Numerical Analysis And Research On Vibration Fatigue Of Automotive Rear Torsion Beam

When structure is suffering from dynamic load which is close to the naturalfrequency, stress level is lower than the static fatigue stress limit in the project. But itmay produce crack. In other words, the actual life of the structure is lower thanprediction results on the basis of the static fatigue analysis method. It is one ofproblems difficulty resolved by the traditional theory of fatigue life analysis so far.This paper has introduced the basic concept between vibration fatigue and thetraditional ordinary fatigue, and pointed out the primary difference whether theexcitation frequency covers work modal frequency between vibration fatigue and thetraditional ordinary fatigue. Aiming at vibration fatigue damage problem on reartorsion beam when a certain type of vehicle with rear torsion beam suspension is inservice condition, a method used to predict vibration fatigue life of complex structurewith rigid-flexible coupled was proposed in the paper. Based on vibration fatigueanalysis of a rear torsion beam which was a typical chassis component in a passengervehicle, key techniques for vibration fatigue analysis were described by usingmulti-body dynamics, finite element method and the strain operational modal test andanalysis, including load identification, operational modal analysis, numericalprediction method of dynamic stress and the influence analysis of dynamic stressfrom excitation frequency.1、The finite model of rear torsion beam was established and its free modalfrequencies were acquired. The three possible boundary conditions were studiedaccording to assembly relationship of rear torsion beam in working status. Theoperational modal parameters of rear torsion beam were identified by using thestochastic subspace method based on the strain modal test platform. Studies haveshown that the boundary condition of structure in the state of serving is morecomplicated.2、In order to solve the problem of insufficient data acquisition channel in largescale testing, the strain signals on structure surface were tested by two-direction straingauge instead of three-direction strain gauge. The principal stress calculation formula of thin-walled structure was conducted based on the strain gauge. The U-shapedthin-walled structure specimen was designed and processed. The numerical principalstress was verified by surface strain testing system of the thin-walled structure. Thestudy has shown the principal stress of thin-walled structure calculated based on thestrain gauge can reflect the real stress level, and provide reference for subsequentengineering application. Then the research result was applied into stress analysis ofrear torsion beam, and the correctness and precision of the method was validatedthrough the real vehicle road test.3、In the dynamic analysis of rear torsion beam, the nonlinear characteristics ofrubber lining tube in spindle head were considered. Based on displacement signal ofthe suspension spring on the real vehicle road test and multi-body dynamic model ofthe torsion beam with rigid-flexible coupled via the ADAMS of dynamic simulationsoftware, the load of left and right spindle noses of rear torsion beam were simulatedvia iterate. In order to verify the accuracy of the method, the load of spindle nose wasverified based on the test bench. Experiment has shown that the simulation resultshad high accuracy and the error was in10%.4、The quasi-static method, the direct-integral method and modal-superpositionmethod used to analyze the dynamic stress of structure were presented. And thedynamic stress of the key parts of transom was paid much attention. In the analysis ofdynamic stress, the relation between load frequency and the operational modalfrequency of structure was especially studied. The applicability of the method ofvibration fatigue analysis was analyzed. Finally the calculation results ware verifiedon the bench test of whole vehicle. Tests has showed that the quasi-static method isdifficult to directly express the actual dynamic stress of structure, but the modalsuperposition method and direct integration method can accurately describe thestructure dynamic stress.5、 Considering with the influence of material fatigue characteristic fromstructure size and processing technology, the S-N curve of part was obtained byamendment. The fatigue life analysis of rear torsion beam in time domain method ondifferent road was calculated by using the quasi-static method, the direct-integralmethod and modal-iteration method. And analysis results were verified by bench strain test. Test has showed that the quasi-static method can not accurately predictvibration fatigue life of structures, but the direct integral method and modalsuperposition method can accurately predict vibration fatigue life of structure.6、The frequency range and energy distribution information of load spectrum ofspindle nose was given through spectrum analysis of load of spindle nose. In order toget the frequency response characteristics of structure, a method used to impel the teststructure on spindle nose by way of sine sweep method in indoor test bench ispresented. Based on method of Dirlik, the fatigue life of rear torsion beam waspredicted by using fatigue frequency domain analysis method. The fatigue life of reartorsion beam was analyzed in the whole road, resonance-three section and therock-road. In order to study the influence of the dynamic stress and fatigue life fromload frequency, the zoom factor is presented, which is used to the contribution of thedynamic stress from resonance excitation frequency. The results have shown that thefatigue damage of structure from resonance excitation is about four times from thesame energy non-resonant incentive.