| Ductile fracture generally undergoes significant plastic deformation and is the most common form of steel failure.With the rapid growth of the application of steel structures in China,it has important theoretical significance and practical value to study the toughness fracture mechanism of steel and accurately predict the toughness fracture behavior of steel.The fracture prediction method based on the microscopic void theory has good applicability for studying the ductile fracture behavior of steel structures.The theoretical basis is to study the interaction between stress,strain and the microstructure of the material,reveal the cause of crack initiation and expansion from the evolution process of the void inside the material,and establish a ductile fracture damage model based on the fracture mechanism.It is one of the effective methods to accurately predict the fracture of metal materials.In this paper,a combination of theoretical models,finite element analysis and fracture experiments is used to study the ductile fracture mechanism and ductile fracture model of steel based on void theory.In this paper,the finite element analysis of 15 groups of different high-stress triaxial stress states and63 groups of low-stress triaxial stress states was carried out to study the ductile fracture mechanism of steel under complex stress states,the law of void evolution with stress triaxiality and Lode parameters is quantified.From the microscopic point of view,two meso-mechanisms that depend on the stress state of the steel are verified,namely the tensile fracture mechanism and the shear fracture mechanism.Secondly,this paper converts the evolution mechanism of voids on the micro scale to the overall material properties of the material on the macro scale.Based on the Rice-Tracey model and the VNISFM model,a ductile fracture model that comprehensively considers the effects of stress triaxiality and Lode parameters is proposed.And this model is suitable for the fracture performance analysis of metal materials under tensile stress state,shear stress state and tensile-shear composite stress state.Then introduce the damage factor into the ductile fracture model,write the USDFLD subroutine in Fortran language,and insert it into the finite element program Abaqus / Standard for fracture prediction analysis.Thirdly,the finite element analysis of three groups of Q345 steel material properties tests performed by existing scholars is carried out.The simulated information of crack initiation,crack propagation and load-displacement curve of the specimens is in good agreement with the test results.The parameter verification of the ductile fracture model is completed according to the stress state of the fracture starting position of the specimen.On this basis,the axisymmetric tensile stress state(T = 1/3,L =-1)is introduced as the boundary point assumption between the tensile fracture branch and the shear fracture branch,which reduces the difficulty of parameter verification of the fracture model.Finally,the author conducted two sets of cross-type rigid joint specimens uniaxial tensile test,using the calibrated fracture model to predict its fracture.The calibrated fracture model was also used to predict the fracture of a group of square steel tube welded joint specimens completed by other scholars.The results show that the prediction accuracy error of the ductile fracture model in this paper is less than 15%,and it can accurately capture the steel fracture start position and crack propagation path.The innovation of this paper is to propose a new steel ductile fracture model based on micro-macro multi-scale method.Compared with the existing ductile fracture model,this model has the advantages of microscopic fracture mechanism support,fewer model parameters,simplified parameter verification process,and is suitable for the analysis of complex stress state of steel joints. |
