Dynamic fracture properties and criterion of cyclic freeze-thaw treated granite subjected to mixed-mode loading

Rock masses in high-elevation or cold regions are vulnerable to the combined effects of freeze-thaw (F-T) weathering and dynamic mixed-mode loading, posing a serious threaten to the safety and stability of geotechnical engineering. In this study, a series of dynamic fracture tests were conducted on notched semi-circular bend (NSCB) granite specimens subjected to different mixed-mode loading and F-T cycles using a split Hopkinson pressure bar (SHPB) test system. The effects of F-T treatment and dynamic mixed-mode loading on the fracture properties of granite, including effective fracture toughness, progressive fracture process, and macroscopic morphology of fracture surface, were comprehensively revealed. The experimental results suggest that the dynamic effective fracture toughness of NSCB specimens is dependent on the loading rate, particularly when the mode I loading is dominant. Additionally, the fracture toughness decreases as the number of F-T cycles increases, with an inflection point at 30 F-T cycles. All granite specimens subjected to mixed-mode loading exhibit a curved fracture path, with faster crack propagation speed and more fine cracks in specimens exposed to higher F-T cycles. Macroscopic morphology of fracture surface obtained using three-dimensional (3D) scanning indicates that the fractal dimension of the fracture surface increases with increasing F-T cycles, and the increment is more pronounced for specimens subjected to a higher mode II loading component. Moreover, this study compared the fracture resistance of F-T treated granite subjected to dynamic mixed loading using the maximum tangential stress (MTS) criterion and the generalized maximum tangential stress-based semi-analytical (SA-GMTS) criterion. Compared with the MTS criterion, the SA-GMTS criterion shows a more reasonable consistency with the experimental results, with a root mean square error within ±7%.