On Nonlinear Mechanical Characteristics Of Ballastless Track Interface Under Train And Temperature Loads

Ballastless track is a modern technology adapted to the construction and operation of high-speed railways,which comprehensively improves the smoothness and stability of railway lines.As a typical multi-layer composite structure,the ballastless track is a solid whole with each structural layer closely connected by physical or chemical means to achieve rapid load transfer and diffusion as well as deformation coordination of adjacent material layers,among which the mechanical properties of interlayer interface play a crucial role for the service condition of ballastless tracks.Long-term operation practice indicates that the deterioration of interlayer bonding performance represented by the track slab–mortar,track slab–self-compacting concrete,etc.has become one of the most common diseases,which alters the force transmission characteristics of multi-layered composite structure,deteriorates the structural load-bearing state,affects the smooth of railway lines in severe cases and thus jeopardizes the vehicles’ running safety.What’s more,the weak granular ballast bed is removed from the ballastless track jointless line,and the longitudinal resistance of railway lines mainly comes from the rail–rail pad contact interface.Although the track buckling phenomenon is effectively avoided,however,longitudinal deformation incoordination and local stability problems still emerge occasionally.The longitudinal deformation accumulation and axial additional stress residual of the continuously welded rails(CWR)under cyclic loads are often accompanied by a change in the stress-free temperature,which poses a potential threat to the stability and service life of CWR.With the continuous increase in the driving speed and more complicated service environment,the deformation coordination and retention of mechanical properties of the interlayer interface of ballastless tracks will face more serious challenges.On the one hand,the longitudinal interaction between the interlayer interfaces will be intensified under frequent traction or braking operations,especially in perilous mountain lines characterized by complex and large gradient sections.Meanwhile,the multi-source(train,temperature,etc.),multi-dimensionality and non-linearity of external loads lead to more complex static and dynamic coupling mechanisms between trains and railway lines,railway lines and the environment.On the other hand,accurate evaluations and predictions of the evolution of the interfacial service state also encounter insufficient understanding of the mechanical behavior of the interface and difficulties in characterizing the constitutive relation and numerical solution.To this end,funded by the major project of the National Natural Science Foundation of China entitled “Evolution of dynamic performance,damage mechanism and control of high-speed rail transit infrastructures(11790283)”,the National Natural Science Foundation of China entitled “Research on damage identification and safety assessment method of high-speed ballastless track based on CNN and probability density evolution(51978587)”,the National Natural Science Foundation of China entitled “Study on fatigue cracking expansion mechanism and evolution model of ballastless track interface under cyclic complex loading(51708457)” and the Cultivation Program for the Excellent Doctoral Dissertation of Southwest Jiaotong University(2020YBPY01),this dissertation takes the bonding interface between track layers and the contact interface between CWR and rail pad as the breakthrough points,focuses on the core issues involving the influence mechanism of the longitudinal coupling effect of high-speed trains and three-dimensional(3D)elastic ballastless track on the dynamic interactions,the failure mechanism of the interlayer bonding and performance maintenance of CRTS Ⅲ ballastless track,and the characterization of friction mechanics behavior of rail–rail pad contact interface and the longitudinal service state evolution law of the jointless line,integrates multiple disciplines including vehicle–track coupling dynamics,railway engineering,vibration theory,damage mechanics and heat transfer theory,as well as multiple technical means including theoretical studies,numerical simulations,indoor experiments and field tests,and the following research work has been carried out:(1)Taking the classical vehicle–track coupled dynamics as the theoretical framework and introducing more refined ballastless track modeling theories,a train-3D elastic ballastless track dynamics model considering the longitudinal coupling effect is established.The "longitudinal coupling effect" on the one hand refers to the influence of the longitudinal absolute velocity difference between the wheel and rail at the contact point on the wheel–rail creep force,on the other hand,refers to the consideration of longitudinal interactions between vehicles and track layers induced by the introduction of longitudinal degrees of freedom.The "3D elastic" aims to emphasize that the physical models of track structures are formulated based on the theory of linear elastic continuum dynamics,which comprehensively considers the three-dimensional elastic mechanical characteristics of track structures,and breaks through the limitation of the neglect of the longitudinal vibration and rigid body assumption for the lateral motion in the traditional modeling method of ballastless tracks.In addition,due to the introduction of new degrees of freedom,the interaction relations among subsystems are also supplemented or re-derived.(2)Through a large number of numerical examples and comparisons with the results of classical models,existing literature and commercial software,the accuracy and superiority of the analytical solution methods and the developed train–track coupled dynamics program are demonstrated.The dynamic response characteristics,difference boundaries and application scope of different theoretical models are revealed from the perspective of numerical stability,shear effect and rotary inertia,curvature effect,3D elastic mechanical characteristics,etc.,and as a result,the general principles of dynamic modeling of track structures are presented,which lays a foundation for the efficient and accurate evaluation of train–track dynamic interactions.(3)To address the nonlinear interaction between track layers and the deterioration of bonding performance due to train and temperature loads,the non-homogeneous unsteady heat conduction differential equations of the ballastless track correlated to the geographical and meteorological factors are established first,and the analytical solution and distribution characteristics of the time-varying temperature field are obtained by the integral transformation method and verified by the measured data.Subsequently,an efficient method for solving small-deformation-based nonlinear static problems is creatively proposed to solve the deformation and interface responses of track structures induced by thermal loading.Finally,the deterioration process and failure mechanism of the interlayer bonding performance between CRTS Ⅲ track slab and self-compacting concrete due to train and temperature loads are revealed,and the optimized layout of the door-type reinforcement is proposed to effectively inhibit the initiation and expansion of the interlayer damage,which is a feasible technical solution for maintaining the interlayer mechanical performance of ballastless tracks under complicated and harsh service environment.(4)Aiming at the friction mechanical behavior of the contact interface between the rail and rail pad as well as the longitudinal nonlinear interactions between CWR and substructures due to train and temperature loads,we first characterize the non-linear mechanical properties of fastener longitudinal resistance considering the hysteresis effect and loading history via improving and extending the classical Dahl friction model.Subsequently,the accuracy of the proposed model and its superiority over the traditional models are illustrated based on an experimental study,and the algorithms for the longitudinal resistance under complex load conditions are developed.Finally,the stress/deformation residual and accumulation phenomena of CWR in different high-speed railway sections(subgrade,tunnel transition and bridge)under train and temperature loads are revealed and the evolution process is predicted,which provides theoretical guidance for improving the design and maintenance strategies of ballastless track CWR.