Bifurcation analysis on formability coupled with microstructural evolution of dual-phase steels subjected to various stress states

Forming limit serves as a critical metric in evaluating the formability of advanced sheet metals. Unlike the plane stress state in conventional sheet processing technologies, compressive normal stress and even more intricate stress states play a significant role in certain forming processes, such as hydroforming and incremental sheet forming. In this study, a bifurcation approach is utilized and modified to evaluate forming limits in both necking and ductile fracture modes, considering various stress states of dual-phase steels. This approach proves effective in predicting the conventional mode of necking-based forming limits for dual-phase steels in the Nakajima test and hydroforming process. The modified bifurcation approach, coupled with a damage-accumulation-based constitutive model, can predict ductile fracture forming limits in incremental sheet forming process. The derived theoretical results are corroborated with experimental fracture forming limits for dual-phase steels and commercial pure aluminum, demonstrating distinct beneficial effects of compressive normal stress in materials and processes. This study proves that the modified 3D bifurcation approach can work as an effective guideline in evaluating the formability of sheet metals subjected to various stress states, with support of meso-macro scale simulations and microstructural evolutions. This approach provides a comprehensive understanding of the material behavior under different forming conditions, thereby enhancing the predictability and control of the material formability.