No abstract

No abstract

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.Energies 2020, 13, 2061 2 of 17 reported the effect of cyclic temperature on the physical and mechanical properties of rocks. Rong et al. [11,12] investigated the influence of thermal cycling on the physical and mechanical properties of rock (i.e., marble and granite) through a uniaxial compression test and conducted microscopic observations to investigate the thermal damage of rock sample induced by the treatment of different thermal cycles. Battaglia et al. [15] measured the linear thermal expansion of marbles subjected to thermal cycles utilizing a high-sensitivity apparatus. Lin et al. [16] investigated the variation of residual strain and micro-crack density of granite with cyclic temperature in the range of 600 • C, and explained the mechanism of permanent deformation caused by micro-cracks from the microscopic perspective. Fang [18] analyzed and compared the effects of mechanical properties of marble subjected to thermal treatment and thermal cycling. They showed that the physical and mechanical properties of rock samples deteriorate gradually with an increasing number of temperature cycles. Further, other researchers considered that the deterioration of the mechanical properties of building materials and engineering structures in cold regions with harsh climatic conditions was due to freeze-thaw cycles [19][20][21].Most of the earlier studies on the effect of cyclic loading on the mechanical properties of rocks did not consider the effect of temperature, i....

In geothermal mining, the mechanical properties and micro‐structure of the rock under different temperature or cooling modes are highly relevant to the stability of the artificial reservoir. For reflect the actual working conditions, an experimental study is carried out on high‐temperature granite that has been cooled both naturally and rapidly with water. The following conclusions are reached. (a) The wave velocity, elastic modulus, and uniaxial compressive strength of the test specimen decrease with an increase in its heating temperature under both cooling modes. (b) The characteristics of the stress‐strain curve of granite are different after different temperatures and cooling modes. With natural cooling, the stress‐strain curve features double peaks in the range of 200‐300°C, whereas such phenomenon occurs at temperatures greater than 300°C with water cooling. (c) When the temperature is less than 300°C, no obvious change occurs in the crystal structure of the sample. When the temperature is more than 300°C, the original micro‐cracks gradually expand and eventually develop into larger cracks.

No abstract