Thermal damage and acoustic emission characteristics of granite based on Brazilian splitting test

Rapid and non-destructive evaluation of the thermal stability of surrounding rocks and rock strength is important in field construction. Physical properties and acoustic emission (AE) characteristics of granites can be changed at high temperatures. Thus, investigations on the relationships between rock strength and physical properties could help to evaluate the thermal stability via a physical measuring method. In this study, granite selected from a geothermal drilling project was heated at 25-600 degrees C and different physical properties and tensile strength were measured. Moreover, the thermal damage degree was evaluated and the tensile failure pattern was quantitatively characterized. Notably, the tensile strength of granites could be indirectly acquired through P-wave velocity, hardness, colour, mass loss and volume expansion. The experimental results showed that the variations could be roughly divided into two stages. (1) At 25-400 degrees C, the mass, volume, P-wave velocity, hardness, tensile strength, colour and AE parameters changed slightly. The failure mode and damaged section did not differ significantly within this temperature range. (2) At 400-600 degrees C, these physical and mechanical properties and AE parameters changed significantly. The formation of micro-cracks was aggravated by the loss of structural water and phase transition of quartz, leading to complex failure modes and jagged morphology of damaged sections. The thermal damage factor and the compressive damage factor defined by the P-wave velocity at different conditions increased greatly. In summary, there was no major damage and the internal structure was still relatively intact at 25-400 degrees C. However, the granite showed serious damage in the high-temperature environment (400-600 degrees C), resulting in great changes in physical and mechanical properties and AE characteristics.