Crystal Plasticity Finite Element Study of Necking Behavior of Aluminum Alloy Sheet Subject to Thickness-Stress

In sheet metal forming, thickness stress may affect the formability of sheet metal. In this paper, effect of through-thickness pressure stress on the formability of aluminum alloy AA6022-T4 sheet is investigated based on crystal plasticity finite element analysis. A self-hardening behavior is considered for the strain hardening behavior of the slip systems. Further, for prediction of necking initiation and growth, maximum shear strain criterion is used for damage initiation and evolution. In order to implement the model in Abaqus finite element package, a VUMAT was developed based on the discretized equations and forward Euler integration scheme. After verification of the developed code, the parameters of the model were calibrated against the tensile test results. For simulating tensile test of 1 mm thick sheet, a representative volume of 3x1.5x0.5 mm3،was partitioned into 14790 grains through a python code in ABAQUS/CAE environment and then discretized using 50 μm tetrahedral linear elements. Using the experimental data available in literature and considering appropriate texture for the simulation domain, the crystal orientations were assigned through Euler angles. Then, tensile tests were performed on the sample in the presence of the thickness pressure stress. The results show that application of the through thickness stress increases the strain corresponding to the necking initiation and thus postpones necking. Correspondingly, a decrease in tensile load is observed in this case.