| With the rapid development of high-speed railway and the large-scale construction of passenger dedicated line,CRTSⅡslab ballastless track has been widely used in the construction of high-speed railroad in China.This is because of its mature structure and easy construction.The main structure of CRTSⅡballastless track with the slab and CA mortar is difficult to avoid various damages such as separation between slab and CA mortar and CA mortar voids under the influence of high speed train load.These damages can have a great impact on the stress of the track and the vibration of the wheel-rail system,thus affecting the long-term service life of the track.Therefore,it is necessary to establish an analytical model of CRTS Ⅱ ballastless track slabs to study the damage extension and further investigate the stress characteristics under CA mortar voids.It is of great practical significance for the maintenance and treatment of various defects during track operation.The main research work is as follows.（1）The background and importance of the study are introduced.High-speed railway is currently adapted to national economic development,and to meet the convenience and safety of travel transportation.The ballastless track（slab track）,which is commonly used in high-speed railway,is a new type of track system with the advantages of low maintenance cost,high stability and high durability,and has been widely adopted in many countries.Ballastless track or slab track system is the replacement of typical ballasted track by concrete slab or asphalt surface.Some of the factors of its rapid development are due to its better stiffness,higher operational stability and lower maintenance costs.Nevertheless,one of the main challenges in building flat racks is the initial investment cost,which is mainly evaluated for new projects.However,in terms of adopting whole life cycle costs,slab ballastless track is more attractive.The purpose of this study is to investigate the static and dynamic response patterns of CRTS Ⅱ ballastless track under interlayer damage using Abaqus finite element technique,and to investigate the key impact indicators of interlayer damage on ballastless track service performance and ripped vehicle operational safety.（2）The finite element method used in this paper and the analysis environment of the commercial finite element software Abaqus,which was developed in 1978,are introduced.After decades of development,it has gradually developed into a powerful finite element analysis software covering all fields of engineering.Currently,it is one of the most widely used finite element simulation software in the world.In this work,the simulations were performed with Abaqus commercial finite element analysis software.Because the track model can deform plastically,the nonlinear capability of the software allows a reasonable assessment of the possible stress-strain behavior of the track component materials.Since a railway track is a complex system consisting of multiple components with a large number of contact interfaces,the realism of the contact interaction behavior of the model contributes significantly to the validity of the numerical solution.Such contact-induced interactions must be accurately considered to obtain accurate results.The software has the ability to simulate various contact interactions,such as self contact between two objects,and surface-to-surface contact.In addition,it is able to include the coefficient of friction between two contact surfaces and automatically detect contact between two objects,since most railroad components are in contact with each other.In general,static analysis can include linear and nonlinear effects for checking systematic and formal,e.g.static stress-induced deformations.Stability is a prerequisite for performing the analysis.Static methods do not use time increments as dynamic methods do,but rather a percentage of the applied load.The time period is set by default to 1.0 unit,which corresponds to 100%of the applied load.The NLGEOM function can be used if large displacements,material nonlinearities,boundary nonlinear effects,contact or friction,and other nonlinear effects are expected.When instability is encountered,such as buckling or collapse,a risk reduction method can be used.It solves the simulation of loads and displacements by including the magnitude of the load as a second uncertainty factor.This approach produces a solution even if the problem is nonlinear.In ABAQUS,the modeling technique can be divided into different modules,each of which has a different role.For example,the PART module is used to sketch the geometry of each part in the model;the PROPERTY module allows the user to specify the features or material properties to be utilized in the model;and the STEP and MESH modules allow the user to select the type of analysis and create a mesh.The last section is the JOB module,which is used to generate the input files and perform the analysis.Finally,post-processing will run the VISULAIZATION module and output the results of the analysis.Abaqus includes two solver modules:the ABAQUS standard solver and the ABAQUS explicit solver.It is an extensive analysis module in the ABAQUS standard.It is capable of solving a wide variety of linear and nonlinear problems,including static,dynamic,thermal,and complex nonlinear coupled physical field analysis.Numerous solvers are included to reliably solve large-scale computational problems.Abaqus/Explicit is a specialized analysis package that employs a dynamic explicit finite element model.It is well suited for solving rail-foundation interaction systems with large degrees of freedom.The models established in this paper include rail system,fastener system,CRTSⅡrail slab system,CA mortar infill layer,and concrete base system,all of which are established using solid unit C3D8.