Analysis of crack path for subcritical crack growth in rocks

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, the 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 the 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.