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For the characterization of the deformation and damage behavior of the automotive lightweight steels ZStE340 and DP1000, high speed tension tests for various stress triaxialities with shear tensile specimens, smooth tensile specimens and notched specimens and static and dynamic Nakajima tests were performed. The deformation up to failure was recorded using high speed video cameras with high resolution in the necking zone and evaluated with 2D and in part 3D digital image correlation (DIC). The maximal achieved effective strain at the location of failure, determined by video analysis and fractography, was specified as failure strain. The results for various specimen geometries were plotted versus stress triaxiality as determined by FE simulation in order to receive failure diagrams. Overall, both materials show descending failure curves from shear loading up to tensile loading of notched specimens. Up to equibiaxial tension, failure curves are rising again. For uniaxial and equibiaxial tension, with increasing strain rate larger failure strains occured for DP1000, while no significant influence of strain rate on failure strain was observed for ZStE340. However, under shear loading with increasing strain rate, decreasing failure strains occur for both materials. This is probably due to an adiabatic temperature rise in a small localized zone which promotes shear failure. For shear crash tests a temperature rise of more than 200 K was measured by a high speed infrared camera. These results show the necessity of dynamic tests under different stress triaxialities to provide reliable input data for crash simulations and increase the ability to accurately predict failure. Especially shear failure must be investigated very carefully under crash loading, because here failure occurs at smaller failure strains than it would be expected under quasistatic loading.

Deformation and damage behavior of lightweight steels at high rate multiaxial loading