Constitutive formulation of strain gradient plasticity for geomaterials via a
thermomechanical approach is investigated in this paper. It is demonstrated that,
by defining two thermodynamical potentials (a free-energy function and a
rate of dissipation function), the entire constitutive behavior of a decoupled
strain-gradient-dependent material may be determined. The elastic relations are
dependent on the free-energy function, while the plastic yielding and flow rule are
determined by the dissipation function in conjunction with the free-energy
function. Yield surfaces in both dissipative stress and true stress spaces
may be derived without difficulty. Nonassociative flow rules and possible
micromechanical mechanisms for the difference between plastic work and
rate of plastic dissipation are interpreted for gradient-dependent materials.
Using the obtained formulations and choosing appropriate thermodynamical
functions, a wide variety of strain gradient plasticity models in the literature are
recovered. Typical features associated with geomaterials, such as pressure and
Lode-angle dependency, are addressed in detail. This paper provides a general
thermodynamically-consistent framework of developing strain gradient plasticity
models for geomaterials.