| At present,China is in the large-scale operation stage of high-speed railway.Ballastless track is one of the key technologies of high-speed railway.In recent years,various damages that have occurred in the ballastless track pose a serious challenge to the normal service of the ballastless track,especially in the area of interlayer fracture that has a great influence on the support state of the track structure.An in-depth analysis of the shear damage mechanism of the interlayer fracture in order to take more targeted repair measures is a frontier research topic related to the long-term safety service of the ballastless track.In this thesis,through the combination of theoretical analysis,model test and mesoscopic simulation and starting from the microscopic structure of the material and the microscopic morphology of the fracture surface,the dynamic evolution process and mesoscopic mechanical mechanism of the ballastless track interlayer fracture are explored.Therefore,the law of fracture development and its influence on structural stability performance can be obtained,which lays a theoretical foundation for the prediction and evaluation of the ballastless track service state.1.The reconstruction and description method of the fracture surface shape of the composite structure in the ballastless track is determined.According to the material and structural characteristics of the composite structure of the double-block ballastless track,the composite structure specimen with track slab/supporting layer was poured.The non-contact 3D imaging technology was used to extract the fracture surface morphology data after initial separation and reciprocated shearing-abrasion.By means of reverse reconstruction engineering,the fine digital reconstruction of the interlayer fracture surface was realized.Based on the geometric morphology characteristics and mechanical shear properties of the reconstructed fracture surface,and combined with the heterogeneous characteristics of concrete and the interlayer state variation of repeated shearing and abrasion,a modified fracture surface morphology characterization parameterθ_a~*/(C+1)was proposed from the perspective of the integrity and accuracy of the characterization and applicability for the research.This parameter takes into account both the directional characteristics of the interlayer shearing and the distribution characteristics of tiny convex parts,which can quantify and characterize the fracture surface morphology after reciprocated shearing more accurately.This parameter can also establish a connection with the mechanical properties of shearing between the layers more conveniently.Meanwhile,the concept of“interlayer conformal ratio”is proposed to describe and quantify the interlayer matching degree,which provides a prerequisite for the analysis of the damage characteristics and void evolution of the interlayer.2.Theoretical and experimental exploration of the fractured interlaminar damage mode and shear capacity of composite structures are carried out.Based on the mesoscopic morphology of the reconstructed fracture surface and the heterogeneous characteristics of the concrete composite structure,the shear damage mode and shear capacity of the interlayer fracture surface were explored from the aspects of the model test and theoretical analysis.(1)It is clear that the shear capacity of the interlayer is the result of the coordination of“convex-part fracture”and“surface friction”.Based on this,the formula of the interlayer peak shear force was deduced,and the quantitative distinction of the composition of the interlayer shear capacity was realized.(2)The N-V_N curve that describes the relation of the interlayer shear force with the normal constraint load does not start from the zero point theoretically.The starting value varies due to the fracture surface morphology.Based on this,Barton and Grasselli’s interlayer shear model can be locally modified.(3)The shear force transfer coefficient of the interlayer varies with the normal load and fluctuates in the range of 1.1～1.8.The fracture surface after bidirectional roughness treatment has a better shear force transfer ability.(4)There is a good positive correlation between the fracture surface morphology parameterθ_a~*/(C+1)and the peak shear force between the layers.The shear damage and morphology degradation of the interlayers derive from the shearing and cut-off of the tiny steep convex parts.There is faster damage and a larger variation of the fracture surface morphology on the fracture surface after bidirectional roughness treatment for the interlayer during the shearing process.3.The development law and mesoscopic mechanical mechanism of interlayer fracture under repeated loads are clarified.Through the reciprocated shear model test after the initial fracturing of the composite structure interlayer,the evolution law of the interlayer contact state with the number of loads was discussed,and the variation law of the force transferring and damage characteristics of the interlayer under different contact states was analyzed.(1)The variation of the interlayer contact state stems from the continuous accumulation of the damage on the fracture surface.The shearing-abrasion of the interlayer can be divided into three stages:rapid abrasion,stable abrasion and residual abrasion.(2)Tiny convex parts being cut off is the main source of interlayer damage in the preliminary stage of shearing.With repeated shearing processes,surface friction gradually becomes the main source of the shear capacity.Based on this,the evolution formula of the shear capacity of the interlayer under repeated shearing processes is deduced.Moreover,the corresponding relationship between the number of loads or the roughness of the fracture surface and the mechanical behavior of the interlayer and the contact state is established.(3)The interlayer fracture can be regarded as a special thin layer that can only withstand the pressure and shear force but the tension.The stress-displacement evolution behavior of the interlayer in different directions can be described by the interlayer conformal ratio,the morphological parameter and the height parameter.The stress-displacement constitutive equation reflecting the interlayer fracture evolution characteristics can be established to predict the mechanical behavior of the interlayer under different contact states and abrasion degrees.4.A mesoscopic finite element analysis of the shear characteristic and damage mechanism of interlayer is carried out.Based on the micro-structure of the material and the micro-morphology of the fracture surface,the micro-finite element model of the composite structure with fracture surfaces was constructed,and the mechanical behavior and progressive damage characteristics of the interlayer fracture in the shearing process were explored.(1)By embedding zero-thickness cohesive elements in all the original finite elements,the simulation of progressive damage of the interlayer during the shearing process,such as the convex-part fracturing,fracture extension and coalescence,and material shedding and migration,was realized.(2)The shearing process of the interlayer fracture can be divided into four stages:pre-peak linear stage,pre-peak nonlinear stage,post-peak softening stage and residual shearing stage.The pre-peak nonlinear stage and post-peak softening stage are the main stages of interlayer damage initiation and development.The peak shear stress occurs when the number of damaged elements and the shear expansion increase the fastest.(3)The normal load changes the initial embedding and occlusion degree of the interlayer,while the interlayer contact state changes the contact area and stress of that.They both have a significant influence on the shear performance,damage degree and dilatancy behavior of the interlayers.(4)The interlayer damage is mainly shown as the shear damage,and the tensile damage is subsidiary.The tensile fracture energy in the entire shearing process is maintained in the range of 10～20%of the total fracture energy. |
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