A constitutive model for describing coupled damage and plastic behavior ofDrucker-Prager geomaterials is developed in this study. Considering both plasticstrain and damage variable as the role of internal variables, the formulation ofconstitutive equation is based on thermodynamic theory and Ziegler’s orthogonalitycondition. The entire constitutive response is derived from two thermodynamicpotential functions. The flow rules of both plasticity and damage are constructed byusing a single yield function of generalized stress space. It was found that thecoupled model is non-associated because of frictional and dilative properties of D-Pmaterials. The relationship between plastic and damage variables is deduced and it isdetermined by parameters of potential functions. The model naturally satisfies thefundamental laws of thermodynamics and overcomes the limitation ofphenomenological methods. The formulation of the model presented here can beused as a basic theory for geomaterials mechanical response analysis of coupleddamage and plasticity.
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