Investigation of the dislocation slip assumption on formability of BCC sheet metals

This paper explores the right-hand side of the forming limit diagram (FLD) for a BCC material in order to test the crystallographic slip assumptions. BCC crystals are considered with either 24 or 48 slip systems (BCC24 or BCC48). Identical uniaxial stress responses are assumed in order to compare the predicted FLDs. FLDs are performed using a rate-dependent polycrystal viscoplastic model together with the Marciniak–Kuczynski (M–K) approach. It is verified that the predictions of the limit strains carried out with the full-constraints (MK-FC) model are strongly affected by the selected deformation modes, showing unrealistically high limit strains in balanced-biaxial tension. Much more reliable values are found with the viscoplastic self-consistent (MK-VPSC) approach using either a BCC24 or BCC48 assumptions, enhancing the relevance of the selected transition scale model. Discrepancies between the numerical results, obtained using MK-FC and MK-VPSC, are interpreted in terms of the differences in the active slip systems selected by each model, and consequently, in the predicted lattice rotations and local curvature of the yield locus. Finally, it is found that the calculation of the FLD with MK-VPSC, using 48 slip systems, successfully predicts the right-hand side experimental tendency observed in a low carbon steel sheet metal obtained by bulge test.