# Study on Mechanical Properties of Fractured Sandy Mudstone Based on Triaxial Compression Experiment
## 3 Analysis of Triaxial Test Results  
### Analysis of Stress-Strain and Acoustic Emission Characteristics of Specimens  
The failure of rock under loading is often accompanied by a large amount of strain energy release and fracture expansion, which can be captured by triaxial compression experiment and numerical simulation [24; 25; 26; 27]. The axial and circumferential stress-strain curves of fractured sandy mudstone were derived by summing the data collected under various confining pressure conditions in order to investigate the mechanical properties and energy evolution of the material. The acoustic emission characteristic parameters are subject to change as a result of the axial strain, as illustrated in Figure 3. The tensile strain is assigned a negative value, while the compressive strain is assigned a positive value. Numbers W100-1 and WL100-1 in Figure 3 correspond to stress-strain curves under a confining pressure of 0 MPa, and so on. There are five groups of test curves under different confining pressures. In these curves, \(\epsilon 1\) represents the axial strain value of the specimen, \(\epsilon 2\) represents the circumferential strain value of the specimen, and \(\epsilon v\) represents the volumetric strain value of the specimen. Additionally, \(\epsilon b\) represents the crack closure stress of the specimen, \(\alpha c\) represents the yield stress of the specimen, and \(\alpha\) represents the peak stress of the specimen. The figure also includes three acoustic emission characteristic parameters: impact count rate, AE accumulated energy, and ringing count. As shown in Figure 3, when rock specimens with cracks are subjected to an axial stress loading less than \(\sigma b\), they undergo crack compaction stage where both axial strain (\(\epsilon 1\)) and volume strain (\(\epsilon v\)) increase while generating tensile strain (\(\epsilon 2\)) perpendicular to the maximum principal stress in circumferential direction. The number of acoustic emission events during this stage is small and cumulative AE energy increases slowly. When axial stress loading exceeds \(\sigma b\), there is an approximately linear relationship between axial stress and axial strain indicating that rock is in elastic deformation stage where compressive strain as well \begin{table}
\begin{tabular}{c c c} \hline
**Test Piece Number** & **Whether There Is Crack** & **Confining Pressure (MPa)** \\ \hline WL100-4 & Yes & 24 \\ WL100-5 & Yes & 30 \\ W100-1 & No & 0 \\ W100-2 & No & 8 \\ W100-3 & No & 16 \\ W100-4 & No & 24 \\ W100-5 & No & 30 \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 2: Triaxial mechanics experimental equipment and specimen layout. as tensile strain maintain a linear increase. Before the stress loading reaches the yield stress point \(\propto\)c, the number of acoustic emission specimens is small. The micro-cracks in the rock do not expand, and the cumulative AE energy increases faster than that in the compaction stage. When the stress is loaded to the yield stress point \(\propto\)c, it enters the plastic deformation stage. The strain curve deviates from linearity and reaches its peak at \(\sigma\)t. After reaching the yield point, both toroidal strain and volumetric strain turn from compression to tensile deformation, indicating that rock dilatancy occurs after yielding. Subsequently, as fissures rapidly develop post peak, elastic strain energy is released leading to a sharp increase in acoustic emission events and an abrupt rise in cumulative AE energy when fissure expansion occurs rapidly. Finally, both toroidal and volumetric tensile strains reach their maximum value near \(\sigma\)t.  
Figure 3: _Cont._  
The mechanical properties and acoustic emission characteristics of intact rock specimens are essentially the same as those of fractured rock during the compaction stage and elastic deformation stage. Based on the series of accumulated AE energy, it is observed that the strain energy stored by intact rock specimens during deformation is greater than that of fractured rocks.