A fundamental question in thermodynamics for rate-independent plasticity

Hi everyone!

    I am recently studing thermodynamics for rate-independent plasticity.  Fundamental statement in this subject is that "in the rate-independent materials,  dissipation function must be homogeneous of degree one  in rate of internal variables "。I can't understand this statement! I am wondering that why dissipation function must be homogenerous of degree one in rate of interal variables(in the rate-independent materials)? Is it just a assumption? Or does it have a mathematical or physical justification? I spend a lot of time to think this problem, but with little success. I appreciate your kindness if anyone give me some comments or suggestion in this question!

Thank you!