Effects of cyclic freeze-thaw treatments on the fracture characteristics of sandstone under different fracture modes: Laboratory testing

For engineering projects in cold regions, the failure behaviour of jointed rocks is affected by freeze-thaw (F-T) cycle action. To reveal the fracture characteristics of rock under the influence of cyclic F-T treatments, fracture tests on semicircular specimens with different F-T cycle numbers were conducted. The fracture toughness values in different modes were investigated by SCB specimens with different crack inclinations (0 degrees, 5 degrees, 15 degrees, 30 degrees, 45 degrees and 54 degrees). The experimental results show that cyclic F-T treatments can lead to degradation in three kinds of fracture toughness values. Specifically, the value of K-IC (fracture toughness in mode I) decreases by 3.1, 7.2, 15.0 and 24.3% after 20, 40, 60, and 80 F-T cycles, respectively. When the number of cycles increases from 20 to 80, the value of K-IIC (fracture toughness in mode II) of the sandstone decreases by 8.43, 12.0, 18.8 and 27.9%. For the specimen with crack inclinations of beta = 5 degrees, 15 degrees, 30 degrees and 45 degrees, after 80 F-T cycles, the K-eff (effective fracture toughness) value decreases to approximately 83.7, 78.3, 79.1 and 76.7% of the initial value, respectively. The mixed-mode fracture toughness ratios K-eff/K(IC )were calculated and compared with the theoretical solution. The experimental results of K-eff/K-IC agree well with the theoretical results calculated based on the MMTS criterion. Moreover, after F-T treatments, the fracture paths of the SCB specimens are more tortuous than those in specimens without F-T treatment. In addition, the fracture surface morphological characteristics of the SCB specimen were obtained by a 3D laser scanner. Overall, the fractal dimension value increases gradually with additional F-T cycles. This indicates that the F-T treatments lead to rough fracture paths and fracture surfaces.