Fracture mechanism and precursors of thermally damaged granite uniaxial compression based on acoustic emission information

To determine the impact of high-temperature-induced thermal damage on the acoustic emission characteristics and fracture mechanism of granite during various stress stages, uniaxial compression tests and real-time acoustic emission monitoring of thermally damaged granite at 25, 200, 400, and 600 ℃ were performed. The peak frequencies of thermally damaged granites, the distribution characteristics of RA-AF (AE rise time/AE amplitude-AE average frequency) data, and the distribution patterns of energy concentration $\rho $ at various loading stages were investigated. The results show that the stress stages of granite under uniaxial compression conditions after thermal damage at each temperature can be divided into the following stages: Stage Ⅰ corresponds to the crack compaction stage; Stage Ⅱ corresponds to the crack emergence and stable development stage; Stage Ⅲ corresponds to the crack unstable development stage; and Stage Ⅳ corresponds to the post-peak damage stage based on the acoustic emission development characteristics. Further, the more severe the thermal damage to the granite, the earlier the granite enters Stage II and the longer the duration of this stage. The acoustic emission peak frequency of thermally damaged granite at different temperatures exhibits a band distribution across four principal frequency regions. With increasing severity of thermal damage, the generation of medium- and high-frequency fracture signals occurs earlier, resulting in a wider distribution range of the primary frequency bands. In addition, there is a decrease in the occurrence of ultrahigh frequency signals during the failure stage. The distribution characteristics of the acoustic emission RA-AF data of the thermally damaged granite at different temperatures can provide insights into the cracking mechanism observed during different stress stages. Subsequently, Stage I generates a small amount of tensile and tension–shear cracks; Stage II generates mixed tension–shear cracks; Stage III generates tensile and mixed tension–shear cracks while shear cracks continue to develop and enlarge; and Stage IV generates numerous shear cracks. The thermally damaged granite is more likely to produce shear cracks under pressure. The higher the temperature of thermal damage, the more shear cracks develop. The curve of energy concentration based on acoustic emission data during uniaxial compression of granite contains three major stages: the initial irregular fluctuation, stable, and abrupt drop stages, which have obvious correspondence to stages I, II, and III, respectively. The abrupt change point between the stable and abrupt drop stages of the energy concentration curve can be used as a failure precursor for granite samples under uniaxial compression.