Research On Mechanical Properties Of Sleeper Based On Digital Image Correlation And Finite Element Analysis

The sleeper plays an important role in the track structure.In order to ensure its normal function,it is required to keep the structure intact under various complex environments,different axle loads and speeds,so as to ensure the safety of the railway and the comfort of the passengers.However,the sleeper in the actual railway is in the bending state under the load and the ballast,because of rail defects,transmitted to the sleeper load increase.Under the long-term action of these loads,the sleeper produces cracks,even destroys and affects the safety of the railway line,increases the maintenance cost,causes rail damage,derailment and other hazards.In recent years,with the enhancement of people’ s environmental awareness and the development of new materials,composite sleeper and fiber reinforced concrete have emerged.The new material provides a new method for strengthening the design bearing capacity of sleeper and reducing the cost of railway maintenance.The research on mechanics and crack resistance of sleeper produced by new material is less.In addition,the comparison of sleeper crack simulation is also relatively lacking.Based on digital image correlation,this paper analyzes the stress and crack development characteristics of prestressed concrete sleeper,FFU composite sleeper and bamboo fiber concrete sleeper through three-point bending test.Secondly,combined with the test results obtained by three-point bending and digital image correlation technology,the concrete damage plasticity model,brittle cracking model and extended finite element model are compared and analyzed to determine the mechanical model suitable for the static analysis of sleepers.Finally,a reasonable model is used to analyze the mechanical properties of the sleeper under the change of the support caused by tamping,migration and deterioration of the ballast,and to compare the bending moment under the design load specified in the Chinese standard.The main results and conclusions are as follows:1.Mechanical and crack characteristics of prestressed concrete sleeper,FFU composite sleeper and bamboo fiber sleeper were tested by three-point bending test based on digital image correlationWith the development of new materials and technologies,people ’ s awareness of environmental protection and railway transportation put forward higher requirements for the force of sleepers,so that composite materials are applied in railway,called composite sleeper.At the same time,with the research of concrete technology,fiber concrete appears.The addition of fiber can change the mechanical properties of concrete,bamboo fiber can be added to concrete and produce sleepe.At present,there are many researches on macroscopic cracks of concrete sleepers at home and abroad,but less in composite and other materials sleeper.In this paper,the deflection,stiffness,crack propagation form and crack opening displacement of prestressed reinforced concrete sleeper,FFU composite sleeper and bamboo fiber sleeper were tested by digital image correlation technology combined with three-point bending test.The results show that FFU composite sleeper is always in elastic state,mainly transverse crack,crack opening displacement is small;The bamboo fiber sleeper and prestressed concrete sleeper are elastic at the beginning,with the increase of load,the stiffness decreases and the increase rate of crack opening displacement increases.The early strength of bamboo fiber sleeper is higher than prestressed concrete sleeper The addition of bamboo fiber can improve the ductility of sleeper,but due to the poor bonding performance of bamboo fiber and concrete,the subsequent strength is lower than that of prestressed concrete sleeper.2.Comparative analysis of concrete damage plasticity model,brittle cracking model and extended finite element model with digital image correlationAlthough most studies have proposed the simulation method of concrete sleeper,the analysis of micro-cracks and crack path is less or more complex,Only the loaddeflection curve is used to verify the model,which cannot explain the accuracy of the model in simulating the crack of sleeper.Digital image correlation technology provides a basis for checking various damage and cracking models in FEM.In this paper,the three-point bending models of the sleeper are established by using the concrete damage plasticity model,the brittle cracking model and the extended finite element model,respectively,and the results are analyzed and compared with those obtained by the three-point bending.The conclusions are as follows,the concrete damage plasticity model can accurately simulate the displacement results,and the damage location is determined in advance through the damage cloud image,which is a continuous medium,but the damage description is not clear enough.The brittle cracking model and the extended finite element model regard the structure as linear elasticity before cracking,so that the displacement simulation is not accurate enough and can only describe the crack situation after cracking,which is relatively clear.The brittle cracking model structure is also a continuous medium,and the extended finite element model structure is a discontinuous medium.3.The mechanical properties of sleepers with different supports are studied by damage plasticity modelIn practice,the interaction between sleeper and ballast is very complex.Under the action of upper cyclic load and other factors,settlement,degradation and migration often occur in ballast bed,which makes the support state uneven and constantly changing.The unfavorable supporting state will cause sleeper cracking and even destruction.At the same time,the deterioration of ballast is intensified,forming a vicious cycle,affecting the safety of the railway and the comfort of passengers.In this paper,the stress and damage of the sleeper under 9 supporting conditions and the damage without supporting at railseat under cyclic loading are analyzed.The main conclusions are as follows: with the decrease of the support in the mid-span of sleeper,the positive bending moment at the railseat is basically unchanged,and the bending moment at the mid-span decreases and becomes positive from negative;With the decrease of the support at the railseat,the negative bending moment at mid-span and the positive bending moment at railseat increase.With the decrease of the support at the sleeper end,the negative bending moment at mid-span is increased,and the bending moment of the section under the rail is changed from positive to negative.The decrease of this support is the most unfavorable to the force of the sleeper.When there is no support under the railseat,the failure form of the sleeper is the tensile crack at the bottom of the sleeper,and then the damage inside the sleeper,and the internal damage expands along the width of the sleeper.