Research On The Static And Dynamic Property Of Ballastless Track Based On Fracture And Damage Mechanics

Ballastless track has become the main type of high-speed railway infrastructure due to its advantages such as high smoothness, high stability and low maintenance. However, damaged cracks of the main concrete structure of the ballastless track was unavoidable taking into account a variety of factors such as the vehicular load or temperature influences, and using elastic beam or slab to simulate the track could not reflect its actural state. In this thesis, on the basis of study at home and abroad, static and dynamic property of ballastless track with damaged cracks was researched by the theory of fracture mechanics, wheel-rail system dynamics, damage mechanics and finite element method. The research work and main conclusion were divided into following areas:(1) Classify the damage of ballastless track main stuctureThe damage of track main structure of China's first ballastless track experimental section was investigated; the damage of slab, bi-block and continuous ballastless track were classified respectively. The characteristics and rules of the damage of track main structure were summarized, and the reason of the damaged cracks was analyzed.(2) Establish the spatial finite element method of ballastless track preliminarily based on fracture mechanicsAccording to the structural characters of ballastless track, a spatial finite element method of ballastless track with damaged cracks was established with finite element method based on the theory of elastic fracture mechanics. Rail was simulated by Euler beam. Concrete and longitudinal bars of track bed were simulated as 20-node hexahedron isoparameteric element and truss element, respectively. Constraint of the crack's opening by bars was also considered. The singularity of crack tip was simulated by 20-node hexahedron isoparameteric singular element. A calculating program of the method was coded and verified by some known results.(3) The mechanical analysis of ballastless track with damaged cracks under vehicular loadFirstly, the mechanics of ballastless track without crack under vehicular load was given, and then for the same track but with full-depth crack of 20mm at the track bed bottom face, some cracking parameters at the crack tip such as the concrete stress, stress intensity factor (SIF), crack opening displacement (COD) and bar stress were calculated. The effects of crack depth, crack location and vehicular location on the cracking parameters were studied. Finally, the crack stability was evaluated according to the fracture toughness of concrete. The results show that the crack has little effect on the global stress and deformation of the track, and the constraint of the crack's opening by bars is significant. The crack is not likely to propagate by general vehicular load. The bar stress at the crack has a sudden increase, and increase with crack length increasing. The most unfavorable crack for the track mechanics is that at the middle of the track bed, and the crack belongs to opening mode crack.(4) The mechanical analysis of ballastless track with damaged cracks under temperature gradientThe mechanics of bi-block track with full-depth crack at the track bed top face under minus temperature gradient were calculated, and SIF, COD and bar stress at the crack tip were also given. The effects of crack depth, crack location and temperature gradient magnitude on the cracking parameters were studied, and the crack stability was also evaluated. The results show that the constraint of the crack's opening by bars under temperature gradient is not significant. The crack is likely to propagate cause of temperature gradient. The bar stress at the crack has a sudden increase, and it increase with crack length and temperature gradient magnitude increasing. The most unfavorable crack for the track mechanics is that at the middle of the track bed.(5) Dynamic analysis of ballastless track with damaged cracksAccording to the crack characters of the bi-block track bed, a damagefunction was introduced to simulate deterioration of the bending stiffness, the dynamic coupling analysis method of vehicle and bi-block ballastless track with cracks was established. The dynamic responses of track were given, and the effects of vehicle velocity and crack location on the track dynamic responses were also studied. The results show that crack has little influence on the vehicle and rail vibration responses, while more on the track bed and substructure. Dynamic bending stress of track bed and dynamic stress of subgrade increase obviously with the increase of vehicle velocity. The crack at the middle of the track bed has more effect on dynamic bending stress of track bed and dynamic stress of subgrade than crack at other location.