Characteristics of dynamic mode Ⅱ fracture of frozen saturated sandstones under varying low temperatures and loading rates

BackgroundInvestigating the mode Ⅱ fracture of frozen saturated sandstones under dynamic disturbances, including changes in their critical stress intensity factor (SIF; i.e., fracture toughness) and energy dissipation mechanisms, is crucial for the safe excavation of frozen wellbores through drilling and blasting in coal mines. MethodsUsing a split Hopkinson pressure bar (SHPB), this study conducted experiments on the dynamic mode Ⅱ fracture of a sandstone specimen using the short core in compression (SCC) method (also referred to as dynamic SCC experiments) under varying temperatures and loading rates. Accordingly, this study analyzed the SIF versus time curves of the frozen saturated sandstones, as well as the evolutionary patterns of their fracture toughness and energy dissipation mechanisms varying with the temperature and loading rate. Furthermore, it summarized the characteristics of the shear fracture planes of the sandstone specimen at macroscopic and microscopic scales and revealed the microscopic mechanisms behind the low-temperature strengthening effect using nuclear magnetic resonance (NMR). Additionally, this study further analyzed the effects of the loading rate on fracture characteristics using finite element simulation. Results and ConclusionsThe results indicate that with a decrease in temperature, the frozen saturated sandstones exhibited intensifying fracture toughness. Accordingly, energy absorbed during sandstone fracture increased, and the energy utilization ratio showed an increasing trend. With an increase in the loading rate, the fracture toughness increased linearly, energy absorbed during sandstone fracture also showed an increasing trend, but the energy utilization ratio exhibited a decreasing trend. Two different fracture modes were observed on the shear planes of the SCC specimen: stepped and fracture-shaped shear slips. The NMR data revealed that the pore water freezing of the sandstones can be divided into three stages: supercooling, rapid freezing, and slow freezing sequentially. In the first stage, the unfrozen water content remained unchanged. In the rapid freezing stage, the free and capillary water roughly experienced phase transition, and most of the bound water froze, leading to rapidly enhanced fracture toughness. In the slow freezing stage, a small amount of bound water froze, with fracture toughness increasing slowly. The strain versus time curves derived from numerical simulation agreed well with those obtained using experiments, suggesting reasonable and reliable simulation results. The simulation results revealed significant stress concentration on the rock bridge of the SCC specimen and that the shear stress showed an increasing trend with the loading rate. Moreover, both the fracture toughness of the sandstones and energy absorbed during sandstone fracture increased with an increase in the loading rate. The results of this study can serve as a guide for the excavation of frozen wellbores through drilling and blasting in water-rich soft rock formations in West China.