Binder wrap forming for automobile body panels can be evaluated by finite element analysis. Sometimes, however, numerical instabilities occur in the implicit nonlinear solution process. While this instability may be an artifact of the numerical model, it may also signal the onset of a real physical phenomenon, such as bifurcation.
This paper presents a numerical experiment to examine bifurcation in binder wrap forming, using an experimental decklid die. Numerical instability occurred early in the deformation process. Two stable solutions for the binder wrap surface were obtained, demonstrating that the instability resulted from bifurcation. One of the deformation modes is saddle like, and the other is cylindrical. In practice, the blank will deform to one of the above modes under the clamping actions generated by the closing of the binders. If the deformation takes the other mode, substantially different strains will result in the subsequent punch forming.