Study On Fatigue And High Temperature Mechanical Properties Of Bolted Spherical Joints

Grid structure is a new spatial structure with high-order statically determinate,which is composed of multiple rods in accordance with certain rules and connected into a network by joints.Joints are considered to be the weakest part of the grid structure,and its stability and reliability are closely related to the safety of the whole grid structure.As one of the common joints of grid structure,the stress concentration at the thread notch or the change of mechanical properties in high temperature will affect the overall bearing capacity and eventually lead to structural collapse of the grid structure.Therefore,it is of great practical significance to study the fatigue notch stress concentration and the high temperature mechanical properties of bolted spherical joints with full-threaded model.The main contents of this paper include:（1）Study on the constant amplitude fatigue test of bolted spherical joints in grid structures.The constant amplitude fatigue test data of high strength bolts in bolted spherical joints are analyzed and analyzed,and taking?_maxax and（35）?as design parameters,the constant-amplitude fatigue design method of bolted spherical joints of grid structure is established.（2）The effect of thread notch on stress concentration of bolted spherical joints is studied.The finite element analysis is carried out from five aspects:lead angle,bolt diameter,bolt-sphere diameter ratio,fillet radius of thread bottom and multi-direction stress state.The results show that no matter how to change the specification parameters of bolted spherical joints,the maximum stress always occurs at the first thread where the high-strength bolts mesh with the bolted spherical joints.By calculating the influence coefficients under different conditions,a general formula for stress concentration factor K of bolted spherical joints is obtained.（3）The mechanical properties change law of bolted spherical joints at high temperature is explored.The high temperature mechanical test data of bolted spherical joints M20 and M24 are analyzed,and the change law of yield load a nd ultimate load of bolted spherical joints are studied to provide test and data support for the safe load-carrying of high temperature grid structures.Finite element analysis of bolted spherical joints is carried out by ABAQUS,and it is found that the f ailure of high-temperature bolted spherical joints is located at the bolt threads.When the temperature T is less than 300℃,the mechanical properties of bolted spherical joints generally change little;When the temperature T is more than 300℃,the yield load and ultimate load decrease obviously;When the temperature T is more than 700℃,the bolted spherical joints basically lose the bearing capacity.The results show that the temperature has a great influence on the mechanical properties of bolted spherica l joints.It is suggested that the working temperature of bolted spherical joints should not exceed 300℃.（4）The fatigue life of bolted spherical joints is predicted based on fracture mechanics theory.The fatigue failure of bolted spherical joints first occurs on high strength bolts,the fatigue life of bolted spherical joints is predicted by the test data of high strength bolts.By changing the initial crack length and fracture toughness of materials,the fatigue life of high strength bolts is calculated.The theoretical values are basically in consistent with the test results,and the average error is about 30%.It shows that the accuracy of calculating the fatigue life of high strength bolts by using fracture mechanics theory is higher,and this study m ethod has a certain reference value for the life prediction of bolted spherical joints.