Influence of strain rate on the beginning of instable deformation and failure behavior from shear to multiaxial loading for a DP1000

The influence of strain rate on the beginning of instable deformation and failure behavior of a DP1000 steel is investigated for a wide range of stress states with experimental methods. Therefore quasistatic and high speed tests have been performed for four different loading situations, shear loading, uniaxial tension loading, plane strain loading and equi-biaxial tension loading. The deformation of the specimens up to fracture in the highly deformed zones has been captured with high speed video recording and evaluated with digital image correlation (DIC). The beginning of instable local deformation behavior designated as beginning of instability has been detected with one uniform procedure. For tensile dominated loading situations the development of the local thinning rate in the necking zone on the surface of the specimen has been analyzed. For the determination of the beginning of shear instability, the development of the major and minor strain rate in the shear zone has been investigated. The difference between strain at beginning of instability and failure strain, determined as the largest strain at the location of failure prior to fracture, gives hints to the materials crash performance under the investigated stress state. The largest difference has been observed for uniaxial tension loading and increases with increasing strain rate. However, under dynamic shear loading, fracture occurs without previous instability and at significant lower strains than under quasistatic shear loading. The proposed evaluation procedure to determine the beginning of instability for a wide range of stress states including shear loading is applied to the investigated DP1000 and strain rate effects are discussed.