## 3 Results and Discussion
### Surface Damage Characteristics
#### 3.2.2 Fissure Expansion and Fissure Evolution Processes  
The pattern of developing surface cracks in the sample can be analyzed by combining the stress-strain curve and the principal strain field. The horizontal displacement cloud graph clearly shows the crack propagation during the failure process of the sample. Sample S-20%-3 was used as an example for the analysis. Figure 5 shows the main strain cloud chart for the stress-strain curve in combination with sample destruction, and the corresponding time points (t1-t6) are marked on the stress-strain curve. Figure 4: Cloud chart showing principal strain and horizontal displacement of the sample during failure. In Figure 5, t1 = 0 represents the starting time of the compaction stage, corresponding to the strain field at 0% Fp, as a reference for subsequent DIC processing. In the compression phase, the curve appeared to be abrupt; with an increase in stress, the increase in pressure gradually decreased, indicating that the sample was compressed. At t2, the sample entered the elastic phase, where the stress level corresponded to 25% of the maximum stress, and the stress-stress curve was approximated as a straight line. At the bottom and end of the main stress field of the sample, there was a local zone of changing stress, at which time the deformation was very small. Subsequently, the sample continued to be under pressure, and microcracks developed steadily until the end of the elastic phase at time t4. At that moment, the stress is 7.53 MPa, reaching 75% of the peak stress. Furthermore, at time t4, there was a marked concentration of stress at both ends of the sample, as shown on the left, and the sample began to crack. When subjected to continuous loading, the crack expanded rapidly at both ends, and the contact surface of the coal sample and the slurry stretched rapidly along the back of the cluster, resulting in a rupture when the maximum bearing force was reached. During continued loading, the tip of the crack rapidly expanded and consolidated, extending along the contact surface between the coal and the slurry. When the rock body reaches its ultimate bearing capacity, it ruptures, causing a decrease in axial stress. With a continuous increase in stress, the cracks between the coal sample and the slurry contact surface were closed, the sample was again compacted, and the bearing capacity increased again. At t5, the peak stress of the sample was reached; although the cracks expanded to the upper and lower ends of the sample, their appearance suggests that the sample remained basically intact. Figure 6 shows the process of evolution between the level shift of the sample and the macrodestruction at six different loading levels, including 0%, 25%, 50%, 75%, and 100% of the failure load (Fp) and the post-peak stage. The horizontal shift is set from left to right and negative. During the phase below 75% peak stress, the horizontal displacement of sample S-20%-3 along the stress load direction gradually increases, and there is noticeable layering in the ladder, but the shift value gap is small. In this process, the small holes are compressed, but there are no noticeable cracks. At 75-100% peak stress, the cracks split from the upper end of the sample and develop rapidly along the direction of the maximum main stress, with the maximum shift region gradually changing from the top left corner to the bottom. The shift cloud chart shows the marked deformation area. After the peak of the destruction phase (Figure 6e), the cracks quickly passed through the sample, the damage  
Figure 5: Stress–strain curve for the specimen under 20% early damage. was more severe, and the lower left corner of the sample appeared as a small separate piece. The fissure width increased significantly. During the evolution of the displacement field, the variation in the difference (relative displacement) between the maximum and minimum global displacements at each time point can reflect the change in the fracture propagation speed in the sample [30].