| The grid is widely used in large-span structures,grid node is the key structure to connect each rods.Affected by the coupling effect of static load and dynamic load,the grid is prone to collapse.Therefore,the safety and stability of the node is the key to ensure the safety of the grid.Because the joint defects are inevitable in engineering,It is of great engineering significance to explore the static and dynamic mechanical properties of joints.In this thesis,the bolted spherical joints and welded hollow spherical joints in steel grid joints are taken as the research objects,and the numerical simulation analysis is carried out based on experiments.The main work is as followsAided by the finite element software ABAQUS,the three-dimensional solid model of the bolted spherical joint and the welded hollow spherical joint was established,contact state and weld structure between rotating threads were simulated.The material specimens of high-strength bolts and bolt-ball materials were made and uniaxial tensile tests were carried out to obtain the constitutive model of bolt-ball materials.Constitutive model of bolt-ball materials were obtained by literature acquisition.The formula was transformed into the real plastic relationship model in finite element software,and the material damage parameters were set based on the material ductile damage model to simulate the failure mode of the joint.Based on test,six different screwing depths of the high-strength bolts were set in the five kinds of bolt spherical joints,stress distribution and stress concentration factor of high-strength bolts.The results show that the stress value of the first ring screwed into the thread bottom of the high strength bolt is the largest.The larger the false screw defect of the high strength bolt,the more serious the stress concentration phenomenon.The load-displacement response curve and tensile failure process of bolted ball joints are obtained by numerical simulation of axial tensile test,and the ultimate bearing capacity and failure mode of the joints were verified according to the test.Eight kinds of weld defects were set for welded spherical joints,and the influence of weld defects on stress distribution was analyzed.The results show that the local weld defects will change the stress distribution of the weld.The maximum stress is always at the weld defect,and the stress at the weld toe is also affected by the weld defects.The numerical simulation of axial tensile and axial compression tests on welded spherical joints was carried out,and the tensile and compressive load-displacement response curves and failure modes of welded hollow spherical joints were obtained and verified.Franc3D was used to analyze the fatigue crack propagation of welded hollow spherical joints,and the initial cracks with different lengths of 0.1-0.5 mm were set.It is found that the stress intensity factor at the crack tip increases gradually with the crack propagation.When the crack of all lengths extends to 0.5 mm,the change trend of the stress intensity factor at the crack tip is basically the same.The distribution of stress intensity factor along the crack front presents ’ groove type ’,it shows that the stress intensity factor on the wall surface is larger,and the growth rate of stress intensity factor at both ends of the crack is greater than that in the middle of the crack,and the growth rate of the crack along the surface of the steel pipe is greater than that in the direction of wall thickness.According to the results of crack propagation,the fatigue life of welded spherical joints is predicted and fitted to S-N curve,which is verified by experimental results.Crack growth is slow in the initial stage,and the same length of crack propagation requires more stress cycles.When the initial crack is set to0.1 mm,the correlation with the test is the best.With the increase of initial crack size,the fatigue life of the joint decreases significantly.At the end of the article,the research is summarized and makes an outlook for further research. |
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