| Heavy-duty vehicle transmissions need to withstand harsh working conditions such as high-speed ratios and large torques.The welded structure of the intermediate shaft is under multiaxial stress during torque transmission,which is easy to cause fatigue crack initiation and propagation fracture during the working process.Therefore,it is necessary to evaluate the multiaxial fatigue life of the welded structure of the intermediate shaft.This thesis proposes a novel approach based on notch stress to evaluate fatigue damage in the weld seam of the intermediate shaft welding structure in heavy-duty vehicle transmissions.The fatigue life evaluation model for the intermediate shaft structure is established based on the multiaxial equivalence criterion and the multiaxial critical plane criterion,respectively.This model is then used to conduct multiaxial fatigue life evaluation.The accuracy of the proposed method is validated by comparing its results with those obtained from bench fatigue tests.The specific work are as follows:(1)The three-dimensional model of the intermediate shaft structure is established.Considering the constraints and loading conditions of the intermediate shaft in the transmission,the notched finite element model of the intermediate shaft welding structure is created by using sub-model technology.By conducting finite element simulation,the stress distribution cloud diagrams of the intermediate shaft structure are obtained,which is fundamental to the subsequent multiaxial fatigue life analysis.(2)A multiaxial equivalent stress evaluation model for rotational structures is proposed.For the multiaxial equivalent criterion,a rotational coordinate system and a local arc radius coordinate system are established to consider axis rotation and the influence of arc radius,respectively.The equivalent stress of dangerous points is calculated by the Von Mises criterion and IIW criterion.While for the critical plane criterion,the analysis is done under the global coordinate system,and the directions of critical planes are obtained through the longest chord method for different critical plane criteria.Based on the calculated shear stress range and the normal stress range on the critical plane,the equivalent stresses of the dangerous points are calculated based on MWCM criteria,C-S criteria and Findley criteria,respectively.(3)Fatigue data of welded joints for different joint types,loading conditions and failure modes are collected,and the equivalent stresses of each welded joint under different multiaxial criteria are calculated by the notch stress concentration coefficient matrix which can be obtained by finite element analysis.The multiaxial S-N curves based on the Von Mises criterion,IIW criterion,C-S criterion,and Findley criterion are fitted respectively by using the least squares method,and the S-N curves based on the MWCM criterion are corrected based on the stress ratio.These five multiaxial S-N curves can be used for multiaxial fatigue life evaluation,this can avoid the drawbacks of the traditional approach which uses the same uniaxial S-N curve for different multiaxial criteria.The fitted multiaxial S-N curves can provide a reference for multiaxial fatigue life assessment of welded structures.(4)The fatigue life of the intermediate shaft structure is predicted based on the Von Mises criterion,IIW criterion,C-S criterion,Findley criterion,and MWCM criterion by the calculated multiaxial equivalent stress at the critical position of the intermediate shaft and the Miner cumulative damage criterion.By comparing the prediction results with the bench fatigue test results,it is found that the evaluation results based on Findley’s criterion are closest to the test results,while the evaluation results of IIW criterion deviate the most from the test results.Meanwhile,the accuracy of the evaluation results is compared and analyzed with the width of the dispersion band of the fitted S-N curve to verify the reliability of the fitted S-N curve. |
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