Experimental characterisation of the localisation phenomenon inside a Vosges sandstone in a triaxial cell

The behaviour of a Vosges sandstone is studied, including quasi-homogeneous deformation, incipient strain localisation and localised rupture. The homogeneous behaviour is first presented from about 60 experiments in triaxial compression with two slenderness ratios (H/D = 1 and 2), in triaxial extension, and in isotropic compression. A large range of confining pressures (0-60 MPa) is investigated, showing a significant evolution of material response. A strong positive dilatancy is observed at lower pressure, decreasing to become negative (contractancy) at higher pressure. Simultaneously, the strength decreases with increasing confining pressure. The localisation is described in terms of onset of localisation, sheer band orientation and patterning. The volumetric strain is analysed inside the band with computed X-ray tomography and electron microscopy. We observed the formation of a gouge layer, and around it, a dilating shear band at lower confining pressure and a compacting shear band at higher pressure. This compacting shear band seems to be the transition mechanism between the brittle and semi-brittle regime. (C) 2000 Elsevier Science Ltd. All rights reserved.