In this paper we present an experimental approach aiming at assessing the correlation between fracture roughness and fracture toughness (KIC) of two granitic rocks exhibiting significant fracture toughness anisotropy. Fractured surfaces obtained under mode I testing method from three orthogonal planes with respect to microstructural fabrics in Barre and Stanstead granites were analyzed for their degree of roughness. There exists a clear correlation between roughness and toughness within each rock examined along the three planes. Stanstead being coarser revealed a higher roughness but lower toughness in comparison to Barre granite.


Preferentially oriented microstructural fabric and microcracks in granitic rocks are largely responsible for the anisotropic behaviour of physico-mechanical properties of otherwise apparently isotropic medium. In a previous attempt to study the fracture toughness anisotropy and its relation to the microstructural fabrics in granitic rocks, (Nasseri et al. 2006, Nasseri & Mohanty 2006) it was concluded that fracture toughness is linked more closely to the presence of oriented microcracks than grain size and distribution. The former was also found to have good correlation with measured differences in seismic velocities. The study showed that the measured value of fracture toughness can show variations by a factor ranging from 1.3 to 2.4 in the same rock type. This anisotropy has been conclusively shown to be linked directly to the microstructural anisotropy present in the rock, as represented by the size of dominant microstructural sets, grain shape ratio and their orientation with respect to the direction of propagation of the induced fracture. For instance, a fracture propagating at right angle to the major set of weakness plane yielded the highest fracture toughness value. Such fractures, in contrast to the fracture propagating parallel to the dominant weakness planes, are characterized by increasing roughness, segmentation, and higher deflection.

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