Research On Ultra-low Cycle Fatigue Fracture Of X-shaped Round Steel Pipe Intersecting Joints Based On Micro-mechanism

Steel is widely used in building structure due to its many advantages.In recent years,with the continuous development of China's national economy and construction market,the application scope of domestic steel structure is constantly expanding,and the development of construction steel structure science and technology is also constantly improving.However,researches on ultimate bearing capacity and ultra-low cycle fatigue fracture of tubular joints are not sufficient in the field of bu ilding science.In this paper,the circular steel pipe intersecting node is taken as the research object,and the research idea is from simple to complex,starting from the plane node,the common circular steel pipe X-shaped intersecting node is studied.There have been many studies on the Void Growth Model(VGM for short)and the Cyclic Void Growth Model(CVGM for short)in dog bone specimens and beam and column nodes,but few in the intersecting nodes.In this paper,finite element analysis and experimental data are combined to find a fracture model suitable for ductile fracture of round steel pipe joint.The main contents and results of this paper are as follows:(1)X round steel tube are nodes in the finite element analysis of fracture prediction point orthogonalization Lode Angle values indicate that point stress state for the combination of stress state,and the shear stress is larger,and eta VGM fracture toughness in the model parameter is measured in the axisymmetric tensile test,did not consider the effect of Lode Angle fracture prediction results,in the VGM model considering Lode Angle parameters affect the model can be improved in the round steel tube are the applicability of the node fault prediction,the preliminary analysis of the existing test data,this paper proposed a revised X-ray VGM model for fracture prediction,It can better predict the ductile fracture of X-shaped round steel pipe intersecting joint and meet the actual demand.(2)The stress amplitude at the node fault will change with the change of initial displacement(loading amplitude),so the initial displacement(loading amplitude)has a great impact on the number of load cycles of the node under the action of reciprocating load.In order to consider the energy dissipation capacity of joints under strong seismic load,the joints should be designed with both rigidity and softness,and the welding quality should be strictly controlled.(3)The hysteretic performance of the joint can be improved by increasing the height of the welding foot.When the weld strength matches the stiffness of the joint,the hysteretic performance of the joint can be greatly improved.(4)Although the main concave-convex deformation is the main energy consumption of the joint,the thickness of the branch wall limits the height of the welding foot at the joint,and the strength of the joint needs to match the main stiffness.Therefore,the matching relationship between each member of the joint should be strictly controlled in the design.(5)The weld configuration has a great impact on the joint fracture prediction and hysteretic performance.The effect of the weld configuration on the joint hysteretic performance should be evaluated according to the different characteristics of the joint.It is easy to use butt weld when is large and the main stiffness is high.