Experimental investigation on mode I fracture characteristics of granite after cyclic heating and cooling treatments

Rock in the geothermal reservoir is frequently subjected to cyclic heating and cooling, and fracture characteristics are closely related to the stable operation of Enhanced Geothermal Systems. To examine impacts of cyclic heating and cooling treatments on mode I fracture characteristics of granite, fracture tests of semi-circular bend specimens were conducted, acoustic emission (AE) events were monitored, and morphology characteristics of fracture surfaces were analyzed. Meanwhile, the open porosity was measured, and the scanning electron microscope (SEM) was applied to identify variations of microstructure in granite affected by cyclic treatments. Failure pattern and modifications of microstructure in treated granite specimens were discussed. Results indicated that cyclic treatments can lead to serious degradation in granite, manifested as the increase of open porosity, the decrease of fracture toughness and cumulative AE counts with both increasing temperature and cycles. The smoother post-peak stage in load-displacement curves and the prolonged active period of AE events of granite specimens after cyclic treatments implied the enhancement of ductility. Fractal dimensions of fracture surfaces exhibited evidently negative correlation with the brittleness index, which demonstrated that the enhanced ductility promotes the generation of rougher fracture surfaces. The failure pattern of treated granite specimens revealed that the propagation path of the main crack is strongly affected by the thermal cracks. SEM observations verified the increased, extended and deepened micro-cracks in granite after cyclic treatments, which can be attributed to the comprehensive effects of heating, rapid cooling, thermal cycling, and water-induced weakening.