Dynamic mechanical behaviours of frozen rock under sub-zero temperatures and dynamic loads

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Dynamic mechanical behaviours of frozen rocks are essential for dealing with the design and stability of various engineering structures in cold regions. A dynamic-cryogenic testing system integrated with high-speed Digital Image Correlation (DIC) is adopted to investigate the dynamic mechanical behaviours of rock subjected to ambient room and sub-zero temperatures, along with high-rate loading. Dynamic compression, tensile and Mode I dynamic fracture tests were performed at 20, -20, −50 and −70 °C using dry and saturated sandstone from Yulong Copper Mine at high-altitude cold region. A DIC-based approach using pixel-based virtual extensometers is proposed to accurately capture the real-time cracking speed with high precision. The proportion of ice and unfrozen water in rock pores across various temperatures is quantified using the nuclear magnetic resonance (NMR) method. The results indicate significant temperature and rate dependencies in dynamic fracture toughness, crack propagation velocity, dynamic tensile strength, dynamic compressive strength, and failure modes; These parameters exhibit an increase with decreasing ambient temperature as unfrozen water transitions into ice, demonstrating the frozen effect of ice filling pores, supporting the rock skeleton, and bonding ice-rock interfaces. With decreasing temperature, crack branching becomes more pronounced in dynamic notched semi-circular bending (NSCB) tests. Additionally, dynamically compressed frozen rocks tend to transition from pulverization (Type II) to splitting (Type I) with multiple elongations, attributed to the strengthening effect. The findings elucidate the failure mechanisms observed in a higher proportion of larger blast blocks and hold valuable implications for blasting designs and dynamic disaster prevention in cold regions.