Study On Semi-coupled Ductile Fracture Criterion And Its Application In DP590 High Strength Steel

Under the background of automobile lightweight,high-strength steel sheet is used more and more widely in automobile.Fracture is one of the three defects in high-strength steel sheet forming.With development of ductile fracture criterion theory and computer technology,the application of ductile fracture criterion in the failure prediction of sheet metal forming has become extensive.Compared with the coupled ductile fracture criterion,the uncoupled ductile fracture criterion is more popular in industrial applications because of its simple parameter calibration process.However,the uncoupled ductile fracture criterion cannot describe the damage-induced softening in the process of material forming.In order to solve this problem,this paper aims to explore a semi-coupled ductile fracture criterion by combining the advantages of coupled model and uncoupled model,and then apply it to DP590 high strength steel.Taking the 1.5mm thick DP590 high-strength steel sheet as the research object,six types of tensile specimens were firstly designed,and then used to measure the mechanical properties and fracture parameters of DP590 respectively.In the experiment,the resistivity method was used to measure the initial damage and fracture of specimens,and DIC technology was used to measure the strain on the surface of specimens.The experimental results show that the yield strength anisotropy of DP590 sheet is not obvious,and the anisotropy of r-value is also weak.The fracture of DP590 sheet metal tends to be isotropic.It was also found that DP590 have obvious damage-induced softening phenomenon and the damage evolution was slow.The correct description of the plastic behavior of DP590 is the premise of the study of ductile fracture criterion.In this paper,a reduced Yld2004-18 p anisotropic yield function(called RYld2004)and Swift+Voce linearly combined hardening model were selected to characterize the plastic deformation of DP590.According to the experimental data of different orientations,the RYld2004 yield function was calibrated.And Based on the true stress-true strain relation along rolling direction,the parameters of the hardening model were fitted.Based on ABAQUS finite element software,the constitutive model was implemented as VUMAT subroutine by Fortran programming language and applied to the tensile simulation of specimens.The comparison of experiment and simulation results show that the constitutive model accurately describes the plastic deformation behavior on each orientation of the DP590.Based on the above constitutive model,a semi-coupled ductile fracture criterion was proposed based on DF2012 uncoupled ductile fracture criterion.The semi-coupled model is composed of three parts: the plastic model that describes the deformation process before initial damage,the initial damage criterion and the damage evolution model.The plastic deformation of the material before initial damage is described by the constitutive model in this paper.The DF2012 uncoupled ductile fracture criterion was modified as the initial damage model and used as the criterion for the occurrence of initial damage.And then a damage variable used to quantitatively describe the damage evolution degree of materials was established and coupled to the plastic model to establish the damage evolution model.In addition,the finite element algorithm for semi-coupled ductile fracture criterion was introduced.In order to verify the semi-coupled ductile fracture criterion,based on the DP590 fracture experiment,the initial damage model and damage evolution parameters were calibrated by the experiment-simulation hybrid method and the inverse analysis method respectively.The center hole specimen,specimen with notched of radius five and shear specimen were used to calibrate the parameters of the initial damage model,and then constructing the initial damage loci of DP590.Based on the load-displacement curve of the center hole specimen,the damage evolution parameters were calibrated by the inverse analysis method.Since the reverse analysis method requires repeated tensile simulation of corresponding experiments,this paper built a joint optimization platform based on ISIGHT optimizes software,MATLAB scientific computing software and ABAQUS finite element software.Based on the ABAQUS/Explicit module,the VUMAT user subroutine of the calibrated semi-coupled model was applied to the tensile simulation of each specimen.The results show that the semi-coupled ductile fracture criterion can accurately predict the fracture behavior of DP590 materials under different stress states and describe the damage induced softening phenomenon in the DP590 forming process.