Abstract
Thermally induced damage has an important influence on rock mechanics and engineering, especially for high-level radioactive waste repositories, geological carbon storage, underground coal gasification, and hydrothermal systems. Additionally, the wide application of geothermal heat requires knowledge of the geothermal conditions of reservoir rocks at elevated temperature. However, few methods to date have been reported for investigating the micro-mechanics of specimens at elevated temperatures. Therefore, this paper uses a cluster model in particle flow code in two dimensions (PFC2D) to simulate the uniaxial compressive testing of Australian Strathbogie granite at various elevated temperatures. The peak strength and ultimate failure mode of the granite specimens at different elevated temperatures obtained by the numerical methods are consistent with those obtained by experimentation. Since the tensile force is always concentrated around the boundary of the crystal, cracks easily occur at the intergranular contacts, especially between the b-b and b-k boundaries where less intragranular contact is observed. The intergranular and intragranular cracking of the specimens is almost constant with increasing temperature at low temperature, and then it rapidly and linearly increases. However, the inflection point of intergranular micro-cracking is less than that of intragranular cracking. Intergranular cracking is more easily induced by a high temperature than intragranular cracking. At an elevated temperature, the cumulative micro-crack counts curve propagates in a stable way during the active period, and it has no unstable crack propagation stage. The micro-cracks and parallel bond forces in the specimens with elevated temperature evolution and axial strain have different characteristics than those at lower temperature. More branch fractures and isolated wider micro-cracks are generated with increasing temperature when the temperature is over 400 °C. Therefore, the total number of cracks is almost constant when the temperature is below 400 °C; next, it linearly increases when the temperature is over 400 °C. This trend is the same as that observed by experimentation.
Highlights
When rock is subjected to thermal load, thermal damage and thermal cracking usually occur, which may affect underground engineering and geological projects, such as enhanced geothermal systems, high-level radioactive waste repositories, geological carbon storage, underground coal gasification, and hydrothermal systems [1,2,3,4]
The thermally induced strains are produced by particle radii and the force carried in each parallel bond, and the clumped particle model cannot simulate the intragranular cracking induced by the thermal load
The macro-mechanical behavior of granite variations with temperature can be simulated by the cluster model in particle flow code, and the results obtained by the numerical simulation are similar to those obtained by experimentation
Summary
When rock is subjected to thermal load, thermal damage and thermal cracking usually occur, which may affect underground engineering and geological projects, such as enhanced geothermal systems, high-level radioactive waste repositories, geological carbon storage, underground coal gasification, and hydrothermal systems [1,2,3,4]. The first explanation for the variation in rock mechanics under thermal treatment is that the thermal stress within rocks subjected to a thermal load is induced due to the different expansion rates of mineral grains, resulting in the initiation of new intergranular and intragranular cracking and failure at elevated temperatures [9,10,11]. The widely accepted mechanism explaining thermal cracking is that the minerals in continually heated rocks expand at different rates, inducing strain at the grain boundaries and inter-granular cracking [9,10,13,15] Based on this theory, Yu et al [9] and Zhao [16] used rock failure process analysis (RFPA) and particle flow code in two dimensions (PFC2D ) to simulate the effect of thermal expansion on granite, but they were not able to simulate the intragranular cracks induced by thermal expansion. According to the simulation results, the distribution of micro-cracks and parallel bond forces in the granite specimen at different elevated temperatures, the micro-crack numbers with temperature, the crack evolution process, and the parallel bond force fields were analyzed in detail
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