Study On Ductile Fracture Behaviors Of Diaphragm-through Joints Considering Effect Of Defect And Access Hole

Large cumulative plastic deformation in steel structure will occur due to repeated loading under the action of strong earthquakes.In particular,under the influence of long-period ground motion,the cumulative damage effect of super high-rise steel structures will be amplified to some extent.With the development of destructive mechanics such as fracture mechanics and damage mechanics,the ductile fracture problem of steel-structured beam-column joints driven by large cumulative plastic deformation has attracted much attention,and the related research has made breakthrough progress.This thesis focuses on the effects of welding defects and the shape of access hole on the ductile fracture behavior of the diaphragm-through joints.According to the existed experimental model,the finite element model is established by ABAQUS finite element software.The Chaboche combined hardening constitutive model,the micro-fracture criterion and the implicit analysis method guarantee the realization of accurate ductile fracture prediction.In addition,the fracture performance of the critical part after ductile cracking to a certain extent is analyzed.The main research contents and achievements are as follows:(1)The UMAT subroutine for material model and fracture criterion is improved which can achieve material nonlinearity and ductile fracture simulation.The method of joint fracture performance evaluation based on fracture index is proposed after ductile fracture prediction to a certain extent.(2)The finite element simulation of the diaphragm-through joints in the existing test is carried out by using an extended subroutine containing the constitutive model of Chaboche mixed hardening material and the micro-fracture criterion.The calculation model is included in the butt weld of the beam flange and the transverse baffle.The analysis results show that the defective model is finally destroyed at the butt weld of the upper flange and the transverse partition because the defective model can simulate artificial cracks at the junction between the arc runner and the base metal,which is in good agreement with the test results.Moreover,the defect is not only the cause of stressconcentration,but the large equivalent plastic strain value of the defect driven by the large cumulative plastic deformation sometimes,which will accelerate the ductile cracking of the heat affected zone near the weld or weld.(3)The effect of the shape of the access hole on the ductile fracture behavior of the joint shows that the access hole form has little effect on the first cracking position,but has a great influence on the first cracking moment;the two diaphragm-through joints are subjected to the equivalent plastic strain and stress concentration at the first cracking.The effect of the shape of access hole has little effect on the equivalent stress in the second cracking,but it has little effect on the equivalent plastic strain;the failure parameters of equivalent plastic strain and the stress triaxial interaction induced cracking.And the difference in the form of the access hole sometimes causes one side of the equivalent plastic strain and the stress triaxiality to play a leading role,and the other side has a relatively small influence.(4)The different access hole forms have certain influence on the hysteretic behavior of the joint.The energy consumption of the ?-type access hole joint is lower than that of the ?-type access hole joint.In terms of bearing capacity,the ?-type access hole joint is better than the ?-type access hole joint.The stiffness degradation of the ?-type access hole joint is earlier than that of the ?-type access hole joint,and the stiffness coefficient is slightly smaller than that of the ?-type access hole joint during the subsequent cracking.(5)The analysis method of fracture performance of key parts after ductile cracking to a certain extent has the effect of assisting fracture prediction.The analysis results show that the I-type access hole joint is more likely to break along the width of the flange at the butt weld of the upper flange.The ?-type access hole joint is more likely to break along the width of the flange at the butt weld of the lower flange;the cracking tendency along the width of the flange at the intersection of the over-weld and the flange will continue to expand from the crack to both ends.The occurrence of local buckling reduces the fracture index and alleviates the development of cracking.