Study On Vibration Fatigue Performance Of An SUV Front Subframe

As an important component for bearing the sprung mass and connecting the suspension system,the front subframe is prone to fatigue failure under random alternating vibration loads.The front sub-frame has a relatively low natural frequency.On some high-frequency random roads,the problem of resonance fatigue needs to be considered when the frequency spectrum is close to the low-order natural frequency of the front subframe.However,the current fatigue damage analysis of the front subframe mostly adopts the time domain method,which is not suitable for solving the dynamic response problem,especially the resonance fatigue problem.The time-domain analysis method is based on the principle of linear superposition to deal with complex multi-directional load excitation,and fails to consider the influence of the interaction between loads on fatigue damage.Therefore,taking a certain SUV front subframe as the research object,the fatigue damage in the frequency domain based on the durability test data of the proving ground is analyzed in this thesis.And compared with the time-domain fatigue analysis,the effect of the change of load spectrum characteristics and the superposition effect of interaction between loads on the fatigue performance of the front subframe is studied.First clarify the basic principles of fatigue damage analysis of the front subframe.From the perspective of loading system and loading form,analyze the load acquisition and processing methods suitable for the front subframe.In order to accurately express the fatigue performance of the front subframe material,the S-N curve that characterizes the fatigue characteristics of the material is discussed.Based on the S-N curve of the front subframe material,the average stress correction method is used to consider the superimposition effect of the average stress on the fatigue of the components.At the same time,the linear fatigue cumulative damage criterion is introduced to calculate the fatigue damage in the elastic deformation stage of the component.The test field was designed and carried out to strengthen the road durability test.And the road spectrum and vehicle dynamic response information of the test field were measured.Through disassembly and measurement of the vehicle chassis,modal verification of key components and flexibility,a rigid-flexible coupling multi-body dynamics(MBD)model of the entire vehicle is established.In order to verify the accuracy of the model,static calibration and dynamic calibration of the MBD model were carried out respectively through suspension K&C test and real vehicle bench vibration test.Through virtual iteration and wheel center loading method simulation,the load spectrum of the front subframe connection point is obtained.In the time-domain fatigue analysis of the front subframe,according to the road spectrum frequency range reaching 1/3 of the natural frequency of the front sub-frame,the dynamic response needs to be considered.Using the modal superposition method and linear cumulative damage criterion in the fatigue analysis software n Code,combined with the modified S-N curve of the front subframe,the strain time history information of each node of the front subframe is obtained.Compared with the strain signal of the monitoring point,the RMS value in the comparison result is similar.However,through the statistical processing of rain flow data,it is found that the simulated signal makes less fatigue damage than the experimentally collected signal.Considering that the design of components should follow the principles of safety and reliability.Therefore,the time-domain fatigue analysis method based on the front subframe has certain limitations.Comparing the PSD spectrum characteristics of the vertical acceleration at the connection point of the front subframe and the lower arm under hi gh-frequency rando m road speeds of 20km/h,30km/h,and 50km/h,it is found that the increase in vehicle speed causes the peak of the spectrum to move to high frequency and increase the peak values.In order to obtain the stress frequency response function(FRF)of the front subframe,the modal frequency response finite element analysis under unit acceleration excitation is carried out.According to the characteristic parameters of the stress FRF,the Dirlik method is used to calculate the stress response probability density function(PDF).Combining the component S-N curve and the linear fatigue cumulative damage criterion,the fatigue cumulative damage of the front subframe under the 20km/h strengthened road and the standard speed of the test field is analyzed.The results show that time domain analysis and frequency domain analysis are similar because the load is not easy to arouse structural resonance under the velocity of 20km/h.At higher velocity(standard vehicle speeds at test field),the damage of frequency domain analysis is generally higher than that of time domain analysis.The increase of vehicle speed causes the peak of the load spectrum to move to the higher frequency to produce resonance fatigue.It reflects the effect of the change in the characteristics of the load spectrum.The frequency-domain fatigue damage of the front subframe under different vehicle speed conditions is greater than the time-domain analysis result.Reflects the superposition effect of interaction between loads.The comparison of time-frequency methods shows that frequency-domain fatigue analysis can perform fatigue damage analysis on this type of front subframe with a higher safety factor.It can be more intuitive to analyze the weak area of the structure to facilitate the reliability optimization design of the product.