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Experimental investigations indicate that the third stress invariant; Lode angle α affects significantly the behavior of pressure sensitive materials. The present communication presents a formulation to account for α in isotropic pressure-sensitive elastoplastic materials. Seven Lode dependences are reviewed. A new one, referred to as LMN, in proposed to generalize Lade and Duncan, and Matsuoka and Nakai failure surfaces. The formulation is general enough to introduce α into the isotropic elastoplastic modes which are only developed in terms of first and second-stress invariants. As an illustration, several Lode dependences are introduced into Roscoe and Burland model. The performance of the modified model is estimated by comparing experimental and analytical results in the case of true triaxial loadings on normally consolidated clay.

The structure of persistent shear bands in granular materials is investigated by numerically simulating an idealized assembly of two-dimensional particles. Flexible stress-controlled boundaries are used instead of periodic boundaries to avoid constraining the motion of particles within the shear bands. The displacement, volumetric strain, void ratio, rotation of the particles, rotation of their neighbourhoods and contact orientation are examined within the shear band. The volumetric strain determined from local deformation gradients is found to overestimate dilatancy. The particle rotations are related to the rigid rotation of their neighbourhoods. The importance of rotations inside shear bands justifies the micropolar description of granular materials. Dans cet article, on étudie la structure des bandes de cisaillement permanentes en utilisant un modèle numérique d'un ensemble de particules dans un espace de dimension deux. On a remplacé les frontières périodiques par des frontières aux contraintes imposées afin d'éviter de restreindre le déplacement des grains dans la bande de cisaillement. On montre le déplacement, la déformation volumique, l'indice des vides, la rotation des grains, la rotation de leurs voisinages et l'orientation des contracts à l'intérieur de la bande de cisaillement. On trouve que la déformation volumique calculée d'après le gradient de déformation surestime la dilatance et que la rotation des grains est reliée à la rotation d'ensemble des particules voisines. L'importance des rotations confirme les hypothèses des milieux micropolaires de la théorie de Cosserat.

A viscoelastic model is proposed to describe the dynamic response of the saturated poroelastic materials that obey the Biot theory (1956). The viscoelastic model is defined from the velocity and attenuation of dilatational and distortional waves in poroelastic materials. Its material properties are defined in terms of the elastic moduli, porosity, specific gravity, degree of saturation, and permeability of the soils. The proposed model is tested by comparing its response with the one of poroelastic materials in the case of axial and lateral harmonic loadings of one-dimensional columns. The viscoelastic model is simpler to use than poroelastic materials but yields similar results for a wide range of soils and dynamic loadings.

SUMMARYA numerical technique is proposed to obtain stress-strain response curves from rate-type and incremental constitutive equations during generalized loadings. The proposed method linearizes the loading constraints of laboratory experiments, links them to the constitutive relations, and forms a linear system of ordinary differential equations. It circumvents the difficulties associated with the non-uniqueness and bifurcation of boundary value problems. The method is illustrated for the elastoplastic von Mises and Roscoe and Burland models subjected to torsion, circular stress path, and undrained triaxial compression. The approach pertains to most stress-strain relationships and laboratory experiments of geomechanics. It is useful for research on material modelling, engineering practice and computational mechanics.