Determination of anisotropy and material hardening for aluminum sheet metal

Sheet metal forming, crushing simulations of thin-walled structures, and other large deformation processes require knowledge of the material stress–strain behavior to large strains. The material response of sheets measured in the traditional uniaxial tension test usually terminates at strains of a few percent due to necking. It can be extrapolated to some degree using results from biaxial tests (e.g., equibiaxial tension) but not to sufficiently large strains. This work shows a systematic methodology that uses a combination of experiment and analysis to extract the material response at much larger strains. This is achieved by accurately following the deformation in the necked region of a custom tensile test specimen. The test is simulated numerically using a 3D FE model and the material response is iteratively extrapolated until the calculated and measured force-elongation match. For the Al-6061-T6 sheet metal of interest, the process is complicated by inherent anisotropies introduced during the rolling of the sheets. The anisotropy is characterized by a set of uniaxial and biaxial tests conducted in parallel. The results are used to calibrate the 18-parameter non-quadratic Yld2004-3D yield function. The calibrated yield function is then used to simulate the tensile test. The material hardening is iteratively adjusted by comparing the measured and calculated force–displacement response and validated by the corresponding measured strains and shape of the neck.