Failure prediction in anisotropic sheet metals under forming operations with consideration of rotating principal stretch directions

An approximate macroscopic yield criterion for anisotropic porous sheet met als is adopted to develop a failure prediction methodology that can be used to investigate the failure of sheet metals under forming operations. Hill' s quadratic anisotropic yield criterion is used to describe the matrix norm al anisotropy and planar isotropy. The approximate macroscopic anisotropic yield criterion is a function of the anisotropy parameter R, defined as the ratio of the transverse plastic strain rate to the through-thickness plast ic strain rate under in-plane uniaxial loading conditions. The Marciniak-Ku czynski approach is employed here to predict failure/plastic localization b y assuming a slightly higher void volume fraction inside randomly oriented imperfection bands in a material element of interest. The effects of the an isotropy parameter R, the material/geometric inhomogeneities, and the poten tial surface curvature on failure/plastic localization are first investigat ed. Then, a non-proportional deformation history including relative rotatio n of principal stretch directions is identified in a critical element of a mild steel sheet under a fender forming operation given as a benchmark prob lem in the 1993 NUMISHEET conference. Based on the failure prediction metho dology, the failure of the critical sheet element is investigated under the non-proportional deformation history. The results show that the gradual ro tation of principal stretch directions lowers the failure strains of the cr itical element under the given non-proportional deformation history. (C) 20 00 Elsevier Science Ltd. All rights reserved.