International audienceGranular materials are constituted of an assembly of particles. In spite of the simplicity of this assembly, its mechanical behaviour is complex. In the first stage we propose a framework to establish correlations between parameters of the supposedly continuous medium and grain properties which are assumed to be constant. However, this hypothesis is no longer valid in the case where physical (shape, size…) or mechanical properties (Young modulus Eg, Poisson's ratio νg…) of grains evolve during loading, causing the behaviour of the assembly to modify. We study the influence of the physical and mechanical parameters on grain breakage. We subsequently propose a way to model the influence of the grain breakage on granular materials and we introduce this influence in an elastoplastic constitutive model. Validations are made on two kinds of sands under isotropic and triaxial loading. Since the results of numerical computations corresponded well with the experimental data, we believe that the new model is capable of accurately simulating the behaviour of granular materials under a wide range of stresses and of taking into account, through new parameters, the individual strength of grains

Granular materials are the most recurrent form of solid-state matter on Earth. They challenge researchers and engineers in various fields not only because they occur with a broad variety of grain sizes, shapes and interactions in nature and industry, but also because they show a rich panoply of mechanical states. Despite this polymorphism, all these different types of soils, powders, granules, ores, pharmaceutical products, etc., are instances of the granular matter with the same least common denominator of being sandlike (psammoid in Greek), i.e., solid grains interacting via frictional contacts. This review describes milestone contributions to the field of granular materials since the early elastic-plastic models developed for soils in the 1950s. The research on granular materials has grown into a vast multidisciplinary field in the 1980s with increasing focus on the microstructure and owing to new experimental tools and discrete simulation methods. It turns out that the granular texture, particle-scale kinematics, and force transmission are far more complex than presumed in early micromechanical models of granular materials. Hence, constitutive relations cannot easily be derived from the particle-scale behavior although advanced continuum models have been developed to account for anisotropy, intermediate stress, and complex loading paths. The subtle elastic properties and origins of bulk friction will be discussed, as well as the effects of particle shape and size distributions. The review covers also recent developments in macroscopic modeling such as the thermomechanical approach, anisotropic critical state theory, nonlocal modeling approach, inertial flows, and material instabilities. Finally, a brief account is given of open issues and some new frontiers and challenges in the field.

This paper presents a synthesis of the works performed by various teams from France, Italy and Canada around the question of second‐order work criterion. Because of the non‐associative character of geomaterials plastic strains, it is now recognized that a whole bifurcation domain exists in the stress space with various possible modes of failure. In a first part these failure modes are observed in lab experimental tests and in discrete element modelling. Then a theoretical study of second‐order work allows to establish a link with the kinetic energy, giving a basis to explain the transition from a prefailure (quasi)static regime to a postfailure dynamic regime. Eventually the main features of geomaterials failure are obtained by applying second‐order work criterion to five different constitutive rate‐independent models—three being phenomenological and two micromechanical. As a whole this paper tries to gather together all the elements for a proper understanding and use of second‐order work criterion in geomechanics. Copyright © 2010 John Wiley & Sons, Ltd.