Orientational evolution of plastic orthotropy in sheet metals

Recent experimental findings show that when orthotropic sheet metal samples are subjected to uniaxial tensile loading at an angle to the orthotropic axes, a drastic re-orientation of the axes takes place at moderate tensile strain levels, while orthotropic symmetries are preserved. This phenomenon is explained and simulated within a simple theory of plasticity, which combines Hill's quadratic yield criterion for orthotropy with the concept of plastic spin as a necessary constitutive ingredient for the orientational evolution of anisotropic tensorial internal variables. Detailed analytical investigation, corroborated by the experimental data, shows a trend of anisotropic axes rotation towards alignment of a pertinent minimum yield stress with the stretching direction. This-imposes a constitutive coupling relation between the anisotropic parameters and the plastic spin coefficient. Similarities and differences with other works as well as unresolved questions are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.