Research On Vibration Fatigue Analysis Method Of High-speed Train Carbody Under Random Loads

As the speed of trains increases,the frequency range of the load is continuously expanded and the energy of the load is increased.Aluminum alloy is adopted in carbody lightweight design,which has changed its own modal characteristics.In this case,the design method of vibration fatigue is conducive to the discovery of the problem.So as to improve the dynamic fatigue resistance design of the carbody and its fatigue durability.Firstly,the vertical multibody system dynamic model of a certain vehicle is established and each response spectrum is calculated according to the track irregularity spectrum in the standard,and then the frequency response function of each response of the dynamic model is calculated.The vertical load spectrum of the carbody is obtained by frequency-time domain transformation based on the power spectral density(PSD)of the secondary suspension force.After that,based on the traction and braking characteristics of the high-speed train motor and the unit running resistance of the high-speed train,the speed-time curve under the typical working conditions of the train is calculated,and then the longitudinal load spectrum of the vehicle body is derived.The speed-time curve of the train under a typical working condition is calculated,and the longitudinal load spectrum of the carbody is obtained.The upper and lower limits of the analysis frequency during the compilation of the carbody load spectrum have a great influence on the simulation result of the time domain signal.When the lower limit of the analysis frequency is selected to be smaller,the simulation result has obvious wideband signal characteristics.When the lower limit of the analysis frequency is selected to be larger,the simulation result has obvious narrowband signal characteristics.And different track irregularity grades have a great influence on the vertical load of the carbody.After that,the finite element model of the carbody is established.Based on the inertial release method,a finite element boundary condition suitable for the calculation of carbody strength is discussed.Two kinds of boundary conditions are used to calculate the static strength of the carbody.The correctness of the boundary conditions used in this paper is verified by the comparison of two results.The modal simulation results of the carbody are in good agreement with the test results,and the correctness of the modal simulation and the carbody finite element model is verified.On this basis,the suspension stiffness of the carbody hanging equipments is further calculated.The rigid connection and the elastic connection finite element model of the hanging equipments are established and analyzed respectively.The rationality of the hanging equipment elastic connection carbody mode is verified by the comparison of carbody modes and the hanging equipment elastic connection carbody mode is used in the modal superposition method.The modal superposition method is used to evaluate the vibration fatigue damage of the carbody.Firstly,the effect of modal truncation on the calculation accuracy of carbody fatigue damage is calculated and analyzed.The error analysis results show that the maximum error caused by choosing the 25 th mode as the truncation mode is 2.5%,which meets the engineering analysis accuracy requirements.The error analysis results can provide reference for calculating the carbody modal response by modal superposition method.Furthermore,the effect of longitudinal load on vibration fatigue of carbody is analyzed.The results show that the longitudinal load has a great influence on the damage of underframe,and the maximum error caused by ignoring the longitudinal load is up to 63%.The analysis of the effects of each mode on vibration fatigue of the carbody can be concluded that the modal viberation of 1st,8th,12 th,and 24 th order modes have a greater impact on the vibration fatigue of carbody.By further comparing and analyzing the modal parameters of each mode,it can be found that the modal frequency,the modal displacement response range and the modal stress are the three main parameters that characterize the effect of the mode on the structural vibration.Through the optimal design of the structure,the effect of harmful modes on carbody vibration fatigue can be eliminated or reduced,and the fatigue durability of the carbody can be improved.Finally,the modal superposition method is used to evaluate the vibration fatigue life of carbody base metal and weld line based on nominal stress and structural stress.The analysis results show that the fatigue dangerous zones have a good consistency with the actual failure position of the carbody in the existing literature,which verifies the effectiveness of the vibration fatigue analysis method.In addition,the structural stress method and the nominal stress method are used for life assessment of the weld line of the carbody.The structural stress life prediction results are smaller than the nominal stress results.The reasons are the dispersion of weld line S-N existing in the nominal stress method is eliminated in the structural stress method and the stress calculation in weld toe section is more accurate in the structural stress method based on grid insensitive.The fatigue life of the carbody calculated by the modal superposition method is more in line with the actual service life of the carbody,which provides a reference method for the evaluation of the fatigue life of the carbody.