## 4 Discussion
### Fracture Evolution  
The digital image correlation analysis method can reveal surface deformation and rupture, along with enabling an analysis of rock strain and the identification of the tension shear rupture type, to explain the mechanisms of crack initiation and extension evolution. This method obtained the axial strain for specimens with varied immersion durations, as depicted in Figure 10. To examine the alterations in strain and the development of fracture extension in immersed specimens during the compression process, we analyzed the strain from various compression stages while combining this with stress-strain curves. The chromaticity bar in the cloud diagram registers tension as positive and compression as negative. The deformation of the coal rock sample can be classified into four distinct stages: the compression-density stage, the linear elasticity stage, the yielding stage, and the post-peak softening stage. For the purpose of analysis and comparison, this study defines the stress at the junction point between the compressive and linear elastic phases as the compressive stress, the stress at the junction point between the linear elastic phase and the yielding phase as the initiation stress, the stress at the junction point between the  
Figure 10: (a) 1d; (b) 3d; (c) 5d; (d) 7d. Axial strain of long coal samples under different immersion durations. post-peak softening phase as the peak stress, and the last surface damage map captured by the high-speed DIC camera as complete rupture stress. This approach ensures consistency in the stress terminology. Figure 10a presents the axial strain cloud of the specimen after one day of immersion in water. The compaction pressure of the specimen is 2.60 MPa, resulting in the overall blue coloration of the strain cloud map. The strain exhibits negligible fluctuations around the value of zero, implying limited deformation at this stage. The initial stress required for cracking was 17.20 MPa, with a red area of strain present in the lower-left region of the strain map. The maximum strain observed was 0.5%. As the specimen reached its peak stress of 23.23 MPa, the red area expanded further, and the crack significantly grew, resulting in an increase in strain up to 0%. In the post-peak softening stage, the specimen incurred the maximum strain of 0.78% in the red strain area, which extended from the lower-left part to the upper-middle section. The maximum strain remained at 0.78%. The complete rupture stress amounted to 18.72 MPa. Consequently, the specimen sustained damage at this point, with cracks appearing and small coal rock fragments dislodged in the above area. The specimen initially experienced uniformly distributed strain during 1-day water immersion, followed by the gradual formation of a strain concentration area and the occurrence of damage. The final area of damage coincides with the strain concentration area, suggesting that the damage process of coal rock undergoes a transition from uniform to non-uniform deformation. Figure 10b-d display the axial strain for specimens submerged in water for 3, 5, and 7 days. The samples examined during the compaction stage are similar to those immersed in water for 1 day, exhibiting more uniform deformation. During the remainder of this stage, samples with immersion durations of 3, 5, and 7 days show no visible surface cracks and display a strain concentration area in the form of a band oriented vertically along the laminae. This band is believed to be associated with the direction of the internal joints. The critical stress of specimens with different immersion lengths reached various stages. Specifically, the specimen immersed in water for 3 days experienced compressive stress of 2.11 MPa with strain of 0.44%. The cracking stress reached 15.56 MPa with strain of 0.53%, while the peak stress was 28.04 MPa, and the strain was 0.95%. The cracking stress reached 15.56 MPa with strain of 0.53%, while the peak stress was 28.04 MPa, and the strain was 0.95%.