No abstract

Forming modern advanced high strength steels poses challenges that were not of real importance in the previous decades. These challenges are the result of the steels’ complex microstructures and hardening behaviors, and the problems directly related to the high strength of the material, especially springback. New methodologies and processes are required to overcome these challenges and to produce formed panels via optimized forming processes. This paper reviews the key developments in the fields of numerical simulation of sheet forming processes, the material models required to obtain accurate results, and the advanced stamping presses and approaches for shaping modern steel sheet materials into desired shapes. Present research trends are summarized, which point to further developmental possibilities. Within the next decade, it is predicted that numerical simulations will become an integral part of the developmental and optimization process for stamping tools and forming processes. In addition to predicting the strains in the formed panel and its shape after trimming and springback, the simulation technology will also determine the optimum displacement path of the forming tool elements to realize minimum springback. Toward those goals, digital servo presses are expected to become an integral element of the overall forming technology.

SUMMARYTo enhance the reliability and efficiency of implicit finite element (FE) sheet forming simulations, a contact search method consisting of a global search procedure and a newly developed local search procedure was formulated to work with the N-CFS solution algorithm, described in the literature. The method enforces and searches for contact search along the mesh normal direction using an irregular triangular mesh-patch tool description. To take into account sheet thickness in contact analysis for shell elements, the contact distance error was reformulated. These improvements were implemented into the implicit FE program SHEET-3 and tested numerically with several forming problems. The results show that the contact algorithm for general tools, including sheet thickness, is as accurate and efficient as special closed-form approaches using analytical tool descriptions. Taking the sheet thickness into account can make improvements in the solution, in terms of shapes and forces, depending on the details of the problem.

Equivalent drawbead models associated with drawbead types, with which the geometrical modeling of drawbeads can be replaced in a numerical simulation of the stamping process, are introduced. The accuracy of the equivalent drawbead models rapidly computing the drawbead restraining and opening forces, based on the hybrid membrane/bending method accommodating Barlat's anisotropic yield function for the plane stress state and the modified Chaboche model considering the Bauschinger effect and transient behavior under the plane strain condition, is verified by comparing the equivalent drawbead forces with those obtained from the finite element simulation. Moreover, the drawbead forces computed by equivalent drawbead models are employed in a finite element simulation of the automotive fender stamping process, from which the effectiveness and applicability of equivalent drawbead models are observed by comparing strains and draw-ins with those measured in the tryout panel.