Analysis Of Influence Of Welding Defects On Fracture Failure Of Aluminum Alloy Car Body

Aluminum alloy welded structures are widely used in the lightweight design of the EMU body.Although the welding has the advantages of stable connection and low cost,the aluminum alloy welding is more likely to cause welding defects such as inclusions,pores and hot cracks in the weld portion.In the course of service,these defects will become a source of cracks,putting the car body structure in a dangerous situation.Welding residual stress is another welding defect that is unavoidable in the welding process.The tensile residual stress increases the crack driving force and reduces the fracture resistance of the material.It will pose a great threat to the vehicle body strength reliability during the service.Therefore,the analysis of the train body is carried out by using the analysis method of fracture mechanics.The influence of welding residual stress and cracking defects on the fracture failure of the vehicle body is studied.It is of great significance to the service safety evaluation and welding quality control of the vehicle body.The welding defect is studied by the combination of thermo-elastic method,linear fracture mechanics,crack characterization,isoparametric inverse mapping,finite element method,inherent strain method,reliability safety factor method,structural stress method and FAD method.The influence of vehicle body failure failure provides a method reference for welding quality control and vehicle body safety assessment.The research content mainly consists of four parts.Firstly,the welded joint model was numerically simulated by the thermo-elastic method.According to the form of each welded joint on the aluminum alloy car body,the corresponding geometric and finite element models are established,and the welding temperature field and thermal stress are calculated by sequential coupling method.Secondly,the stress intensity factor is calculated by the linear elastic fracture mechanics method for the welding defect cracking characterization model.The finite element model of the I-type crack welded joint was established,and the crack stress intensity factor under the influence of welding residual stress was calculated.A full-scale vehicle finite element model is established.The inherent strain method is applied to the vehicle body to calculate the stress of the vehicle body under the influence of residual stress,and then the influence of welding residual stress on the structural strength of the vehicle body is analyzed.Finally,the FAD method is used to evaluate the fracture failure of the vehicle body welding defects when the quantitative cracks are used.Calculate the failure assessment curve of the welded joint of aluminum alloy,determine the car body focus,calculate the load ratio and fracture ratio of the point of interest and complete the fracture failure assessment.From the results,the following conclusions can be drawn.the longitudinal residual stress of each welded joint is much larger than the transverse residual stress,and the yield limit of the material has been reached;The residual stress of welding has different fracture properties for cracks of different lengths;when the crack is shallow,the residual stress has a greater influence on it;on the contrary,the influence is smaller;In the structure of the car body,the stress caused by the welding residual stress and the external load is not a simple superposition relationship,which is mainly due to the difference in the influence of the two on the principal stress under different working conditions;The cracks at the welds and the cross-weld joints at the geometrical singularity have a low allowable failure value,and the allowable crack size corresponding to the fracture failure is small.In this paper,the numerical simulation method of welding residual stress,the equivalent simplified calculation method of residual stress,the influence of residual stress on vehicle body fracture failure and the FAD method under the cracking characteristics of welding defects are studied.It provides a reference method for welding quality control and vehicle safety assessment.