Derivation of the constitutive equations of elastic-plastic and elastic-viscoplastic solids at finite deformations is discussed. The deformation is uncoupled by using the Lee-Freund three-configuration deformation model. By assuming elastic properties to be independent of plastic deformation, the elastic and plastic (or viscoplastic) constitutive equations are essentially uncoupled. The normality condition of the plastic strain-rate vector to the yield surface in stress space is obtained by incorporating the concept of internal variables in the energy equation.

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