Research On Fracture Resistance Of Welded H Section Steel Beam-to-column Connections Based On Micromechanical Fracture Models Of Q460 Steel

High-strength steel structure connections have obvious advantages compared with ordinary steel structure connections.They have been successfully applied in practical engineering at home and abroad,but there are still problems such as difficulty in welding,increase in yield strength ratio and decrease in ductility compared with ordinary strength steel structures.At present,there are few studies on high-strength steel structures at home and abroad,and most of the research at home and abroad is based on traditional fracture mechanics methods.The traditional fracture mechanics methods assume that defects and cracks exist,and there is high strain constraint at the initial crack tip,therefore they are mainly suitable to study the brittle fracture problem or the pseudo-brittle fracture problem with extremely limited local plastic damage,but not suitable to study the ductile fracture problem at the position with no obvious defect and in large-scale yielding.In addition,since traditional fracture mechanics does not consider the influence of stress triaxiality,it cannot be applied to the prediction of connection fractures where there may be significant three-dimensional stress states.The micromechanics-based fracture models do not assume initial defect.They simulate the stress and strain field at the microstructural scale,establish the relationship between the macroscopic fracture process and the microstructure of the material,and then apply the criteria to conduct prediction.Micromechanics-based fracture models make it possible to predict ductile fracture of steel structures.In order to study the ductile fracture mechanism of high-strength steel structural connections,the following work was carried out:(1)The research status of high-strength steel structural connections at home and abroad in recent years was combed,the research methods of these researches were summarized,and they were classified into two categories.The steel structure connections based on traditional fracture mechanics were studied in the first category,and the steel structure connections based on the micromechanical model criteria were studied in the second category.The limitations of these two methods,the development and application of high-strength steel structure connections in recent years were summarized.(2)A finite element model for the high-strength steel connection partial specimen under the action of existing monotonic loads was established,and the VUMAT subroutines with the calibrated micromechanical fracture criteria under monotonic loading was embeded into the finite element model.The fracture path and the post-crack curve of the high-strength steel structure connection partial specimen under monotonic load were compared with the test results to verify the applicability of the VUMAT subroutines with micromechanical fracture models.(3)Finite element models for high-strength steel structure connection specimens under cyclic loading were established,the micromechanical fracture model under cyclic loading was used to predict the fracture of high-strength steel structure connections,and the hysteretic curves before fracture were extracted and compared with the test results to verify the accuracy of the finite element modeling and the micromechanical fracture model under cyclic loading.(4)By verifying the accuracy of finite element modeling,four high-strength steel structure connections with different structures were designed,and the finite element models under cyclic loading were established.The calibrated micromechanical fracture model under cyclic loading was used to predict the fracture and ultimate bearing capacity of the four high-strength steel structure connections with different structures.The fracture moment and ultimate bearing capacity of the high-strength steel connections with different structures were compared.A high-strength steel structure with good bearing capacity and late fracture moment was obtained.(5)Variable material analyses on the four connections with different structures designed in this paper were performed.The properties of the ordinary-strength steel structure material Q345 B steel were given to the connections,and the finite element analyses of the connections under cyclic loading were performed.The calibrated micromechanical fracture criterion was used to compare the fracture performance differences between high-strength steel structure connections and ordinary-strength steel structure connections of the same structure.