Comparison of the microfracture localization in granite between fracturation and slip of a preexisting macroscopic healed joint by acoustic emission measurements

Experiments of fracturation and slip of a preexisting macroscopic healed jo int have been performed under triaxial deformation on granite from Mayet de Montagne (France). This granite shows high grain-scale inhomogeneity. Acou stic emissions have been recorded and hypocenters have been determined duri ng the entire experiments. For both rupture experiment and slip experiment, precursory localization of microfractures in the final rupture plane has b een observed in the early stage of deformation, well before the dilatancy. It is likely that not only initial closure of favorably oriented cracks but also breaking of partially cemented grains or slipping between grains may occur in the pseudoelastic phase and are already localized on the final rup ture plane where the shear stress seems to be concentrate. This behavior is observed in both cases where stress heterogeneity and rupture nucleation a re controlled by (1) medium-scale heterogeneity at the grain scale (HS samp le) or (2) macroscopic heterogeneity in the form of a preexisting healed jo int (JS sample). The sample with the healed joint exhibited similar to 1.6 times more acoustic emission events than the intact sample. The presence of the healed joint significantly weakened the sample.