Necking behavior of AA 6022-T4 based on the crystal plasticity and damage models

In order to analyze necking behavior with a continuum level approach, initial imperfection or bifurcation algorithm should be employed. The initial imperfection is arbitrary and has a great effect on the necking behavior of sheet metals. Most polycrystalline materials have natural imperfection such as orientation mismatches across the grain boundaries. In polycrystal plasticity, orientation mismatch is considered as material imperfection instead of geometric imperfection including thickness unevenness. The main idea of this study is to investigate the possibility that the stress concentration on a grain boundary (caused by orientation mismatch) works as imperfection and finally causes necking. For the consideration of stress concentration along grain boundaries, a crystal plasticity model was introduced in three-dimensional finite element analysis of a tensile test. A small region of a tensile specimen was divided into sufficient number of octahedral grains and each grain was discretized by fine tetrahedron elements. The same orientation angle was allocated for all the elements in each grain, but different orientations for grain by grain. Using this crystal plasticity-based analysis, stress concentration in the grain boundaries can be predicted. Also, four damage models based on the critical strain and stress were proposed to consider material softening due to void initiation and growth under the framework of crystal plasticity theory. The damage parameters for each model were determined based on the stress-strain relations obtained from experiment. Void nucleation, growth and coalescence behaviors during necking were reasonably predicted. Finally, the predicted necking behaviors of AA 6022-T4 were compared with the experimental results in terms of necking strain, deformed shape, and necking direction.