What's the meaning of "the maximum dissipation principle"?

Hi everyone

I am a graduate student.I am trying to formulate a constitutive equation for rock salt based on irreversible thermodynamics theory. Recently,I encounted two prolems:

（1）how to define the damage variable?

(2)what's the meaning of "the maximum dissipation principle"?

could everyone give me some advice?

thanks a lot.

Chao Du