| In recent years,with the continuous advancement of modernization and the general promotion of environmental protection awareness,the application of steel structures in the engineering field has become more and more extensive.The construction of high-rise and largespan spatial structures in various places puts forward higher requirements for the improvement of steel strength and performance.The research on steel structures in my country started relatively late,and there is less research on high strength steel.Connection is an important part of steel structures,but because of the low elongation of high strength steel,it is difficult to meet the requirements of plastic design,and the fracture problem of high strength steel under strong earthquake is more prominent.The fracture of steel structures under strong earthquake is a problem of extremely low cycle fatigue,and its failure property belongs to typical ductile fracture.The ductile fracture prediction model based on micromechanism can describe the influence of stress-strain field on the internal microstructure characteristics of the material,which can be used as a criterion with clear physical significance to realize more accurate prediction of ductile crack in steel structures.Based on the micromechanical fracture prediction model CVGM(Cyclic Void Growth Model),the extremely low cycle fatigue fracture performance of Q460 high-strength steel frame cruciform beam-column welded connections was studied in this paper,and the main work carried out is as follows:(1)The current research status of the configuration and fracture mechanisms of steel frame beam-column connections by domestic and foreign scholars in recent years is reviewed,there are two main types of connections commonly used at present: one is that the connections are strengthened by structural measures to make the weak position transferred to a certain position away from the end of the beam;one is to weaken the beam section,so that the weak part is transferred to the weakened position.There are two main methods to study the fracture mechanism: one is the stress intensity factor method,the J-integral method and the CTOD method.This type of method is mainly suitable for brittle fracture or pseudo-brittle fracture problems with extremely limited yield range;the one is the fracture prediction method based on micromechanism,which has good applicability for ductile fracture.(2)Based on the finite element analysis(FEA)and the CVGM model,the cracking moment of the connections was predicted.The cracking time of TS-1 and TS-2 predicted by the CVGM model differs by 2.96 cycles and 0.87 cycles from the experimental cracking time respectively,they are within the acceptable range,which verifies the CVGM model based on the micromechanism has good applicability for predicting ductile fracture of Q460 highstrength steel frame cruciform beam-column welded connections.(3)According to relevant specifications,three Q460 high-strength steel frame beamcolumn connection specimens were designed,namely “dog-bone” connection specimen,flange plate reinforced connection specimen and cover plate reinforced connection specimen,and the cracking time was predicted for each connection specimen based on the FEA and the CVGM model.Compared with the standard connection specimen,the cracking time of the reinforced connection specimen is relatively earlier,and the “dog-bone” connection specimen is relatively later,indicating the connection structure has a great influence on the fracture performance of the connections.The bearing capacity of the reinforced connections is obviously greater than that of the “dog-bone” weakened connection;the stiffness degradation trend of the connections with different configuration forms is basically the same,which is fast at first,then slow,and finally tends to be stable.(4)Based on the two connection specimens in the test,two mixed strength section beam connection specimens were designed by changing the steel grade of the beam section web,and the connection cracking moment was predicted based on the CVGM model.The CVGM model predicted the crack of TS-6(mixed strength section)at 9.26 cycles,and predicted the crack of TS-1 at 9.29 cycles,with a difference of 0.03 cycles.The CVGM model predicted the crack of TS-7(mixed strength section)at 10.84 cycles,while the crack of TS-2 was predicted at 10.38 cycles,with a difference of 0.46 cycles,it can be seen that the change of the strength matching of beam section has limited effect on the fracture performance of connections;the hysteresis curves of mixed strength section beam connections and ordinary section beam connections basically coincide with each other except for some hysteresis loops,and the stiffness degradation curves are basically the same,it can be seen that the change of strength matching of beam section has limited influence on the bearing capacity and stiffness of the connections. |
PDF Full Text Request