Experimental and numerical investigation on forming limit curves of AA6082 aluminum alloy at high strain rates

The influence of high strain rate on the forming limit curves of AA6082-T6 aluminum alloy sheets, 1 mm in thickness, was investigated by means of dynamic tensile and hemispherical punch tests carried out until the onset of sample failure using a Split Hopkinson Bar (SHB) system, at a punch speed of 7500 and 12000 mm/s. The equipment used was designed according to the standard EN ISO 12004-2 but properly scaled to adapt it to the SHB apparatus. A finite element analysis was performed to quantify the strain rate values reached during tests (around 600 and 1000 s-1 associated at the punch speeds of about 7500 and 12000 mm/s, respectively). The limit strains were determined on different sample geometries undergoing deformation in the range from the uniaxial and balanced biaxial stress states typical for sheet metal forming processes. The major and minor strain distributions, acquired through an accurate image analysis by analyzing the deformation of a previously imprinted grid on the specimen surface, allowed to plot the dynamic forming limit curves at different high strain rate values. Such forming limit curves were compared with those provided by samples deformed under quasi-static loading condition, using the same equipment and specimen geometries, in order to evaluate the influence of the loading rate on the formability. It was observed that the formability of AA6082-T6 aluminum alloy strongly improves as the strain rate increases from quasi-static to dynamic conditions.