  This study proposes a modification of the yield condition that overcomes the
mathematical constraints of the Directional Distortional Hardening models
developed by Feigenbaum and Dafalias. This modified model surpasses the
mathematical inconsistency of the complete model and the limitation of the
r-model of Feigenbaum and Dafalias. In the complete model, the inconsistency of
distortional term in the yield surface and the plastic part of free energy
appears in the absence of kinematic hardening. In addition, the simplified
r-model fails to capture the flattening of the yield surface in the opposite
direction of loading due to the absence of a fourth-order internal variable in
the distortional term. Hence, this study incorporates a decoupled distortional
hardening term in the equation of yield surface that makes it possible to
simultaneously capture the flattening of the yield surface and allows isotropic
hardening with distortion, even in the absence of kinematic hardening. The
mathematical formulation of the proposed modified model is also developed,
which sets the stage for future experimental investigations and validation
efforts.
