Pad Force Required in Shear-deformation Sheet Forming of Curved Hat Channels for Ultra-High-Strength Steels

One of the latest challenges in sheet forming of crashworthy lightweight automotive parts is the cold stamping of curved hat channels. Neither cracking in convex bottom nor wrinkling in concave bottom can be avoided in conventional drawing. Tanaka and others have proposed in-plane shear forming technology, in which shear strains are given to vertical walls by pad draw-bending for the case of curved channels. This requires a sufficient pad force to clamp the bottom face area; however, the minimum pad force for successful forming can be known only by trial and error. In this study, we focused on cracking in convex bottom and examined a method to estimate the required pad force. Finite element simulations and forming experiments for a 980-MPa-grade steel led to a theoretical model in which three types of force were applied to the flat bottom during forming: a force given by in-plane shear of walls, a frictional force given by the pad force for clamping, and a resistance against the flow to the convex bottom. The required pad force could then be obtained as a solution of the equilibrium of these forces. Furthermore, the validity of the proposed method was confirmed.