Fracture toughness anisotropy in granitic rocks

Four relatively fine grained and homogeneous granitic rocks have been studied to investigate the relationship between their microstructural properties and fracture toughness behaviour. The apparently homogeneous rocks have been found to exhibit strong anisotropy in terms of microcrack distribution and their orientation. The fracture toughness values for these rocks, measured with the cracked chevron notch Brazilian disc method (CCNBD), varied from 0.71 to 1.89 MPa m0.5, with similar variation observed within the same rock type depending on the specific fracture plane studied. The relationship between these variations and the corresponding microstructural properties along specified planes has been investigated in detail to show a strong correlation between the two. Further confirmation is provided by taking into account the seismic velocity anisotropy observed along the same specified directions. The nature of these variations and their effect on the resulting fracture path and surface are discussed. The results clearly show the difficulty in assigning a unique value of fracture toughness to rocks and rock-like materials without cognizance of their microstructural characteristics.