Effects of environment and rock fabric on subcritical crack growth in granite were investigated. Double Torsion test was used. It was shown that the relation between the crack velocity and the stress intensity factor was anisotropic. Under the same temperature and humidity, the crack velocity depended on the crack opening direction, and it was the highest when the crack propagated parallel to Rift plane. From the results under different temperature and humidity, it was shown that the crack velocity was higher when the temperature and humidity were higher. The crack velocity in water was higher than that in air. These results agreed well with the concept that stress corrosion is the main mechanism of subcritical crack growth in rock. From the experimental results in this study, it is concluded that subcritical crack growth in granite is affected by water and pre-existing microcracks.
In the classical fracture mechanics, it was postulated that the crack propagated rapidly once the fracture toughness has been reached. In fact, the crack can propagate even when the stress intensity factor is lower than the fracture toughness. This phenomenon is called subcritical crack growth (Atkinson 1984), which is one of the main causes of time-dependent behavior in rock. Knowledge of time-dependent crack propagation is important to consider the long-term stability of structures in rock mass, such as underground power plants or repositories for radioactive wastes in underground. In this study, subcritical crack growth in granite was investigated. Especially, the relation between the crack velocity and the stress intensity factor was determined experimentally by Double Torsion (DT) test (Williams & Evans 1973, Sano & Kudo 1992), which is the typical fracture mechanics test used to measure subcritical crack growth. Dependence of subcritical crack growth on the environmental conditions and rock fabric was investigated.
Under low homologous temperatures and atmospheric pressure, stress corrosion (Anderson & Grew 1977) is considered to be the main mechanism of subcritical crack growth. Stress corrosion is a weakening process due to a chemical reaction between the siloxane bond structure near the crack tip strained by the tensile stress and water (Michalske & Freiman 1982).
In this study, Double Torsion test was used. The specimen and the loading configuration for DT test are shown in Figure 1. The experimental apparatus was set in the room where the temperature and the relative humiditywere controlled and kept constant.All experimentswere conducted under the controlled temperature and humidity. For the apparatus used in this study, the temperature and the relative humidity can be controlled within 0.1K in the range of 283–333K and within 1% in the range of 40–75%, respectively (Nara & Kaneko 2005). 4 ROCK SAMPLES Oshima granite, Westerly granite, and Inada granite were used as rock samples. It is known that granite has orthorhombic elasticity (Sano et al. 1992). In Table 1, the P-wave velocities in three orthogonal directions are shown.
