On the Elastic and Plastic Strain Decomposition in Granular Materials

An impressive number of constitutive relations have been developed in the past few decades. With respect to the class of elastoplastic phenomenological models, elastic and plastic strain decomposition is generally stated as a basic assumption, so as to treat the elastic (i.e., recoverable) and plastic (i.e., unrecoverable) parts of the strains separately. For incrementally nonlinear relations, this decomposition is not possible. In the first part of this paper a detailed discussion of elastic and plastic decomposition is presented. Then the paper expands the debate on this crucial point by addressing the question of defining elastic (or plastic) deformation specifically for granular materials, considering two complementary approaches. An incrementally nonlinear model is used first and then a multi-scale approach is considered to examine the compatibility of this partition from a micromechanical point of view, with the usual definition of both elastic and plastic incremental strains. Finally, micro-structural considerations show that only a fraction of the elastic strain energy can be recovered, whatever the unloading path, after an incremental loading path inducing both elastic and plastic mechanisms.