Research On The Load Spectra Establishment Method Of A Subway Bogie Frame

In China,the real life of some subway vehicle bogie frames is far less than the designed life due to fatigue problem.The root cause is that due to the objective factors of the complex and changeful subway operating condition,large amount of passengers and lacking of maintenance.Because of these particularities,the current international standards for bogie frames design cannot fulfill the real requirement in China.Therefore,bogie frame load spectra which can reflect the real condition of China are very important references for subway vehicle design which in turn ensure its operational safety.The research object in this paper is a B-type bogie frame of a real subway line.The main focuses of this paper are the load identification and generalized load spectra derivation of the subway vehicle bogie frame.The main work in this paper is as follows:(1)Carry out the load decoupling identification and fatigue strength evaluation of the bogie frame.Firstly,analyze the force type of the bogie frame and establish test plan for each kind of load.Then,find the load identification positions where each kind of load is decoupled from other kinds of load through experiments,and obtain the “load-stress”transfer coefficients at the load identification positions by indoor calibration.At last,fulfill the identification of the load-time histories of all kinds of load.At the same time,measure the stress data at typical fatigue positions in order to carry out fatigue strength evaluation.The measured stress value is also used as the basis for the calibration of load spectra in the end.(2)Reducing the amplitude amplification effect from the load measurement signal with elastic vibration is very important to generate a generalized load spectrum.When analyzing the load measurement signal in time domain and in frequency domain together with the finite element modal analysis,it is clear that the bogie frame resonated during operation.As a consequence,the measured load signal has an obvious amplication effect compare to the real external load.This fact has not been considered in load spectra research of the bogie frame so far.Instead,the relationship between measured load signal and the real external load is considered to be independent of frequency and modal,resulting in a load spectrum which can only be used for analyzing the same bogie frame.It is difficult to solve the problem of amplitude amplification in load identification test.In this paper,a semi-adaptive load data post-processing method is raised,which can effectively reduce the amplitude of the load response signal amplified by the elastic vibration.The processed load measurement data will be closer to the real external load,which decouples the load spectrum with the modal characteristics.This is the first attempt to study the amplitude amplification caused by elastic vibration in load spectra research of a bogie frame.(3)Due to the limited measurement length in most engineering applications,load extrapolation is needed.Two load spectrum extrapolation methods are introduced:1)Load spectrum extrapolation based on diffusion-based kernel density estimation.The highly accurate exrtrapolated load spectrum can be achieved by using the diffusionbased kernel density estimation combining with transcendent probability method.The accuracy of this method is verified in terms of various criterion,including the goodness of fit of the probability density curve,damage calculation and the estimated maximum load amplitude in the life cycle.2)Improved standardised cumulative frequency load spectra and its extrapolation method.This method solves the problem in traditional standard cumulative frequency load spectra that the error caused by removing undamaged load is not considered.The accuracy improvement of this method is proved by the real cases.The advantages and disadvantages of the above two extrapolation methods are compared and analyzed.(4)After load spectra extrapolation,a genetic algorithm is used to execute the damage consistent load spectra calibration.Furthermore,the calibrated optimal load spectra of different kind of loads are combined from the diffusion-based kernel density spectra and the improved standard cumulative frequency load spectra.In the end,generalized full life cycle load spectra of full kinds of load that can cover all critical fatigue damage of the bogie frame is derived.The accuracy of the derived load spectra is proved comparing with current international standards.The load spectra derivation method could serve as a good reference for China specific subway vehicle design and fatigue strength assessment.