Improved associated flow rule for anisotropic viscoplasticity in thermoplastic polymer systems

An elasto-viscoplastic constitutive model was previously developed and successfully used to describe the anisotropic time-dependent response of short fiber reinforced thermoplastics. It is demonstrated that when it is applied to systems with a higher level of anisotropy, the apparent anisotropy of the rate-dependent yield stress deviates from the Hill parameters that are used to define it. This becomes more apparent if the level of anisotropy is increased. The origin of this discrepancy is mathematically investigated and shown to be related to the viscoplastic multiplier of the associated flow rule employed. A solution is proposed that completely restores consistency and extends the applicability of the model to systems with high anisotropy.