Research On Fatigue Strength Of Weld Based On Solid-Shell Element Sturcture

Large numbers of fatigue tests and operational practices show that fatigue failure is one of the major forms of failure of engineering welding structures.Fatigue failure usually occurs at the weld toe of the weld,the weld toe cracks occur under the alternating load,and extends along the thickness direction of the weld toe until the fracture happens.Using shell element modeling method can greatly reduce the computation and analysis period when the fatigue strength of welded structures is evaluated.However,due to the complexity of the actual industrial structure,the geometric model is usually discretized into solid element,and several fatigue strength theory widely used in the world also have some limitations.Therefore,it is of great practical significance to study the fatigue strength analysis of welded joints based on solid-shell modeling method.A solid-shell finite element model of welded frame on C0 locomotive bogie is established based on Hypermesh software.With the difference of solid and shell element's degrees of freedom,transfermation of force and displacement at the interface between soild element shell emement is inaccurate.Four solid-shell element interface modeling methods are adopted to verify the accuracy of fatigue strength analysis,including nodes coupling method,MPC contacts,RBE3 flexible connection and RBE2 rigid connection method.The result shows that the accuracy of RBE2 rigid connection method is better than the others with significant higher accuracy.The nominal stress method is widely used in engineering project and does not take into account the stress concentration effect of the welded joint,which results in the deviation of stress calculation.The equivalent structural stress method based on master S-N curve proposed by Dong Pingsha is adopted by this thesis,due to the stress concentration effect of welded joint is considered,the fatigue strength analysis is not sensitive to the mesh size and shape.In this new method,a single master S-N curve is obtained,based on fracture mechanics theory and a large number of fatigue tests and actual operating data,which can improve the accuracy of fatigue strength assessment.The equivalent structural stress method is difficult to be widely used in engineering because of the complexity to obtain the nodal force and moment of welded joint in the local coordinate system.In this thesis,the automatic recognition method of the local coordinate system of weld joint is realized based on FORTRAN language.The solid-shell finite element model of welded frame on C0 locomotive bogie is established in Hypermesh software,and analysed by ANSYS software,using APDL language to extract shell and solid element's stress and coordinate information in the global coordinate system.The results date of finite element analysis is processed based on the FORTRAN language to identify the local coordinate system of weld nodes,both on straight line or space curve of weld joint.Then the nodal force and moment in the local coordinate system can be obtained by coordinate transformation.According to the equivalent structural stress method proposed by Dong Pingsha and ASME standard,the 4-node shell element and 8-node solid element's calculation formula of structural stress ?s and equivalent structural stress Ss are deduced.Finally the fatigue strength analysis of weld on welded structure is assessed combine with the master S-N curve.Compared with the fatigue strength evaluation results of traditional nominal stress method,this research shows that there is a slight difference between the equivalent structural stress method and the nominal stress method on material utilization distribution,but the overall distribution tends to be uniform.The fatigue strength material utilization of equivalent structural stress method is relatively smaller with greater dispersion,the fatigue strength analysis result on engineering project using equivalent structural stress method is not conservative.