Study On Ultra High Cycle Fatigue Characteristics Of Structural Steel Connection Joints

With the rapid development of the current society,China bridge,China Road,China car and so on,"Chinese pride"are constantly"produced",there are more and more high requirements for the function of the components,especially the requirements for the reliability and durability of the metal components,which has become an urgent demand for the development of important projects and the key technical of field of aerospace,vehicle and ship,high-speed trains,MEMS,and biomedicine fields.In order to save energy and protect environment and protect more green hills and green hills,the metal structure components in these works usually require a certain length of service time.On the other hand,all kinds of components used in the project are gradually developing towards miniaturization and light weight.These small mechanical components of these lightweight components are often under the high frequency alternating load,so the safety,reliability and stability of these small components are important for the whole project.In the bridge structure and high rise structure,the connection between the components is often used by welding connection or bolt fastening.However,because the mechanical properties of the welded material or bolt material are generally different from the mechanical properties of the parent material itself,the connection point is complex,which is often the energy gathering point under the load of the whole structure,and it is easy to get fatigue damage firstly,resulting in the economic loss.Therefore,the fatigue behavior of welded joints and bolted joints is studied,especially the super long life fatigue performance of more than 10~7 times,which is of great engineering significance to ensure the reliability,safety and durability of the structure.In this paper,the raw material components of Q345 low alloy materials are analyzed and studied,and the finite element analysis software is used to model the parts to meet the needs of the experiment.The Shimadzu Corporation USF-2000 ultrasonic fatigue testing machine is used to do the experiment,The experiment has set up the tensile stress ratio of-1 and the vibration frequency of 20kHz,and the experiment is carried out at normal temperature,to study the super high cycle fatigue performance of the parent material.The results show that the ultra high cycle fatigue cycle of the material itself decreases with the increase of the stress value,and the S-N stress life curve is distributed logarithmically,and there is no fatigue limit.The location of fracture is observed.Most of them are cracked in the middle section,and a small number are broken at the transition section of the arc.An electron scanning microscope is used to observe the fracture of the ultrasonic fatigue test specimens.It is found that the cracks in the specimen of the parent material are sprout from the surface,especially at the place where the scratch,rough or infiltration of the inclusions stayed on the surface.Two common welded joints of steel members were simulated,and the size design was carried out by finite element software,and the ultrasonic fatigue testing machine was used for the experiment.The influence of various dimensions on the natural frequency and stress displacement ratio of Cs is studied.The results show that the size of each part has a linear relationship with its natural frequency,The size change of the arc length L3 and the thickness of the plate t has the greatest influence on the frequency,so the control accuracy is needed for the machined specimen.The influence of each dimension on the force displacement coefficient Cs is approximately proportional.but generally has little effect.Ultrasonic fatigue vibration tests show that the fatigue properties of lap welded joints are lower than that of base metal,which is about 57%of the strength of base metal.The fracture position of the specimen is observed,and most of them are broken at the weld toe,and some of them are broken in the parent material which is greatly affected by the bending.Therefore,the effect of bending should be considered when the size is designed and processed.The observation section shows that there are more than one crack initiation,and so many inclusions and pores.In order to explore the ultrasonic fatigue performance of bolt fastening components,the finite element method is used to carry out the size design,the contact element is used to simulate the fastening between the screw cap and the connector,and the interaction between the screw rod and the screw cap is simulated by the pre-tightening force.Finally,the natural frequency of the thirteenth order axial tension and compression mode is 20.15 kHz,and the size of the specimen is determined.We continue to carry out the ultrasonic fatigue test,and draw the conclusion that the ideal model is needed first.The bolt must be tightened tightly.The bolt can be fixed with professional instruments to ensure that there is no clearance in the whole specimen,or the specimen can be made into a whole,the middle tapping and the bolt are installed;Secondly,due to the easy accumulation of energy between the threaded rod and the connector,the temperature increases rapidly,so the interval loading is adopted to ensure that the specimen has enough heat dissipation time.Finally the specimen vibrates successfully.