On the prediction of strength and deformation anisotropy of automotive sheets for stamping formability analysis

Today, sheet metal process simulations based finite element (FE) analysis became an indispensable part of tool design engineering in automotive and related stamping industries. In this context, an analytical description of the inherent directional strength and deformation variations in these sheet metals are conducted by means of an orthotropic yield criterion. In practice, an appropriate criterion can be determined using directionality parameters such as r-values and yield stress ratios from simple tension tests to predict material strength and deformation anisotropies analytically. When yield criteria together with the computed anisotropy parameters are implemented into the finite element software, however, it should be also investigated whether the finite element (FE) model could capture the actual anisotropic behavior of the material and assess the analytical model accurately. One way of ensuring this condition is to use single finite element tests in order to simulate uniaxial deformation behavior of material in simple tensile tests. In this study, FE analyses of simple tension test with sheet specimens were conducted for specimens from seven evenly spaced directions for two widely used sheets in the automotive industry, namely DP600 and AA2090-T3 aluminum alloy. Lankford parameters and the yield stress ratios were predicted with analytical approach and FE analysis for different material orientations. It is determined that, while plasticity model analyses are quite successful in terms of computed deformations and flow curves, Barlat's yield functions family has significant strength and deformation differences between analytical and numerical results, especially for steel sheets. It is assessed that these discrepancies are caused by plasticity implementation into FE software.