In order to investigate the influence of water vapor pressure of surrounding environment on mode I fracture toughness of rock, a series of Semi-Circular Bend (SCB) tests under various water vapor pressures were performed. The water vapor is the most effective agent which promotes stress corrosion of rock. The rock used in the test was Africa Rustenberg granite which is known as anisotropic and the region of water vapor pressure used was 10-2 to 103 Pa. The tests revealed that two types of crack propagation do exist and that fracture toughness is dependent on water vapor pressure of surrounding environment and anisotropy.
Failure of rock materials is a process of crack propagation. Crack initiation takes place when the crack tip stress intensity factor K reaches the critical value, called fracture toughness KC. The rock fracture toughness is known to be affected by the surrounding environment, such as temperature, humidity and so on. In order to examine the effect of temperature, the double torsion test has been performed under a condition of temperature 20 to 400 °C for granite and gabbro¹). They showed that the fracture toughness of gabbro increases below 100 °C and decreases over 100 °C, and at 400 °C it reaches 50 to 60 % of that of 20 °C. Granite also has the same characteristics. In order to examine the effect of water vapor, a series of Semi-Circular Bend tests was performed under various water vapor pressures2). It was shown that the fracture toughness of Kumamoto andesite is dependent on water vapor pressure. On the other hand, Al-Shayea et al.3) estimated the fracture toughness of limestone at in situ conditions of temperature and confining pressure. The uniaxial compressive strength and the tensile strength of rock are mechanical properties similar to the rock fracture toughness. It is also known that they are dependent on the surrounding environment, particularly water vapor pressure in the atmosphere. The water vapor promotes stress corrosion of rock4). Therefore the strength decreases with increasing water vapor pressure. Jeong et al.5) showed that the uniaxial compressive strength and tensile strength of rock increase with decreasing water vapor pressure. However, considering the significance of the influence of water vapor on rock properties such as fracture toughness further research is needed. In addition, some rock materials are anisotropic, which can result in properties such as fracture toughness depend on the orientation of cracks with respect to planes of anisotropy6). Several tests exist to determine the fracture toughness, namely Chevron Bend test7), Single Edge Notched Round Bar in Bending test8), Short Rod test9), Central Straight-Through Brazilian Disk test10), Central through Chevron-Notched Brazilian Disk test11 and Semi-Circular Bend (SCB) test 12. Among many different testing methods for the fracture toughness, the SCB specimen is chosen for its simplicity of specimen preparation, equipments and testing procedure.
In this study, Africa Rustenberg granite was selected as a specimen. Africa Rustenberg granite is composed of plagioclase, amphibole, biotite, etc.