Evolution of strain localisation with stress in a sandstone: Brittle and semi-brittle regimes

The experimental characterisation of strain localisation in a porous sandstone is presented for the brittle and the transition between the brittle and semi-brittle regimes. Localisation occurs through bands and the observations show a continuous evolution from a dilative strain type inside the bands at low mean stress to a compactive strain type at higher mean stress. The orientation of the bands changes also with the stress level and the onset of localisation is systematically detected before the peak stress. Microscope observations suggest two steps during the localisation process. A theoretical analysis of the localised strain inside bands is presented using the framework of bifurcation theory. The notion of dilating and compacting shear bands is introduced and this allows to show that a continuous transition from pure extension bands to dilating and compacting shear bands and to pure compaction bands is expected. These theoretical predictions are in accordance with the experimental observations, providing an interesting framework to investigate the brittle to semi-brittle transition regime.