Hybrid strengthening-weakening effects of microdefects on mechanical properties and energy evolution of granite

Rock structures at different scales, including macroscopic structural planes, mesoscopic joints, and microscopic defects, significantly influence rock properties. To investigate the effects of microdefects on granite, a series of laboratory experiments and numerical simulations were conducted, including micro-computed tomography scanning, uniaxial compression test, Brazilian disc test, uniaxial loading-unloading test, and discrete element method simulation. The results demonstrate that microdefects exert complex influences on the mechanical properties and energy evolution of granite, exhibiting both strengthening and weakening effects. The volume, density, and structural characteristics of microdefects are key controlling factors, while variations in size and shape further reflect these effects. With the increasing development of microdefects, the uniaxial compressive strength, tensile strength, initial elastic modulus, and tangent elastic modulus all exhibit a trend of initially increasing and then decreasing, indicating a hybrid strengthening-weakening effect. At low microdefect volumes, the closure-induced reduction in microdefect density enhances mechanical properties, whereas at higher volumes, the decrease in effective load-bearing area leads to mechanical deterioration. Moreover, numerical simulations reveal that increasing the number of microdefects promotes the development of total and tensile cracks during brittle failure, accompanied by higher energy storage and dissipation rates. This suggests that microdefects weaken mechanical strength while enhancing energy evolution by intensifying stress concentration and crack interaction. These findings improve the understanding of microdefect characteristics in granite and their effects on mechanical behavior and energy evolution.