To examine the fatigue process of granite, cylindrical Westerly granite specimens were subjected to a cyclic loading test under uniaxial compression with a maximum of 140–160MPa at room temperature, and crack growth patterns within them were analyzed by microscopic observation and image analysis. The fatigue process is generally divided into three characteristic stages. A series of prefailure specimens were prepared, and their stages in the fatigue process were decided by monitoring the strain behavior during the test. Cross-sectional thin sections were prepared for detailed observation. The fluorescent method was applied to identify microcracks within the specimens. At the initial degradation stage, distinguishing crack growth was identified in quartz grains. It is estimated that the candidate portions of crack growth were damaged at the first or early loading. At the second steady stage, crack development portions were shifted from quartz grains to feldspar grains. It is estimated that a gradual progress of microcracks within feldspar grains was dominant during the second stage. At the final accelerated stage, many intergranular cracks running parallel to the loading direction were observed. Their formation takes the fatigue process from the steady stage to the final stage with a sharp increase in strain.
The structures constructed in underground spaces are generally used over a prolonged period. It is therefore important to reveal the deterioration characteristics of rocks under repeated stress changes over the long term for the stability evaluation of many rock structures. In recent years, several new proposals for the usage of underground spaces have drawn increasing interest. For example, the base rock surrounding the compressed air energy storage system has to bear cyclic stress changes in the first planning stage. It is well-known that many materials deteriorate due to repeated stress changes over a prolonged period and then finally reach failure even if the change is below their static breaking strength. This phenomenon is generally known as ‘fatigue’. Fatigue occurs in many kinds of materials, and rock is no exception. Many studies on the fatigue characteristics of rocks have been carried out. Attempts to estimate the fatigue life were also conducted (e.g., [3]). In many geological engineering problems, granite is one of the most important materials to investigate. The fatigue characteristics of granite have also been studied. However, the detail process in granite fatigue, i.e., the phenomena occurring in phases from the onset of degradation until the final collapse, has not been clarified. It is necessary to examine the growth of cracks during the fatigue process, since the initiation and elongation of microcracks play an essential role in the granite failure. Therefore, to examine the fatigue process in granite, cylindrical Westerly granite specimens were subjected to the cyclic loading test under uniaxial conditions at room temperature with a maximum applied stress of 140–160 MPa, and the patterns of microcracks at three characteristic stages during the test were observed microscopically, applying the fluorescent approach [4]. In addition, microcrack growth patterns were investigated by the digital image analysis technique.
