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Abstract

With the requirement of vehicle performance and fuel economy, dual phase (DP) steel as one of the advanced high stress steels (AHSS) is increasingly used in the automotive industry due to its excellent mechanical behaviour: excellent combination of tensile strength and ductility. On a microscale the ductile fracture is governed by the void nucleation, growth and coalescence mechanisms. In the dual phase steels this damage mechanism exhibits a rather complex situation: voids are generated by the debonding of the hard phase from the matrix and the inner cracking of the hard phase besides by inclusions. On a macroscale fracture of these materials is observed in the automotive industry with the absence of strain localization or minimal post-necking deformation. Consequently the failure during the forming process is caused by a competitive or combined mechanism of internal damage evolution and metal instability. In this study, the target is to predict the failure behaviour of a DP steel sheet (DP600) by combining a recently proposed damage plasticity model, modified Bai-Wierzbicki (MBW) model, and the conventional localized and diffuse necking criteria by Hill and Swift, respectively. With this approach, the insight of the damage initiation and evolution of this damage-dominant material is revea led as well as the relation of the damage and instability of sheet metals is investigated. It will on the numerical side decease the effort to determine the material parameters in the model, and on the application side, give a more comprehensive understanding about the forming limits of this steel. To quantify the relation of these two factors, Nakajima tests were performed experimentally and numerically. The prediction of forming limits by different criteria are compared to the experimentally measured results. It is concluded that for the investigated material damage induced softening triggers the necking and final fracture of the Nakajima tests.