Dependence of dynamic tensile strength and fracture toughness of Longyou sandstone on heat-treatment temperature and loading rate

The mechanical properties of Longyou Sandstone (LS) (a material for Longyou rock caverns) under the combination of high loading rate and high temperature are important parameters for the protection of the caverns. Thus, the LS sample were first heat treated by various temperatures (150°C, 250°C, 350°C, 450°C, 600°C and 850°C) and the thermal damage was then characterized by the 3D X-ray Micro-Computed Tomography (CT) method. Meanwhile, the density and P-wave velocity of heat-treated sample and reference sample were measured. The values of CT value, density, and P-wave velocity decrease with the elevated heat-treatment temperature except for 450°C. Thereafter, according to the suggested method for determining dynamic tensile strength and dynamic fracture toughness of rock by International Society of Rock Mechanics (ISRM), Brazilian Disc (BD) and Notch Semi-Circular Bend (NSCB) samples were employed in the Split Hopkinson Pressure Bar (SHPB) system to measure the dynamic tensile strengths and the Mode-I fracture toughness of these LS samples under different dynamic loading rates. The dynamic tensile strength and fracture toughness of LS increase with the loading rate at a given heat-treatment temperature, and both the tensile strength and fracture toughness at the same loading rate decreases with the heat-treatment temperature except for 450°C. This phenomenon can be explained by the variation of the numbers of micro-cracking in LS sample for the different heat-treatment temperatures. With the thermal damage variable derived from the average CT value, empirical formulas were established to relate the dynamic tensile strength and fracture toughness of LS to the loading rate and the heat-treatment temperature. The results demonstrate that the dynamic tensile strength and Mode-I fracture toughness of LS are significantly influenced by the heat-treatment, especially under higher loading rate.