A Study of Anisotropy and Post-Necking Local Fracture Strain of Advanced High Strength Steel with the Utilization of Digital Image Correlation 2011-01-0992

The automotive industry has a strong need for lightweight materials capable of withstanding large mechanical loads. Advanced high-strength steels (AHSS), which have high tensile strength and formability, show great promise for automotive applications, yet if they are to be more widely used, it's important to understand their deformation behavior; this is particularly important for the development of forming limit diagrams (FLD) used in stamping processes. The goal of the present study was to determine the extent to which anisotropy introduced by the rolling direction affects the local fracture strain. Three grades of dual-phase AHSS and one high-strength low-alloy (HSL A) 50ksi grade steel were tested under plane strain conditions. Half of the samples were loaded along their rolling direction and the other half transverse to it. In order to achieve plane strain conditions, non-standard dogbone samples were loaded on a wide-grip MTS tensile test machine. Strain measurements were made using Digital Image Correlation (DIC). DIC is an optical method which delivers whole-field noncontact measurement of strain, providing the ability to accurately determine peak deformation as well as gradients over the entire area of interest. DIC uses gray scale intensities of light reflected from the surface area of interest as recorded by image detector(s) to determine local strain. It was found that HSLA 50ksi steel exhibits, on average, a small but noticeable increase in post-necking local fracture strain values if loaded in the rolling direction, as opposed to the transverse. The dual-phase steel samples tested, i.e. DP600, DP780, and DP980, exhibit negligible post-necking strain variations along the rolling as opposed to the transverse directions.