2.1 Granite material
The granite used in this research was collected from Rizhao city in
Shandong province, China. According to the results of X-ray diffraction
(XRD), the minerals in the granite are feldspar (59.85%), biotite
(21.56%), quartz (11.12%), amphibole (6%), chlorite (1.01%) and
dolomite (0.46%). This granite has a crystalline and blocky structure
with a connected porosity of 0.828 % (Yang et al., 2017b). The average
density of the tested granite at room temperature is approximately 2594
kg/m3, and its P-wave velocity is about 4200 m/s.
Before beginning the triaxial cyclic loading-unloading tests, the
granite specimens were first placed in the furnace and the heating rate
was set as 5°C/min to minimize thermal shocks. The predetermined
temperature was kept constant for 2 h to ensure uniformity in
temperature across the specimen. For triaxial cyclic loading-unloading
tests, the target temperatures were set as 300 and 600°C. Finally, the
heated specimens were left in the furnace to cool down to the room
temperature.