Load Spectrum Extrapolation Of Vehicle Front Sub-frame And Fatigue Damage Analysis Based On Ground Durability Test

As an important load-bearing part of automobile chassis system,fatigue failure is the main failure mode of the front sub-frame,which usually durability test verification.Due to the shortcomings of long period and high cost in real vehicle durability test,the full-life load spectrum of components is obtained by extrapolation of measured load parameters,which is used for durability simulation evaluation and bench test.However,there are some limitations in the description of load distribution characteristics by parameter extrapolation,which seriously affects the accuracy of load fitting and fatigue life estimation.Therefore,based on the characteristics of load distribution and the fitting of load data,this paper selects the parameter method and the non-parametric method to extrapolate the load spectrum,and analyzes the applicability of the two extrapolation methods from the point of view of goodness of fit and probability map distribution.First,combined with the proving ground durability road test,multi-body dynamics and finite element simulation,the stress spectrum of the dangerous point of the front sub-frame is extracted as the basic data of the extrapolation method.The model parameters are obtained and the multi-body model of the vehicle is constructed and verified,including the hard point and the quality characteristic parameters of the parts are obtained by scanning the chassis and parts,the centroid coordinates of the test vehicle are calculated by longitudinal elevation method,the finite element simulation model of the front sub-frame is verified by comparison of test and simulation.By comparing the K&C characteristics with the dynamic simulation and test of shaking table,the precision is verified from two aspects: static and dynamic response of the whole vehicle.The multi-body simulation of the vehicle is carried out based on the signal obtained by real vehicle test l on the durability road in the proving ground.The damage cloud map of the front sub-frame is obtained by displacement backloading method and modal superposition method,and the time domain signal of the stress at the dangerous point is extracted.The adaptability and accuracy of the load spectrum extrapolation method are discussed,the stress spectrum distribution are fitted by parameter method and non-parametric method,and the extrapolation method is compared and analyzed from the angles of goodness-of-fit and probability map distribution respectively.The goodness of fit of mean value and amplitude of parameter extrapolation is 96.6% and 97.5% respectively.the mean value is consistent with the fitting function in the [-80,54],and there is an error in the description of the mean distribution in the other interval fitting functions.the small load,the other parts of the amplitude data agree with the description of the fitting function.The goodness of fit of equivalent amplitude extrapolated by non-parametric method is 98.6%,and the analysis of probability graph shows that the fitting function is very consistent with the characteristics of equivalent amplitude distribution.Based on the above comparative analysis,the non-parametric extrapolation in complex road conditions can better describe the distribution of load spectrum.The extrapolation spectrum is obtained based on the optimal probability density function and divided into eight stages.According to the phenomenon of small load cycle in the eight stage spectrum,Haibach model and NASA model are used to estimate the fatigue damage of of front sub-frame.The comparison shows that the proportion of small load damage based on NASA model is obviously higher than that of Haibach model,which results in the fatigue life of NASA model is less than that of Haibach model,the model is conservative,which is beneficial to the reliability design of components.Haibach model is more suitable for lightweight design of components.The results show that the non-parametric extrapolation endurance road load spectrum is more accurate than the parameter extrapolation method,which can provide more accurate extrapolation spectrum for component fatigue analysis and improve durability evaluation accuracy.Based on the fatigue analysis of Haibach model and NASA model,the NASA model is suitable for component reliability design,Haibach model is suitable for lightweight design of components,which provides reference for reliability design and lightweight design of automobile chassis components.