On the impact of local rotation on the evolution of an anisotropic plastic yield condition

Isotropic and kinematic hardening is combined with the local, average lattice rotation in order to describe the hardening behaviour of a polycrystalline metal in terms of continuum mechanics. This approach is adapted by means of parameter identification to experimental results by Boehler and Koss (In: Bruller O., Mannl V., Najar J. (Eds.), Advances in Continuum Mechanics. Springer, Berlin, (1991) pp. 143) Koss (Ecrouissage isotrope et anisotrope des aciers laminees soumis a de grandes deformations. Thesis, Universite Joseph Fourier, Grenoble I. (1992)) as well as to the experiments of the authors; a fair to good agreement is achieved. Although in particular, kinematic hardening fits the experimental data quite well it will be seen that the generalised, linear Prager hardening rule does not hold. Moreover it is found that the local rotation may differ considerably from the classical rotation, induced by the plastic flow field. Its magnitude, obtained by the authors, agrees well with the values, obtained via a direct texture analysis by Bunge and Nielsen using a similar material. Finally a conjecture by Boehler and Koss is checked, according to which an initial orthotropy vanishes completely under a secondary stretching along an axis, different from the axes of material symmetry, while subsequently a new orthotropy is generated along the axis of secondary stretch. This mechanism contradicts another one postulated by Kim and Yin (J. Mech. Phys. Solids 45 (1997) 841) according to which the axes of orthotropy rotate continuously from the initial to the asymptotic new position.