Editing equationin at Expression in Variables from Equation views

Hi all,
I’m playing with thermo-mechanic coupling. In the mechanical part, I use solid mechanics with Norton Creep. In this creep, I would like to have nD=1. That’s why I’ve changed Expression in Variables from Equation views. Is it a valid process to edit Equation? You may have look in this attached model. It seems not working. Your contribution is appreciated.

Thanks

Md. Tariqul Islam
Department of Earth Science
University of Gothenburg, Sweden
tariqul.islam@gvc.gu.se