Study On Behavior Of CHS K-joints Based On Micromechanical Fracture Mechanism

CHS K-joints are widely used in large-span space structures because of their excellent performance.It has been generally found that the failure of ductile fracture is an important failure mode of CHS joints in previous earthquakes.However,the influence of the ductile fracture has been hardly considered in the ultimate bearing capacity and seismic performance evaluation reasonably in current finite element analysis,resulting in a certain degree of error in the evaluation of the CHS joints' performance.Firstly,finite element modeling method of solid weld in CHS joints was established in this paper.Then,fracture load,fracture position and ultimate load bearing capacity of CHS K-joints under monotonic and ultra-low-cycle fatigue loading was analyzed based on micromechanical fracture mechanism.The influences of geometrical parameters on the seismic behavior of CHS K-joints were discussed.Fracture propagation was simulated by finite element analysis with the use of VUSDFLD subroutine of ABAQUS.Finally,combined with CVGM model,the constant amplitude loading was adopted to discuss the ultra-low-cycle fatigue behavior and the rule of damage accumulation for CHS K-joints under different amplitude loading and geometrical parameters.The results show that:(1)More accurate ultimate load bearing capacity can be obtained by considering the ductile fracture of CHS K-joints under monotonic loading with VGM.The smaller the gap of branches is,the greater tendency of ductile fracture failure is.(2)Failure mode can be obtained more accurately by considering the ductile fracture of CHS K-joints under cyclic loading with CVGM and the ductile fracture has a significant impact on the seismic performance of the joints.(3)Element size has a significant impact on fracture bearing capacity in the analysis of fracture propagation with subroutine,and suggested size can ensure the accuracy of the calculation and save computational time.(4)Under constant amplitude loading,the ultra-low-cycle fatigue behavior of CHS K-joints is greatly affected by loading amplitude and geometric configurations.Ultra-low-cycle fatigue life of CHS K-joints is inversely proportional to the load amplitude nonlinearly.The ultra-low-cycle fatigue behavior is better for CHS K-joints with bigger brace to chord diameter ratio ? and gap size between branches.(5)Ultra-low-cycle fatigue life of CHS K-joints can be predicted accurately based on the rule of damage accumulation obtained for constant amplitude loading.