Effect of sub-zero temperature on dynamic mode II fracture properties of saturated porous rocks

Investigating mode II (shear) fracture properties in alpine and arctic rock engineering is crucial. However, the effects of sub-zero temperatures on these fracture parameters of rocks remain unexplored. Thus, short core in compression (SCC) specimens are fabricated to investigate dynamic mode II fracture parameters of porous white sandstone (WS) across varying ambient temperatures (-55 degrees C, -40 degrees C, -25 degrees C, -10 degrees C, 0 degrees C, and 20 degrees C). Both dry and fully saturated WS SCC specimens were tested, with dry specimens as the control group. A high-speed camera was used with a dynamic-cryogenic testing setup to perform the dynamic tests on the rock specimens. Additionally, the fracture surface of recovered specimens was quantitatively characterized by laser scanning. The findings illustrate that the dynamic mode II fracture toughness and dynamic fracture energy (DFE) of WS specimens rise with the loading rate at several ambient temperatures. At a specific loading rate, the dynamic mode II fracture toughness of saturated WS demonstrates an initial growth as the temperature drops from 0 degrees C to -10 degrees C, subsequently decreases at -25 degrees C, and finally increases again as temperature continues to decrease below -25 degrees C. Nevertheless, the sub-zero temperatures slightly influence the mode II fracture toughness of dry WS. The fracture surface roughness of saturated and dry WS specimens in terms of the ambient temperature is consistent with the tendency of fracture toughness. However, the DFE of saturated and dry WS specimens remains nearly identical when the temperature is above 0 degrees C. In contrast, the DFE of dry WS specimens is larger than that of the saturated specimens when the temperature is below 0 degrees C. These phenomena are attributed to the water-ice phase transition in pores and the impact melting of ice at microcrack tips. Furthermore, it is found that the fractal dimension of shear fracture surfaces of SCC specimens decreases with rising loading rates, which is attributed to the enhanced occurrence of transgranular fracture during the dynamic failure process.