## 3 Test Results and Discussion
### Deformation and Cracking Process  
By observing the evolution of the strain field on the specimen surface, the strain localization and failure characteristics can be captured. Figures in Table 4 show that the strain localization zone can characterize the final crack formation area; therefore, the strain evolution process can be used to analyze the formation and propagation of cracks. The high-speed cameras of the DIC observation system are used to record the global deformation field during the static and dynamic loading stages. In this study, the strain evolution and crack propagation process are analyzed using specimens with different strain rates (D6 and D10). (1) Specim D6  
A typical stress-strain curve with five key stress points (a-e) under superimposed static and dynamic loads is given in Figure 11. The principal strain fields are compared and analyzed at different stress stages. The stress-strain curves are consistent and can be divided into four stages: compaction, elastic deformation, pre-peak yield and post-peak failure. Point \"a\" is the dividing point between static loading stage and dynamic loading stage, point \"b\" belongs to elastic deformation stage, point \"c\" belongs to pre-peak yield stage, point \"d\" corresponds to the peak stress point and point \"e\" belongs to post-peak yield stage. Figure 12 shows the evolution of the principal strain field of specimen D6 at five selected stress stages. We can summarize the evolving relationship between crack propagation process and stress condition by comparing the strain results and the corresponding stress stages. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Static Load/** & \multirow{2}{*}{**No.**} & **50\% of the UCS** & \multirow{2}{*}{**Failure Image**} & **70\% of the UCS** & \multirow{2}{*}{**Failure Image**} \\
**Strain Rate** & & **DIC Analysis** & & **No.** & **DIC Analysis** \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The failure patterns of rock specimens under superimposed static and dynamic loads. During the static loading stage (stress \(\leq\) 38.55 MPa, 50% of the SUCS), the principal strain field remains uniform, after which strain localization zones (Figure 12b) begin to appear in the upper right and lower left of the specimen, indicating the development of damage. As the applied stress increases, the strain concentration area in the upper right corner of the rock specimen (Figure 12c) is further expanded, and the principal strain gradient is much larger than that in other areas. Subsequently, the stress increases to the peak stress under superimposed static and dynamic loads, and the strain localization zone (Figure 12d) is fully formed. Finally, the cracks are created quickly after the peak stress (Figure 12e), and the strain concentration area is distributed along the crack position. Figure 13 illustrates the failure process of specimen D6. The crack propagation pattern is simple; two single cracks extend rapidly, and the specimen immediately loses its carrying capacity. Figure 11: The stress–strain curve and key stress points of specimen D6. Figure 12: The principal strain fields corresponding to point (**a–e**) under different loading stages in Figure 10.  
The deformation and crack propagation process of rock specimens can be obtained by the DIC technique. It can be found from the strain distribution that the damage accumulates gradually with increasing stress, and the principal strain distribution tends to be concentrated in a narrow area at peak stress point, indicating the localization of rock damage. After the peak stress, the cracks propagate rapidly from the strain localized area to the top of the specimen, as shown in Figure 12e. We observe that the identified crack corresponds to the path of the strain localization zone. It can be concluded from the entire loading process that the damage continues to accumulate and strain before the peak stress, while cracks rapidly extend and form after the peak stress.