Anisotropic fracture of advanced high strength steel sheets: Experiment and theory

Sheet metals such as advanced high strength steels often exhibit varying degrees of anisotropy due to the existence of preferred orientation of their grain structures from the rolling process. While extensive research has been devoted to characterize and model anisotropic plasticity behavior over the past decades, only recently people began to tackle their anisotropic fracture behavior. The focus of these studies is almost always on the in-plane anisotropy, and ignores out of-plane fracture behavior. The reasons are two-folds: (1). Sheet metal deformation during a stamping or crash application is mostly thought as in a plane stress state and modeled as such, so there is no need to consider out-of-plane fracture behavior; and (2). It is very challenging to develop experimental techniques to reliably measure out-of-plane fracture strengths for sheet metals with a thickness only about 1 mm. In this paper, we theorize, based on strong empirical evidence, that the fracture strength of a thin sheet metal in out-of-plane shear is inherently weaker than that under in-plane shear, analogous to the interlaminar properties of composite laminates. This weakness is responsible for the so-called shear fracture behavior, often observed during the stamping operations of advanced high strength steels when the sheet metal flows around a tight radius. A new double-notched shear specimen with shear plane perpendicular to the thickness direction is carefully designed and tested to accurately measure the out-of plane shear fracture strength. Simulations are executed to validate the rationality of the out-of plane shear specimen design. Test results of a DP980 sheet show that the out-of-plane shear fracture strength is indeed about 15% lower than that measured under the in-plane shear condition. A new anisotropic ductile fracture model based on linear transformation of stresses is proposed to account for the out-of-plane fracture strength for AHSS sheets. The model is calibrated with experimental data, and the fracture surface is compared to the isotropic Mohr Coulomb (M-C) fracture model to illustrate its effectiveness.