（3）CA mortar as the weakest part of CRTSⅡslab ballastless track,under the long-term effect of train load,temperature load,rainfall and other harsh environment,it is very easy to have cracks,interlayer debonding,concrete slab arching and CA mortar stripping,etc.,when the separation becomes more and more serious,it will evolve into the bottom of the slab debonding.When the bottom of the track slab appears out of the air defects,in the high frequency train load and various other external environmental factors,the gap will gradually expand,until the transverse through the entire track slab.The appearance of these damage phenomena will easily destroy the integrity of the track structure,deteriorate the structural performance,and also have adverse effects on the long-term service of the track structure.Therefore,the finite element software ABAQUS will be used in Chapter 3 to analyze the different damage lengths and different damage locations under static loading.The bending stress and deformation of CRTS Ⅱ slab ballastless track under static load are calculated theoretically to analyze the separation mechanism of track slab and CA mortar.The void is used to simulate the working condition,which is divided into two models:the void at the center of the slab and void at the center of the edge.Assuming that the width of the void does not exceed 0.5 m and the length of the void is set to 0.63 m,1.26 m,1.89 m and 2.52 m for four working conditions,the analytical results of the displacement,stress and strain in the center of the slab and the center of the edge under the static load are given by theoretical calculations respectively,and the following conclusions are made:the displacement,stress and strain increase sharply with the increase of the void length.The maximum transverse tensile stress of the slab track is 1.358 MPa while the maximum transverse compressive stress is 2.094 MPa.however,compared with CA mortar,the maximum transverse tensile stress is 1.176 MPa while the maximum transverse compressive stress is 1.766 MPa.For the slab end debonding,we obviously find that the effect is slightly less than the intermediate debonding length as the debonding length increases.The deformation and stresses in the track structure remain constant as the debonding length increases.However,the displacements and stresses of the track structure tend to increase when the debonding length is greater than 1.89 m.The maximum transverse tensile stress of the slab is 0.644 MPa,while the maximum transverse compressive stress is 1.081 MPa.As for the CA mortar,its response is smaller than that of the slab track.the maximum transverse tensile stress of the CA mortar is 0.167 MPa and the maximum transverse compressive stress is 2.87 MPa.It can be seen that the maximum tensile stress of the track slab did not exceed the tensile strength of the concrete,and the transverse tensile stress and longitudinal tensile stress of the slab track increased with the increase of the gap length interval.At the location of the track slab,the transverse tensile stress of the track slab reached the maximum value when the debonding length was 2.52 m,and the maximum tensile strain was3.73×10-5;at the edge of the track slab,the transverse tensile stress of the track slab reached the maximum value when the debonding length reached2.52 m,and the maximum tensile strain was 1.69×10-5.From this study,the maximum debonding size of the track slab was 3.73×10-5.（4）A coupled vehicle-track dynamics model was established using the large commercial finite element software ABAQUS.The model consists of two subsystems:the multi-rigid body dynamics of the vehicle and the dynamics of the ballastless track substructure.The actual contact conditions are determined by the nonlinear Hertzian contact.In this section,the vehicle model,the wheel-track model and the CRTSⅡslab ballastless track structure are presented.The first section is the vehicle model development.The vehicle model in this thesis consists of a vehicle body,two bogies,four wheelset assemblies and a two-stage suspension system.In addition,suspension springs and punches connect each component of the vehicle.The model has 31 degrees of freedom and lists the parameters specified in the following table.The vehicle model takes into account the mass and pitch moment of inertia of the vehicle body,the mass and pitch moment of inertia of the front and rear bogies,the mass of the four wheel sets,the vertical stiffness and damping of the primary suspension,and the vertical stiffness and damping of the secondary suspension.Section 2 presents the wheel-track contact model.This section uses the finite element software ABAQUS to model the wheel-track model contact.The vehicle and the track are connected vertically by wheel-track contact.The vertical wheel-rail forces are calculated using