index are the same sort with higher underground space use intensity. This means that the space syntax method is suitable and feasible for calculation of the quantitative accessibility index of subway lines. The higher the accessibility index is, the more underground space will be needed potentially in future. The areas with high accessibility of subway lines and low intensity of public underground space develop are the key districts under potential exploitation of underground space.

Modern railway tracks for high-speed traffic are often built based on a slab track design. A major disadvantage of slab track compared to conventional ballasted track is that the environmental impact of the construction is higher due to the significant amount of concrete required. In this paper, the dimensions of the rectangular cross-sections and the types of concrete used in slab tracks are optimised with the objective to minimise greenhouse gas emissions, while considering the constraint that the design must pass the static dimensioning analysis described in the European standard 16432-2. The optimised track design is also analysed using a three-dimensional (3D) model of vertical dynamic vehicle–track interaction, where the rails are modelled as Rayleigh–Timoshenko beams and the concrete parts are represented by quadratic shell elements. Wheel–rail contact forces and the time-variant stress field of the concrete parts are calculated using a complex-valued modal superposition for the finite element model of the track. For the studied traffic scenario, it is concluded that the thickness of the panel can be reduced compared to the optimised design from the standard without the risk of crack initiation due to the dynamic vehicle load. In parallel, a model of reinforced concrete is developed to predict crack widths, the bending stiffness of a cracked panel section and to assess in which situations the amount of steel reinforcement can be reduced. To reduce the environmental impact even further, there is potential for an extended geometry optimisation by excluding much of the concrete between the rails.

A recently developed Flexible Track System Model integrated with a multibody dynamics software tool was used to simulate the dynamic interaction between a vehicle and two innovative slab track designs, comparing their performance with respect to conventional ballasted track. The design concepts are presented and the modelling assumptions are given. Simulations are then carried out to quantify, for example, the impact of ballasted track degraded state using the case of a hanging sleeper. In comparison, the benefit of the two innovative track designs is highlighted as they prevent this type of localised defect from occurring. The alternative track designs were also shown to be capable of carrying a vehicle safely and with less impact on the system over a localised weakened formation support, by simulating a washout event.