## 2 Experimental Investigation of Structural Evolution of Rock Specimens
### Experimental Results  
Five specimens are loaded in the experiments. Although the evolution details and forms of DLB of these specimens are different, the law of the structural control is consistent. In order to analyses the characteristics of structural evolution in detail, the result of one of them is shown as follows. Figure 2a shows the stress-strain curve of the rock specimen, and in the x-axis \(\mu\varepsilon\) is the micro-strain. The maximum shear strain fields (points 1, 2, P, and 3 on the stress-strain curve) and failure form at point 4 are shown in Figure 2b. At point 1, the rock is still in the stage of elastic deformation, and the deformation field is almost homogeneous. At point 2, the stress-strain curve becomes non-linear, and strain concentration areas appear in the lower and middle part of the rock specimen. Thereafter, the existing strain concentration areas continue to increase, and develop into two obvious DLBs, shaped as \"X\" at point P, which is the peak point of the stress-strain curve. In the process of stress adjustment, one DLB can develop into the dominant DLB, while others will slow down or even stop developing. In the deformation localization process, the unbalanced evolution of the DLBs can be explained as the competitive process of forming the DLB. However, the DLB to the left becomes the dominant DLB at point 3, as shown in Figure 2b. Finally, a macro-crack appears at the location of the dominant DLB, and the rock specimen fails at point 4. The above deformation process analysis reveals a considerable difference in the evolution of DLBs before and after the peak stress point (point P). It is worth noting that there may be more than one DLB in the early stage of formation. In the evolution process of DLBs, there is a very strong competition mechanism between them. The competition is positive feedback, that is, when one of the DLBs' development gains an advantage, the degree of deformation concentration will further strengthen. However, the development of other DLBs will slow down or even stop. This complex competition mechanism leads to the fact that not all DLB locations could show macro-cracks. Figure 1: Schematic of specimen and testing system: **(a)** Sketch of specimen: 1. specimen, 2. acoustic emission (AE) sensors, 3. area of interest; **(b)** schematic of testing system: 1. loading equipment, 2. specimen, 3. AE amplifier, 4. AE system, 5. computers, 6. charge-coupled device camera, 7. light sources. In addition to the deformation fields under different stress levels, different deformation indexes, including the normalized value of deformation field variance S, the dislocation displacements of dominant DLB, and the accumulative AE ringing counts during the loading process, were determined. The variance S of the maximum shear strain field was obtained using the deformation field data, which can be used to describe the non-uniform degree of deformation field [38]. The accumulative AE ringing counts, which can reflect the damage inside the rock specimen, were obtained from the AE signals [39]. Using the deformation field and the failure form of the rock specimen, the location of the dominant DLB can be determined. Two small areas perpendicular to the crack direction at different sides of the dominant DLB were selected, as shown in Figure 3a. Using the DIC method, the relative displacements \(\Delta u\) and \(\Delta v\) of the two center points (\(P_{1}\) and \(P_{2}\)) of the small areas in the u and v directions were calculated, respectively. The sum of components \(d_{1}\) and \(d_{2}\) of \(\Delta u\) and \(\Delta v\) along the direction of the dominant DLB were defined as the dislocation displacement [40], and the calculation principle is shown in Figure 3b. Further, the curves of the four deformation indexes are plotted in Figure 4.  
Figure 2: Response of rock specimen under uniaxial compression: (**a**) Stress–strain curve; (**b**) maximum shear strain fields under different stress levels and failure forms of the rock specimen.