| In recent years,with China’s growing economic power,population to concentrate in large cities and the rapid growth of the motor vehicle number,lead to a surge in demand for smooth ground traffic.The emergence of urban rail transit has effectively improved the situation of traffic congestion on the ground.Chongqing is the first city in the western region to open rail transit,and has been opening new rail lines.The trapezoidal sleeper used by some line stations is a new sleeper,which has many advantages such as vibration and noise reduction,fast construction,etc.Relevant technicians have fully studied its static performance.However,there are few researches on the fatigue performance of sleeper structure under train load and temperature cycle.This thesis mainly takes the trapezoidal sleeper prototype used in some stations as the research object,the fatigue performance of sleeper is analyzed by theoretical calculation and test data,and obtains the fatigue damage law of the trapezoidal sleeper:With reference to other rail transit routes that have been opened in Chongqing at present,combined with the expected passenger flow and vehicle flow,the operation plan of the line where the research station is located is estimated.By Midas/Civil rail-trapezoidal sleeper structure analysis model is established,and the trapezoidal sleeper under moving load bending moment of the envelope and the most unfavorable moment influence line position were calculated to obtain the bending moment history curve of the most unfavorable section.Then,the bending moment amplitude and the number of cycles on the bending moment history curve were extracted by the rain flow counting method and the drainage method.Combined with the estimated operation plan,the bending moment amplitude and the total number of cycles within the 100 years of operation were determined.In view of the extreme high temperature in Chongqing in summer,the influence of temperature gradient was introduced into the sleeper fatigue analysis for the first time.The temperature field model of the track plate was used to calculate The temperature difference between the surface and under surface of the trapezoidal sleeper by using the temperature and radiation amount obtained from the analysis of the local area for many years.The positive and negative trapezoidal sleeper upper and lower surface temperature gradient of each month and the bending moment cycle of the most adverse section were obtained.Through reading a large number of literatures,it is found that most of the previous studies only considered the train load and ignored the influence of temperature gradient on the structure.However,this thesis combined the effect of train load and temperature gradient on the trapezoidal sleeper.Two cases of train load alone and train load plus temperature gradient are considered respectively.Paris formula was used to equivalent the variable bending moment cycle to constant bending moment cycle,and the constant bending moment under 2 million fatigue loads under two conditions was calculated by S-N equation of concrete and Miner’s linear damage accumulation theory,and then the bending moment amplitude was converted into the test load amplitude acting on the trapezoidal sleeper.A three-dimensional solid model of ANSYS Workbench was established to analyze the static,dynamic and fatigue life of the trapezoidal sleeper.The prestress in the sleeper was simulated by cooling and shrinking the prestressed steel bar,and when the tensile stress in the edge of the most unfavorable section reaches concrete tensile strength design value,the concrete tensile strength corresponds to the bending moment.The cracking moment is basically consistent with 41 k N·m measured in the static failure test of trapezoidal sleeper,and slightly larger than 38.15 k N·m calculated according to the code.The calculation results of natural frequency provide a basis for the final determination of fatigue test loading frequency of 3.5Hz.The calculated life of trapezoidal sleeper in the above two cases is in line with the standard requirements.According to the calculation of constant bending moment under 2 million fatigue loads under two conditions,a series of fatigue tests are carried out on 5 pieces of trapezoidal sleeper considering the combination of bending moment classification and number of cycles.Based on only considering the bending moment amplitude of train action,the initial cycles are 2 million,1.5 million,750,000,500,000 and 250,000 times respectively.The bending moment amplitude is successively increased to 1.11 times of the foundation amplitude(combined action of train load and temperature gradient),1.3times,1.45 times,and then continue to carry out fatigue test with different cycles until failure.The variation laws of structural stiffness,neutral axis position of section,representative measuring point strain,fatigue life and damage degree of trapezoidal sleeper were obtained during cyclic loading classification combination until fatigue failure.Most of the previous fatigue tests did not consider the load classification,therefore,it is the innovation of this thesis to use the combination of moment amplitude classification and cycle number to study the damage variation law of sleeper. |
