Experimental Reaserch And Mechanical Analysis On The Influence Of Welding Structure Of Weld Backing Bars On Fracture Properties Of Welded Beam-To-Column Joints

The investigation of major disasters at home and abroad showed that the common welded steel frame is prone to be brittle cracking at the joints of beam and column under earthquake load.The case of joints failure caused by the cracking of the butt weld is especially common,which is a weak link in the joint design.It has been found that ameliorating the stress state at the welded backing bars has a significant effect on relieving the cracking of the butt weld.However,the welding structure of the welded backing bars between column flange and beam flange,which is commonly used in engineering and proposed by scholars at home and abroad,needs to be further improved and supplemented.In this paper,a new type of construction of four-sided circumferential welding on welded backing bars is proposed,and the comparison between the two-sided welding on welded backing bars and the one-sided welding on welded backing bars is carried out.The experimental test and finite element numerical simulations are carried out to study the effect of the weld seam on welded backing bars on the fracture behavior of the beam-column connections.The main research contents include:(1)Three welded and bolted moment connections were designed for low cycle reciprocating loading tests.The failure modes,bearing capacity and energy dissipation performance of the specimens were analyzed.The results show that the welds on welded backing bars have certain influence on the ultimate bearing capacity and energy dissipation capacity of the joints.The ductility coefficient,the equivalent viscous damping coefficient and the ultimate bearing capacity of the the specimen with two-sided welding on backing bars respectively increase by 29%,31%,23%than the specimen with one-sided welding on backing bars;and the ductility coefficient,the equivalent viscous damping coefficient and the ultimate bearing capacity of the the specimen with four-sided welding on backing bars respectively increase by 70%,47%,33%than the specimen with one-sided welding on backing bars.The structure of two-sided welding of weld backing bars and four-sided welding of weld backing bars can change the stress state at the butt weld,and make the stress peak move to beam flange to slow the cracking of butt weld.(2)Based on ANSYS finite element analysis software,a three-dimensional thermal analysis model was established for three welded and bolted moment connections.The welding transient process of three specimens,which has different welding structure of the butt-welded,was simulated.And the welding temperature field and residual stress distribution of each specimen were calculated.Based on the results of finite element simulation of welding residual stress,cyclic loads is applied to the model,and the stress triaxiality,plastic equivalent strain index and rupture index of each model under specified path are extracted and analyzed to evaluate the effect of different welding structure of the welded backing bars on the fracture behavior of joints.It was found that a fillet weld on the right side of the welded backing bars can shift the peak value of the rupture index of the welded steel moment joints.The structure of four-sided welding on weld backing bars is more effective to mitigate the cracking of butt weld area than the structure of two-sided welding on weld backing bars.The structure of the four-sided welding on weld backing bars can also alleviate the cracking at the toe position of the weld access hole.(3)Based on the classical fracture theory,a three-dimensional equivalent T-section fracture analysis model is established by ANSYS finite element software.Elastic stress intensity factor(K_I)and J integral of mode I at the crack tip are used as evaluation indexes to compare and analyze the influence of three kinds of welding structure of the welded backing bars of the joint.The results show that the fracture toughness of butt weld can be improved by changing the welding structure of welded backing bars,and the mode I stress intensity factors K_I and J_I at the defect tip can be reduced by both the structure of two-sided welding on weld backing bars and four-sided welding on weld backing bars.But the structure of four-sided welding of weld backing bars is more effective to prevent brittle fracture of the butt weld.And it can significantly reduce mode?elastic stress intensity factor(K_I)and J-integral(J_I)of the cracktip.