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This paper investigates the variation of mechanical properties of granite during temperature and stress cycling, which is an important part of evaluating the long-term thermal and mechanical stability of thermal energy storage. Cyclic temperature and loading tests were conducted where the upper limit of cyclic temperature was 100–600 °C, and the upper stress limits were 70% and 85% of the average uniaxial compressive strength (UCS) at the corresponding temperature. The response of stress–strain characteristics of the granite samples to changes in temperature, and cyclic load upper limit, while the number of temperature and loading cycles was comprehensively analyzed. The results show that the temperature and stress cycles have significant effects on the mechanical properties of granite (i.e., stress–strain curve, strength, elastic modulus, and deformation). The elastic modulus of the sample during loading increases gradually. The strain corresponding to the upper loads of the granite samples decreases with an increasing number of cycles. Additionally, the UCS of samples after 10 cycles at 70% loading stress is greater than that at 85% loading stress. The mechanical properties of samples change dramatically during the first and second cycles at 85% loading stress, whereas at 70% loading stress, the mechanical properties change gradually in the first few cycles, and then tend to stabilize. Cyclic hardening is observed at temperatures below 500 °C, where post cyclic UCS is greater than the uncycled average UCS. This phenomenon requires further research. [ABSTRACT FROM AUTHOR]

Highlights • Two different thermal cycling treatments were applied to granite samples. • Both slow cooling and rapid quenching induced damage to granite samples. • Rapid quenching has a more significant effect on granite samples due to thermal-gradient cracking. • The thermal deterioration mechanisms due to different thermal cycling were revealed by optical microscope observations. Abstract Two different types of thermal cycling treatments (slow heating followed by slow cooling or rapid quenching) were carried out on the granite samples in the present research. The responses of physical, mechanical and transport properties of the granite after various thermal cycling treatments were compared and analyzed. Non-destructive tests (mass, volume, longitudinal wave propagation and permeability) and destructive tests (uniaxial compression and Brazilian splitting) were utilized to characterize these properties. Variations of all the properties demonstrate that thermal cycling (heating and cooling) can induce damage in the samples. Mass, density, P-wave velocity, UCS, Young's modulus and tensile strength decrease, whilst volume and permeability increase with rising temperature. Additionally, the results also demonstrate the importance of cooling method. Rapid quenching has a more significant effect on granite samples, showing lower density, P-wave velocity and strength, but greater permeability, which is due to the generation of thermal gradient cracks. Moreover, the difference between P-wave velocity and UCS of slow cooling and those of rapid quenching increases with temperature up to 500 °C, which can be attributed to the more significant thermal shock cracking induced by higher temperature-gradient stress in the samples during rapid quenching. The microstructural analyses from thin sections show that microcracking in granite subjected to thermal cycling treatments was induced in different severity depending on peak temperature and cooling method. [ABSTRACT FROM AUTHOR]

• The mode-I fracture toughness of granite at high temperature was tested in real time. • The increase in temperature in the low-temperature range universally toughens the rock. • Heat treatment method affects the mode-I fracture toughness of granite. • The temperature threshold of granite fracture toughness at real-time high temperature is 200 °C. • Fracture characteristics of granite at high temperature are similar to those after cooling. The mode-I fracture toughness of semicircular bend granite specimens at high temperature was tested in real time, and the failure specimens were examined using X-ray computed tomography. The results showed that with increasing temperature, the fracture toughness increased from room temperature (20 °C) to 200 °C, then decreased beyond 200 °C. Quasibrittle failure was observed up to 500 °C, followed by plastic characteristics at 600 °C. The cracks tended to propagate along the boundaries of mineral crystals. With increasing temperature, the main cracks became rougher and tended to propagate perpendicular to the tensile stress, and more branch cracks formed. The increase of temperature within a certain range had toughening effect on a variety of rocks and temperature-treatment method affected the mode-I fracture toughness of granite. Compared with the mode-I fracture toughness after cooling to room temperature, the temperature threshold of mode-I fracture toughness of granite at real-time high temperature was higher. The results provide guidance to determine fracture toughness and stability for high-temperature rock engineering. [ABSTRACT FROM AUTHOR]