The Research On The Drawbead Models And The Optimization Of Drawbead Parameters In The Forming Of Automotive Cover Panels

In forming field, defects such as wrinkles and fractures are often caused owing to non-uniform material flow in the complex surfaces. In the forming operations of an automotive cover panel, the setting of drawbeads in the blank holder is an effective technique to supply a frictional resistance force in the sheet contact area and produce tensile stress in the sheet, which controls the material flow to obtain a more uniform deformation. In the past several decades, works of the drawbead have been studied by many researchers, but there still exist many problems in this field. In this study, the drawbead analytical model, the equivalent drawbead model in FEM and the optimization of drawbead parameters, have been proposed based on the former researches. All of the research results have been applied successfully in forming self-devolped software CADEMâ…¡. This paper includes the following topics:1. A numerical analysis drawbead model with isotropic-kinematic harding law is proposed to calculate drawbead restraining force. The equivalent drawbead restrained force models are used to describe the effects on the mechanics of the deformation of the sheet experiencing cyclic bending/unbending with isotropic cyclic hardening rule, and does not consider the incline of neutral layer and the Bauschinger effect. A numerical analysis model is proposed to calculate drawbead restraining force based on the Love-Kirchhoff assumption and plane strain assumption. The proposed model adequately considers the valuation of sheet thickness, the anisotropy and the incline of neutral layer. A Stress-strain law and a mixed isotropic-kinematic harding law under cyclic bending are described through the introduction of a strain memory factor to take into account the Bauschinger effect. The proposed model is compared with experimental ones and the validity of this model is verified.2. An equivalent drawbead model is established, which is based on the particle to segment contact algorithm. The particle to segment contact algorithm is applied to quickly build and update the contact pair between the equivalent drawbead nodes and sheet metal elements, to reduce the time consuming for contact search.3. A metamodel based inverse method of the drawbead geometrical parameters is suggested. The building of reasonable criterions to form objective functions such as fracture, wrinkle and insufficient stretching, which can be used to evaluate the formability of automobile panels. The first approach was for an optimization of the design variables of sheet forming with the region of the interest moved across the design space and the uniform latin square designs of experiment. The method can effectively overcome the shortcoming of low calculation accuracy compared with the conventional response surface method. And the obtained optimization procedure is very efficient due to decreasing the number of FEM evaluations. In addition, Multi-objective particle swarm optimization method is applied in the research on the forming optimization. It extends the applications of the particle swarm optimization method. numerical examples are presented to demonstrate the usefulness of the proposed optimization procedure for the design of mould.4. Based on the adaptive method and subcycling algorithm, The FEM model with true drawbeads is established. Because the variable cross-section of the transition drawbead, the effect of drawbead end, the cable-stayed extension drawbead and the material properties change of local sheet are not be taken into account in the equivalent drawbead models usually used in numerical simulation, The strain of the sheet metal having passed the true drawbead is accurate access by the true drawbead simulation model compared with the equivalent drawbead models. However, the sheet element must be enough small to run through the drawbead, leading to the critical time step too small to significantly reduce computational efficiency. To solve the problem, we establish the true drawbead model based on the adaptive method and the subcycling algorithm. We first refine the elements accessing the drawbead and coase these having passed over the drawbead, the sheet metal elements mesh is divided into subdomains with the different time-steps, and then the elements in different domains are updated with their time-steps to increase the simulation efficiency. numerical examples are presented to demonstrate the usefulness of the proposed procedure for the finite element simulation with true drawbeads.