## 5 Experiment Results and Discussion
### Scenario 2: Loading Rate at 0.3 MPa/s
#### 5.2.1 Results on X-Direction Strain Dynamics  
The same experiment procedure was followed. Four typical points A, B, C, and D were chosen to study the failure mode when the loading speed equals 0.3 MPa/s. Points A, B, and C represent the three point positions where failure occurs at different times, and D indicates the point location where no failure occurs during the whole loading process. Figure 12 shows the strain variations in the x direction throughout the compression process at the loading rate of 0.3 MPa/s. Figure 12a-d plots the strain profiles at t = 58, 71.5, 72.5, and 78 s, respectively, characterizing the failure time of three points of A, B, C, and the compression end event. In the early stage of the compression course, no failure occurs at any point, while local strain values slowly increase but still with linear elastic deformation. At t = 58 s, as shown in Figure 12a, the strain value at point A exceeds the linear elastic strain range extremely and failure occurs. From Figure 12a, it can be clearly seen that the strain at A point is relatively large. As the compression course proceeds further, the crack continues to expand from point A and a new crack occurs at the upper side, as shown in Figure 12b. Point B is right on the new crack, and at t = 71.5 s, the new crack turns significantly larger, and then failure occurs at point B. As shown in Figure 12b, the strain value at point B is relatively high. Quickly, in the following one  
Figure 11: Y-direction strain–time relationship of sandstone (loading rate = 0.1 MPa/s). Figure 10: Shear strain–time relationship of sandstone (loading rate = 0.1 MPa/s). second, there appears a fresh crack over the old fracture, and as shown in Figure 12c, point C, which is right on the new crack, fails at t = 72.5 s.  
Figure 13 shows the temporal dynamics of strain at points A, B, C, and D with the loading speed of 0.3 MPa/s. It can be learned from Figure 13 that the strain values at points A, B, and C increase over time, displaying a linear or near-linear trend in the early stage. Up to this point, the specimen is undergoing elastic change and the failure does not occur. As the loading process continues, all the three points A, B, and C may be subjected to sharp strain changes where the tested specimen generally goes beyond its elastic range and eventually fails. In order to further uncover the detailed information of failure processes at individual points, individualized deeper analyses were executed to each single point. Figure 12: Strain dynamics along the x-direction (loading rate = 0.3 MPa/s): (**a**) 58 s; (**b**) 71.5 s; (**c**) 72.5 s; (**d**) 78 s.  
In Figure 14, before t = 58 s, the variation of strain values at point A is relatively even and in a linear pattern relative to the time dimension without presenting sharp turns. However, starting from t = 58 s, the strain values change nonlinearly, and the strain values increase sharply after t = 60 s. A strain limit may present at t = 58 s. As a result, it can be inferred that the failure strain value for the sandstone sample may lie between 0.0048-0.0093. As seen from Figure 15, before t = 71.5 s, the strain values of point B vary little and display a near-linear elastic mode. At t = 71.5 s, the strain value of point B is 0.0048, while at t = 72 s, it reaches 0.0086. During this period, the strain values increase more dramatically than before; it is then determined that the failure strain value at B point lies within the range 0.0048-0.0086. Figure 14: X-direction strain–time relationship at point A (loading rate = 0.3 MPa/s). Figure 13: Strain–time profile along the x-direction (loading rate = 0.3 MPa/s). From Figure 16, before t = 72.5 s, the strain values of point C vary slightly, showing a near-linear elastic mode. From t = 72.5 s to t = 73 s, the strain values rise significantly, and it is likely that the failure begins at this point. The failure strain at point C is estimated to be within the range 0.0049-0.0071. The failure occurs successively at points A, B, and C during the loading process.