Prestraining effect on hole-expansion behavior of AA6022-T4 sheet to replicate multi-step forming processes

Yannis Korkolis, University of New Hampshire, Durham, United States
Jinjin Ha, University of New Hampshire, Durham, United States
Moritz Dirian, Universität der Bundeswehr, Munich, Germany
Dunn Chris, University of New Hampshire, Durham, United States

This study investigates the effect of prestrain on the hole-expansion (HE) test of AA6022-T4 aluminum alloy sheet. This is meant to replicate a “material chain”, i.e., the multi-step stamping operations used in industry. The material is subjected to approximately 10% uniaxial tension in the rolling direction (RD), and then, HE test is conducted in a fully-instrumented double-action hydraulic research press. The HE test is conducted within 3 days, to minimize time-dependent mechanical property changes after prestraining. Digital image correlation (DIC) is used to acquire the surface strain field during the tests. Besides the experimental investigation, finite element simulation is also performed in the same sequence as the experiments, i.e., uniaxial prestraining in the RD followed by HE test. To take into account the plastic anisotropy and hardening behavior of the material in the simulations, the non-quadratic anisotropic yield function Yld2000-2d and homogeneous anisotropic hardening (HAH) model are used. The material parameters of Yld2000-2d are calibrated by uniaxial tension, plane-strain tension, and disk-compression tests of monotonic loading, while the ones of HAH model are determined by tension-compression and two-step tension tests with loading path changes. The predicted punch force-punch stroke and thickness strain distribution are compared with experiments.

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