Micro-mechanical bases of some salient constitutive features of granular materials

Although phenomenological constitutive models have the ability to exhibit most salient macroscopic features of granular materials, they generally do not provide any convincing interpretation of them: their basic origin remains hidden. It is now well established that the micro-structure of granular materials plays a significant role in their overall constitutive behavior. In the past few years, a great deal of theoretical and experimental research has been devoted to this domain, giving rise to efficient micro-mechanical approaches. First, this paper reviews the micro-directional model, which is a micro-mechanically based constitutive relation. Then an extension is proposed to describe the possible collapse of force chains. This micro-structural adjunction is shown to be sufficient to simulate work-hardening and softening mechanisms. A granular assembly, containing a multitude of frictional contacts whose orientation is generally anisotropically distributed, exhibits various other typical features such as the nonassociated character of plastic strains. The micro-structural origin of this feature is investigated, and further conclusions related to the existence of a regular or a singular flow rule are drawn.