## 3 Experimental Results and Analysis
### Full-Field Strain Evolution Analysis  
The generation, propagation, and stress concentration of microcracks are not easily visible to the naked eye but can be captured by DIC technology. By solving the full-field displacement and strain data obtained from DIC, the Lagrangian full-field strain information, as shown in Figure 8, can be derived. This provides a clear representation of the entire process of crack initiation, propagation, and through-going failure during the creep failure process of anisotropic coal rock. The figure clearly demonstrates the influence of bedding angle on the strain field evolution of the coal samples. The regions of concentrated displacement changes are typically located at the intersections between bedding planes, where cracks generally initiate and propagate along the bedding interfaces. Throughout the compressive strain process of the coal rock, the specimen remains in a compressed and densified state for a relatively long duration in the early stages. However, in the final few seconds before failure, as the stress approaches the peak value, and the strain  
Figure 7: Strain evolution patterns of coal rock at different creep stages: (**a**–**d**) Bedding angles of 0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\), respectively.  
exhibits a sharp change, leading to the formation of a crack strain band that eventually results in overall macro-scale failure.  
For the coal sample with a bedding angle of 0\({}^{\circ}\), as shown in Figure 8a, it can be observed that during the loading process, the coal sample remains in a long-term compressed and densified state. Stress concentration exists at the horizontal bedding interface, but  
Figure 8: Lagrangian strain field evolution contour maps of coal rock with different bedding angles: (**a–d**) Bedding angles of 0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\), respectively.  
no obvious crack initiation or propagation occurs. Just before failure, cracks suddenly initiate and propagate, leading to localized spalling of the coal sample, which results in brittle failure. For the coal sample with a bedding angle of 30\({}^{\circ}\), crack initiation typically concentrates at the junction points of the X-shaped shear strain damage zone. At these junctions, the stress concentration is most pronounced. After reaching the yield limit, local tensile stress is released, causing crack initiation perpendicular to the direction of maximum tensile stress. Often, after cracks initiate, spalling of the material occurs due to the reduced constraint between blocks at the edges of the coal rock sample. The energy release during crack propagation causes the blocks to \"bounce\" and spall off. The continued propagation of these cracks and their connection with the locally spalled regions contribute significantly to the overall macroscopic failure of the specimen. For the coal sample with a bedding angle of 60\({}^{\circ}\), the strength of the specimen is the lowest, and a shear failure surface, consistent with the bedding angle of 60\({}^{\circ}\), is clearly visible. This indicates that the coal sample with a 60\({}^{\circ}\) bedding angle fails along the structural weakness of the interface, where the coal rock is more likely to form tensile or tensile-shear type fractures at the bedding junction, eventually leading to single-shear plane failure or X-shaped conjugate shear failure. For the coal sample with a bedding angle of 90\({}^{\circ}\), the final macroscopic failure mode of the coal sample is longitudinal tensile splitting failure. During loading, longitudinal tensile cracks first initiate at the edges of the specimen and extend inward. After local penetration, the sample experiences localized spalling. As the longitudinal cracks gradually develop, the coal rock sample undergoes overall failure.