| The stress concentration area of beam column joints will be subjected to large plastic strain and fracture due to large-scale yielding under strong earthquake.In 1994,the post earthquake survey of the Northridge earthquake and the 1995 Kobe earthquake in Japan found that this ductile fracture occurred in many steel frame structures,which usually occurred under less than a dozen cyclic loads,and the failure mode of the joints was known.It is ultra-low cycle fatigue(ULCF)fracture failure.For the study of the ultra-low cycle fatigue failure performance of steel structure joints under strong earthquake,it can effectively evaluate the fatigue life and ductility design performance of steel structure joints;at the same time,it can also provide reference for the seismic design of steel structure joints.In this paper,based on the micro fracture mechanism of steel,the test and finite element analysis of Q345 steel round bar are carried out,and the constitutive relationship of steel under cyclic load is calibrated.The experiment of Q345 steel notched plate under different cyclic loads was carried out,and the corresponding finite element analysis was carried out to verify the cvgm micromechanical fracture model under ultra-low cyclic load.According to the existing experiments,the ABAQUS finite element model of the beam column joint with external reinforcement under ultra-low cycle fatigue load is established to simulate the fracture failure analysis of the joint.The specific work of this paper is as follows:(1)Introduce the fracture mode of steel under the action of earthquake,and introduce the microscopic fracture model that can be used to predict the ultra-low cycle fatigue fracture life of steel,mainly including the cyclic hole growth model(CVGM);(2)The test selected Q345 B steel to design a total of three smooth round bar samples.For the three smooth round bar samples(two round bar samples a made of steel plates along the X and direction(A1 and A2),the round bar made along the X direction One test B1,)The cyclic loading test was performed to determine the material properties of Q345 B steel material in different directions,and the mixed strengthening model parameters of the steel were obtained;(3)Using the same batch of Q345 B steel to make smooth round bar samples,8notched plate samples were designed and made,and cyclic tensile and compression tests were carried out,and the obtained mixed strengthening model parameters were applied to the finite element analysis of the notched plate samples,Get the node’s break time.The parameters of the mixed strengthening model were verified by comparison with the experimental results of cyclic tension and compression of the notched specimen;(4)In the finite element,by using the ABAQUS subroutine interface,the VUSDFLD subroutine is prepared to simulate the crack propagation path of the notched plate specimen in the cyclic loading experiment,to obtain the fracture process of the notched plate specimen,and verify the cyclic empty The hole expansion model deletes elements to simulate the accuracy of the crack propagation process;(5)With reference to the existing tests,through the numerical analysis of the ultra-low cycle fatigue fracture of the outer reinforced ring TEDX-2 beam-column joint,the mechanical response of the outer ring plate beam-column joint under the action of cyclic loading is studied.The prediction and test results of fracture of steel pipe flanges and outer ring plates in the core area verify the applicability of the cyclic hole expansion model and explain the significance of ultra-low cycle fatigue failure as a structural limit state in the numerical simulation process. |
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