Modeling of the monotonic and cyclic Swift effects using an isotropic, finite viscoplasticity theory based on overstress (FVBO)

The simulation of free-end torsion effects of a tubular specimen using isotropic, FVBO is presented. The limited capability of isotropic formulations to model deformation induced anisotropy is demonstrated. The logarithmic, the Jaumann and the modified Jaumann co-rotational rates are chosen as objective rates. Stress-like state variables should have the same non-zero components as the Cauchy stress to satisfy stress-boundary conditions once equilibrium is reached. This method will be called Case B. However, this is not always practiced. Some papers use different non-zero components. This method will be called Case A. In this paper, two methods are employed to investigate their influence on the free-end torsion behavior. It turns out that numerical experiments according to Method A can only reproduce the loading behavior. Method B can reproduce the cyclic Swift effect qualitatively. The newly introduced logarithmic spin [Acta Mech. 124 (1997) 124 89] had a positive influence on the shape of the curves.