The Experimental And Theoretical Research On Fatigue Performance Of M30 And M39 High Strength Bolts In Grid Structures With Bolt Sphere Joints

The grid structure with bolt sphere joints is more and more widely used in the industrial plant in our country.When the suspension crane is set up,it will cause repeated alternating load on the grid,which will lead to the fatigue problem of high strength bolts.Because of the discreteness of the fatigue test,the difficulty and the high cost of the experiment,the fatigue design method has not been established yet,which has caused great difficulties to the engineering designers,and seriously restricted its popularization and application.Based on the National Natural Science Foundation(51578357,51178286)and the study of the fatigue performance of high strength bolts such as M20,M22 and M27 in the early stage of our research group,this paper focuses on the fatigue performance and fatigue life evaluation of M30 and M39 high strength bolts in grid structures with bolt sphere joints to solve the problem of fatigue.The main research work and conclusions are as follows:1.A total of 71 constant amplitude fatigue tests of M30 and M39 high strength bolts were carried out by means of the American MTS Lankmark370.50fatigue testing machine.Through the analysis of the test data and based on the fatigue strength S-N curve of the high strength bolts obtained in this paper,the constant amplitude fatigue design method of high strength bolts in grid structures with bolt sphere joints are established.And this method respectively takes the nominal stress amplitude??and the hot spot stress amplitude??_k as the design parameters.2.In the process of constant amplitude fatigue test,the variable amplitude fatigue test data of 20 M30 high strength bolts and 5 M39 high strength bolts were obtained.The S-N curve of the variable amplitude fatigue test was obtained by calculating the equivalent stress amplitude of the obtained variable amplitude fatigue test data.3.A total of 35 fatigue test data were obtained by using the Amsler1200pulsed fatigue testing machine produced in Switzerland to simulate the plane truss of M20,M30 and M36 bolts.The results are compared with the test data of M30 and M39 high strength bolts in this paper.It is found that the fatigue strength of plane truss structure is lower and the test data is more discrete.4.With the help of imported TESCAN Mira3 LMH scanning electron microscope,the fatigue fracture of 65 high strength bolts was analyzed,the fatigue failure mechanism of the high strength bolts is revealed.5.With the domestic SDS100 electro-hydraulic servo fatigue testing machine,the fatigue crack growth rate testing of 40Cr steel materials has been finished.And this paper put forward the fatigue crack growth rate parameters C and m of 40Cr materials based on Paris formula.The selection of the parameters for the fatigue evaluation of high strength bolts in grid structures with bolt sphere joints is given.6.Using the method of fracture mechanics,the fatigue life of 40Cr M30,M39 high strength bolt connection is estimated by using the measured parameters C and m.The correlation between the theoretical value and the test value is good.It is proved that it is suitable to estimate the fatigue life of high strength bolts by using the measured parameters in this paper.