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.
Highlights
A deep understanding of the effects of cyclic temperature and cyclic loading on the mechanical properties of rocks is critical for many high-temperature underground engineering applications, such as thermal energy storage (TES) [1,2], enhanced geothermal systems (EGS) [3,4,5] and compressed air energy storage (CAES) [6]
It is of considerable importance to understand the mechanical properties of granite subjected to cyclic temperature and cyclic loading, which is associated with the long-term stability of TES systems
100 ◦ C to 500 ◦ C, the stress–strain curves of the granite samples gradually shift to the left as the number limit of cyclic temperature is 100 °C to 500 °C, the stress–strain curves of the granite samples of cycles increases, i.e., the strain gradually decreases and the elastic modulus gradually increases gradually shift to the left as the number of cycles increases, i.e., the strain gradually decreases and under this stress condition
Summary
A deep understanding of the effects of cyclic temperature and cyclic loading on the mechanical properties of rocks is critical for many high-temperature underground engineering applications, such as thermal energy storage (TES) [1,2], enhanced geothermal systems (EGS) [3,4,5] and compressed air energy storage (CAES) [6]. Energies 2020, 13, 2061 reported the effect of cyclic temperature on the physical and mechanical properties of rocks. Xiao et al [35,36] reported a suitable damage variable to describe fatigue damage in granite subjected to cyclic loading They suggested an inverted S-shaped nonlinear fatigue damage cumulative model. Few studies considered the effects of cyclic temperature and stress on the mechanical properties of rock samples. In view of the engineering problems of rock under extremely complicated geological conditions, e.g., TES and EGS, cyclic temperature and loading tests were conducted in this study. The results of this study can be used as a reference for the safety and stability of TES and future studies on the fatigue failure of granite subjected to cyclic temperature and stress
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