Nonlinear Characteristics of Granite After High-Temperature Treatment Captured by Digital Image Correlation and Acoustic Emission Technology

In order to understand the nonlinear characteristics of granite after exposure to high temperature, fracture tests were carried out. The fracture process of granite was monitored by combining digital image correlation (DIC) method and acoustic emission (AE) technology, and the changes of microcracks in granite after high-temperature treatment were observed. The apparent fracture toughness of granite decreased significantly after high-temperature treatment and showed obvious size effect, which is consistent with the size effect law of Bažant. The combined observations by DIC and AE showed that the crack of granite at room temperature started to grow significantly only when it approached the failure load, and the fracture process was rapid. However, the crack growth was observed clearly in the early stage of loading for the granite subjected to high temperature, and the process was slow and persistent. After granite was heated at high temperature, the critical crack tip opening displacement (CTOD c ), AE b value, effective fracture process zone length, and many other indicators all increased significantly. This indicates that the fracture types of granite changed from brittle to ductile. The microscopic results showed that the granite microcracks developed after high-temperature treatment, the number of microcracks increased significantly, and the porosity and fractal dimension increased. Finally, the relationship between work of cohesion and cumulative AE counts was estimated based on the fictitious crack model. The results showed that there was a significant linear correlation between the two variables, regardless of room temperature granite or heat-treated granite. The work of cohesion can characterize the activity of microcracks in the fracture process zone and it can be quantified by AE technology. This provides a new way to study crack propagation by AE technology.