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. (C) 2012 Elsevier Ltd. All rights reserved.