Calibration of Barlat Yld2004-18P yield function using CPFEM and 3D RVE for the simulation of single point incremental forming (SPIF) of 7075-O aluminum sheet

In conventional sheet metal forming processes, such as stamping, application of a two-dimensional (2D) plane stress yield function is sufficient as the out-of-plane stresses (sigma(zz), sigma(xz), sigma(yz)) are negligible and the deformation occurs under plane stress condition. However, in incremental sheet forming (ISF) processes, significant through-the thickness shears necessitates the use of a three-dimensional (3D) yield function to account for out-of-plane stress components. However, to calibrate the parameters of the non-quadratic anisotropic 3D yield function Yld2004-18p, out-of-plane normal and shear stresses are needed which are very difficult to obtain experimentally. In this study, the out-of-plane stresses were found using a three-dimensional (3D) representative volume element (RVE) developed from Electron Backscattered Diffraction (EBSD) images. By applying the crystal plasticity (CP) material model to these 3D RVEs, it was possible to perform computational experiments to generate the out-of-plane stresses required for the calibration of the Yld2004-18p yield function. To simulate the single point incremental forming (SPIF) of 7075-O aluminum alloy sheet, two different yield functions namely; Hill's 1948 and Yld2004-18p were used. A detailed comparison of the two yield functions' predictions was made with respect to different parameters, such as the tool force and moment, part thickness, development of stress and strain tensor components, and effective plastic strain distribution.