Study On Vibration Fatigue Properties And Vibration-fatigue Coupling Analysis Of Rail Materials

As an important component that trains run on,rail plays a role for supporting and guiding the train.After a long period of operation,the carrying capacity of the rail tends to drop.Many serious rail damage accidents have occurred in China and abroad.On the one hand,the fatigue damage of the rail is the most dangerous type of damage,and in the most serious case,it will result in a major accident by causing fatality and train destruction.Fatigue is affected by various factors,and little research has been conducted on vibration fatigue due to the complexity of theory and test conditions.However,in recent years,with the rapid develop-ment of trains in the direction of high speed and heavy load,the fatigue problem induced by vibration has become increasingly prominent,and the vibration fatigue of trains can't be ig-nored any longer.On the other hand,with the rapid development of China's railway industry,by the end of 2018,the national railway operating mileage was over 131,000 kilometers;the national railway network density was 136.9 km/10,000 km~2.To ensure safe operation of these existing lines,it is also necessary to conduct research on vibration fatigue.Therefore,it is of great theoretical and practical significance to carry out research on the vibration fatigue of rails.In this paper,the vibration fatigue simulation test of rail is carried out by using the vibra-tion test bench,and the microstructure of rail is studied by using the scanning electron mi-croscope of the analysis center of southwest jiaotong university.The main work is:observe the microscopic damage morphology,observe and analyze the internal microstructure,sur-face morphology and fracture characteristics.The influencing factors and formation mecha-nism of rail vibration fatigue are tested,studied and discussed in detail from the perspective of macroscopic mechanical model and microscopic metal structure.Finally,based on the test data backed by the microscopic morphology analysis of the specimen fracture,the coupling mechanism of vibration and fatigue for railway rail fatigue damage is explored.The main results and research conclusions obtained in this paper are as follows:1.The load,and the geometric characteristics of structure are the main factors affect ing the vibration fatigue damage.By using range analysis,it is found that the effect of load on the vibration fatigue damage is the largest,while the effect of the geometric characteristics of structure on the vibration damage is relatively small.For each affecting factor,(1)As the frequency increases,when the excitation frequency is near the intrinsic frequency of the test structure,the test specimen will suddenly and rapidly break;(2)As the load increases,the vibration fatigue damage will gradually increase;(3)The specimen is not sensitive to the changes in the geometric characteristics of structure,and the vibration fatigue damage will not change because of small changes of geometric characteristics.2.U71Mn and U75V rail materials show different resistances to fatigue fracture under different excitation frequencies.When the excitation frequency is lower,the fracture tough-ness property of U71Mn rail is better,and when the excitation frequency is higher,the U75V rail shows better fracture toughness property.Under vibration conditions,the fatigue crack growth rates in U71Mn and U75V rails is lower than the crack growth rate obtained in con-ventional tensile fatigue tests.In the initial stage after fatigue crack initiation,there is little difference between the two rails,but as the magnitude of change in the stress intensity factor increases,the gap between the two crack growth rates becomes larger and larger.3.U71Mn and U75V rails during the vibration show better tenacity than that in the con-ventional tensile fatigue test,so they are not easily to break.Under different vibration excita-tion conditions,the fatigue cracks exhibit different propagation mechanisms.When the exci-tation load is high,the low cycle fatigue damage mechanism dominates.When the excitation load is low,the high cycle fatigue damage dominates.When the excitation load is normal,the fatigue damage mechanism shows the characteristics of high-low cycle complex fatigue.At the same time,the crack surface contact arises during the fatigue crack propagation.4.The natural frequency has a significant effect on the vibration fatigue damage of U71Mn and U75V rails.The natural frequencies of each order decrease rapidly with the con-tinuous increase of the relative depth of the crack.With the continuous increase of the rela-tive depth of the crack,the natural frequencies of each order are more severely affected.As the cracks approach the fixed end,the effect of the crack depth on the first-order natural fre-quency becomes greater.The deeper the crack,the greater the effect of the crack location on the natural frequency is.However,as the crack position approaches the excitation position,the influence of the relative position and the relative depth of the crack becomes weaker and weaker,until the influence disappears.5.The magnitude of the friction damping loss factor indicates how much the dissipative capacity is due to the contact friction of the crack surface during the fatigue crack propaga-tion.On the one hand,the closer the fatigue crack is to the fixed end of the specimen arm,the greater the effect of the friction damping loss factor is.On the other hand,the friction damp-ing loss factor increases with the increase of the fatigue crack length integral along the prop-agation direction,and the fatigue life of the specimen also increases.