Numerical Analysis Of Micromorphic Damage Consitutive Models In Sheet Metal Forming

Due to the urgent demands in energy-saving,reducing Greenhouse gas emissions,and automotive lightweight,the technology of using high-strength steel and complex hot forming processes has been widely used in automotive manufacturing industry.The numerical simulations based on the finite element method(FEM)are playing importan roles in producing automotive body parts,in which the sheet metal experiences extreme forming conditions,high speed,high temperature or complex loading paths etc.,and results in large plastic deformations which prompts the evolutions of micro-voids or micro-cracks.A group of complex thermo-mechanical constitutive equations is essential to provide formability analysis and precisely model the material’s behaviors of large deformation,elasto-visco-plasticity,strain softening,fracture etc.In this thesis,the main contents are as follows:1).The pseudo minimum area(PMA)method is proposed to construct the general 3D sliding surfaces of middle configurations by solving a quadratic programing.The initial solutions of the middle configuration is generated by using the mapping technique while mainting its area coordinates.The penalty method is applied to constrain the movement of material points during the equilibrium iterations.The static-implicit multi-step FEM solver is developed based on the KMAS platform,with the help of the new derived nonlinear kinetics,to provide efficient formability analysis of the pre-designed forming components and to improve the poor estimations of stress in one-step FEM.2).Based on the generalized continuum mechanics,a complete set of thermomechanically consistent constitutive equations coupling with elasto-visco-plasticity,local ductile damage,isotropic and kinematic hardening,and temperature as well as micromorphic variables is formulated to avoid the ill-posed initial and boundary value problems(IBVP).The new extra micromorphic state variables lead to additional balance equations taking into account the nonlocal effects and involving characteristic length scales as representative of the materials’ microstructure.The model is implemented in Abaqus user subroutine and applied to the uniaxial tensile and forming processes of material DP1000.3).The micromorphic approach is applied to heat transfers and is shown to deliver new generalized heat equations as well as the nonlocal effects.A thorough discussion of existing extended heat equations is presented in five types:(ⅰ)the classical Fourier model,(ⅱ)the hyperbolic type with relaxation time,(ⅲ)the double temperature model,(ⅳ)the temperature/entropy gradient theory and(ⅴ)the micro-temperature model,according to the used different theoretical frameworks.Several existing extended heat equations could be retrieved from constrained micromorphic heat equations with suitable selections of the Helmholtz free energy and heat flux expressions.As an example the propagation of plane thermal waves is investigated according to the various generalized heat equations.Possible applications to fast surface processes,nanostructured media and nanosystems are also discussed.