Study on dynamic fracture characteristics and fracture process zone evolution mechanism in sandstone under freeze-thaw cycles

This study, through laboratory experiments, investigates the effects of freeze-thaw (F-T) cycles on the fracture characteristics of sandstone under seasonal freezing conditions on the Qinghai-Tibet Plateau. Sandstone samples were first pretreated with F-T cycles in a temperature range of 20 ℃ to −20 ℃ and then subjected to impact three-point bending tests with the aid of a SHPB system. In the meantime, the evolution of the fracture process zone (FPZ) was analyzed via high-speed camera recordings and Ncorr digital image correlation software. The results demonstrate that F-T cycles induce mechanical property degradation in sandstone, which in turn affects its dynamic fracture behavior. As F-T cycles increase, the time required for pre-existing crack penetration extends, the crack opening width within the same time interval increases, and the time needed to achieve an equivalent opening width decreases. Crack opening velocity is significantly suppressed by F-T cycles before 111 ms, while this influence dininishes afterward, indicating that F-T effects predominantly govern the initial crack tip propagation stage. The evolution of the FPZ can be divided into two stages: expansion and contraction. To be specific, in the initial stage, microcracks cluster near the crack tip, and the FPZ expands with the increase in load. After main crack penetrates, the FPZ contracts and eventually dissipates due to energy release. F-T cycles promote the formation of a substantial number of pores within the sandstone, enhancing its energy absorption capacity. As a result, FPZ peak values are reduced and peak stresses occur earlier, accompanied by a transition in failure mode from brittle to ductile, characterized by prolonged crack propagation time and reduced initial opening velocity. The research results can provide basic experimental data reference for the dynamic disaster relief of engineering rock masses in cold regions.