Subcritical crack growth is one of key factor to cause the time-dependent behavior of rock. Therefore, the knowledge of subcritical crack growth is important for evaluation of long-term stability of structures In rock mass. In this paper, the experimental results of the Double-Torsion (DT) test and crack observation are presented, and the relation between crack growth and the rock fabric is discussed. A series of DT tests was carried out using granite. For granite, it was shown that the crack growth behavior was anisotropic. When the crack propagated parallel to the direction in which the density of the pre-existing microcracks is the largest, he stress level at the crack tip was the smallest. After the DT tests, polished thin sections were prepared from the rock specimens to observe the stress-induced crack paths. Using the images from the electron probe micro-analyzer (EPMA), the geometry of the crack path was measured and the fractal dimension of the crack path was determined by the spectrum method. It is found out that the fractal dimension of the crack path was also dependent on the direction of crack propagation. When the crack propagated parallel to the direction in which density of the pre-existing microcracks is the largest, fractal dimension was the smallest. These results Suggest that crack growth behaviors are influenced by pre-existing microcracks. It can be considered that the crack propagates nonlinearly and intermittently by interconnecting to microcracks existing ahead of the crack front. It is concluded that the orientation distribution of pre-existing cracks causes anisotropy of subcritical crack growth.


In fracture mechanics, the crack growth is considered to occur when the stress intensity factor at the crack tip reaches the fracture toughness (Griffith 1920, Irwin 1957). However, the crack can propagate even when the stress intensity factor is lower than the fracture toughness. This phenomenon is called subcritical crack growth. The controlling process of subcritical crack growth in rocks is considered to be stress corrosion, and the corrosive agent is water (Anderson & Grew 1977, Atkinson 1984).

Rocks consist of different minerals and posses microcracks and pores. For granite, a lot of microcracks exist, and anisotropic properties have been reported due to the preferred orientation of the preexisting. microcracks (Sano et al. 1992). Therefore, the crack growth behaviors can be affected by the microcracks in granite. To make it clear, it is important to measure the crack growth behavior and to observe and assess the crack path quantitatively.

In this study, subcritical crack growth in granite is studied experimentally by Double-Torsion (DT) test (Williams & Evans 1973, Sano & Kudo 1992). Additionally, the stress-induced crack path is observed, and the geometry of the crack path is estimated by fractal analysis.

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Rocks studied are Oshima granite and Westerly granite. It is known that granite has orthorhombic elasticity. In Table 1, P-wave velocities in three orthogonal directions for these granites are listed.

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