## 3 Analysis of Experimental Results
### Effect of Dynamic Disturbance on the Failure Pattern of Siltstone Specimens  
Figure 5 shows the stress-strain curves of the specimens under various dynamic disturbances. When the initial average stresses of the specimens are similar, the stress-strain curves are also similar. However, there are differences in the stress-strain curves when the initial average stresses are different. It can be seen from the figure that the fracture compaction occurs mostly at the beginning of the loading, and the curve is upward convex in the compaction stage. After that, when the sine wave cyclic disturbance load \(\sigma_{\rm c}\) for the siltstone specimens is greater than 60% of the uniaxial compressive strength, the specimens could be failed before the preset number of cycles (1000) is completed. During the cyclic dynamic disturbance loading, the hysteresis loop area indicating the energy loss of the specimen during the cyclic loading changes with \(\sigma_{\rm c}\). When the initial average stress value is 5.48 MPa (30% of \(\sigma_{\rm c}\)), the hysteresis loop is \"thin-lobed\", and when the initial average stress value is 6.39 MPa (35% of \(\sigma_{\rm c}\)), the hysteresis loop becomes \"full\" and its area increases. This phenomenon occurs due to the large deformation of the primary fractures in the specimen at the beginning of the cyclic dynamic disturbance stage. The hysteresis loops become \"sharp-lobed\" when the initial cyclic load is 7.31 MPa, 9.13 MPa (i.e., 40% and 50% of \(\sigma_{\rm c}\)). When the initial cyclic load is 10.05 MPa (55% of \(\sigma_{\rm c}\)), the hysteresis loop is \"banded\" and its area gradually decreases, indicating that the fractures inside the specimen are compacted and closed, and the energy dissipation is reduced. When the average stress is 10.96 MPa (60% of \(\sigma_{\rm c}\)), the hysteresis loop becomes \"full\" and the specimen undergoes greater irreversible deformation, resulting in unstable failure. When the average stress is 11.87 MPa (65% of \(\sigma_{\rm c}\)), the specimen is failed before the cycle starts and there is no hysteresis loop. From the axial stress-strain curves of these seven specimens, it can be found that the deformation of the siltstone specimens increases as the initial average stress increases, indicating that the damage of the specimens during the cyclic disturbance also increases. When the axial stress exceeds the upper limit threshold of stress, the cracks continue to extend and intersect with each other, which in turn leads to the destabilization of the specimens. Figure 6 shows a typical specimen failure plot under different initial average stresses. When the initial average stress value is 5.48 MPa, its impact on the damage of the specimen is low, and its damage characteristics are generally similar to that from uniaxial compression tests. Due to the low initial average stress, the specimen does not show obvious macroscopic damage after 1000 cycles of disturbance, and only new microfractures are created. The failure mode is shear, and there is partial damage in the middle of the specimen. When the initial average stress is 6.39 MPa, due to axial compressive stress, the shear stress on the fracture surface exceeds the shear strength and the tensile stress in the radial direction. The failure mode is mainly a shear failure, and there is a small part of splitting failure, and the specimen becomes cone-shaped inside after the failure. When the initial average stress is 7.31 MPa, the axial compressive stress causes the radial tensile stress of the specimen; once the radial tensile stress exceeds the tensile strength of the rock, split failure occurs; at this time, the extension direction of secondary cracks changes from longitudinal to dendritic dispersion. The initial average stress of 9.13 MPa has a greater impact on the overall failure mode of the specimen: a large number of secondary cracks are created in the specimen, and large pieces of rock fragments are peeled from the surface, and the failure mode is a  
combination of shear and splitting.