The Mesoscopic Damage Mechanism Research And Numerical Simulation In The Process Of Sheet Metal Forming

Sheet metal forming technology is an important plastic processing technology,and it has been widely used in automotive ships,aerospace,instrumentation equipment and other industries.Due to the extremely complex stress state of material in plastic deformation zone during sheet metal forming process,there is a large deviation between the prediction of theoretical model and experimental results in some special conditions,and the theoretical studies on the damage and fracture mechanism of metal sheet in complex environment are not systematic and complete enough.Therefore,it is necessary to study the damage failure mechanism of materials under complex stress conditions in sheet metal forming process,which not only makes some theoretical innovation,but also has very important engineering application value to prevent and improve the fracture and shearing failure of parts during sheet metal forming.The main research work of this paper is as follows(1)In view of the extremely complicated stress state of sheet metal plastic deformation region during sheet metal forming,and considering the deformation process of the plate mainly by shearing(such as:fine blanking,punching,drawing,etc.),the shear modified GTN damage model proposed by Zhou was introduced,which shear plays a key role in the tearing failure form.The paper deduced the damage constitutive equation and the numerical algorithm of the shear modified GTN model.The numerical algorithm of the modified damage model is implemented by developing the user material subroutine(VUMAT)via the finite element explicit Solver ABAQUS\Explicit.(2)The material parameters of shear modified GTN damage model were determined by using the microstructure analysis method and the reverse method.The in-situ tensile tests with three different shapes were carried out and the microstructure observation was analyzed.The theoretical derivation formula of the damage constitutive model and the damage model parameters are verified by the tensile tests,the SEM observation and the FE calculation results.The numerical simulations were carried out to analyze the stress triaxiality,equivalent plastic strain,void volume fraction distributions,the load-displacement curve and stress state curve of three tensile specimens with different stress triaxial states during the crack initiation.(3)Taking the fine-blanking forming as the research object,the stress-strain distributions of the local plastic shear zone under the complex stress triaxiality conditions were analyzed based on the shear modified GTN damage model,and the damage evolution of the porous materials and the effect of shear damage on the failure behavior of ductile materials were studied.By embedding the bonding elements into the 2D Voronoi polycrystalline model,the crack simulation was carried out by means of the representative volume element model and the cohesive model,and the damage and fracture behavior of the plates in the process of fine blanking was predicted.(4)Aiming at the anisotropic behavior of AZ31 Mg alloy and considering the influence of temperature on the formability of metal sheet,the modified damage constitutive model,which can describe the plastic anisotropy behavior of sheet metal,was presented based on the continuum damage mechanics theory and the meso damage mechanics theory,respectively.The constitutive equations and numerical algorithm expressions of the modified damage model were derived in the paper.The damage failure behavior of the warm stamping process for AZ31 Mg alloy sheet was studied by FEM numerical simulation and experimental tests.