| In the case of rare or extremely rare earthquakes,the steel frame beam joints are prone to low cycle fatigue fracture due to the low frequency of seismic amplitude.For steel structure fatigue research methods,the fatigue fracture damage model of traditional fracture mechanics,crack tip opening displacement(stress intensity factor K CTOD and J integral)can be used to predict the initial crack tip is high strain constraints and limited plastic damage fracture problem,however,the fatigue model based on traditional fracture mechanics is the premise condition of initial crack,the crack and the expansion of the two stages of crack,the traditional fracture mechanics method without considering the influence of triaxial stress degree,can not accurately reflect the actual component or actual structure of the true state of fatigue,and low cycle fatigue damage with a wide range of yield,so traditional fracture mechanics method is not applicable to solve the problem of low cycle fatigue.The cyclic growth model(CVGM,the hole nucleation,expansion and poly-nucleation of fine micro-mechanics)can be used to predict the initial crack and crack propagation of components or structures under cyclic loading.In addition,the fatigue evaluation and life prediction of beam column welding joints can be carried out by using CVGM fracture theory for ultra-low cycle fatigue fracture behavior of steel structure beam column welding joints.Firstly,CVGM theory is used to analyze the mechanism of ultra-low cycle fatigue and discuss the parameters affecting the CVGM model.On this basis,establish finite element model of beam welding,secondly,based on the simulation results to write CVGM FORTRAN computer program of the theory of calculating the under constant amplitude and variable amplitude load index of fracture,and evaluate the welding beam-column joints under different loading mechanism of ultra low cycle fatigue properties,the analysis results show that the loads on the weld toe of beam-column welded joint and hole stress concentration,the first crack,increase with the displacement amplitude loading mechanism,and gradually increase,reduce the less fatigue life.CVGM theory in order to compare the effect of different section size on the low cycle fatigue,the modeling method and the size of column section and beam section made a corresponding adjustment,different unit with the same unit of beam-column joints,found that the results of simulation show the same unit of beam-column jointsobvious fracture behavior,the index of fracture,fracture demand,fracture parameters such as fracture ability;The pillars(beam)of section size unchanged,the(post)the section size of beam becomes large or small,to analysis the fracture index,find that the change of the beam section size will cause a transformation of the fracture position,and change the column section size,will not affect the position of the break.Steel frame model is set up,after a FORTRAN program calculation,found the beam-column node location in the weld fracture happens,and through the comparative analysis of the fracture parameters show that the early crack point of the final fracture index value and the fracture is one of the biggest demand value,CVGM model is verified by the effectiveness of fracture in the earthquake prediction,also for the earthquake under the action of steel structure or component provides the basis for ultra low cycle fatigue fracture prediction. |
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