Calibration And Application Of Ultra-low Cycle Fatigue Fracture Of Cast-steel Based On Micromechanical Fracture Models

Cast steel joint has good adaptability and can reduce the influence of the weld heat-affected zone in the joint region.It has a good application prospect in the seismic resistance of beam-column joints.Beam-column joints will happen large plastic deformation and will happen ultra-low cycle fatigue fracture under large earthquake.Cast steel is prone to defects during the manufacturing process,and the defects are often the beginning area of the fracture.It’s necessary to study the ultra-low cycle fatigue fracture performance of cast steel joints.In winter or low temperature environment,the ductility of steel will reduce,and steel structure or steel members are more likely to fracture under earthquake.Based on the micromechanical model of steel,this paper studied the ultra-low cycle fatigue fracture performance of G20Mn5 QT and calibrated the micromechanical model parameters of G20Mn5 QT at different temperatures by experimental research and finite element analysis.These parameters were applied to the actual joints to verify the applicability.And this paper evaluated the performance of the ultra-low cycle fatigue of the cast steel joints at low temperature.The specific work of this paper is as follows:(1)The fracture forms of steel under earthquake were introduced.The micromechanical models suitable for prediction of ultra-low cycle fatigue fracture of steel were introduced,including cyclic void growth model(CVGM)and degraded significant plastic strain model(DSPS);(2)Designed and tested 12 smooth round bars of G20Mn5 QT and determined the properties of G20Mn5 QT cast steel at 20 ℃,-20 ℃,-40 ℃ and-60 ℃.Four groups of mixed hardening model parameters were obtained at corresponding temperature;(3)Cyclic tests at 20℃,-20℃,-40℃ and-60℃ were carried out on 48 specimens with three kinds of notches.The damage degradation parameters of CVGM and DSPS of G20Mn5 QT at four kinds of temperatures were calibrated with finite element analysis method.And the characteristic length of the cyclic void growth model of G20Mn5 QT was obtained through sem test of notched specimens.;(4)The mixed hardening model parameters and the damage degradation parameters of CVGM were applied to the beam-column joints,and the fracture moment of the joints was obtained.The accuracy of the calibration parameters was verified by comparing the test results of the existing literature.After that,the influence of temperature on the ultra-low cycle fatigue fracture of the joints was analyzed.and the performance of the joints at different temperatures was then evaluated.(5)The VUSDFLD subroutine was compiled to simulate the crack growth path of the joints under the ultra-low cycle fatigue load.The fracture process of the joints in this paper was obtained,and the influence of temperature on the crack growth of the joints was studied.