Abstract: Based on irreversible thermodynamics, the paper elaborates two fundamental constitutive issues: (1) existence or inexistence of the flow potential; (2) the flow potential can or cannot be decomposed into a damage potential and a plastic potential, namely, whether the two mechanisms can be decoupled. Within this framework, sufficient and necessary condition for the existence and the decoupling of the flow potential are given, and they are strictly proved in theory. According to the mechanical property of the geomaterial, the inexistence and the un-decoupling of the flow potential are also proved; meanwhile, it is shown that the existence and decoupling of other solid materials can theoretically be obtained from the two fundamental theories.

Key words: geomaterial, constitutive relation, irreversible thermodynamics, damage, flow potential