A brittle fracture of rocks occurs under subjected stress state. What parameters govern fracture in rock? It is considered that the fracture is dependent on not only distribution, orientation, size, shape and deformability of grains but also fracture toughness of grains and grain boundaries. The final one is called microscopic fracture toughness (MFT) and is an important parameter. In this paper, a new mechanical testing machine is developed to make clear MFT of mineral grains within rock, based on the mechanical testing machine in the field of metallurgical engineering. Then, MFT of micro-sized specimens made of plagioclase, alkali feldspar, and quartz within Iksan granite (Korea) is evaluated at room temperature, using the testing machine. A micro-sized specimen of a cantilever beam type has dimensions of 10 × 10 × 50 μm. The specimen is made by Focused Ion Beam (FIB) machining. From the test results, it is shown that the mechanical weak plane can influence MFT of mineral grains of the granite. Then, it is concluded that the developed testing machine is available to evaluate MFT of mineral grains of rocks.
It is known that a brittle fracture of rock under a stress state occurs through a following process: pre-existing cracks initiate and propagate, then connect to other cracks and a fracture surface is finally developed. In order to explain this process, fracture mechanics, which was established in the field of metallurgical engineering in the 1970s, was introduced to rock mechanics in the 1980s. Some textbooks for fracture mechanics of rocks were published at that time (Rossmanith 1983, Atkinson 1987, Whittaker et al. 1992). Structures made of high strength metal sometimes failed at stress below the designed value when only small pre-existing cracks existed. Few cracks play an important role in the fracture in metals. On the other hand, rock can be considered an inhomogeneous material. For example, granite consists of a completely crystalline assemblage of minerals. The structure contains lots of micro-cracks within mineral grains and at grain boundaries, representing mechanical weakness. Therefore, fracture mechanism in rock is different from that in metal. Accordingly, it is difficult to introduce fracture mechanics for metal to rockmaterials directly. However, the fundamental principles of fracture mechanics shall be used in understanding hydraulic fracturing in the field of developing geothermal energy, petroleum, shale gas, and so on. It is desired that fracture mechanism in rock be clarified.