Mechanical Threshold Stress model for 6061-T6 aluminum alloy

I have attached a copy of my McMat07 talk below.  The talk is about a modified mechanical threshold stress (Follansbee-Kocks) model that can be used for strain rates > 1000 /s and high homologous temperatures.  In the model, the pressure dependence of the flow stress arises from the pressure dependence of the shear modulus.  Even after using the most accurate models for the shear modulus (and the melting temperature) we find that the pressure sensitivity of the flow stress is underestimated by our model (by quite a bit).

An open question in this regard is how does one incorporate the correct pressure dependence in a model for the flow stress.  Does pressure dependence have to be included in the initial yield stress model, the strain hardening model, or elsewhere?

Any pointers are welcome.