This paper aims to investigate the effect of cyclic thermal shock on mechanical properties and brittleness of granite. Uniaxial compression tests and brittleness evaluation was performed on granite samples after cyclic heating and water-cooling treatment. Further, characteristics of microscopic thermal cracking was investigated by SEM observation to reveal the temperature-dependent response of macro mechanical properties. Experimental results show that uniaxial compression strength, Young's modulus, and brittleness index slightly decreased with the increasing of number of cycles when the thermal treatment temperature was 300 °C. As the thermal treatment temperature increased to 400 °C, obvious thermal cracking was induced and network of cracks with apparent aperture was created. Consequently, significant degradation in uniaxial compression strength and Young's modulus occurred and the plasticity was enhanced due to the thermal damage. Additionally, the degradation in mechanical strengths and brittleness tends to be mainly induced in the early cycles of thermal treatments. Experimental results may be beneficial for understanding the exploitation process of hot dry rock geothermal energy.

1. Introduction

During the stimulation and the heat extraction processes in hot dry rock geothermal formations, temperature field variation caused due to the cold-water injection commonly occurs (Ghassemi, 2012; Olasolo et al., 2016; Jin et al. 2019). According to the study of Jansen et al. (1993), the difference in thermal expansion coefficients between adjacent crystalline grains and the temperature gradient can induced thermal stress. When the thermal tensile strength exceeds the cohesion strength of adjacent grains or the strength of grain, inter- or intragranular cracks tend to be created, which further influence the mechanical properties of the rock (Liu et al. 2010; Heap et al. 2017).

A large number of researches have been carried out to investigate the influence of temperature on mechanical properties of granite (Xu et al. 2010; Brotóns et al. 2013; Kong et al. 2016; Yin et al. 2016; Yang et al. 2017; Kumari et al. 2018; Wu et al. 2019). Generally, laboratory experiments were performed on sample under heating condition or after heating and cooling treatment. Zuo et al. (2011) studied the thermal cracking characteristics of Beishan granite by SEM, and characterized the thermal cracking behavior using fractal analysis model. It was indicated that with the increasing of temperature, the thermal cracking behavior was dominated by from intergranular cracks to intra-granular or mixed cracks. However, the temperature-dependent response of mechanical properties may be different among various types of granite. This can be accounted for by the fact that the thermal cracking characteristics of rock are related to not only the thermal treatment temperature but also the heating or cooling rate, mineral composition, and microstructure of rocks, etc. (Finnie et al. 1979; Kumari et al. 2017). Wang et al. (2013) compared the mechanical properties of preheated granite after air-cooling and rapid water-cooling treatments. Experimental results show that the mechanical properties of granite samples after rapid water-cooling treatment degraded more severely than that after air-cooling treatment. Shao et al. (2015), Zhang et al. (2017), and Kumari et al. (2017), also investigated the thermal shock effect on mechanical properties of granite after heating and water-cooling treatments. In summary, tensile strength, uniaxial compression strength, Young's modulus, and fracture toughness tend to be reduced with the increasing of thermal treatment temperature. However, these experiments performed after single-cycle thermal treatment. To date, investigation into the cyclic thermal shock effect on mechanical properties and brittleness of granite are still scarce.

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