Experimental Study on Damage and Fracture Characteristics of Beishan Granite Subjected to High-temperature Treatment with DIC and AE Techniques

The effect of high temperatures up to 800 °C on the physical and mechanical characteristics and fracturing behaviour of Beishan granite is experimentally studied with a combination of acoustic emission (AE), digital image correlation (DIC) and optical microscope observations. The experimental results show that the responses of the P-wave velocity, the effective porosity, Young’s modulus, and the uniaxial compressive strength (UCS) of the granite to temperature are different. The critical temperature for the brittle–ductile transition of Beishan granite is between 500 and 600 °C. The counts, cumulative energy, b value, and waveform from AE and the full-field deformation evolution from DIC are combined to investigate the damage evolution and fracture mechanism in heated granite. It is found that a rise in temperature increases the number of AE events but reduces the cumulative energy release. Tensile microcracking mainly occurs in granite exposed to low temperatures, while shear fracturing gradually dominates in granite exposed to higher temperatures. With increasing temperature from 25 to 800 °C, the failure mode of granite specimens changes from being controlled by longitudinal splitting cracks to a single shear fracture and finally to multiple conjugate shear fractures. Microscopic observation of granite thin sections is conducted to further reveal the essential mechanism driving the physical–mechanical response and fracturing behaviour of heated Beishan granite.