A viscoplasticity model with an enhanced control of the yield surface distortion

A new model of metal viscoplasticity, which takes combined isotropic, kinematic, and distortional hardening into account, is presented. The basic modeling assumptions are illustrated using a new two-dimensional rheological analogy. This demonstrative rheological model is used as a guideline for the construction of constitutive relations. The nonlinear kinematic hardening is captured using the well-known Armstrong–Frederick approach. The distortion of the yield surface is described with the help of a so-called distortional backstress. A distinctive feature of the model is that any smooth convex saturated form of the yield surface which is symmetric with respect to the loading direction can be captured. In particular, an arbitrary sharpening of the saturated yield locus in the loading direction combined with a flattening on the opposite side can be covered. Moreover, the yield locus evolves smoothly and its convexity is guaranteed at each hardening stage. A strict proof of the thermodynamic consistency is provided. Finally, the predictive capabilities of the material model are illustrated using the experimental data for a very high work hardening annealed aluminum alloy 1100 Al.

► A new model of viscoplasticity with directional distortional hardening is proposed.
► The model is based on a simple two-dimensional rheological interpretation.
► Any convex symmetric yield surface can be captured in the saturated state.
► Simple mathematical structure, since no higher-order tensor variables are used.
► The model is thermodynamically consistent.