Description of the transient and stationary hardening behaviour of steel by means of a boundary layer model

A simple bounding surface model is developed with special regard to the hardening behaviour of polycrystalline metals under varying loads. Hereby also transient phenomena, as to appear in the course of alternating loading conditions, are considered by means of an additional isotropic hardening mechanism in the subspace of microstresses. The model is tested by simulating cyclic strain-controlled loading histories, which have been obtained from axial/torsional experiments on Ck 15 steel tubes. A special procedure has been developed, by using polynomial splines of grade three to quantify the amount of isotropic and kinetic hardening during axial loadings. Moreover nonproportional loading histories with rectangular, circular and triangular shapes have been applied, and the hardening behaviour under such conditions is compared to the uniaxial case. (orig.)