| Fire is one of the main natural disasters faced by steel structures.The uniaxial tensile mechanical properties and cyclic mechanical properties of high strength steel structures are obviously changed by high temperature fire.The damage of the whole steel structure may be caused by the deterioration of the mechanical properties of individual steel members.Therefore,it is particularly urgent to study the mechanical properties of high-strength steel structures after high temperature and the fracture properties of steel structures after fire.The fracture models based on the micro mechanism can be used to accurately predict the ductile fracture of steel structure,and whether the micromechanical fracture models can be used to accurately predict the fracture of steel structure after fire deserves further study.Therefore,the following work was carried out in this paper:(1)By analyzing the research status of mechanical properties of high strength steel after the high temperature cooling,the uniaxial tensile tests were carried out on the smooth round bar specimens made of Q460D steel and ER55-G welding material which were naturally cooled after 900?and at room temperature to obtain the uniaxial tensile mechanical properties and cyclic loading mechanical properties of each material,as well as the basic material property data required by the later finite element analysis(FEA).(2)The cyclic loading tests were carried out on the smooth round bar specimens made of Q460D steel and ER55-G welding material which were naturally cooled after 900?and at room temperature to obtain the cyclic loading mechanical properties of each material.The differences of cyclic mechanical properties of the materials were analyzed from the failure modes,hysteretic properties and ductility characteristics.(3)By analyzing the research status of micromechanical fracture models,the void growth model(VGM)suitable for monotonic tensile loading and the cyclic void growth model(CVGM)suitable for extremely low cycle fatigue loading were selected as the theoretical basis,and the parameters of the VGM model and CVGM model of Q460D steel and ER55-G type welding material after 900?high temperature cooling were calibrated.(4)Three base metal notched round bar specimens and three welding material notched round bar specimens were extracted from Q460D steel plate and welded Q460D steel plate(using ER55-G welding wire)perpendicular to the weld length direction.After the specimens were extracted and manufactured,they were firstly treated at 900?,then naturally cooled to room temperature,and then monotonic tensile tests were carried out on them on the MTS test machine.Then,combining with the ABAQUS FEA,the deformation of the guage length part corresponding to the slope change of the load-guage length part deformation curve obtained from the test was regarded as the fracture deformation,and the stress and strain data corresponding to the fracture deformation at the notched section of the specimen were obtained by FEA.Combined with the VGM criterion formula,the fracture toughness parameter?in the VGM model of Q460D steel base metal and ER55-G welding material perpendicular to the weld length after 900?cooling were calibrated.What's more,the scanning electron microscope tests on fracture surfaces of smooth notched tensile specimens were performed to obtain the characteristic length parameters7)*.(5)Six base metal notched round bar specimens and six welding material notched round bar specimens were extracted from Q460D steel plate and welded Q460D steel plate(using ER55-G welding wire)perpendicular to the weld length direction.After the specimens were extracted and manufactured,they were firstly treated at 900?,then naturally cooled to room temperature,and then extremely low cycle fatigue loading tests were carried out on them on the MTS test machine.Then,combining with the ABAQUS FEA,the deformation of the guage length part corresponding to the slope change of the last tensile cycle in the load-guage length part deformation hysteresis curve obtained from the test was regarded as the fracture deformation,and the stress and strain data corresponding to the fracture deformation at the notched section of the specimen were obtained by the FEA.Combined with the CVGM criterion formula,the damage degraded parameter ?CVGM of Q460D steel base metal and ER55-G welding material perpendicular to the weld length after 900?cooling under the extremely low cycle fatigue loading were calibrated.(6)The calibrated VGM model was introduced into the ABAQUS subroutine VUMAT,which is suitable for explicit dynamic analysis,to simulate the actual loading process of the Q460D steel base metal and the ER55-G type welding material notched round bar specimens under the action of uniaxial tensile load,the crack development path and load-displacement curve of the whole process of each kind of specimen were obtained. |
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