On the implementation of anisotropic yield functions into finite strain problems of sheet metal forming

In this article the implementation of anisotropic yield functions into fini te element investigations of orthotropic sheets with planar anisotropy is d iscussed within a plane-stress context. Special attention is focused on the proper treatment of the orientation of the anisotropic axes during deforma tion into the finite-strain range. As an example problem the hydrostatic bu lging of a membrane is considered in conjunction with a recently proposed a nisotropic yield function. It is shown that the aspects of the plane-stress assumption, which do not come into consideration in isotropic analyses, ca n play an important role on the accuracy of the solution when the rotation of the orthotropic axes enters the computation directly due to the presence of material anisotropy. When the material anisotropy is considered and whe n the deformation of the workpiece is not limited to the plane of the undef ormed sheet (such as cup drawing, hydrostatic bulging, etc.), the numerical experiments indicate that the only correct formulation is the one based on numerically imposing the requirement that for the plane-stress application , the in-plane material axes have to remain in the plane of the sheet durin g the deformation. (C) 1999 Elsevier Science Ltd. All rights reserved.