# Evaluation of Rock Brittleness Index under Dynamic Load
## 3 Results  
### Brittle Failure Characteristics of Rock under Static and Dynamic Loads  
Figure 2 shows the stress-strain curves of rocks with different loading methods, and Figure 3 shows the changes in the compressive strength of five kinds of rocks. It can be seen that fine granite had the highest compressive strength under static load, followed by coarse granite, shale, marble, and sandstone. Under dynamic load, the compressive strength of the five kinds of rocks was greatly improved compared with that under static load.  
Figure 2: Stress–strain curves with different loading modes.  
[PERSON] [34] conducted a large number of uniaxial compression tests on granite, schist, and sandstone, and divided the failure modes of all rock types into six types: axial splitting, shearing along single plane, double shear, multiple fracture, along foliation, and Y\(-\)shaped failure, as shown in Figure 4.The images of rock failure under different loading modes in this paper are shown in Figure 5. It can be seen that under static load, the failure mode of fine granite, marble, and sandstone was shearing along single plane, while the failure mode of coarse granite and shale was axial splitting. The failure modes of five kinds of rock under dynamic load were mainly axial splitting failure. Under the action of load, the elastic deformation of the rock was transformed into sudden fractures; this process is called brittle failure. In the test, none of the five kinds of rock showed obvious deformation after failure under different loading modes, demonstrating the characteristics of brittle failure.  
Figure 4: Schematic representation of different failure modes under uniaxial compression.  
Figure 3: Change in compressive strength of rock samples.  
Brittle failure is often the result of crack formation and development in rocks. Under static load, the loading rate is low and deformation occurs simultaneously, during which the whole rock specimen is compressed. At this time, the original microcracks in the specimen first close with the increase in pressure. Subsequently, when the compressive tensile stress exceeds the local tensile strength of the deficient tips, cracks are generated from these tips and propagate parallel to the direction of loading [35]. The crack propagation is affected by the microstructure of the specimen [36; 37]. When the microstructure of the specimen does not hinder the crack propagation, the specimen is destroyed in axial splitting mode. However, the specimen microstructure restricts crack propagation, adjacent cracks, and close\(-\)range cracks generated by microcrack tips with appropriate orientation merge, releasing strain energy in the form of shear fracture and resulting in higher uniaxial compressive strength than that described in general axial splits [34]. In this study, fine granite had a higher uniaxial compressive strength than coarse granite, and the failure mode changed from axial splitting to shearing along a single plane. The uniaxial compressive strength of sandstone and marble was higher than that of similar rocks, so they also produced shear along a single plane, which is consistent with [PERSON]'s conclusion. In addition, when the specimen itself has a bedding structure, the crack propagation will be affected by the bedding structure direction due to the large difference in local tensile strength. The direction of shale bedding structure used in this paper is basically consistent with the direction of loading, so axial splitting failure occurs. Under dynamic load, the loading rate is high and the deformation of rock specimen propagates in the form of compression waves. Deformation occurs sequentially, and the specimen cannot be considered as a whole compression. Under the action of incident waves, cracks are generated along the loading direction of the rock. The crack propagation is affected by the compression wave, and the restriction of the microstructure on crack propagation becomes smaller, resulting in axial splitting failure.