## 3 Test Results and Discussion
### Analysis of Cooperative Failure Characteristics in Jointed Rock Mass during the Instability Process  
#### 3.2.1 Characteristics of Accelerated Interaction Zones during the Instability Process  
In conventional uniaxial compression tests, the stress-strain curve for rock can be divided into four segments: the compaction phase (from a to b), elastic phase (from b to c), plastic phase (from c to d), and post-peak phase (from d to e), as illustrated in Figure 8.  
Figure 6: Axial stress vs. time curve of jointed rock mass.  
Figure 7: Failure forms of jointed rock masses.  
Based on prior research [19; 20], it is established that fracture interaction predominantly occurs during the strong non-linear deviation stage and the instability stage. The jointed rock mass exhibits varying physical field information, such as stress and strain, during these different stages. The emergence of cooperation during this phase is attributed to the interaction between strong and weak segments. Typically, before the applied force reaches a certain threshold, the various regional units within the jointed rock mass undergo spontaneous and irregular independent movements with minimal mutual influence. However, as it enters the strong non-linear deviation stage and the instability stage, the correlation between strong and weak segments becomes more pronounced than the independent movement of individual units. This correlation becomes the primary driving force behind cooperative motion among different regions. Consequently, interaction demonstrates zone-specific characteristics.  
The study integrates methodologies from prior research to classify the strong non-linear deviation stage, sub-instability stage, and instability stage. Figure 8 illustrates the stress-time curve derived from the stress-strain curve obtained via the loading test machine. Critical stress values at key moments are identified with letters.  
The LM (b-c) segment signifies the linear deformation stage, characterized by elastic deformation in the rock, with the elastic model remaining essentially constant during this phase.  
The MO (c-d) segment marks the departure from linearity, where the stress-strain curve exhibits an upward concave trend. This phase involves microcracking and plastic deformation in the rock, accompanied by changes in the stress field. The NO subsegment represents a significant departure from linearity, featuring a slower rate of stress increase.  
The OB segment represents the sub-instability stage, featuring a post-peak drop in the curve as internal rock fractures expand and connect. Within this stage, the OA subsegment represents a static sub-instability stage, with stress changes transitioning to a plateau, decline, or stable fluctuations in the stress-time curve. The AB subsegment represents a dynamic sub-instability stage, characterized by a rapid decline in the curve.  
The sub-instability stage OB lies between the stress peak point and the rapid instability point. During this stage, the stress-strain relationship may exhibit non-linear fluctuations. This phase is pivotal as jointed rock masses approach rapid instability, serving as a period of strain accumulation leading to strain release.