New Method Research And Application Of Vibration Fatigue Life Prediction For Welded Structures In Railway Vehicle

Welded structures are very important to railway vehicle due to their many unique advantages.However,there are several defects belong to welded structures such as their anti-fatigue performance is much worse than base metal.Welds are always the weak locations of railway vehicle structure.It has been proved by many fatigue failure cases of welded structures.Therefore,in order to improve the fatigue resistance of welded structures,any progress of theory,method or technology has engineering value.In this context,two new concepts refered as modal structural stress and frequency-domain structural stress are proposed in this paper based on the latest structural stress method.Among them,the modal structural stress technology is used to predict the structural modal vibration related fatigue life of welded structures in railway vehicle.The frequency-domain structural stress technology is used to predict the vibration fatigue life of welded structures in railway vehicle under random loads.A series of studies were performed in this paper to develop these two technologies.Its main contents are summarized as follows:First of all,a lot fatigue related literatures of welded structures were studied.The adaptabilities and limitations of widely used fatigue life predicton methods based on nominal stress method were analyzed according to the structures and loads characteristics of railway vehicles.Such as the S-N curve datas have limitation and the calculated results of stress are sensitive to mesh.Then,the fatigue particularities of welded structures were summarized according to the comparison between base metal and welded structures.Such as there is no crack initiation process in the beginning,the influences of mean stress stress cycling characteristic and yield strength of the base metal can be neglected.The S-N curve slopes for different types of welded joints are identical.All welded structures have the same fatigue failure mode.The fracture mechanics could be to use to study the cracking process of welded structures due to the existence of micro-cracks in weld root and toe.In order to research the mechanism of welded structures fatigue failure,not only the basic concept and the mesh-insensitive property of structural stress method invented by Dong but also the equivalent structural stress obtaining procedure and the master S-N curve equation(ASME2007)was studied in this paper.The concepts of frequency-domain structural stress and the modal structural stress cannot be proposed without the deep.understanding about the structural stress method and the main S-N curve equation.Based on the above,the concept of modal structure stress was proposed according to the modal characteristics of railway vehicle.Procedure of modal structural stress calculation was derived based on the modified Craig-Bampton modal algorithm.Then fatigue life prediction route based on modal structural stress was presented.The effectiveness of load frequency to fatigue can be considered in modal structural stress technology which was proved by some cases.An engineering application of modal structural stress technologgy on weld in an exported freight wagon was carried out in this paper.The fatigue life of weld at side wall of this wagon was predicted based on the modal structural stress technology.The calculated results were consistent with actual results.The engineering validity of the modal structural stress concept was verified.Suggestion of improvement for original design was proposed based on the calculated results of the modal structural stress technology.The fatigue life of the improved structure meets the application requirement.The statistical characteristics of random response of welded structures in railway vehicle were also studied in this paper.The basic concept of frequency-domain structural stress was proposed based on structural stress method,random vibration theory and the concept of transfer function.The basic processes of fatigue life prediction based on the frequency-domain structural stress technology are as follow.First of all,the response spectrum of nodal force,structural stress and equivalent structural stress in frequency-domain can be acquired by frequency-domain structural stress technology under random load(?)spectrums.Then the random vibration fatigue life prediction of welded structures can be performed according to the Dirlik probability statistic method.Its effectiveness was proved by some cases.The fatigue failure at weld in railway container flat car was taken as the analysis object.Measured acceleration spectrums of carbody under actual track incentive were used as input loads.The coupling effects between loads of front and rear center plates were taken into considered through the acceleration cross-spectrals.The predicted fatigue life results show that the crack location and fatigue life from frequency-domain structural stress technology are consistent with actual facts.The suggestion of improvement has been accepted by the factory.The effectiveness of the modal structural stress and the frequency-domain structural stress are proved by the studies above.Therefore,the fatigue life prediction system of welded structures in railway vehicle was compiled.Vibration response of welded structures can be considered in this system directly.General finite element and dynamic simulation softwares could be integrated in this system.The data interaction,the division of labors and collaboration between fatigue prediction system and business software were introduced.The weld definition,nodes and elements number sorting method,obtaining method of local coordinate transformation matrix along weld line,structural stress calculation program,statistics of the modal structural stress and frequency-domain structural stress were carefully described in this paper.Finally,three innovations and some research prospects were given in the end of this paper.