Strength dependence of siltstone under coupled cyclic-monotonic loading tests and the evolution of three-dimensional acoustic emission source

The purpose of this paper is to reveal the influence mechanism of various cyclic loads on siltstone strength, by conducting a series of coupled cyclic-monotonic loading tests. Firstly, it was found that the peak strength of rock depends on the upper limit and cycle number of cyclic load. The peak strength of siltstone shows low-stress weakening effect, low-cycle weakening effect, strengthening effect, chaotic effect, and fatigue effect. Secondly, the ultimate strain of siltstone under cyclic load is controlled by the peak strain of uniaxial compression test. The peak strength is linearly with the peak strain, and the peak strain under uniaxial compression can be regarded as the critical threshold for fatigue failure. Then, the decrease of AE b value reflects the aggregation of micro cracks, which can predict the damage and failure of siltstone earlier than the burst of AE signals. The propagation paths of cracks under these strength effects show significant differences, by comparing the evolution of threedimensional AE sources with different energy levels. Finally, the fracture mechanism of the strengthening and weakening effects was discussed through electron microscopy scanning. The results indicate that the micro structures are adjusted by cyclic loadings, its fracture and energy release mode was changed, resulting in the increase of peak strength.