Fracture roughness and gouge distribution of a granite shear band

Localization of deformation during fracture mechanical tests leads to the development of shear bands. We performed triaxial tests using Sidobre granite at four different confining pressures (from 20 to 80 MPa). We compared two sets of tests: one set was stopped immediately after the formation of the shear band; a second one included additional shear deformation. From the analysis of thin sections of these laboratory samples, we characterize the typical microstructures in the shear band (mode I and II cracks, Riedel cracks, cataclastic flow). Statistical properties of rupture surface roughness and gouge grain size reveals scaling invariance. Using a mechanical profiler, the fracture roughness is measured along parallel profiles and shown to be correctly described over up to 3 orders of magnitude by self-affine geometry with a roughness exponent close to zeta=0.80. This property is very similar to tensile crack even if local processes are different. The influence of the slip is observed. Fracture surfaces are rougher along the slip direction (zeta=0.74) than perpendicular to it (zeta*=0.80). The confining pressure is shown to have a weak effect on the fracture roughness. It smoothes the surface: slight increase of the roughness exponent. Gouge particles extracted from the shear band present a power law distribution with an exponent ranging from 1.44 to 1.91. This exponent appears to increase with the shearing displacement and the confining pressure. When a significant shear of the band is combined with a high confining pressure (i.e., impeded dilation of the band), the hallmark of fragmentation is observed for the particle distribution and related to a smoothing of the band boundary.