We present a new continuum damage mechanics constitutive model for brittle geomaterials based on irreversible thermodynamics with internal state variables. Damage is incorporated by a vector-valued internal state variable that can be interpreted as the local void density in a cross-section with normal n within a representative material volume element. The thermodynamic basis, free energy function, damage evolution equation, and damage-induced inelastic compliance are derived. It is shown that damage causes strain-softening and positive dilatancy, as well as decrease of the material stiffness. Comparisons of the model predictions with well-known experimental data on rocks and concrete under uniaxial and triaxial compression stresses are given. This model could be valuable in appropriate Petroleum Geomechanics cases.

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