PDE

I am currently working on COMSOL 4.0a.
Can anybody tell me how the variables for stress are defined in COMSOL?
Since I want to define a Coefficient PDE, which its diffusion coefficient (c) is coupled with Mechanical -Stress( such as Von Mise Stress) through decay exponential law:
C= exp(-0.2*s),
where s represent the yield stress such as Von Mises Stress.

Thanks so much in advance.
please answer my question I really need it.