Determination of Fracture Strain of Advanced High Strength Steels Using Digital Image Correlation in Combination with Thinning Measurement 2017-01-0314

Fracture strain data provide essential information for material selection and serve as an important failure criterion in computer simulations of crash events. Traditionally, the fracture strain was measured by evaluating the thinning at fracture using tools such as a microscope or a point micrometer. In the recent decades, digital image correlation (DIC) has evolved as an advanced optical technique to record full-field strain history of materials during deformation. Using this technique, a complete set of the fracture strains (including major, minor, and thickness strains) can be approximated for the material. However, results directly obtained from the DIC can be dependent on the experiment setup and evaluation parameters, which potentially introduce errors to the reported values. To evaluate the capability of the DIC for fracture strain measurements, a validation study was performed to compare the fracture strains of a 980GEN3 steel measured with a DIC and a microscope, respectively. Briefly, Marciniak cup tool was employed to deform the specimens under different strain conditions (i.e., uniaxial tension, plane strain, and equi-biaxial stretch). Deviations in the DIC measurements from the thinning measurements were identified and understood via comparing the thinning at fracture measured with the DIC and the microscope, which demonstrated the limitation of the current DIC setup used in this work. In an effort to improve the accuracy of fracture strain measurements with current DIC configurations, a combined approach integrating the DIC with the thinning measurement was practiced. Using this combined method, fracture strain data for two advanced high strength steel grades were successfully generated.