Strength change mechanism of siltstone under near fatigue strength cyclic loading

Through the cyclic loading unloading to monotonic loading test, combined with acoustic emission and CT scanning technology, the micro fracture evolution law, fracture structure characteristics and strength change mechanism of argil-laceous quartz siltstone under near fatigue strength cyclic loading are revealed. The results show that: (1) the damage stress of argillaceous quartz siltstone is less than the fatigue strength, so the stress level between the damage stress and the fatigue strength can be called as near fatigue strength. (2) The near fatigue strength cyclic load has obvious deterioration or strengthening effect on siltstone, the peak strength of rock decreases slightly with the increase of the number of cycles, then increases continuously and finally tends to be stable. (3) Cyclic loading and unloading changed the fracture mode of argillaceous quartz siltstone, the rock changed from tensile failure under uniaxial compression to shear failure under low cyclic loading and unloading to monotonic loading, and with the increase of the number of cycles, it transited to tensile shear conjugate failure. (4) When the number of cycles is low, the weak cementation structure of siltstone breaks during cyclic loading and unloading, and the microcracks further expand, after monotonic loading, the rock fracture scale in-creases, and the internal cracks concentrate locally, leading to deterioration. (5) When the number of cycles is high, siltstone particles are broken and rearranged during cyclic loading and unloading, the compactness increases, the effective bearing area increases, and the rock fracture size, fracture density and fragmentation degree decrease after monotonic loading, resulting in strengthening.