On the Influence of the Through-Thickness Strain Gradients for Characterization of Formability and Fracture of Sheet Metal Alloys

The main focus of this thesis is to investigate the effect of through-thickness strain gradient on sheet metal formability. Current technology for formability analysis has relied upon deformation modes of in-plane stretching in which localized necking and unstable strain localization become the dominant failure mode. The formability of sheet metal is often explained with a forming limit diagram (FLD) that the onset of necking indicates the upper end of the formable range. However, it is practically known in the industry that a material can often be formed beyond the conventional forming limits under out-of-plane deformation. The through-thickness strain gradient generated by out-of-plane deformation suppresses the onset of necking and thus improves formability. In this thesis, novel out-of-plane formability tests were designed and performed to investigate the correlation between the severity of bend and material formability. Three deformation modes considered in the research: a pure bending under plane strain, equi-biaxial stretching with severe bending, and a combination of stretching and bending. These formability tests have been specially designed to employ Digital Image Correlation (DIC) to obtain full-field strain measurements until fracture. The advent of DIC has revolutionized the field of experimental mechanics and allows for a better understanding and quantification of fracture strains compared to indirectly inferring them from inverse finite-element simulations based upon a measured displacement to the failure of the test sample. Various automotive sheet metal alloys are considered in this thesis. However, the focus materials of interest were advanced high strength steels such as DP980, and an aluminum alloy, AA5182, since they have distinct mechanical properties in aspects of strength, ductility, and hardening behavior. In the field of formability, the VDA238-100 bend test introduced in 2010 has rapidly gained acceptance since it has reliable and intuitive test parameters for bendability analysis. In contrast to the most of traditional formability tests in which in-plane stretching is the dominant failure mode, the VDA bend is designed to investigate material response under the pure bending condition in which the most severe through-thickness strain gradient is expected. The result is reported in terms of bend angle which is a direct and straightforward representative of bendability. However, the relatively simple setup of VDA bend test provides