Damage characteristics of thermally treated granite under uniaxial compression: Insights from active and passive ultrasonic techniques单轴压缩下热处理花岗岩的损伤特征：来自主/被动超声技术的见解

To explore the effects of thermal treatment on cracking processes in granite, granite samples were thermally treated at 25–400 °C and then loaded under uniaxial compression. Active ultrasonic testing and passive acoustic emission (AE) monitoring were combined to monitor the damage characteristics of the samples. The uniaxial compression strength (UCS) of the sample treated at 200 °C shows no apparent change compared with that of the nonheated sample, while the UCS increases at 300 °C and decreases at 400 °C. As the temperature increases from 25 to 400 °C, the initial P-wave velocity (Vp) decreases gradually from 4909 to 3823 m/s, and the initial Vp anisotropy ε increases slightly from 0.03 to 0.09. As the axial stress increases, ε increases rapidly in the crack closure stage and unstable cracking stage. The attenuation of ultrasonic amplitude spectra also shows an obvious anisotropy. Besides, the main location magnitude of AE events decreases after thermal treatment, and low-frequency AE events and high-amplitude AE events increasingly occur. However, there is insufficient evidence that the treatment temperature below 400 °C has a significant effect on the temporal characteristics, source locations, and b-values of AE.