| Sheet metal stamping is an extremely important process for the manufacture of sheet metal parts and is widely used in automobile, aircraft, electronic instrument and other industries. Nowadays, traditional methods are still used in panel die designing and manufacturing in domestic automobile industry. Process planning and die designing depend mainly on the experience of designers and a trial-and-error procedure is often adopted to get qualified dies. As a significant part of the Key project of National Natural Science Fund of China, called the Basic Theories, Simulation Methods and Key Technique of Sheet Metal Stamping and Tool's Design (19832020), the research of the present thesis focuses on the elementary optimization of stamping direction and several critical topics in the one step simulation.First, the optimization of stamping direction is studied when sheet metal forming is simulated by incremental one step method. As a result, "average normal method" is proposed and the program is implemented according to the theory of computer graphics. Second, a new finite element approach is introduced for the direct prediction of blank shapes and strain distributions for desired final shapes in sheet metal forming, i.e. one step simulation algorithm. The one-step forming simulation approach is kind of FEM inverse algorithm based on total theory, which is the research hotpot in the field of international stamping forming in recent years, and it has higher calculation efficiency such as foretells formability quickly during preliminary design stage, foretells the thickness distribution of stamped parts in the process of body structural analysis so as to establish more accurate model of the integral car and obtain more precise intensity analysis results and foretells blank shape rapidly in the stage of manufacture. Consequently it is a method with bright applicable future. One-step simulation simplifies sheet metal forming into simple loading deformation process. Only two configurations are considered: the initial flat blank C0 and final 3D workpiece C while neglecting intermediate configurations with regard to the change of workpiece. According to the Euler configuration, when applied to the principle of virtual work on the overall final workpiece, the formulation can be expressed as Eq. (4): (1)where , are the internal virtual work and the external work, respectively, {} are the virtual displacements, {} are the virtual membrane strains,{} are the external forces and {} are the Cauchy stresses. Using FEM numerical computation, Eq. (1) always can't be so strictly satisfied that the Newton-Raphson method is introduced to solve the non-linear equations. Assuming the difference (i.e. residual force) between external and internal force is , it may be written as: (2)where is nodal external vector and is nodal internal vector. The Newton-Raphson iteration format of Eq. (2) is given by the following expressing: where is relaxation factor, is the tangent stiffness matrix of the th iteration. Most of researches on this field, however, commonly adopt triangular membrane element at present. Though triangular membrane element has certain advantages, for example, element model simplicity, it is constant strain element mainly applies to the prediction of blank shape and is hard to satisfy the need for the rapid prediction of formability as well as pre-analysis of structural CAE. This shows the choice of element plays an important role and is related to the accuracy and effectiveness of solving the problems. This paper applies the quadrilateral membrane element to one step method. As a result, formulations of a one step simulation are also revised. Results from the simulations of L-shaped cup and certain front fender using the quadrilateral and triangular membrane element are compared with those obtained by experiments or by the incremental approach. At the end of the pape...
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