Norton Creep parameters

Hi experts/users,

I'm going to simulate creep of polymer by using Norton model in situation which temperature and applied force are constant.
I've got question regarding parameters in Norton creep model. I appreciate any hints which could help me.


I need parameters 'creep rate coefficient', 'reference stress for creep' and 'Stress exponent'.

First, I found the ''stress exponent'' from graph of natural logarithm of strain rate versus natural logarithm of stress.

Then I used the attached relation to calculate 'Creep rate coefficient'. But the point is that the unit of the calculated
creep rate coefficient (A) is (1/(s.pa^n)).

If the procedure is right, what is the right value for A with unit (1/s) and the value for ''reference stress for creep'' (pa)?


Another question is that, is it possible that stress exponent be less than 1?


Best regards,
Samira