Analysis of fracture damage zone in anisotropic granitic rock using 3D X-ray CT scanning techniques

This paper documents the testing of Barre granite specimens for fracture toughness under mode I, along two different directions with respect to microstructural fabrics. The morphology of cracks was measured by applying a 3D Objects Counter plug-in technique on micro Computed Tomography (CT) images of specially prepared tested specimen for two directions of crack propagation (approximately 500 images of the failed profile were taken along three orthogonal planes). The research objectives of this study were determination of volumetric variation of induced fracture surfaces, analysis of the nature of their structures, and evaluation of mineralogical contribution toward the total test crack passage area along the two specified planes characterized by maximum and minimum fracture toughness values. The results obtained justify the fracture toughness anisotropy in Barre granite, reflecting differences in the structure of its damage zones. The damage zone for the situation in which the test crack was forced to propagate perpendicular to preferably oriented preexisting microstructural fabrics (case 1) is characterized by twice as much fracture surface area, and contains ten times more induced fracture porosity than that observed in the second situation, in which the test crack was forced to propagate parallel to the plane processing the pre-existing microstructural fabrics (case 2). This conclusion further explains why the fracture toughness in case 1 is almost twice that observed in case 2. The present study has potential applications in the fields of rock mass stability, oil reservoir stimulation, and rock fragmentation and damage in blasting and emplacement of dykes.