Research On The Influence Of Strain Paths Of Damage Evolution Of Sheet Metal Based On Fully Coupled Theory

High strength alloy sheet has been widely used in industrial production However,due to its limited plastic deformation ability,high-strength alloy sheet has to face many technological problems in producing parts,which will highly increase the manufacturing cost of the products.Therefore,it is of great theoretical and industrial value to accurately describe the mechanical deformation behavior of alloy sheets,and then provide a reasonable process scheme in metal forming processes.Numerous research results show that the value of metal forming limit is not constant,and the damage evolution under different strain paths or strain state changes radically.Finite element method(FEM).as an important simulation tool,can simulate and analyze the deformation behavior of materials at any time and positions with accuracy and efficiency.However,the obtained simulation results depend heavily on the chosen constitutive equations.The fully coupled damage model,which emphasizes the full-coupling relationship between damage and other state,can accurately describe the damage behavior of materials in plastic forming.So it has attracted lots of attentions.In order to better predict the elastoplastic and damage behavior under different strain states,two important parameters are considered in the proposed model:The stress triaxiality and Lode parameter.In our work,the effects of strain paths characterized by stress triaxiality and Lode coefficient on damage mechanical behavior will be studied by means of experiments and simulation.The main research contents are shown as follows(1)Based on continuum damage mechanics and full coupling theory,a constitutive model considering the influence of Lode parameter is established,which involves the initial anisotropy,isotropic and kinematic hardenings,as well as the isotropic ductile damage.The coupling between the ductile damage and other state variables is possible to achieve a true reflection of damage mechanics of high strength alloy sheet.The microcracks closure and the Lode parameter are introduced to influence the damage evolution,and then affect the final obtained fracture strain surface.The given constitutive model is implemented in the finite element software Abaqus with user subroutine Vumat by Fortran programming language(2)High strength Aluminum alloy AA7055 and dual-phase steel DP900 are selected as the research objectives.The experiments under different strain paths are designed,including the uniaxial tensile test,pre-notched specimen tensile test,butterfly specimen tensile-shear test and Nakazima deep drawing test.Based on the above mechanical experiments,the basic mechanical parameters of the two materials are obtained.The influence of the strain states on the damage and plastic forming behavior is discussed(3)Together with the obtained experimental data,the inverse method is studied aiming at achieving the consistency between simulation results and experimental data.The damage parameters are identified by the optimization tool in Matlab by various optimization algorithms.The accuracy of the optimized damage parameters is verified(4)Using the finite element analysis method,the study on influence of strain path is carried out.The numerical predicted force-displacement curves under different loading paths are obtained,and are compared with experimental data.In the space of stress triaxiality and Lode parameter,the fracture strain surface obtained by simulation is analyzed,and the accuracy of damage prediction under different strain paths is verified.