the Hertzian nonlinear elastic contact theory.The Hertz nonlinear elastic contact equation gives the relationship between wheel-rail forces and elastic displacements,which can be entered into ABAQUS as a force-displacement table.The model treats the wheel-rail tangential contact as pure sliding friction,which is represented in the contact characteristics of ABAQUS by a penalty function with a friction coefficient of 0.3.Section 3 presents the structural model of CRTSⅡballastless track.the structure of CRTSⅡslab ballastless track is built by finite element software ABAQUS.One complete model with a total length of 120 m is simulated with solid elements.The structure consists of five layers,i.e.rail,elastic fasteners,cement asphalt（CAM）and concrete substrate.1.Rail is indispensable and one of the most important components of CRTS Ⅱ slab ballastless track,which plays a connecting role in bearing the forces from above and transmitting them.The rails are made of steel with high hardness and high tensile capacity.In order to obtain more accurate dynamic response results in the simulation analysis,the finite element model uses eight-node linear solid units to model the rail.2.The main function of the elastic fastener is vibration isolation and damping.It absorbs the forces of the rail and wheels in an elastic manner and transmits them downward to the concrete slab.The vibration damping effect is obvious and it is an indispensable part of the whole CRTSⅡslab ballastless track structure.In this model,WJ-8 elastic fasteners are used.WJ-8 fasteners are used to connect the sections between the track and the track slab slabs.Which is simulated by connectors.The fasteners have a spacing of 0.65 m and a vertical stiffness of 35 k N/mm.In the finite element modeling process,the fasteners are simulated in ABAQUS by Cartesian connectors,which are linear in elasticity and damping.The linear elastic parameters are input in three directions and the damping is also input in three directions to achieve constraints in three different directions of freedom.3.Slab track;CRTSⅡslab track is modeled by solid elements.The slab track in this study is composed of precast reinforced concrete of C55 concrete type with design dimensions of 4950 mm in length,2400 mm in width and 200 mm in thickness.The slab is modeled by linear solid elements with 8 nodes.The model was calculated by the ABAQUS explicit solver with a fine mesh of0.1 m.Table 4-5 shows the slab track parameters.4.CA mortar;CA mortar was placed between the track slab and the support layer.Due to its material properties,it is often used as an adjustment layer for the track,which also contributes to the safe and stable operation of trains.After the slab is placed on the CA mortar,the basic function of the CA mortar is to carry the static and dynamic loads caused by the wheel-track interaction and then transfer them down to the subgrade.The mortar has a certain degree of elasticity.CA is mostly classified and modeled as a series of elastic springs that provide the basic elasticity to the track system.CA mortar is a composite material consisting of cement,emulsified asphalt,sand and admixtures.Its design modulus of elasticity is between 100 MPa and 300MPa.In the finite element modeling process,the CA mortar has a length of 4950 mm,a width of 2400 mm and a thickness of 30 mm.It is a linear material,simulated by solid elements with 8 nodes.5.Concrete base slab;The base slab is the support layer,which is the bottom foundation of the track structure.It is made of a hydraulic mixture.The concrete base slab is simulated as a solid element.The strength class of the base slab is C40.the dimensions are based on the actual CRTSⅡballastless track parameters.Section 4 is the model validation.In order to ensure the correctness of this model,the same track structure parameters as the vehicle-track coupled dynamic model in ABAQUS are used for the MATLAB calculation,and the vehicle speed is set to 300km/h to ensure the accuracy of the simulation results.The excitation input of the solid track structure model is the point response of the fasteners obtained from the vehicle-rail coupled dynamic model,and the displacement and acceleration under the same fastener in the middle of the two track slabs are extracted.It can be seen that the acceleration waveforms at the same point of the track slab are practically consistent and show the same tread changes,and the displacement waveforms show consistent changes with essentially the same magnitude.（5）The equations of motion and parameters of the vehicle model,as well as the wheel-rail contact model parameters,are introduced.The structure of CRTSⅡballastless track is briefly introduced,and the structure of each track is set.Rolling,yawing and pitching motions of the front and rear bogies of the vehicle,as well as wheel set parameters are added according to the relevant parameters of the structure.It explains in depth the wheel-track interaction model.The Hertzian nonlinear elastic model is used here to determine the pressure-displacement contact coefficients by studying the connection between the contact forces and the elastic compression between the wheels and the track.This thesis contains 43 figures,8 tables and is well documented by93 references... |
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