To accurately predict the occurrence of ductile fracture in metal forming processes,the Gurson-Tvergaard(GT)porous material model with optimized adjustment parameters is adopted to analyze the macroscopic stress-strain response,and a practical void nucleation law is proposed with a few material constants for engineering applications.Mechanical and metallographic analyses of uniaxial tension,torsion and upsetting experiments are performed.According to the character of the metal forming processes,the basic mechanisms of ductile fracture are divided into two modes:tension-type mode and shear-type mode.A unified fracture criterion is proposed for wide applicable range,and the comparison of experimental results with numerical analysis results confirms the validity of the newly proposed ductile fracture criterion based on the GT porous material model.
The one-step finite element method(FEM),based on plastic deformation theory,has been widely used to simulate sheet metal forming processes,but its application in bulk metal forming simulation has been seldom investigated,because of the complexity involved.Thus,a bulk metal forming process was analyzed using a rapid FEM based on deformation theory.The material was assumed to be rigid-plastic and strain-hardened.The constitutive relationship between stress and total strain was adopted,whereas the incompressible condition was enforced by penalty function.The geometrical non-linearity in large plastic deformation was taken into consideration.Furthermore,the force boundary condition was treated by a simplified equivalent approach,considering the contact history.Based on constraint variational principle,the deformation FEM was proposed.The one-step forward simulation of axisymmetric upsetting process was performed using this method.The results were compared with those obtained by the traditional incremental FEM to verify the feasibility of the proposed method.
