In order to investigate the influence of several factors on the mode-I fracture toughness and fracture behavior of rocks, fracture toughness tests were conducted under various conditions of specimen shape and size, loading rate, confining pressure and water vapor pressure. The influence of not only these extrinsic factors but also an intrinsic one - namely, the microstructure of rocks, was also investigated to understand the fracture toughness and fracture mechanism of rocks, as well as the microscopic fracture toughness of mineral grains. In this lecture, the influence of some factors of them is discussed, based on our various experiments.
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).
In order to understand the fracture behavior of rocks, fracture toughness, which indicates the resistance to pre-existing crack initiation, is an important parameter. Several tests have been developed to estimate fracture toughness of rocks. The International Society for Rock Mechanics (ISRM) suggested the Chevron Bend test, Short Rod test (Ouchterlony, 1998), Cracked Chevron Notched Brazilian Disc test (Fowell,1995), and Semi-Circular Bend (SCB) test (Kuruppu et al., 2014). These testing methods assume that rock materials are of homogeneous elasticity. However, the fracture toughness is influenced by not only microstructure, such as grains, matrix, micro cracks, pores, but also extrinsic factors, such as specimen shape and size, loading rate, confining pressure and water vapor pressure.
Kataoka (2015a) estimated fracture toughness of rocks in order to investigate the influence of several factors on it. Then Kataoka et al. (2014) also evaluated microscopic fracture toughness of minerals within granite. In this lecture, the influence of these factors and the microscopic fracture toughness of minerals within granite are discussed, based on our various experiments.