Sub-critical crack growth in anisotropic rock

In this study, subcritical crack growth in granite was investigated experimentally using the double torsion (DT) test. The orthorhombic elastic properties of granite, caused by the preferred orientation of pre-existing microcracks, were used to estimate the crack velocities and the stress intensity factors. The results showed that the crack velocity in granite varied depending on its direction of propagation. The fastest crack velocities for a given water vapor pressure were obtained when the crack propagated in the direction parallel to the plane in which the crack density of the pre-existing microcracks was the highest. Hence, subcritical crack growth in granite is affected by pre-existing microcracks. It was shown that the crack velocity was high when the water vapor pressure was high. From the results obtained under different temperatures and water vapor pressures, values of the activation energy for subcritical crack growth were estimated. The crack velocity under any given temperature, water vapor pressure, and stress intensity factor can be predicted theoretically if the activation energy is accurately estimated. The activation energy in granite is higher than that in glass. Cracks propagate in a straight line in glass, whereas they do not form a straight line in rocks due to the heterogeneity. The total length and surface area of cracks in rocks are therefore greater than those of straight cracks in glass. The larger activation energies in rocks are due to their heterogeneity.