Fracture Prediction And Parameter Calibration Of Q355B Steel Based On Microscopic Fracture Mode

In recent decades,steel structures have been widely used in civil engineering construction due to their advantages of light weight,high strength,and convenient processing and construction.The widespread application of steel structures also carries certain risks,which may lead to fracture and failure under overload and some unexpected situations.Therefore,the fracture mechanism of existing steel should be summarized and further studied.At present,the micro fracture models used to study the fracture of steel structures under static load mainly include VGM and SMCS models.But the latest research shows that these models are only used to determine the parameters of steel fracture,such as stress triaxiality and equivalent plastic strain,and are no longer applicable to structures with complex stress states.Smith introduced the Lode angle parameter on the basis of the CVGM model and obtained the SWDM model.This model can better describe the ductile fracture of metal materials under complex stress states.Therefore,based on previous research,this article will use the SWDM model to predict and calibrate the ductile fracture of Q355 B steel base material and weld metal under uniaxial tension.The main content is as follows:(1)Five monotonic tensile specimens with different stress states were designed,including smooth round bar specimens for obtaining material properties data required for finite element simulation,circumferential notch specimens with high stress triaxiality,grooved plate specimens with planar stress states,rectangular notch specimens with different Lord’s angle parameters,and shear specimens with low stress triaxiality.A monotonic tensile test was conducted and the influence of various variables on the ductile fracture of the specimen was analyzed.(2)The PW,Voce,Swift,and Swift Voce mixed models were used to fit the true stress-strain data of the smooth round bar specimens of the base metal and weld metal before necking,and the true stress-strain data after necking was extrapolated to obtain the required material data for finite element simulation.Conduct finite element modeling analysis on various notched specimens,compare the load displacement curves obtained from tests and finite element simulation,and analyze the advantages and disadvantages of the four constitutive models.Finally,the fracture stress state parameters of various specimens were analyzed.(3)Extract the stress triaxiality,Lord’s angle parameter,and equivalent plastic strain of the first fracture point of each specimen,and calibrate the unknown parameters of the VGM model and the SWDM model.Using the VGM model and the SWDM model expression as fracture criteria,a VUSDFLD user subroutine was written using Fortran language.Finally,ductile fracture prediction and post crack path analysis were performed on the Q355 B steel base metal and weld metal through finite element simulation.