## 2 Experimental Materials and Methodology
### CT Scanning Points Selection and Scanning Procedure  
Unlike the sudden brittle failure observed in homogeneous rocks [4], natural sandstone usually undergoes progressive failure due to the existence of its anisotropic microstructure. Key scanning locations were strategically selected at distinct loading stages to monitor microstructural evolution. Group 1 specimens underwent CT scanning at three critical stress-strain curve inflection points: loading initiation (1-1), peak stress (1-2), and failure (1-3) (Figure 5a). The parameter \(k\) was defined as the slope of the linear elastic region in the stress-time curve, which quantifies the sandstone's resistance to elastic deformation under uniaxial compression. Considering the marked mesostructural disparities observed in Group 1, Group 2 specimens received 1-2 additional scans near each of the three key points identified in Group 1. Specific additions included point 2-2 (near loading initiation 2-1), dual points 2-3/2-5 (adjacent to peak 2-4), and point 2-6 (close to failure 2-7) (Figure 5b). Accounting for crack propagation, the fracture process of Group 3 specimens was segmented into five loading stages (compression I, elastic II, yield III, pre-failure IV, instability V) with six stress levels, informed by Group 1/2 stress-strain curves and CT imagery. The six stress levels aligned with six real-time CT scanning nodes (3-1 to 3-6) illustrated in Figure 5c [32]. Integrated CT real-time scanning and SEM experiments were conducted at critical transition points during Group 3's continuous uniaxial compression and AE monitoring.  
Figure 4: Crack characteristics derived from CT imaging: (**a**) Sandstone pores/cracks (orange) via threshold segmentation; (**b**) crack eigenvectors (\(\delta_{1}\), \(\delta_{2}\), \(\delta_{3}\)) and projection angles (\(\theta\) for axial, \(\varphi\) for vertical); (**c**) 3D pore-crack network model with equivalent spheres (isolated micropores, connected micropores, and throat microcracks).  
The experimental procedure for Group 3 of specimens is outlined as follows: (1) Baseline Scan: As demonstrated in Figure 3b, the initial CT scan captured the pristine microstructure of Group 3 samples (Point 3-1 in Figure 5c). (2) Loading and Scanning Cycle: Following the reset of the CT platform, the servo-controlled loading system and AE monitoring module were activated. The scanning parameters were dynamically optimized based on real-time stress-strain curves, AE activity, and specimen deformation. The initiation of CT scans was contingent upon predefined stress thresholds (see Points 3-2 to 3-5). (3) Failure Stage Scan: A final CT scan (Point 3-6) was obtained during the post-failure stabilization phase. For the final scanning point (3-6) of Group 3 sandstone specimens, CT scanning was conducted post-failure after complete specimen damage. At this stage, the sandstone no longer sustained axial load, as the specimen had reached its failure state. The CT scanning system captured the post-failure internal configuration of the damaged specimen. The implementation of AE-driven emergency shutdown was undertaken to facilitate the observation of crack propagation. The mitigation of scanning induced stress relaxation was achieved through the optimization of scan scheduling. The loading system was programmed to autonomously halt when the strain-hardening load fell below 50% of the peak load [54]. Integrated experimental workflows yielded time-series data, including stress-strain curves, AE activity records, and 4D-XCT images for the Group 3 samples.