Assessment Method And Application On Low-cycle Fatigue Of Welded Structures

The heavy axle-load gondola car is one of the significant freight in bulk goods railway transportation.For improving the efficiency of heavy axle-load gondola cars,dumper is commonly used to unload the multiple sets of unhooked gondola cars.When the gondola cars enter the dumper,the car bodies are subjected to the high impact from the push machine.Part of the welded structures in the car body enters a yield state,which causes failure in low-cycle fatigue.Aiming at this failure in low-cycle fatigue,this paper studies the method of fatigue assessment.The main work includes the following aspects:Firstly,it particularly focuses on the numerical calculation method of structural strain which is combined with the classical plate theory and the computational plastic theory under plane strain condition.For different strain hardening types of the base metal,three material models are used to describe the method:(1)the modified Ramberg-Osgood material model is suitable for base metal considering nonlinear strain hardening effect;(2)the bilinear material model is suitable for the base metal considering the linear strain hardening effect;(3)the elastic perfectly-plastic material model is suitable for the base metal without considering the strain hardening effect of the material.Secondly,numerical calculation program of the structural strain is developed by using the MATLAB App Designer.The results of structural strain calculation program are verified by a finite element example,which shows that the calculation results are effective.The comparative analysis of the structural strain calculated by using the modified Ramberg-Osgood material model,bilinear material model and elastic perfectly-plastic material model shows that the elastic perfectly-plastic material model can be used for approximation when the relevant parameters of strain hardening cannot be obtained accurately.Thirdly,for the common types of welded joint in heavy load railway wagon and by analyzing a large number of fatigue test data of welded joints with different types of load mode,base metal and joint,it can be concluded that the fatigue test data of welded joints are unified in a master E-N curve with equivalent structural strain.The calculation parameters of the master E-N curve are fitted through the least square principle,and the low-cycle fatigue assessment process of welded structures based on the equivalent structural strain is formulated.Finally,the practical cases in the field of rail vehicles are analyzed by using this method.(1)The low-cycle fatigue life of welded plate specimens made of high-strength weathering steel used in the heavy axle-load gondola car is analyzed.The predicted fatigue life obtained by this method is close to the fatigue life obtained by test,and the measured fatigue life can fall in the range of the master E-N curve fitted in this paper.(2)The low-cycle fatigue life prediction of the key welded structure at the connection between the shifting seat and the upper cover plate of the middle beam of the heavy axle-load gondola car.According to the requirements of 200 shifting conditions and 200 stopping conditions a year,the service life of the welded structure is 16 years.