  In order to enhance the modeling of metallic materials behavior in non
proportional loadings, a modification of the classical elastic-plastic models
including distortion of the yield surface is proposed. The new yield criterion
uses the same norm as in the classical von Mises based criteria, and a
"distorted stress" Sd replacing the usual stress deviator S. The obtained yield
surface is then "egg-shaped" similar to those experimentally observed and
depends on only one new material parameter. The theory is built in such a way
as to recover the classical one for proportional loading. An identification
procedure is proposed to obtain the material parameters. Simulations and
experiments are compared for a 2024 T4 aluminum alloy for both proportional and
nonproportional tension-torsion loading paths.
