# Rock Crack Types Identification by Machine Learning on the Sound Signal+
## 3 Results and Discussion
### Identifying the Development of Tensile and Shear cracks during Rockburst
#### 3.2.1 Rockburst Test on a Granite Specim  
In our study, the true-triaxial rockburst testing machine developed at Guangxi University, China, is used to simulate the rockburst process. As shown in Figure 21, a loading path, \"one free face, five stressed faces\" is used, and the tested cuboid specimen has dimensions of 100 mm (width) \(\times\) 100 mm (length) \(\times\) 200 mm (height) [39]. The loading plan can be described as follows: \(\sigma_{x}\) and \(\sigma_{y}\) are increased to 30 MPa and 5 MPa; simultaneously, \(\sigma_{z}\) is increased at a rate of 0.3 MPa/s until the rock specimen fails.  
During the testing process, the voice recorder for collecting the sound signal is located approximately 20 cm from the tested specimens. The failure form of the tested specimen is shown in Figure 22. The tested specimen shows a binary failure characteristic, where more serious failure occurs at the free faces. A rockburst pit can be observed, and the surface of the rockburst pit is covered by rock fragments and powders. In addition, near  
Figure 21: Rockburst test (**a**) size and shape of the rock specimen and loading device, (**b**) stress state of the specimen, (**c**) loading path.  
Figure 20: Tensile and shear crack development process under biaxial compression test.  
the free face, a vertical tensile (splitting) crack can be observed. Away from the free face, several oblique cracks develop, and a lot of white powders are observed on the crack face, indicating a shear failure characteristic. It can be concluded that rockburst is characterized by the external tensile crack on the free face and the internal shear crack away from the free face. Consequently, an investigation on the development of tensile and shear cracks can provide beneficial information for the revelation of rockburst mechanisms.  
According to the characteristics of the stress-strain curve, the pre-peak deformation can be divided into four stages, as shown in Figure 23: A: initial compaction stage, B: linear elastic stage, C: elastic-plastic transition stage, and D: plastic stage. From 0 to 519.621 s, there are no sound signals. From 519.621 to 742.447 s, the sounds occur sporadically. The characteristics of the sound signal in the different stages are described in detail as follows.  
In the crack compaction stage (0-167.76 s), pre-existing cracks close, which is an extremely slight rock deformation behavior; in the elastic deformation stage (167.76-519.621 s), there are no crack sounds. In elastic-plastic deformation (519.621-700.92 s), a large number of sound signals with high amplitude occur, which can be detected by not only sensors but also the human ear. The waveform is dense because many cracks continually develop and propagate, and local failure caused by unrecoverable plastic deformation occurs. Then, the plastic deformation stage (700.92-742.447 s) is encountered. In this stage, more high-amplitude sound signals occur, intensive rockburst occurs, and a large number of rock blocks are ejected from the free face.