| As surveyed at home and abroad, the brittle fracture of traditional beam-columnconnections significantly limits the ductility development of components and reducesthe seismic performance of the structure in major earthquake damage. So the design ofsteel connections is an important part of the design of steel structures. Analysis of crackgeometry must presuppose, as the classical theory of fracture strength which does notapply beam-column analysis of elastic-plastic fracture mechanism is on the basis ofinitial components or weld defects.In order to study failure mechanism of beam-columnjoints in steel frames under seismic load.Combining with the quasi-static test on theexisting plate welded moment-resisting connection in steel frame and using a variety offracture indexes, fracture properties of the connection considering welding residualstress is studied, which is apparent as follows.1.The general-purpose finite element software ANSYS and the thermo-structuralanalysis of indirect coupling method were applied to analyze welding processes of fourwelds on the flange-plate reinforced connection. The’ birth and death element ’ methodwas used to simulate the process of transient welding. The article obtained the weldingtemperature field, the residual stress and the distribution and development of theresidual strain by simulating. The main cause of welding residual stress was the limitedlongitudinal contraction along the welding direction. The residual strain value of weldwere equivalent large in both ends of the longitudinal weld, inverted "a few" shapeddistribution. The equivalent residual stress of the plate welded moment-resistingconnection at the weld inconsistencies height occurred the mutation which was over thematerial yield strength. The existence will highly increase the likelihood of brittlefracture of the steel.2.Based on the simulation work of welding temperature field, residual stress andstrain in the enhanced connections, the software ANSYS was applied to analyze theductility and fracture properties of nonlinear finite element of three-dimensionalstructure of the five test specimens. The article compared the test results withoutconsidering welding residual stress of the model. By analyzing the fracture performanceindicators (the equivalent stress, hydrostatic pressure, stress triaxiality degree, theequivalent plastic strain, accumulated equivalent plastic strain increment, PI and RIindices index) in the weak parts of connections, it verified that a various indicators wereconsistent and reliable; Compared with the pros and cons of the fracture properties about five kinds of connections, the plate welded moment-resisting connection andordinary connections were most prone to brittle fracture in the middle and the edge ofthe butt weld of the beam, which was consistent with experimental phenomena; Basedon the destruction phenomenon of the test specimens, the reference value of assessingfracture properties was given: when the cracking index–RI was greater than20, it wasprone to brittle fracture; when the cracking index–RI was40, the structure was verydangerous and brittle failure was very high. Whether to consider the overall weldresidual stress on the force of the structure was not important. But the RI was greatimpacted.3.It should take effective measures to reduce the maximum residual tensile stress,reduce stress concentration, in order to improve the welding quality, improve the highcracking index the site, thereby reducing the possibility of brittle fracture of connectionsto ensure seismic performance and practicality of new connections.4.This paper also studied the connections enhanced plates parameters’ influencelaw to connection properties of force and fracture, and gave the range of plate-enhancedconnections’ parameters: suggesting the length of flange and cover plate to be0.5-0.8times of the length of the beam, the thickness of flange plate to be1.2-1.4times of thethickness of beam flange and the thickness of the cover plate to be0.7-1.2times of thethickness of beam flange. Comprehensively, it comparatively analyzed six differentstructural forms of welding connection: direct extension wing (WFS), enhanced sideconnection (SPS), flange enhanced connection (FPS), cover enhanced connection(CPS), dog bone weaken-connection (RBS) and the common connection (NFS). Themerit order of the fracture properties is FPS>CPS>WFS>SPS>RBS>NFS. The propertyof the flange enhanced connection (FPS) is best, so should be widely used.The research results for the fracture performance analysis and the seismic design ofsteel frame and welding residual stress and strain analysis provided valuable analysisand reference. |
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