| The progress of urbanization has significantly promoted subway construction in China.Although subway trains have brought convenience to residents,their negative influences on the environment along the track,such as vibration and noise,have raised concerns.Previously,ballast tracks were the main structural form of subway tracks because loose ballast bed could effectively reduce the influence of vibration on the surrounding environment.However,local deformation is likely to occur in this type of track beds,which aggravates track irregularity and may even seriously affect track safety.Therefore,ballast tracks have been replaced with ballastless tracks with enhanced structural continuity and regularity.However,the high-strength concrete structure adopted by ballastless tracks is prone to excessive vibration under long-term impact load from trains.Consequently,it is of considerable economic and practical value to either perform appropriate damping measures for the current ballastless tracks or develop new damping ballastless tracks that are practical and economical,to overcome the above shortcomings of ballastless tracks,thereby extending the service life of track structures and improving ride comfort.Based on the analysis and understanding of the characteristics of ambient vibration caused by subway trains and the existing damping measures for the structure of ballastless tracks,this study focused on the construction of an SMA-FRP track structure layer.Based on the theory of layered viscoelastic system mechanics and the principle of vehicle-track coupling dynamics,this study investigated the damping effect of a novel SMA-FRP prefabricated slab ballastless track structure via theoretical deduction,numerical calculation,and model test to reduce the vibration influence of subway trains during operation.The following analyses were performed for this purpose:1.Based on the theory of layered viscoelastic system mechanics,this study established a theoretical analysis model for the layered viscoelasticity of five layers of SMA-FRP prefabricated slab ballastless tracks,which was used to analyze the vertical transfer mechanism of train load theoretically.Subsequently,by converting the rectangular load on the prefabricated track slab to an equivalent circular load,the corresponding viscoelastic theoretical solution was obtained through the elastic-viscoelastic "correspondence rule."2.According to the operational characteristics of subway trains,this study derived a vertical vibration equation with 22 degrees of freedom in calculation for subway vehicles.Subsequently,based on the correlation between the wheel-rail force and the displacement reflected by the wheelset equation,the vibration force of vehicle-orbit coupling vibration is obtained from the displacement compatibility condition to simulate wheel rail force.Subsequently,a simplified equation for the wheel-rail force tailored to this study was proposed,which laid a theoretical foundation for the following numerical calculations.3.A vibration equation for the SMA-FRP prefabricated slab ballastless track was established using modal superposition.Subsequently,by taking the novel SMA-FRP track structure layer as the reference vibrator of the track structure and considering it as a structure between "integrated reinforced concrete" and "loose ballast," a vibration equation of the structure layer of SMA-FRP track was established by introducing shear spring and shear damping between its microcells.4.The numerical calculation model of the SMA-FRP prefabricated slab ballastless tracks was established using the finite element program ABAQUS.Subsequently,using the vertical acceleration and vertical displacement as the key mechanical analysis indexes,this study proposed the optimal topological structure of an SMA-FRP track structure layer.The vibration characteristics of this structure were compared with those of the existing subway prefabricated slab ballastless tracks to identify the damping effect of this novel track structure.Subsequently,the study provided a recommendation on the SMA thickness of SMA-FRP track structures suitable for engineering use.Accordingly,the paper discussed the variation of the damping characteristics of the SMA-FRP track structure under different train speeds.5.From the perspective of qualitative research,the study conducted a subscale test on the damping performance of the SMA-FRP track structure,the result of which was compared with the numerical calculation result,thereby validating the damping effect of the SMA-FRP track structure and the reliability of the numerical calculation model proposed in this study.According to the aforementioned analyses,the major contributions of this study include:(1)By fully utilizing the differences in the stiffness and damping of SMA and FRP materials,a damping measure adopting the SMA-FRP track structure to achieve vibration reduction in subway track structures was proposed for the first time.(2)Using the Newton-Euler method,the vibration equation of two rigid body systems was established,and its validity verified.Accordingly,we considered the operational characteristics of subway trains and comprehensively considered the six degrees of freedom of forward/backward,left/right,up/down as well as pitch,yaw,and roll of trains during their operation,from which we derived the vehicle vibration equation with 22 degrees of freedom in calculation under the condition of small-displacement vibration using the Newton-Euler method.(3)When establishing the vibration equation of the SMA-FRP track structure,it was regarded as a structure between "integrated reinforced concrete" and "loose ballast."Subsequently,using the finite element idea,we introduced shear spring and shear damping between the SMA-FRP microcells to simulate the meshing effect of SMA-FRP structures.Accordingly,the calculation method for the utilization of SMA-FRP as a complete track structure subsystem in the vehicle-track coupled vibration was proposed for the first time.(4)By adopting a method combining numerical analysis with a model test,the study revealed the damping characteristics and laws of SMA-FRP prefabricated slab ballastless track structures and provided a recommendation on the SMA thickness of SMA-FRP prefabricated slab ballastless track structures suitable for engineering use. |
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