Study On Crack Mechanism Of Ballastless Track And Its Impact On Safety Of Train Operation In Beijing Metro

ABSTRACT:Recently, the subway system as a type of public transit is increasly important with the rapid development of cities. The metro has many advantages such as large traffic volume, high speed, less pollution, no level crossing with the urban road, prompt, security and comfort. In big cities of China especially in Beijing, the metro are burdened with heavy operating tasks. Track structure as bearing foundation of the vehicle will inevitably result in the damage under wheel friction, vibratiton and impulsion. The track structure injuries include rail wear and breakage, fastener missing and elastic rod breakage, sleeper and track bed concrete cracks, foundation sinking and so on. Among the many injuries of track structure, track bed concrete cracks is rarely concerned by the manage department of metro. A large amount of track beds cracks exist in Beijing metro, especially in older rail lines such as line land line2. The concrete cracks will weaken the track capacity, reducing the durability and insulation properties, cause irregularity and affect ride comfort and operating safety. So the problems of track bed craks should be sovled without delay. In this paper, the real situation of track bed crack in Beijing metro is investigated and the theoretical studies are done mainly from the following aspects:(1) Firstly, a survey about the injuries of ballastless track in Beijing metro has been done, and the injuries are classified in according to crack and non-crack injury. The crack injury of ballastless track is mainly due to the construction technology and quality, temperatureand concrete contraction and vehicle load. In the meantime, the mitigation measures of track bed crack adopted in Beijing subway have been analyzed and deficiencies have been pointed out.(2) Finite element model of track bed has been established and vehicle loads and temperature loads are applied on it. The stress distribution of track bed under the above loads can be abotained from this model. Then determine the location of the crack initiation by comparing the size of the stress of track bed. The vehicle loads considered are based on the actual situation and the train line conditions. Finally, we can conclude that the location of the crack initiation is located in the corner of thesleeper and extended along the45°Which train loads to be considered based on the actual situation and the train line conditions in three conditions. Through analysis to determine the final location of the crack initiation is located in the corner of the pillow, and extended along the45°direction.(3) Finite element model with the initial crack is established by using the theory of linear elastic fracture mechanics of concrete structures. XFEM is used to simulate the crack development. The values of crack opening displacement (COD) and stress intensity factor (SIF) are obtained from the model with a length of20mm crack. With the expansion of the crack, we can get the relationship between the crack length and COD and SIF.(4) On the basis of damage mechanics, a vehicle-track coupling model with crack injury is built to conduct dynamic analysis. Damage function is introduced to simulate the track bed crack. Comparing dynamic response of track with crack and track without crack, we can conclude that track bed crack makes structural dynamic response of the track increases, particularly for the concrete track bed and its following structure. So the track bed cracks impact the ride comfort and even the operating safety.