On a Phenomenological Plasticity Model with Non-Normality Effects
Prof. Viggo Tvergaard
Dept. Mechanical Engineering, Solid Mechanics
Technical University of Denmark, Kgs. Lyngby, Denmark
An abrupt strain path change has been used to determine the shape
of the subsequent yield surface near a current loading point. When
applying this procedure, both polycrystal plasticity calculations and
experiments have shown non-normality of plastic flow, with a vertex-type
response on the apparently smooth yield surface. The physical reason for
this observation will be carefully discussed, with particular emphasis
on the fact that the prediction appears in spite of the normality rule
built into each slip system of the crystal plasticity model. To represent
this type of behaviour, a phenomenological plasticity model with
non-normality effects has been proposed, such that a smooth yield surface
for an anisotropic solid is combined with a vertex-type plastic flow rule.
Some predictions obtained by using the plasticity model with
non-normality effects will be presented. Here, predictions of plastic
instabilities are of particular interest, due to their sensitivity to
accurate modelling of material response under rapid stress path change.
Results are shown for the onset of necking in biaxially stretched sheets,
and also finite element results for the plane strain tensile test
are presented.
