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Attenuation of fracture toughness and evolution of fracture mechanisms in granite subjected to repeated thermal shocks
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DOI：10.1016/j.engfracmech.2026.112096.png)
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Abstract
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The long-term stability of fracture networks in Enhanced Geothermal System (EGS) reservoirs is governed by the evolution of rock fracture behavior under thermal cycling. This study investigates the cumulative damage effect of thermal cycle number on the fracture characteristics of granite. A comprehensive experimental and numerical investigation was conducted using preflawed Brazilian disc specimens subjected to a fixed high temperature (600 degrees C), different numbers of thermal cycles (1, 3, 5), and different cooling methods (air cooling, water cooling, liquid nitrogen cooling). The key findings are as follows: (1) The mode-I fracture toughness (KIC) of granite exhibits significant degradation with increasing number of thermal cycles, and the extent of degradation is amplified by cooling intensity. After 5 cycles of liquid nitrogen cooling, the KIC value decreases by more than 50% compared to its value after the 1st cycle, indicating severe cumulative damage. (2) The crack propagation path becomes increasingly unstable with cycle number, transitioning from relatively straight to markedly deflected and branched, reflecting the intensification of material anisotropy induced by cyclic damage. (3) Numerical simulations reveal that the mechanical driving force (KI) and energy release rate (J-integral) at the crack tip show a decreasing trend with increasing cycles. This indicates that the macroscopic degradation is primarily caused by the reduction in the material's intrinsic fracture property (KIC) due to thermal cycling, rather than by an increase in the crack driving force. This study systematically elucidates the damage accumulation mechanism and cooling-path dependency of granite fracture behavior under cyclic thermal shock, providing crucial theoretical and experimental basis for assessing the long-term stability of fracture networks in EGS reservoirs.
Keywords:
Granite
Thermal damage
Cooling method
Crack coalescence
Fracture behavior
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