## 5 Experiment Results and Discussion
### Scenario 1: Loading Rate at 0.1 MPa/s  
#### 5.1.1 Results on X-Direction Strain Dynamics  
During compression, the dynamic strain change along the x-direction of sandstone is displayed in Figure 5. Figure 5a-d characterizes the compression profiles at the time points of t = 131, 139, 140.5, and 141 s, respectively. They correspond to the failure time at A, B, C, and complete failure. A slight crack can be seen from the right upper side of Figure 5a and the strain value at the crack is relatively large. The strain value at point A reaches a failure threshold value of 0.0044, so failure occurs at point A. The occurrence of failure at point A can be clearly seen from Figure 5a. At this time (the initial stage of compression), point B has not reached the failure value, so failure does not occur,  
Figure 4: Experiment set-up: (**a**) specimen before painting; (**b**) specimen after painting; (**c**) specimen failure; (**d**) measuring system. but as compression continues, the strain value at point B begins to increase slowly. When it reaches t = 139 s, the crack suddenly turns much larger with a clear indication of failure. The strain value grows rapidly, and failure occurs suddenly at point B, as shown in Figure 5b. Point C does not show any failure during the initial and middle stages of compression, with the corresponding strain value always fluctuating within a reasonable range. Nevertheless, during the later stage of compression, as shown in Figure 5c, the preceding failure trace further expands. A new curved failure segment with greater failure strain values appears at the upper left. Point C is right on this segment, and then failure occurs at point C. Compression continues to t = 141 s at which the whole sandstone test-piece fails, as shown in Figure 5d. Figure 6 shows the full profiles of strain dynamics at the points A, B, C, and D during the whole compression process, which is in accordance with the phenomenon seen from Figure 5. As seen from Figure 6, the process of sandstone failure is almost instantaneous without a substantial transition course, and therefore, it is a challenge to find the specific features of the failure process, not to mention the accurate prediction of the failure of sandstone. To understand the failure process of sandstone  
Figure 5: Strain dynamics along the x-direction (loading rate = 0.1 MPa/s): (**a**) t = 131 s; (**b**) t = 139 s; (**c**) t = 140.5 s; (**d**) t = 141 s.  
more delicately, this paper conducts more detailed investigations on different individual points to examine and visualize the corresponding failure processes. As mentioned before, these individual points were chosen from the four regions of the full-field in terms of the time sequence of failure occurrence. More clearly, four typical points were selected based on the preliminary results shown in Figure 5. As seen from Figure 5, points A, B, and C indicate the tip locations of the first three failure regions of the tested specimen at the three moments of image collection (t = 131, 139, and 140.5 s). Strain dynamics were analyzed in detail at these three points to identify the failure pattern of the rock specimen. Point D represents a typical location that does not reach failure during the whole failure process. This point was intentionally selected to be compared with the other three points to demonstrate strain pattern differences. These four points expect to represent the corresponding regions and moments of specimen failure. In general, for elastic materials, during the early period of failure, the strain-time curve shows linearity or near-linearity, with its slope usually low. Once the change in strain value appears nonlinear or shows an exponential growth with time, the material is most likely to be or has already been destructed. In this study, failure at point A occurs at about t = 131 s. From Figure 7, we can see that before t = 131 s, the strain value changes slightly, and shows a near-linear rise. Between t = 131 s and t = 131.5 s, the strain value undergoes a big change from 0.0044 to 0.011.