Prediction of Forming Limit Diagram (FLD) and Effect of Work Hardening Property on FLD of Sheet Metals

The necking limit diagram NLD and the forming limit diagram FLD under the wide-range bi-axial tensile stress conditions were obtained by using various sheet metals of steels, Al-alloys and a copper. The good correlation between the limited strain values of NLD and FLD and the mechanical properties of total elongation and r-value etc. were obtained. Moreover, the strain dependency of the work hardening coefficient n-value in higher strain area was different according to each material. The complex value of n-value and r-value was defined as the equivalent work hardening coefficient neq-value in higher strain area. And when the neq-value was substituted to Gotoh's modeling of localized necking limited strain, FLD that accuracy is high was able to be predicted.