Acoustic Monitoring of Damages and Shear-Banding in Cemented Granular Material under External Loading

International audienceEarthquakes or fault core sliding occur naturally in response to long-term deformation produced by plate tectonics. The way the damage or fracture process of rocks control the frictional slip is not well understood. It involves materials in very different states: from granular-like materials near the fault [1] to almost cohesive state in distant host rocks. To address this issue, we perform controlled laboratory experiments to study the material evolution from cohesive to granular-like states under external loading. Our porous synthetic rocks are made of cemented glass beads in which the packing density and the cement nature and content are tunable [2,3]. Two mechanical tests have been conducted: i) the oedometric load in a rigid cylindrical cell; ii) the triaxial load in an elastic cell (confined by atmospheric pressure). The fracture processes are monitored by acoustic waves with longitudinal ultrasound velocities [4] and acoustic emissions [5]. In (i) the fracture process is likely associated with the cracks apparition, spatially diffused without shear-band formation. For a ductile cement, the damage induced by load appears likely as an anomalous deviation in the master curve of stress-strain and the combined acoustic detection provides a very clear evidence with an important sound velocity decrease. Upon cyclic unloading-reloading, we find a dependence of the sound velocity with the pressure similar to the power-law in dry granular media but with a non-zero velocity at vanishing pressure depending on the residual cohesion. Acoustic emissions are especially interesting when stress drops occur for a brittle cement. In (ii), for smaller loads, shear-bands appear at the sample scale, accompanied by a strong velocity decrease [4]. [1] Marone, Annu.Rev.EarthPlanet.Sci. 26 1998 [2] Langlois, Jia, Phys.Rev.E 89 2014 [3] Hemmerle et al., Scient.Rep. 2016 [4] Khidas, Jia, Phys.Rev.E 85 2012 [5] Johnson et al., Geophys.Res.Lett. 40 201