Failure Analysis And Numerical Simulation Of Meta Forming Process Based On Ductile Damage Model

From the view of mechanics,metal forming is a complicated dynamic contact process which involves different physical phenomena including:metal plasticity,non-linear contact with friction,and evolving boundary conditions.The ductile damage,fracture,fatigue,abrasion and corrosion often occur due to the extremely complex stress state of material and serviously deteriorate the material property.The critical problem to be solved is estimating the property of the material,predicting the failure of specimen and optimizing the process parameters.In this paper,in order to understand the formability in the metal forming process and the failure mechanism of the processed material,theoretical study,numerical simulation and experimental observation are combined to study the metal forming process.The main work in this paper is as follows:(1)An elastoplastic constitutive equation accounting for isotropic ductile damage is implemented into the finite element code ABAQUS with a user-defined material subroutine UMAT.The simulations of the damage evolution and ductile fracture in a sheet metal blanking process have been carried out by the FEM.The effects of varying technological parameters related to the quality of products are investigated.In order to guarantee computation accuracy and avoid numerical divergence during large plastic deformation,a specified remeshing technique is successively applied when severe element distortion occurs.In the simulation,the evolutions of damage at different stage of the blanking process have been evaluated.It can be observed that the damage localized in a very narrow shear zone along the die edge outflow zone.With the increasing punch displacement and the decreasing of die edge radii,the damage evolution increases rapidly.Distributions of damage obtained from simulation are in proper agreement with the experimental results.(2)A ductile damage model which takes account of anisotropic behavior of Magnesium alloy AZ31B and the influence of different forming temperature are integrated into an explicit finite element frame work to predict the damage evolution during tube hydroforming process.Based on Continuum Damage Mechanics,an elastoplastic constitutive equation accounting for the coupling between the Barlat's anisotropic yield function and anisotropic damage are implemented into a user-defined material subroutine VUMAT.From the numerical results,it can be observed that the circular damage is much greater than the axial damage,and the property of the specimen could be improved by optimizing the load path.The material model constants are determined from stress-strain response which can be obtained by carrying out especially designed ring tensile tests.Furthermore,related experiments of tube hydroforming process with elevated temperature are also performed to validate the proposed model.(3)To get the full knowledge of the failure mechanism of Magnesium alloy,based on theory of Continuum Damage Mechanics(CDM)and meso-damage mechanics,Marciniak and Kuczynski(M-K)model,anisotropic damage model and GTN damage have been introduced to predict the formability for thermal stamping of Magnesium alloy sheet.The numerical algorithms of the damage model are implemented by developing the user-defined material subroutine(VUMAT)into the finite element Solver ABAQUS\Explicit.Moreover,related experiments of thermal stamping are also performed to validate the model.The forming limit diagrams(FLD),the damage evolution and the percentage of cavities have been obtained during the process simulation.It has been verified the damage during the large plastic deformation is generally described as a sequence of events comprising void growth,nucleation,coalescence and propagation of micro-cracks.