Strain rate-dependent characterization of advanced high-strength steels under various multiaxial stress states for the determination of forming and failure limits

In order to draw the greatest benefit from the lightweight construction potential of advanced high-strength steels (AHSS) the knowledge of forming and failure limits under realistic forming and crash conditions is necessary. Therefore, a material characterization considering different stress states and strain rates has to be performed. In this work different tests for a DP1000 and ZStE340 steel, from shear loading up to multiaxial tensile loading, and Nakajima tests were performed at strain rates from quasi-static conditions up to 250 s-1. The deformation of the specimens was observed transiently up to failure with high speed video, and the strain field on the surface was evaluated with digital image correlation (DIC). Failure strains have been determined as the largest strains in the area where fracture occurs immediately before fracture. The beginning of instable deformation behavior has been investigated for the determination of forming limits and as threshold for element size independent FE crash simulation. A generalized evaluation method for the strain limit at the beginning of instable deformation behavior for a large range of triaxialities including shear loading has been conducted by analyzing the development of the strain rate field in the highly deformed zone. This method has been applied to the DP1000 steel. In the biaxial region failure strains and strains at the beginning of instable deformation behavior increase with increasing strain rates. This shows, that high forming velocities increase the forming limit. Both strains decrease in the shear region at a triaxiality around 0 with increasing strain rate. For crash simulation failure strains under dynamic shear loading must be taken into account, as this is the most critical loading situation.