Ductile Fracture Prediction Of Steel Frame Connections Under Equal Amplitude Cyclic Loading With Large Displacement

With development progress of the economic and social,the super-high-rise buildings also reach constantly new height.As the large self-vibration period of super-high-rise building is more sensitive to long-period ground motions,it is a research hot spot to improve the seismic performance of high-rise or super-high-rise structures under long-period ground motions.Because the current mainstream research in China concentrates on the Elastic-plastic time-history response analysis in the overall level of the structure,however,the study of component and connections level is rarely reported.It contributes to the develop the deformation capacity of the steel frame connections that the action effect of long-period ground motions is considered and the corresponding cyclic loading is exerted,as well as the microscopic mechanism method to predict the possibility of ductile fracture and post-fracture path of steel connection is referenced.Therefore,the study has a certain theoretical significance.In this thesis,based on the existing test results abroad and the ABAQUS finite element implicit analysis method is used to establish the finite element model corresponding to the test.And nonlinear analysis is carried by exerting constant amplitude cyclic loading with large displacement and repeating many times,to explore the different factors of the equivalent plastic strain,equivalent stress,strengthening criterion and so on,as well as making full use of post-processing data to carry determination of ductile cracking in which the cracks may occur near the weld.The main research contents and related achievements are as follows:(1)In this thesis,the implicit post-processing method based on the microscopic cracking criterion is proposed.The applicability of implicit and explicit analysis methods is compared and analyzed and the implicit analysis method is adopted.ABAQUS software has certain advantages in terms of nonlinear analysis through the comprehensive analysis.The problem of data storage after the UMAT extended subroutine is used to solve the practical example is put forward,and the method is determine by using the implicit post-processing method based on the microscopic cracking criterion without the UMAT extended subroutines.(2)The effectiveness analysis of post-processing methods is proposed.Based on enlarged-scallop beam-column connections and non-scallop beam-column connections in the existing experiments,two equal amplitude loading systems were established respectively,the related data are extracted in the upper flange in the beam-column connection near the weld.And the microscopic mechanism criteria for cracking connections are used to statistical and computational analysis.The analysis results compared with the test results,and the results show that the post-processing method is more accurate in predicting the first crack time of the relevant part,and due to the absence of unit deletion,the judgment of the subsequent cracking time will be lagging behind and there is a certain calculation bias.(3)The effect analysis of reinforcement criterion.Based on the isotropic hardening criterion and the kinematic hardening criterion,the finite element models of the series of DSC(enlarged-scallop based on the isotropic hardening)and SSC(enlarged-scallop based on the kinematic hardening)were built,and the load equal amplitude ratios of 3.0 and 2.0 were exerted respectively.Comparison and Analysis of Four Hysteretic Curves,we discovery that the stiffness degradation degree of the DSC series is more serious,but the results of the ultimate bearing capacity are relatively close to that of the test;The stiffness degradation degree of the SSC series is similar to that of the test,but the bearing capacity is slightly different from that of the test.The results show that the stiffness degradation degree of the SSC series is similar to that of the test,but the bearing capacity is slightly different from that of the test.For determination of Ductile Cracking,the results of SSC series are closer to the test,so the applicability of the hardening criterion can’t be considered simply from the angle of bearing capacity,bearing capacity and stiffness should be taken into account synthetically.(4)The effect analysis of equivalent plastic strain on ductile fracture.From the crack information of the upper flange near the weld of beam-column connections,the equivalent plastic strain began to show a trend of increasing gradually,but the equivalent plastic strain value of the non-cracked connections on the same path is almost the same,and combined with the experimental phenomenon should be the Inflection points of the cracking speed change,because the limitation of the calculation method only reaches the inflection points.The judgment before the first cracking points has a direct relationship with the equivalent plastic strain(5)The effect analysis of equivalent stress on ductile fracture.The cracking after the above inflection points have small effect on equivalent plastic strain.Therefore,from the angle of the equivalent stress on scallop beam-column connections have carried corresponding analysis.The results show that the equivalent stress on the designated path of the upper flange is the same basically and they is also same with the stress in the judgment of the first crack.However,the local stress near the bottom of the web’ welding hole is too large obviously,which is caused by the stress concentration at the bottom of the web’ welding hole.If the cracking at the end of the upper flange and the cracking at the bottom of the web’ welding hole are together with effect,and the analytical results are consistent with the experimental phenomena.