# 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
#### 3.2.2 Tensile and Shear Cracks Process during Rockburst  
The identification of tensile and shear cracks during rockburst is presented as follows. All the predicted sound signals are processed according to Section 3. Figure 23: Stress-strain curve from the biaxial compression test. Figure 22: Failed specimen after the rockburst test. The related parameters of trained GPC during identification are set as follows. The training sample comprises 48 spectrograms. The GPC initial hyper-parameter is [\(-2\); \(2\)]. The number of repetitions used to optimize the adaptive hyper-parameter is 30. The GPC kernel function is covNNone. The variation in the tensile and shear sound signals obtained using the proposed method is shown in Figure 24. The abscissa indicates the time when the sound signal of the crack occurs, and the ordinate indicates the predicted probability of GPC. When the predicted probability of GPC is greater than or equal to 0.5, a tensile crack is predicted, and when it is less than 0.5, a shear crack is predicted. The proportions of tensile and shear crack sound signals in different deformation stages are shown in Table 6. It can be seen that sound signals caused by tensile cracks occur in the early stage of rockburst, and shear cracks occur in the late stage. In the elastic-plastic stage (519.621-700.92 s), the tensile sound signals account for 92%. The proportion of the shear sound signal rises to approximately 8%. During the plastic stage (700.92-742.447 s), the number of shear sound signals continually increases, and the proportion of the shear sound signals accounts for more than 80%. According to the recognition results of the sound signals using the proposed method, the tensile and shear crack development can be obtained as follows. In the compaction and linear elastic stage, only a few micro cracks initiate, and no macro cracks can be observed. In the elastic-plastic deformation, these cracks will propagate to cause the formation of macro cracks. The sliding along the macro cracks will cause the development of shear cracks, leading to unrecoverable plastic deformation. With more cracks propagating, more shear cracks occur, which causes the formation of the predominant cracks and the final failure. In this process, if the released energy cannot be dissipated by the shear cracks, the residual energy will be translated into the kinetic energy of ejected fragments, leading to the formation of rockburst. Based on the results of the tensile-shear crack evolution and the failure process of the rock specimen during rockburst, the incubation mechanism of rockburst can be better summarized, as shown in Figure 25. In the rockburst test, the failure of the specimen gradually progresses with the loading of the stress. Under the applied stress \(\sigma_{x}\), \(\sigma_{y}\), and \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **A (Initial Compaction Stage)** & **B (Elastic Period)** & **C (Elastic-Plastic Stage)** & **D (Plastic Stage)** \\ \hline tensile crack & 0 & 0\% & 92\% & 20\% \\ \hline shear crack & 0 & 0\% & 8\% & 80\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Proportions of tensile and shear sound signals in different stages under rockburst. Figure 24: Waveform from the rockburst test. \(\sigma_{z}\), the rock specimen gradually becomes dense. At this time, due to the nonuniform grain size, the three-dimensional stresses remain low, and a small number of cracking events occur in the rock. As \(\sigma_{z}\) continues to increase, the rock specimen is fractured by compression, and a large number of tensile cracks form parallel to the \(z\) direction, but, at this time, \(\sigma_{z}\) is still low. Due to the Poisson effect, under the force in the \(x\) and \(y\) directions, the large-scale destruction of the rock specimen on the free surface is suppressed. As \(\sigma_{z}\) continues to increase, the slab-compression-induced splitting phenomenon continues to occur, and tensile cracks also form and expand.