Relation of fracture resistance to fabric for granitic rocks

Double-torsion specimens of two granitic rocks were prepared in several directions with reference to microcracks fabric. Even for the same rock and at the same stress levels, the observed crack velocities in two granitic rocks were dependent on both the propagation direction and the opening direction. The maximum difference by several orders of magnitude was found for both rocks. The highest crack velocity was observed when the subcritical crack was parallel to most of the preexisting cracks. The maximum critical stress intensity factor was about twice as high as the minimum one in different directions. An analysis for a thin plate having anisotropic elasticity under torsional load showed that the observed difference in the crack velocity and the critical stress intensity factor was not an error due to conventional equations derived on the assumption of isotropic elasticity but the true material’s property. As the preferred orientation of microcracks has been pointed out for many granitic rocks, we can conclude that the anisotropic nature of the fracture resistance of the two granitic rocks used in this study was not exceptional. A region of a transport-limited velocity was not found for rocks, even at the velocity of 10−2 m/s, that is almost equal to the theoretical limit of the stress corrosion cracking.

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