Fracture And Low-Cycle Fatigue Assessment Of Beam-Column Connections Based On The Strain Modified Mean Stress Model

Under the action of strong earthquake,cumulative damage will be produced in the steel structure buildings,which leads to ultra-low cycle fatigue and failure.Beam-column connections are the high incidence areas of fatigue and failure.Due to the popularity and rapid development of s teel structure,a large number of steel structure buildings have been established in many frequently-occurring earthquake areas.Therefore,the research on the safety optimization and fail ure mechanism of steel structures has become particularly important i n this era.A lot of studies show that the fracture mechanics based on microscopic mechanism,including void growth model(VGM)and stress modified critical strain model(SMC S)for monotonic loads,and cyclic void growth model(CVGM)and degraded significant plastic strain model(DSPS)for cyclic loadings,can be applied to predict the ultra-low cycle fatigue of beam-column connections caused by large deformation without obvious defects.In this paper,combined with the advantages of the existing micromechanical fracture theories,a fracture model for monotonic loadings is proposed.Then the model is extended to the field of fatigue.The theoretical analysis and experimental research of these models are carried out to better predict the fatigue and fracture of steel structures,which are mainly in the followi ng aspects.(1)The uniaxial tensile test of Q345 steel was completed.Then the modification method of tensile test data and real stres s-strain curves are proposed to propose the dates of the test.The micromechanical fracture model is embedded into ABAQUS finite element simulation by using user subroutine USDFLD,and the applicability of VGM and SMCS model in several specific structures is analyzed.The results shows that the crack initiation points of VGM and SMCS may appear in different positions when there are multiple fracture risk points.At this time,it is suggested that the prediction of VGM shou ld prevail.(2)A strain modified mean stress model(SMMS model)is proposed,which combines the advantages of VGM and SMC S model.The VGM,SMCS model and SMMS model parameters of Q345 steel made in China were calibrated through the existing tensile tests of notched round bars.The characteristic and calculation workload of the each model was analyzed and compared.Finally,the effectiveness of SMMS model in predicting ductile fracture of steel was analyzed by using the existing local welded tensile tests of Q345 beam-column connections.The result shows that the SMMS model can be used to predict the ductile fracture of welded connections in steel structures.(3)A modified degenerate significant plastic strain model(MDSPS model)is proposed by extending SMMS model to ultra-low cycle loading.Compared with DSPS model,the model has a simpler calculation procedure and can be used to predict the fatigue failure of beam-column connections under cyclic loading.The parameters of CVGM,DSPS and MDSPS models are fitted by the existing cyclic loading tests of smooth notched round bars.The damage degradation parameters are calculated by taking the equiv alent plastic strain accumulation and equivalent plastic strain as damage variables respectively.The effectiveness of MDSPS model in fracture prediction of beam-column connections is analyzed.(4)This paper introduces the ultra-low cycle fatigue tests of six steel frame beam-column connections completed by the research group,analyz es the failure forms and laws of beam-column connections,establishes the finite element model of beam-column connections by ABAQUS,and uses the MDSPS model proposed in this paper to predict the fatigue life of beam-column connections under cyclic loads,and evaluates the applicability of MDSPS model to predict the ultra-low cycle fatigue life of beam-column connections.The results show that the MDSPS model has a good effect in evaluating the ultra-low cycle fatigue of beam-column connections.