Investigating the impact of freezing and thawing cycles on the dynamic fracture properties of red sandstone subjected to impact loading

Comprehending the dynamic fracture behaviour of rocks exposed to freeze–thaw cycles (FTCs) is essential for warranting the safety and stability of construction projects and rock engineering activities in frigid regions. In this study, the split Hopkinson pressure bar (SHPB) and nuclear magnetic resonance techniques (NMR) methods are adapted to reveal the dynamic fracture and porosity characteristics of sandstone influenced by FTCs. The dynamic fracture tests were conducted under different impact pressures and the effect of loading rate $\dot{(K})$ on dynamic fracture characteristics is elucidated. The results revealed that there is a non-linearly declining trend between the average dynamic fracture toughness (K_IC(d)) and the number of FTCs. As the number of FTCs increases, the mean values of K_IC(d) decrease, and this loss after each cycle becomes more pronounced. Amongst the FTCs, the K_IC(d) decreased drastically from 0 to 20 FTCs, then stabilized at 40 cycles. As the number of FTCs approaches 80, the K_IC(d) has significantly decreased. The K_IC(d) demonstrated a gradual increase corresponding to the loading rate $\dot{(K})$ during different FTCs. The analysis conducted through NMR revealed a correlation between the total porosity of rock specimens and the number of FTCs, indicating an increase in porosity with each cycle. The number of micropores fluctuates with FTCs, showing no obvious trend. However, macropores consistently increase throughout the freezing and thawing process, indicating that the rock’s microstructural impairment primarily arises from the increase in macropore porosity. Based on the variation law in porosity, the damage characteristics of sandstone under FTCs are demonstrated by introducing a damage factor (D_ft). It has been unveiled from the correlation analysis, that under a specific loading rate $\dot{(K})$ range, the K_IC(d) exponentially declines with an increase of D_ft revealing a good correlation. The results of this research will contribute to the formulation of guidelines aimed at enhancing the safety and stability of rock slope engineering in cold climates.