Research On Equivalent Stress Method Of Spot Welding And Ring Welding

Resistance spot welding is a widely used welding process for stainless steel carbody structures.It is often replaced by plug welding or ring welding when spot welding is not easy to realize.During the operation,the train is subjected to complex alternating loads,spot welding and ring welding are the weak points in the carbody structure,and its fatigue life is directly related to the service life of the train.Using a simple F-N curve to predict the fatigue life of the ring welding and spot welding in the carbody structure,it's not only a tedious process,but it means a huge amount of F-N curves when there are large spot welding types.Therefore,a uniform curve which is S-N curve is needed,so that the simple life prediction can be carried out in the previous finite element analysis.Based on the study of the fatigue of spot welding by Rupp et al.,the finite element models of ring welding and spot welding are established by simulating the nugget with a CBAR element and simulating the two plates with a neutral shell element.The force and moment of the beam element are extracted and the structural stress is calculated according to the structural stress formula.The fatigue life of ring welding and spot welding specimens is tested on the fatigue test machine.In the logarithmic coordinate system of two parameters,the measured life N is the horizontal coordinate,the structural stress amplitude is the vertical coordinate,and the least square method is used for the linear regression analysis.The results show that the maximum principal stress at the nugget calculated from the tensile shear force correlates well with the fatigue life of the ring welding specimen under tensile shear load.Comparing the S-N curves under different load ratios,it is found that the load ratio has little effect on the fatigue characteristics of the ring welding specimens.The S-N curve of the ring welding specimens is obtained by linear regression analysis,this curve covers the test results under different load ratios,and the correlation is good.Most of the data points of the ring welding fall in the 5 times of the dispersion zone of life;By comparing the results of fatigue tests between all load ratios and R = 0.5 only,it is found that the load ratio has a great influence on the fatigue properties of the base metal of the spot welding specimens.The results of fatigue tests under different load ratios should be considered separately.Under tensile shear load,when the load ratio R=0.5,the S-N curve of the spot welding specimens is obtained by linear regression and all the spot welding data points are within 5 times of the dispersed zone of life.The respective curves of ring welding and spot welding are strongly related well and can be used to predict fatigue life.