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 homo geneous behaviour is first presented from about 60 experiments in triaxial compression with two slenderness ratios (H/D = 1 and 2), in triaxial extens ion, and in isotropic compression. A large range of confining pressures (0- 60 MPa) is investigated, showing a significant evolution of material respon se. A strong positive dilatancy is observed at lower pressure, decreasing t o become negative (contractancy) at higher pressure. Simultaneously, the st rength decreases with increasing confining pressure. The localisation is de scribed in terms of onset of localisation, sheer band orientation and patte rning. The volumetric strain is analysed inside the band with computed X-ra y 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 see ms to be the transition mechanism between the brittle and semi-brittle regi me. (C) 2000 Elsevier Science Ltd. All rights reserved.