Anisotropy of the physical properties such as elastic wave velocity, elastic modulus, strength, and fracture toughness of rocks should be understood to design and construct rock structures and estimate its stability. To investigate anisotropy in mode I fracture toughness (KIc) of rocks, semi-circular bend (SCB) tests were performed, using one sedimentary and two types of crystalline rock. The distributions of inherent microcracks and grains have a preferred orientation in these rocks. Therefore, two types of the specimen in different orientations were prepared based on anisotropy of the elastic wave velocity measured in their rock blocks. The SCB test results showed that these rocks exhibit anisotropy of KIc. Following the tests, fracture process was analyzed based on the fracture geometry within fractured specimens, visualized by the X-ray computed tomography method. It was concluded that the dominant factor causing anisotropy of KIc is the distribution of grains in the crystalline rocks.
In diversification and enlargement of underground rock structures such as tunnels, underground mines, power plants, storage facilities for crude oil and liquefied petroleum gas, and high level radioactive waste disposal facilities, understanding of the fracture mechanism of rocks has been an important issue. In order to view the fracture behavior of rocks, many researchers have studied on the fracture toughness of rocks, which is the most useful mechanical property in the fracture mechanics, describing resistance to fracture initiation (Sanford 2003).
Rocks can be considered a complex material. It is known that the mechanical properties, such as the uniaxial compressive strength, tensile strength, and fracture toughness of rocks are decided by the microstructural parameters, such as the shape, size and distribution of inherent microcracks and grains, and the type of matrix and minerals. If there is any preferred orientation of the microstructures, this leads to anisotropy of the mechanical properties of the rock itself (Schedl et al. 1986).
In this paper, in order to investigate anisotropy of the mode I fracture toughness of rocks, the semi-circular bend (SCB) test (Chong & Kuruppu 1984 and Kuruppu et al. 2014) was performed using a sedimentary rock called Kimachi sandstone, and two types of crystalline rock, Rustenburg granodiorite and Iksan granite. Detailed observation of fractures within fractured specimens was performed after the SCB tests, using images obtained by micro-focus X-ray computed tomography (CT) method. The influence of the microstructure of the rocks on the anisotropy of the fracture toughness was discussed.
