Laboratory investigation on physical and mechanical behaviors of granite after heating and different cooling rates

As an important formation of geothermal reservoirs, granite has very complex microstructure and strong heterogeneity which play an important role in the damage and failure mechanical behavior of granite under different temperatures and cooling rates. In order to better understand the influence of microstructure on the multiscale physical and mechanical properties of granite at evaluated temperatures and cooling rates, this paper conducted a series of macroscopic and microscopic experimental investigations on the granite samples after a heating with two cooling rates treatment. The results show that the failure mode changes from brittle to ductile at 400 °C which is the critical temperature for both macroscopic and microscopic properties. The physical and mechanical properties of specimen after air cooling treatment increase at 200 °C due to the transition of macropores into micropores. Transgranular cracks take place in feldspar at 400 °C and numbers of intergranular cracks develop inside all minerals at 500 °C. The number and width of microcracks in water-cooled samples are much greater than those of air-cooled simples. Based on these experimental investigations, a cross-scale characterization method of Young’s modulus is proposed by introducing a thermal damage factor into the Mori–Tanaka method to consider the thermal damage induced by the thermal treatment.