Study On Performance Of CHS T-Joints Based On Micromechanical Fracture Modal

The ductile fracture of steel structure has been already proved to be a common failure mode with great danger,which always occurs in earthquakes.Circular steel tubular joints are widely applied in long-span spatial structure for their excellent performance,among which planar T-joints are one of the most basic and widely-used types.In earthquakes,the ductile fracture of tubular joints always appear in node region.However,there is no acknowledged research method about prediction of joint fracture in current stage,most of existing researches focus on beam columns and mainly adopt the macroscopic fracture theory based on fracture mechanics.But this method needs to follow the initial assumption of cracks,it only applies to brittle fracture with limited yielding,not to ductile fracture of large-scale yielding with no initial cracks.Therefore,the author proposes the micro fracture theory which is more appropriately applied to ductile fracture of tubular joints.This paper first introduces establishment methods of unstiffened tubular T-joint and lines entity weld in finite element and systematically explains principles of external isolated grid sub-modeling technique principles in finite element and discuss about the principles of the selection of unit types;predict the ultimate bearing capacity of six groups of unstiffened tubular T-joints in different sizes by adopting finite element software ABAQUS and VGM model,and analyze the factors that affect the ultimate capacity of joints by combining finite element calculation results and normative numerical value;then predict the cracking properties of two groups of unstiffened tubular T-joints which are under ultra-low cycle fatigue loading in existing tests by adopting CVGM model and further analyze the bearing capacity of joints during the crack extension by adopting subroutine module VUSDFLD combining crack extension model of microscopic fracture criteria.The results show that : 1.Considering finite element model of tubular lines entity weld,the stress state of actual joints can be simulated by precise modeling;2.VGM model can well predict the crack location,time and ultimate bearing capacity of unstiffened tubular T-joint,which are close to the value from calculation;3.CVGM model can obtain crack moment of unstiffened tubular T-joint more accurately,but the crack moment can’t represent the moment that the joints lose their bearing capacity,thecrack moment may be earlier than that of joints destruction.We can use VUSDFLD,the ABAQUS subroutine,combining with extended model of microscopic fracture criteria to better simulate the performance after the crack of joints and obtain more accurate results,which can provide new methods for research about impact on seismic property of joints.