Strength change mechanism of siltstone under near fatigue strength cyclic loading

In this paper, the microscopic fracture evolution law, crack propagation characteristics, and strength change mechanism of argillaceous quartz siltstone under near fatigue strength cyclic loading were revealed by conducting cyclic loading and unloading to monotonic loading tests with different cycles, combined with acoustic emission and CT scanning techniques. Results are summarized as follows: (1) The damage stress of argillaceous quartz siltstone is less than the fatigue strength, and the stress level between the damage stress and fatigue strength is known as near fatigue strength. (2) As the number of cycles increases, the peak strength of the siltstone first slightly decreases, then continues to increase, and finally stabilizes. When the axial (volumetric) deformation changes from compression (expansion) to almost constant after one cycle, the strength of the siltstone changes from deterioration to strengthening. (3) As the number of cycles increases, the bandwidth and density of low- and intermediate-frequency signals of argillaceous quartz siltstone gradually decrease after loading and unloading under near fatigue strength, and the high-frequency band signals become more concentrated. When the stress approaches the peak strength during the monotonic loading stage, the acoustic emission signals in the middle and low-frequency regions of the siltstone significantly increase, which can be regarded as a precursor to rock compression failure. (4) When the number of cycles is low, the siltstone fractures exhibit a weak cementation structure during cycling, and the effective bearing area decreases. After monotonic loading, the acoustic emission vibration frequency range of the siltstone is wide, with a high proportion of low- to intermediate-frequency signals. The rock-fracture scale increases, and internal cracks are locally concentrated, leading to deterioration and a single slope shear failure. (5) When the number of cycles is high, the bonding strength of the siltstone increases, the microstructure becomes dense and uniform, and the effective bearing area increases during the cycling process. Furthermore, after monotonic loading, the acoustic emission signal of the siltstone is mainly a high-frequency signal, whereas the low-frequency band signal decreases. Almost no acoustic emission signal is generated in the middle-frequency band, and the size, density, and degree of fragmentation of the rock fracture decreases. Furthermore, strengthening occurs, forming a tensile shear composite fracture network. The findings of this study can provide a basis for the long-term stability analysis and disaster warning of rock-bearing structures under cyclic loads and are of great significance for guiding the support of surrounding rock tunnels and ensuring the safe mining of coal mining.