Research On Random Vibration Fatigue Problem Of Antenna Beam Based On Frequency Domain

The antenna beam is an important suspension equipment for the bogie of electric multiple unit(EMU)and the subway train,and has the function of installing the on-board signal system.With the continuous increase of the train speed and the complexity of operating environment,the vibration load on the antenna beam continues to intensify,and its fatigue failure events occur from time to time.The existing static strength assessment and fatigue strength assessment based on static load for the antenna beam can no longer meet the actual train operation requirements.From the viewpoint of dynamics,the random vibration fatigue assessment of the antenna beam based on the frequency-domain methods can not only find the influence of the excitation frequency on the stress distribution,but also avoid the resonance phenomenon of the antenna beam,thereby prolonging the service life of the antenna beam and ensuring safe operation of the vehicle.At present,researchers have proposed a variety of frequency-domain methods to evaluate structural fatigue damage,but few literatures study the differences between different frequency-domain methods.In this paper,the nominal stress method which includes the critical plane method and the equivalent von Mises stress method,is introduced to evaluate the fatigue of the antenna beam in the random vibration frequency domain.According to the different stress states,the critical plane method is divided into the two-dimensional damage method,the twodimensional variance method and the three-dimensional variance method.Based on Python language programming,combined with the excitation spectrum specified in the IEC 61373 standard,the above four frequency-domain methods are used to predict the weld fatigue damage of the antenna beam solid element model.The results show that the damage values calculated by the three critical plane methods are basically the same and larger than those calculated by the equivalent von Mises stress method.In addition,for the equivalent von Mises stress method with a single excitation source,under the premise of the same damage values,an improved method is proposed,which simplifies the calculation formula of the equivalent von Mises stress PSD and improves the calculation efficiency.When the structural S-N curve meets the requirements of the equivalent von Mises stress method,the improved von Mises stress method is recommended to improve the calculation efficiency;since the three-dimensional variance method can achieve better calculation accuracy and efficiency without meeting any requirements,it is recommended for random vibration fatigue assessment.In view of the fact that the structural stress of the welded joint is more accurate in describing the fatigue failure mechanism of the joint,the frequency-domain structural stress method is introduced to predict the random vibration fatigue damage of the antenna beam.The programming implementation of this method for shell element and solid element model is proposed,including the sorting of welding line nodes and the identification of straight and curved welds.Using shell elements to simulate butt joints,fillet joints,T-joints and lap joints.Under the random vibration condition,it is verified that except for the fillet joint whose element size is 1.5 times the plate thickness,the equivalent structural stress of the other models is insensitive to the mesh.In order to study the difference between the frequency-domain structural stress method and the nominal stress method in the random vibration fatigue analysis of the antenna beam,the frequency-domain structural stress method is used to predict the weld fatigue damage of the solid element model,compared with the calculated damage value of the threedimensional variance method.The analysis results show that the danger zones predicted by the two methods are basically the same,the damage value of the three-dimensional variance method is larger and the time is shorter,and the frequency-domain structural stress method has certain reliability in calculating the random vibration fatigue damage.In order to study the accuracy of the shell element model in the random vibration fatigue assessment of the antenna beam,an antenna beam shell element model that retains the details of the welded joints is established,and the fatigue damage of the welded joints is predicted by the frequency-domain structural stress method.The calculation results show that the danger areas of the shell element and the solid element model are basically consistent,and the error of the damage results is small.Therefore,it is recommended to use shell elements or solid elements to build the finite element model of the antenna beam.