Non-Cassical Theory Plasticity

Empirical and trial-and-error methods used to be adopted for the design of moulds and technologies for precision or net forming, which usually leads to considerable material consumption, high cost, long trial-producing cycle, and poor product quality. This situation has been changed in recent years by FEM (Finite Element Method) and CAE (Computer Aided Engineering). However, the popular commercial FEM codes are usually based on classical elastoplastic constitutive models for metallic materials and can hardly work well for the materials where there is not a distinct yield surface. Many commercial FE codes reserve the interface for users to use subroutines so as to expand the applied scope, which provides us with the possibility to use a more realistic material model in the analysis of precision or net forming.In this thesis, a non-classical coupled thermomechanical elastoplastic constitutive model for finite strain and deformation and the corresponding numerical algorithm is proposed and applied to the analysis of metal forming. It involves severe non-linearities in the constitutive behavior and thermal properties of materials and the strong non-linearity in geometry. Although non-classical modes of plasticity were reported to be applied to the analysis of metal forming, the application of a non-classical coupled thermomechanical elastoplastic constitutive model to metal forming incorporating plastic deformation induced heat, its transfer, the induced temperature distribution and its effects on further metal forming could not be found in literature.The incremental form of the proposed constitutive model and the corresponding numerical algorithm for FEM analysis based on the Updated-Lagrangian description are suggested. The subroutines related to user defined materials, UMAT, and that related to plastic deformation induced heat, its transfer and temperature, UMATHT, are developed and embedded in the commercial FEM code ABAQUS/Standard. The upsetting and forward extrusion processes are simulated and it shows that the distributions of displacements, stress, strain, and temperature during the metal forming processes can be satisfactorily described. The comparison with the experimental results demonstrates the validity of the proposed model and the corresponding algorithm.