True Fracture Strain Measurement and Derivation for Advanced High-Strength Steel Sheets

The importance of true fracture strain was initially highlighted in the context of local versus global formability considerations used in material selection among advanced high strength steels (AHSSs) of similar tensile strength. Inspired by the relative studies, a precedent work compared the fracture strain results via either digital image correlation (DIC) based method or optical fracture surface measurement on different AHSS samples. It concluded that the DIC-based testing results generally underestimated the fracture strain. As a continued study, the present work further analyzed the DIC-based testing procedure and attributed such an underestimation mainly to the volume constancy assumption. Furthermore, this work pointed out that also because of the same assumption, the optical fracture surface measurement to some extent overestimated the fracture strain. Nevertheless, it was also observed that different AHSS grades were affected discrepantly by the two methods. Therefore, scanning electron microscope (SEM) was applied to inspect the morphology of various micro-voids and dimples on different fracture surfaces to explain the discrepancy. To bypass the volume constancy assumption, this work also proposed two alternative methods, including a DIC-based thinning measurement method and a hybrid method, and discussed their limitations. In addition, the effects of DIC measurement frame rate and anisotropic plasticity based on the Hill-1948 yield function were also studied in this work. Last but not the least, by substituting the different fracture strain results based on all the introduced methods into an early stage of calibration of the Generalized Incremental Stress State dependent damage Model (GISSMO) for forming and crash simulations, the importance of the fracture strain accuracy was further highlighted.