## 3 Experimental Results and Analysis
### DIC Premonitory Information  
Figure 9 depicts the principal strain cloud images obtained by the DIC method during the formation, expansion, and connection of macro-cracks on the surface of the red sandstone specimen with a prefabricated 30\({}^{\circ}\) crack. From the beginning of the experiment to 497 s, no obvious cracks formed on the surface of the specimen, the surface strain was dispersed locally, micro-deformation was evenly distributed on the surface, and obvious strain concentration appeared at both ends of the prefabricated crack. At 499 s, the strain concentration at the two ends of the prefabricated crack became more noticeable, and the strain concentration area at the right end expanded to form a large strain zone. At 500 s, the overall local strain dispersion weakened, and the strain was concentrated around the prefabricated crack. As the compression proceeded, the surface of the specimen fractured along the right end of the prefabricated crack. The specimen suffered macroscopic failure with the appearance of a macro-crack that was still developing and expanding upward. At 505 s, this macro-crack continued to expand along the right end of the prefabricated crack, and a new macro-crack emerged at the left end of the prefabricated crack and developed along the strain concentration area. The speckle image at 535 s is similar to that at 519 s except for the local strain dispersion in it. At 540 s, the macro-cracks emerging at the two ends of the prefabricated crack continued to expand and develop. The strain field was again concentrated at the two ends of the prefabricated crack, and the smaller dispersed local strain weakened again compared with that at 535 s. The strain fields that spread along the left and right ends of the prefabricated crack to the lower left and upper right corners of the specimen basically formed an outstretched trend, and finally, a \"\(x\)\"-shaped fracture occurred. Figure 10 shows the principal strain obtained by the DIC method during the formation, expansion, and connection of macro-cracks on the surface of the red sandstone specimen with a prefabricated 60\({}^{\circ}\) crack. The failure spread along the upper right end of the prefabricated crack to the strain concentration areas at the upper right and lower right corners of the specimen, respectively, and the overall strain field distribution was x-shaped. According to the final failure pattern of the two specimens, six points with high correlation between the red sandstone's surface and the cracking location (i.e., Points a-f) were selected for analysis of the loading process (Figure 11 and Table 1). Figure 9: Variation in the principal strain on the surface of the specimen with a prefabricated \(30^{\circ}\) crack. Figure 10: Variation in the principal strain on the surface of the specimen with a prefabricated \(60^{\circ}\) crack. In Figure 11a, for the specimen with a 30\({}^{\circ}\) prefabricated crack, the principal strain curve at Point a rose more sharply from 501-510 s as compared with those of the other monitoring points, which is attributed to the emergence of a macro-crack at the corresponding time, according to the digital images and principal strain cloud images in Figure 9. From 501-543 s, this macro-crack emerged at Point a and then developed and expanded along the a-c-e path into a large penetrating crack. In Figure 11b, for the specimen with the 60\({}^{\circ}\) prefabricated crack, the principal strain changed slightly at all the monitoring points before 530 s. At 531 s, the strains at different monitoring points began to present different variation characteristics. Specifically, at Point b, a crack emerged at 532 s and expanded rapidly afterward. At 537 s, the deformation no longer belonged to microscopic deformation, and the curve at Point b ended.