Influence of Anisotropy on Edge Fracture of Advanced High-strength Steels

Advanced high-strength steels (AHSS) have been developed with superior properties for wide applications in the automotive industry. Among them, dual-phase (DP) steels and Quenching and Partitioning (QP) steels, as representatives from the first and third generations of development, are attracting great interest. Despite the benefits of the enhanced high strength, the formability of these materials becomes unpredictable in the forming processes, especially in the largely deformed local area. AHSS is more susceptible to edge fracture compared to traditional mild steels, due not only to reduced ductility but also to distinct local fracture behavior. To deepen the knowledge of the edge fracture behavior of AHSS, it is necessary to find a characterization method that accurately describes fracture failure. The study aims to accurately predict edge fracture by addressing often-overlooked factors such as anisotropy evolution, localization, and stress-state-dependent fracture behavior. To achieve this goal, a combination of experimental and numerical approaches is employed. First, a category of tensile tests with various geometries and loading directions was conducted to characterize the anisotropic fracture properties of DP and QP steels. Next, hole expansion tests (HET) were carried out to assess edge formability under both smooth and blanked edge conditions. In addition to the classic fracture dependency on stress states, the spotlight is on the anisotropic behavior in terms of both plasticity and fracture. The evolving non-associated Hill48 (enHill48) model is applied to describe anisotropic plasticity, while the anisotropic fracture models are formulated to represent fracture behavior. The novelty of this study lies in uncovering that the edge fractures of examined DP steel are primarily driven by anisotropy-induced strain localization, while the edge fractures of the investigated QP steel, though less sensitive to anisotropic plasticity, are strongly dependent on anisotropic fracture and stress triaxiality, governed by fracture limits. A partially anisotropic fracture model is found effective for DP steel in capturing anisotropic plasticity and localization but remains limited in accurately describing fracture-dominated local formability for QP steel. Consequently, a fully anisotropic model is proposed for QP steel, based on the linear transformation of plastic strain within the damage initiation criterion. This model is validated for both anisotropic fracture behavior and edge formability with improved prediction accuracy. The findings emphasize the crucial role of anisotropy in edge fracture assessment, which not only enhances modeling precision but also introduces a design concept to elevate material edge formability.