# Experimental Study on the Mechanical Properties of Deep Granite under Gradient-Confining Pressure
## 3 Results and Discussion
### Stress-Strain Curves and Characteristic Stresses  
The rock stress-strain curve is an important tool used to show the relationship between the stress and strain acting on the rock during the loading process. It presents the mechanical properties and deformation laws of the rock in the form of a graph. Through this curve, we can gain an in-depth understanding of the behavior and response of the rock under different stresses. The deformation of the rock mainly includes five stages, as shown in Figure 6.  
The rock fracture compaction stage (the OA segment): The sedimentary or crystallization defects when the rock is formed, crustal movement, temperature changes, weathering, the action of water, mineral expansion, seismic activities, and heterogeneity can all cause small fractures in the rock. It has a significant impact on the mechanical properties and permeability of the rock. The volume of the rock in this stage decreases with increasing load, which is due to the gradual closure of the fractures inside the rock after being compressed and the formation of nonlinear deformation. The stress corresponding to the end of this stage is the closure stress \(\sigma_{\rm cc}\). However, for hard rocks, because of their hard and dense texture, this stage is often not very obvious. The rock elastic stage (AB segment): At this stage, the response of the rock to the external force is linear. After unloading, the rock will return to its size and shape at the initial stage of this stage. The stable development stage of rock fractures (BC segment): After the elastic stage ends, the rock begins to exhibit initial fractures, and the stress corresponding to the initial crack is the initiation stress \(\sigma_{\rm ci}\), which is the critical point for the rock to transition from a stable state capable of bearing the load to a state of producing cracks. The initiation stress is affected by various factors, such as the properties of the rock itself, the loading conditions, and the external environment. By studying the initiation stress, the mechanical properties of the rock can be further understood. In practical applications, the initiation stress has a important significance, and it provides a key indicator for assessing the stability of the rock and predicting the risk of rock fracture. After the rock is subjected to axial loading, it is  
Figure 6: Typical stress-strain curve of the rock. axially manifested as a decrease in volume, while radially, it is manifested as an increase in volume. However, the initial stage of axial strain is often greater than that of radial strain, so the rock macroscopically experiences a decrease in volume. However, with increasing load, when the volume of the rock is compressed to the minimum, the volume of the rock will begin to slowly increase, and the stress corresponding to this point is the dilatancy stress \(\sigma_{\text{cd}}\). The unstable development stage of rock fractures (CD segment): After entering this stage, the rock begins to transform from elastic deformation to plastic deformation, and the stress corresponding to the point where this stage begins is the yield stress \(\sigma_{\text{f}}\). In this stage, the rock fractures continue to develop until they are damaged. The post rock fracture stage (after point D): The post rock fracture stage refers to the state in which the rock is in a state after fracture has already occurred. In this stage, the physical properties and mechanical behavior of the rock will change significantly. After fracture, the integrity and continuity of the rock are damaged, and the bearing capacity is greatly reduced. Cracks further expand and interconnect with each other. At the same time, the permeability of the rock increases, making it easier for water and other substances to penetrate it. This stage holds great significance for geological engineering and geotechnical engineering because it directly affects the stability and safety of the engineering structure.