Abstract: A very important tool in the description of sheet metal formability is the well known Forming Limit Curve (FLC), prediction of which is generally based on models using instability theories. Our experimental observations indicate that the surface Toughness of as-received metal sheets generally does not lead to strain localization. At the same level of effective strain, the internal damage of a metal sheet is the most severe situation in plane strain state. When deformation is at tension-tension state, damage development results in the drifting of the strain state to plane strain state. When deformation is at tension-compression state, the stretch caused by load instability also lead to the development of plane strain state. Based on these results, a united model for the construction of both the left and righ halves of FLC is proposed. FLCs predicted by this new model is consistent ched well with experimental results and the new model is proven to be better than the M-K theory.

Key words: Forming limit, Instability, Sheet metal forming