Publication

Comportement mécanique des milieux granulaires sous sollicitations cycliques : application aux fondations de chaussées

Frédéric Mayoraz
2002
Theses

Résumé

The following work deals with the characterisation of the behaviour of road bases granular material, made of tunnel excavation material and loaded by passing vehicle axles. This granular media, don't correspond generally to the standards demanded for the construction of road pavements. The thesis is a part of the project COST 337 ("Unbound granular materials for road pavements") whose aim is to develop a guideline for the design of road pavements in Europe (material production, laboratory tests, modelling of mechanical behaviour). At the beginning, several loadings of road pavements are described (mechanical, thermal, hydric), concentrating on mechanical loading. Full-scale tests and finite element simulations led to the knowledge of the order of magnitude of stresses and deformations as well as to the understanding of the rotation of the principal stresses, due to the passing of a vehicle axle. This theoretical part is followed by laboratory cyclic tests with sand to estimate the influence of the principal stress rotation on the deformability of granular media and the importance to take this phenomenon into account for the modelling. Further laboratory tests were carried out on high diameter samples (160 and 250 mm) made of tunnel excavation material. Those tests were made by performing a large number of cycles (80'000 to 250'000 cycles) and various stress paths. The aim of the tests was to measure the evolution of elastic (stiffening of material) and plastic (accumulation of permanent deformations) material characteristics during the cycles. Tools have been developed in the framework of image analysis to assess the eventual change of morphology of this non-standard granular media. Finally the modelling of the accumulation of plastic deformations is discussed in accordance to the theory of elasto-plasticity and visco-plasticity. To remove the limits of elasto-plasticity to simulate a large number of cycles, a visco-plastic approach is developed. The elegant approach, replacing the time by the number of cycles, allows to approximate the permanent deformations of a road pavement in dependence on the stress path. It provides a tool to estimate the rutting in road pavements.

Source officielle

Frédéric Mayoraz
2002
Theses

Résumé

Source officielle

Proximité ontologique

Génie chimique

Science des matériaux: Deformation of materials

Concepts associés (28)

Contrainte (mécanique)

vignette|Lignes de tension dans un rapporteur en plastique vu sous une lumière polarisée grâce à la photoélasticité. En mécanique des milieux continus, et en résistance des matériaux en règle générale, la contrainte mécanique (autrefois appelée tension ou « fatigue élastique ») décrit les forces que les particules élémentaires d'un milieu exercent les unes sur les autres par unité de surface. Ce bilan des forces locales est conceptualisé par un tenseur d'ordre deux : le tenseur des contraintes.

Déformation plastique

La théorie de la plasticité traite des déformations irréversibles indépendantes du temps, elle est basée sur des mécanismes physiques intervenant dans les métaux et alliages mettant en jeu des mouvements de dislocations (un réarrangement de la position relative des atomes, ou plus généralement des éléments constitutifs du matériau) dans un réseau cristallin sans influence de phénomènes visqueux ni présence de décohésion endommageant la matière. Une des caractéristiques de la plasticité est qu’elle n’apparaît qu’une fois un seuil de charge atteint.

Deformation (engineering)

In engineering, deformation refers to the change in size or shape of an object. Displacements are the absolute change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain is the relative internal change in shape of an infinitesimally small cube of material and can be expressed as a non-dimensional change in length or angle of distortion of the cube. Strains are related to the forces acting on the cube, which are known as stress, by a stress-strain curve.

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