Investigating physical, mechanical and crack properties of granite in the fatigue process

Joints and cracks are an integral part of rock structures. When load applied to these structures, growth, and interaction of the cracks lead them to failure. This aspect is particularly crucial in cyclic loading because failure occurs at stress levels lower than the yield point due to the fatigue process. This research aims to study the growth and interaction of the cracks in the fatigue process by the fluorescence thin section method. At first, different stages of fatigue for granite rock were determined, then samples loaded up to each stage. Later, the physical, mechanical, and crack properties of the samples were investigated. The results show that physical and mechanical properties declined by increasing the number of cycles. During the first stage of fatigue, crack initiated at a high rate, and in the second stage, the increasing rate descended, and energy mostly consumed to increase the length of the cracks. In the last staged, which is the failure stage, the rate of crack initiation increases again and forms a complicated network of cracks, which leads the sample to failure. Among the constituent minerals, feldspar exhibits the highest crack density, and biotite shows the lowest density of the cracks.