| Steel structures have been widely used in civil,industrial and bridge structures because of their advantages of lightweight,high strength,good construction quality,short construction period and renewable utilization.It is worth noting that steel structures are prone to cracks or major defects during processing,construction and using progress,thereby causing safety issues[1].According to the properties of Q235B and Q345B steels,this paper briefly introduces the problems of brittle fracture,the evaluation methods of defects in various countries in the world,the micro-damage model of steel and its welds,the fracture prediction method based on the micro mechanism to predict the fracture of steel.It will be applied in the research of the fracture performance of steel structures in the future.The specific work is as follows:(1)18 standard round bar samples were designed and produced,which were taken from the Q235B and Q345B steel base metal,welds and heat-affected zone,a total of six kinds of steel,the test loading method using uniaxial tension,through automatic extension.The stress and strain of the test piece were tracked through the entire process until the moment of rupture.The properties of the six steel materials were obtained,and the modified R-O model was used to fit the true stress-plastic strain constitutive relation.The results show that:Q235B steel shows better shape than Q345B steel,engineering strain can reach 54.2%and 53.32%,the area shrinkage is higher than Q345B;Using extensometer tracking can be better for steel mechanical properties and calculation of steel constitutive relations,to avoid the large error caused by the beam loading,the results of the finite element calculation and experimental comparison is more consistent;the use of the microscope from the fracture cross-section observation,the destruction of Q235B and Q345B are microporous aggregation destruction[2],there are more dimples,shear fracture occurred at the edge of the fracture,through mechanical analysis,can be judged by the static load under the destruction of the specimen from the middle,and continue to extend to the surface of the overall destruction;through the Q235B,the study of the mechanical properties of Q345B steel,the establishment of its constitutive relationship and the analysis of the microscopic fracture morphology are expected to be applied in the later research on the calibration of fracture toughness parameters of steel.(2)Thirty-six notched rod specimens were designed and fabricated with notch sizes of 1.25 mm,2.5 mm and 5 mm,respectively taken from the Q235B and Q345B steel base metal,weld deposit metal and heat affected zone.A total of six types of steels were loaded under monotonic loading,together with microscope observations and finite element calculations,and the toughness parameters of VGM and SMCS models for Q235B and Q345B steels were obtained.The characteristic lengths of the steels were observed by electron microscopy and then used.Finite element calculations of VGM and SMCS models predicted the failure displacement and the load of Q235 B and Q345 B steel.The results show that the toughness parameters of weld metal area of common steel Q235B and Q345B welds are greater than those of base metal and HAZ,and the dispersion of corrected toughness parameters is within a reasonable range,the prediction of failure load of Q235B steel by VGM and SMCS models.It can be controlled within 25%,and the prediction of Q345B steel can be controlled within 10%.The prediction of the failure load of the two models is more accurate than the prediction of the failure displacement.The validity of the micromechanical models of VGM and SMCS on the fracture prediction of Q235B and Q345B steels are verified;the toughness parameters in VGM and SMCS micromechanism models are not significantly related to steel strength indexes and yield ratios,however they are related to the plasticity of steel materials.The better the plasticity of the material,the greater the value of the toughness parameter.(3)18 standard round bar samples were designed and produced,which were taken from the Q235B and Q345B steel base metal,welds and heat-affected zone,a total of six kinds of steel were used.The test adopted a cyclic loading system and tracked fully by an automatic extensometer.The stress and strain of the specimen until the fracture moment,its hysteretic properties were calculated to calculate its energy dissipation characteristics,specimen failure morphology and fracture morphology,as well as strength degradation and stiffness degradation characteristics[3].Based on the plastic constitutive model of Chaboche steel,The critical material parameters of the cyclic constitutive model were corrected and compared with the experimental results using ABAQUS.The results show that the Q235B and Q345B steel specimens can be cycled for many times under cyclic load,and there are many dimples in the fracture,indicating that the ordinary steel Q235B and Q345B have good ductility;steel under cyclic loading The specimen will crack before the fracture occurs,expanding from the inside to the inside,and a short break occurs in the middle of the fracture.The entire fracture process is contrary to the failure process under monotonous loading.The hysteretic curves of Q235B and Q345B steel show:after strengthening and deteriorating,the strength and stiffness cycle performance of Q235B steel is higher than Q345B steel.The stiffness degradation degree of Q235B and Q345B steel test curves is greater than strength degradation.The isotropy constitutive parameter bjso has greater influence on the damage of the pretest piece than the parameter.By changing the size of the bjso,the stress range of the hysteresis curve can be adjusted.(4)72 notched rod specimens were designed and fabricated.They were taken from the Q235B and Q345B steel base metal,welds and heat-affected zone sections.A total of six steels were tested.Two cyclic loading methods were used to test the degradation using finite element calculations.The parameters of the effective plastic strain model(DSPS)and the microcavity damage model of the cyclic cavity expansion model(CVGM)were predicted and the failure of the notched round bar was predicted.The results show that the Q235B and Q345B hysteresis curves can be used to calculate the Q345B steel base material and the heat affected zone shows higher energy intensity than the Q235B steel.The heat affected zone of the same type of steel shows higher energy intensity than the parent metal and weld deposit metal;the mixed enhanced model Von Mises flow is used.The rules are used to fit the cyclic constitutive equations of’Q235B and Q345B steels.The load-displacement curves calculated by ABAQUS are in good agreement with the experimental curves.The fracture prediction models CVGM and DSPS can predict the fractures under cyclic loading of steels due to the specimens.The number is limited,data points form a certain dispersion and are affected by the welding effect.There is a certain deviation in the prediction results.On the whole,the CVGM model is better than DSPS model,and the prediction degree under the C-PTF loading system is better than the CTF loading system.It shows that the two models can predict the Q235B and Q345B steels better under the earthquake. |
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