| The failure mode of ductile fracture occurred in steel structure suffered large-strain and low-cycle loading under strong earthquake can be attributed to extremely low cycle fatigue(ELCF)fracture.Micromechanics-based fracture models has high accuracy in predicting ductile fracture of steel structures,which provides basis and prerequisite for preventing from ductile fracture of steel structures under strong earthquake.According to the Cyclic Void Growth Model(CVGM)which is applied to the prediction of ELCF fracture,the ELCF fracture performance of steel frame beam-to-column connections were studied in this paper.The main research contents are as follows:(1)The research situations of the steel fracture toughness and the behavior of steel frame beam-to-column connections were reviewed,and the ELCF fracture performance of steel frame beam-to-column connections were studied.(2)The nonlinear finite element analyses were carried out on the cyclic load tests of three partial specimens originated from steel frame welded beam-to-column connections,and the obtained force-displacement curves are in good agreement with the test curves,which verifies the validity of the finite element models.Then the micromechanical fracture model CVGM was adopted to predict the fracture of the specimens.The predicted results are accurate compared with test results.The accuracy and applicatbility of the micromechanical fracture model CVGM in predicting ductile fracture of the welded connection partial specimens under cyclic loading were verified.(3)According to the current domestic and foreign steel structure design specifications,three beam-to-column welded connection partial specimens with different welded hole forms were designed.The corresponding high precision finite element models were established,the influence of the welded hole form on the ELCF fracture performance of welded connection partial specimens was analyzed based on the micromechanical fracture model CVGM.(4)By giving high-strength steel material properties to beam-to-column welded connection partial specimens in the finite element models,the ELCF fracture performance of the Q460 D high-strength steel beam-to-column welded connection partial specimens were studied.The influence of material properties on the fracture properties of the connection partial specimens under cyclic loading was studied.(5)On the basis of the beam-end cyclic loading tests of a full-scale bolt-welded connection,seven beam-to-column bolt-welded connections including three kinds of structures and two kinds of materials properties were designed according to the current domestic and foreign steel structure design specifications.Taking the CVGM model as the theoretical basis,the influence of connection structures and high-strength steel property materials on the ELCF fracture performance of the bolt-welded connecions was studied through the nonlinear finite element analyses. |
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