Development of a New Index to Depict the Total Forming Capacity of Sheet Metals

Abstract

The forming limit curve (FLC) has been used to represent the stretchability of sheet metal under various forming modes. However, the formability window in stamping depends not only on the material's forming limit, but also on its strain distribution ability. In the present work, a total forming capacity (TFC) index is introduced to describe the global formability of sheet metals, which accounts for the contributions of both forming limit and strain distribution ability in the material during forming. Essentially, this index is constructed by integrating the instantaneous n-value from zero to the effective strain limit of the material. The usefulness of this new index is demonstrated through a comparison of the overall global formability between a cold-rolled (CR) 590DP steel and a CR980GEN3 steel via a dome test coupled with digital image correlation (DIC). Contradicting of its lower forming limit, the 980GEN3 can be formed to a higher limiting dome height (LDH) than the 590DP under a near plane strain condition. This superior overall global formability of 980GEN3 is attributed to its capability to dissipate strain uniformly during forming, resulting in lower strain gradient and delayed strain concentration. While revealing the limitation of using the FLC alone, findings in this study evince the strength and effectiveness of this new index in assessing the overall global formability of sheet metals. The higher index is successfully correlated to the higher LDH of the 980GEN3. Further, the TFC concept is applied to evaluate the global formability of a multitude of high strength steels at different thicknesses. An explicit relationship is established between the calculated index and experimental LDH results. The development and application of the TFC indices promise a straightforward way to assess the overall global formability of a material. It also serves as a more precise yet simple tool for material comparisons and selections.