Vol. 1, No. 5, 2006

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Thermomechanical formulation of strain gradient plasticity for geomaterials

Jidong Zhao, Daichao Sheng and Ian F. Collins

Vol. 1 (2006), No. 5, 837–863

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.

strain gradient plasticity, thermomechanical approach, pressure dependency, lode-angle dependency, geomaterials
Received: 8 December 2005
Revised: 17 January 2006
Accepted: 16 February 2006
Published: 1 September 2006
Jidong Zhao
School of Engineering
The University of Newcastle
Callaghan, NSW 2308
Daichao Sheng
School of Engineering
The University of Newcastle
Callaghan, NSW 2308
Ian F. Collins
Department of Engineering Science
School of Engineering
The University of Auckland
New Zealand