The existence of rotation of principal axes of stress is a phenomenon observed frequently in situ. The models used in order to dimension and verify the stability of buildings, do not take clearly into account, for most of them, this rotation. We wonder about the validity of these models. As part of global research in this direction, we study here the validity of the anisotropic damage model by Dragon. This model is applicable to quasi-brittle rocks (Vosges sandstone in this case). In order to validate Dragon's model, we used the torsion-compression apparatus developed at the Ecole des Mines de Douai. The simulation of the torsional tests was made by finite element analysis. The results obtained, with the same parameters as those used for the triaxial tests' simulation, indicate qualitative validity of Dragon's model under torsional loading.


L'existence de la rotation des axes principaux des contraintes est un phenornene rencontre frequernrnent. Les modeles utilises, dans Ie but de dimensionner et de verifier la stabilite des ouvrages, ne pennant pas en compete explicitement, pour la plupart d'être ex., cite rotation. Nous alones dons studier la validate de cues nodules. La partie des recherches presenters dans cat article concernment lettuce de la validate du model a anisotropies induite introduit par Dragon. Ce modele est applicable a des roches fragiles (un gres des Vosges dans notre cas). Afin de valider Ie modele de Dragon, on a utilise I'appareillage de torsion compression developpe a l'Ecole des Mines de Douai. Les simulations des essais de torsion ont ete effectues par la methode des elements finis. Les resultats obtenus, avec les memes parametres que ceux utilises pour simules les essais triaxiaux classiques, montrent une validate qualitative du modele de Dragon en torsion.


Die Rotation der Hauptspannungsachsen ist eine haufig auftretende Erscheinung. Die meisten der zur Dimensionierung und OberprUfung der Stabilitat der Strukturen verwendeten Modelle berUcksichtigen nicht ausdrUcklich diese Rotation. Wir werden also die GUltigkeit dieser Modelle untersuchen. Der in diesem Artikel vorgestellte Teil der Unter =suchungen betrifft die GUltigkeit des von Dragon vorgestellten Modells von Zerstorungsablaufen mit vorgegebenen Anisotropien. Zur Validierung des Dragons-Modells wurde der von der Ecole des Mines in Douai entwickelte Torsions-Kompressions-Apparat verwendet. Die Ergebnisse, die mit den gleichen Parametern erzielt wurden die denen bei der simulation der klassischen triaxialen Kornpressionversuche, zeigen eine qualitative GUltigkeit des in Betrach gezogenen Torsionsmodells.

I - Introduction

In order to study the validity of Dragon's model, we made a experimental campaign which consisted of several triaxial tests (hydrostatic, deviatoric, proportional) and torsional tests. Some of them permitted to determine the material parameters and the others (proportional and torsional) to validate the model. In particular, the torsional loading tests permit a good validation of the model because the principal axes of stress rotate and the stress state could be tridimensional.

111.2 - Deviatoric tests Similar samples were tested under conventional triaxial tests. They were performed by controlled the axial strain rate equal to 5 10–6 m/mn. The volumetric deformation was deduced from change of pores. The tests, presented in figure 2, were realized for different confining pressures.: 0, 5, 10, 15, 20 MPa. It can be noted that the Young modulus was costant and the Poisson ratio decreases with the isotropic pressure.

111.4 - Torsional tests Torsional tests were carried out on a Vosges sandstone to study the behaviour under a shear loading in three dimensional conditions of stress state. The tests were performed on hollow cylinder samples on which, firstly, an isotropic compression is applied followed by a monotonic torque loading up to failure. The tests were carried out in drained condition with water. The confining pressures are 0, 02, 05, 10,20 MPa. The figure 5 and 6 present the curves torque evolution vs angular deformation and volumetric deformation's evolution vs angular deformation.

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