Experimental determination of the failure surface for DP980 high-strength metal sheets considering stress triaxiality and Lode angle

To meet requirements for reduced fuel consumption of cars, the use of components made of sheets of high-strength steel instead of conventional steel has been on the rise. However, low ultimate elongation of high-strength steel often causes problems during plastic deformation and more research is needed to improve failure predictability. Ductile failure models available in commercial finite element analysis (FEA) packages require the material’s tensile strength and failure strain for failure prediction. For stress states that are more complex than the uniaxial case, accurate prediction of the how, when, and where failure occurs has been become problematic and it has been investigated by numerous researchers. In this study, we investigate the prediction of failure in DP980 sheets under triaxle stress states. We first determine the shapes of specimens using certain triaxial stress states, such as pure shear, uniaxial tension, biaxial deformation, which are induced by corresponding tensile tests. When failure occurs, equivalent strain at the failure locus is obtained by means of digital image correlation (DIC) and then plotted against triaxiality and Lode angle, based on which the triaxiality failure diagram (TFD) is established to implement in the FEA program of LS-DYNA. Validation is made by comparing the numerical results with burring test data. Good agreement was found for failure locus and strain distribution at the time of failure.