A viscoplastic material model of overstress type with a non-quadratic yield function

For a more precise description of the material behaviour experimentally observed the Chaboche-model is modified using a non-quadratic yield function. This yield function is deduced from the equation of Pascal's snail. Thereby internal variables βij are introduced, which describe the distortional hardening and the orientation of the yield surface in the stress space. The verification of thermodynamical consistency of the modified model is provided. The reorientation of the yield surface, which is observed experimentally in the case of a loading change, will be modelled. With the reorientation of the yield surface a change of the back stresses is combined.