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

To investigate the effect of thermal damage caused by high temperature on the acoustic emission characteristics and the fracture mechanism of granite during different stress stages, uniaxial compression tests and real-time acoustic emission monitoring of thermally damaged granite at 25℃, 200℃, 400℃ and 600℃ were carried out. The peak frequencies of thermal-damaged granites, the distribution characteristics of RA-AF data and the distribution patterns of energy concentration ρ=∑N/∑E at different loading stages were analyzed. 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: Ⅰ crack compaction stage, Ⅱ crack emergence and stable development stage, Ⅲ crack unstable development stage, and Ⅳ post-peak damage stage according to the acoustic emission development characteristics. The principal frequency of the acoustic emission peak frequency of thermal-damaged granite at different temperatures shows a band distribution in the four principal frequency regions, the more serious the thermal damage, the earlier the medium and high frequency fracture signals are generated, the wider the distribution range of the main frequency bands is, and the less the ultra-high frequency signals appear at failure stage. The distribution characteristics of acoustic emission RA-AF data of the thermal-damaged granite at different temperatures can characterize the cracking mechanism at different stress stages, and the thermal-damaged granite is more likely to generate shear cracks under pressure. The higher the temperature of thermal damage, the more shear cracks are developed. The abrupt change point between the stable stage and the abrupt drop stage of the energy concentration curve can be used as a failure precursor of granite samples under uniaxial compression.