Pore fracture propagation and unloading failure characteristics of siltite under cyclic loads

In order to reveal the pore fracture propagation and failure mechanism of underground engineering surround⁃ ing rock under cyclic loads，the uniaxial compression and CT scanning were used to carry out constant amplitude cyclic loading and unloading and stepwise linear cyclic loading and unloading tests on siltite. The stress⁃strain curves of siltite，the propagation of pores and fractures，and the occurrence and scale of fractures in the failure stage are studied under cyclic loading and unloading. The results show that the siltite porosity decreases first and then increases under two kinds of cyclic loads，and the porosity of fracture cycle changes suddenly，which has the characteristics of in⁃ stantaneous fracture. Under the constant amplitude cyclic loading and unloading，rock failure occurs in the 25th cyclic loading section， and according to the strain difference， the cumulative strain difference and its first derivative mainly，the fracture propagation in the failure stage of rock samples is subdivided into three stages: fracture initiation， fracture propagation，and accelerated failure. Under the stepwise lineal cyclic loading and unloading，the rock failure occurs in the 4th cyclic unloading section，and presents the characteristics of low⁃amplitude oscillation failure when the strength is lower than the peak load of 1.04 MPa. According to the unimodal fluctuant stress amplitude，frequency， strain difference，cumulative strain difference and its first derivative mainly，the fracture propagation in the failure stage of rock samples is subdivided into five stages: fracture initiation，rapid fracture propagation，metastable fracture propa⁃ gation，fracture regeneration，and accelerated failure. According to the form of stress⁃strain curve，the low⁃amplitude os⁃ cillation failure can be subdivided into a reciprocating cyclic wave and a hysteretic cyclic wave，and the analysis of stress amplitude and wave frequency shows a clear stress adjustment process. The distribution of original pores and fractures of the rock samples plays a major role in controlling the fracture location and failure form. Under the action of two kinds of cyclic loads，the fracture of rock samples is mainly shear fracture and secondary tensile fracture. The rebound and dilatancy effect of the unloading section are the main causes of low⁃amplitude oscillation failure.