Tensile characteristics and fracture mechanism of high-temperature water-cooled granite under cyclic loading-unloading
ZHANG Fan1, 2, ZHANG Yiming1, 2, LI Man1, 2, HU Dawei3
(1. Key Laboratory of Intelligent Health Perception and Ecological Restoration of Rivers and Lakers, Ministry of Education,
Hubei University of Technology, Wuhan, Hubei 430068, China; 2. School of Civil Engineering, Architecture and Environment,
Hubei University of Technology, Wuhan, Hubei 430068, China; 3. State Key Laboratory of Geomechanics and Geotechnical Engineering Safety, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, China)
Abstract： Circulating hydraulic fracturing, a flexible transformation method for hot dry rock, is anticipated to reduce reservoir fracture pressure, enhance transformation volume, and ensure efficient and safe storage. This method holds significant promise for the economic and efficient development of hot dry rock geothermal resources. To investigate the evolution and fracture mechanisms affecting the tensile properties of reservoir rock due to cyclic hydraulic fracturing, we conducted cyclic loading-unloading Brazilian splitting tests on granite treated with high-temperature water cooling. Two test conditions were examined: varying heat treatment temperatures (200 ℃, 300 ℃, 400 ℃, 500 ℃ and 600 ℃) and different cyclic load upper limits (peak loads of 65%, 70%, 75%, and 80%). The fracture surfaces were analyzed using scanning electron microscopy. The results indicate that tensile strength initially increases and then decreases with rising heat treatment temperatures but continues to increase with higher load upper limits. The number of cycles leading to fatigue failure decreases as the load upper limit increases, displaying a trend of first decreasing and then increasing with higher heat treatment temperatures. The peak displacement experiences three stages: rapid growth, slow growth, and eventual failure as the number of cycles increases. Notably, when the temperature exceeds 300 ℃, deformation significantly increases, suggesting that granite transitions from brittleness to ductility. Overall, the fracture mode is characterized by failure along the diameter direction. Beyond 300 ℃, the crack mode shifts from a “straight line” tensile crack penetrating the center of the circle to a “curved line” mixed tensile-shear crack deviating from the center. This change is primarily attributed to the substantial softening of mineral crystals and the formation of microcracks due to high temperatures. Furthermore, microstructure analysis reveals that as heat treatment temperature and cycle number increase, the internal crack propagation mode of granite transitions from transgranular to intragranular. Concurrently, micro-damage accumulates, resulting in decreased fatigue strength and increased plastic deformation. Macroscopically, this manifests as a coupling effect of tensile strength attenuation and enhanced fracture ductility. The findings of this research provide theoretical support for the design and construction of cyclic hydraulic fracturing.
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2025, Vol. 44 
