By analyzing a microelement based on four spheres, equations governing the equilibrium of the microelement are developed. By examining these equations more closely, two primary mechanisms of failure of the microelements, one based on particle sliding and another based on particle rolling, are identified. For each primary mechanism, two separate mechanisms, one based on collapse of the microelement in the vertical direction and the other based on collapse in the horizontal direction, are

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... ed. With the aid of these concepts, constitutive equations are developed for a two dimensional assembly of granular particles. The assembly is considered to consist of the 4-sphere microelements. Taking the plastic strains to be consequence of the collapse of some of the microelements, equations are developed for plastic strains. The formulation yields loading criteria and flow directions. With suitable hardening rules, it is shown that the microstructural model is capable of simulating most of the salient features of the stress-strain behavior of granular materials. In particular, the stress-dilatancy relation with consideration to the phenomena of phase transformation and critical state failure are simulated satisfactorily.