Research On Fracture Prediction Of Steel Joints Under Cyclic Loading

Steel structure is widely used because of its advantages of light weight and fast construction speed.However,several influential earthquakes make people pay more attention to the anti-seismic research of steel structure,which can be roughly divided into two aspects:on the one hand,it is necessary to study the structural system,energy dissipation device,and more effective reinforcement measures with stronger energy dissipation capacity;on the other hand,it is important to study the fracture mechanism and earthquake damage prediction of steel structure,establishing the mathematical model based on fracture mechanism and applying it to the fracture prediction analysis in engineering,and guiding the reinforcement work of steel structure.At present,the accuracy of fracture prediction by using microscopic fracture model needs to be improved.However,steel joints are generally welded,and the welding thermal effect may have impact on the fracture.Therefore,it is significative to study the welding of steel beam-column joint.In addition,it is significative to guide the reinforcement of steel joints and evaluate the reinforcement effect quantitatively by Finite Element Method.The main research work and conclusions of this paper are as follows:(1)The monotonic tensile tests were conducted on round bar specimens to establish constitutive models of different materials from tensile to fracture and to calculate fracture parameters of various materials by user subroutine.The results show that there is no obvious relationship between the values of fracture parameters and geometric dimensions;the fracture parameters of the same type of steel under different test conditions are different,but they are all within a small range;the type and size of finite element mesh have effects on the calibration of fracture parameters.(2)A finite element model of welded steel joints was established to caculate the number of cycles required of initiate cracking at material points by user subroutine.The influence factors about accuracy of fracture prediction were studied in finite element analysis.On the one hand,the mesh size has a significant impact on the prediction accuracy of ductile fracture;on the other hand,through a non coupled function on the Lode angle parameter based on the CVGM fracture model can improve the prediction accuracy of ductile fracture;ductile cracking occurs first at the weld hole,while brittle fracture may occur at the flange weld of the steel beam due to high stress.(3)A thermodynamic finite element model of welded steel joints was established,and a sequential coupling simulation analysis was performed by using a double ellipsoidal heat source model.The entire process of steel joints from welding to the end of cooling was simulated.Meanwhile,the ductile fracture problem of steel joints under cyclic loading considering residual stress was studied.The results show that the number of cycles required for the first ductile crack initiation of steel joints considering residual stress is basically consistent with previous analysis,that is,small amplitude loading has little impact on ductile crack initiation.(4)Four reinforcement schemes for steel joints considering the influence of floor slabs were designed,and a refined finite element model was established.The reinforcement effect was quantitatively evaluated by using the fracture model.The results show that only strengthening the lower flange can delay the ductile cracking of the joint,but it does not change the current situation that the weld hole on the upper part of the joint is prone to fracture;Using the method of attaching steel plates to the upper and lower flanges can move the plastic hinge outward onto the beam member,changing the current situation that welding holes are prone to ductile fracture.