Study On Fatigue Crack Propagation And Residual Life Assess-Ment Of Overline Footbridge Based On Sub-model Method

With the rapid development of high-speed rail construction in China,more and more high-speed railway stations are under construction or have been built,overline footbridge as an infrastructure of the high-speed railway station plays an important role in the passenger distribution and ensuring the safety of the passengers.In some three or four tier cities,there exist the situation of that the high-speed train sometimes passes the high-speed railway station directly without stopping in order to improve the running efficiency,when the high-speed train passes the station with a high speed,the overline footbridge will has a large vibration caused by the train wind load.Since the overline footbridge will occur cracks inevitably during the process of production,transportation and welding,these cracks will propagate gradually until break suddenly under the repeated action of train wind load,this poses a major threat to the safety of the passengers,therefore,it is very necessary to study the fatigue crack propagation and the fatigue residual life of the overline bridge under the repeated train wind load in the aspect of ensuring the safety and normal use of the overline bridge,reducing the loss of the national economy and the casualties.Based on the above ideas,taking overline footbridge of Zhuzhou West Station as an engineering background,this paper makes a deep and systematic study on the dynamic response of the overline footbridge,the fatigue crack simulation and the residual life assessment of the overline footbridge when high-speed train passes the bridge.According to the progress and achievements of fatigue crack propagation and residual life assessment on the overline footbridge at home and abroad,and combined with the actual research,this paper mainly studies the following aspects:(1)Establishing the calculation area model and the train model using CFD pre-processing program GAMBIT,and parting the computational region by the sliding mesh technique,then realizing the dynamic simulation of high-speed train passing through the overline footbridge,obtaining the time history of the train wind load acting on the different points of the overline bridge.(2)Establishing overline footbridge multiscale element model by multiscale analysis method based on sub-model method,including the whole shell model of the overline footbridge and the solid model of the cracked beam,and using 1/4 midpoint method to simulate the singularity of crack tip.(3)The dynamic response analysis of the whole shell model and the solid sub-model is carried out,extracting the cyclic stress amplitude at the boundary of the solid sub-model by using the rain flow counting method,then applying the stress on the solid sub-model with different crack lengths(30groups)after grouping,and establishing stress intensity factor library with different crack lengths.(4)Using MATLAB software programming based on Paris formula and fracture mechanics fatigue crack propagation life evaluation theory to calculate the residual life of fatigue crack under different initial crack lengths and different train speeds.Overall,a systematic method for fatigue crack propagation and residual life assessment of the overline footbridge is put forward combing the multiscale finite element method based on the sub-model method and the residual life assessment theory of fatigue crack propagation based on fracture mechanics in this paper,then determining the fatigue life of the overline footbridge under train wind load,it has very important significance in ensuring the safety of the overline footbridge during service and preventing major accidents.