## 4 Results and Analysis of Impact Tests
### Failure Features of Samples under Impact Loading  
In the tests, the industrial camera was employed to capture the progressive instability process of the samples under impact loading in real time. According to the captured photos, the dynamic development process of the internal cracks and failure states of the specimens at different loading times were illustrated in Table 5. In Table 5, the marked blue numbers mean the crack types, while the superscript letters on the numbers represent the sequence in which cracks of the same kind occur, and the marked cyan texts denote the acting time. During impact loading, three kinds of cracks appeared in the specimen, that is, tensile cracks (1\({}^{\text{a}}\)-1\({}^{\text{d}}\)), shear cracks (2\({}^{\text{a}}\)-2\({}^{\text{e}}\)) and spalling cracks (2\({}^{\text{a}}\)-2\({}^{\text{b}}\), as shown by the arrow). As can be seen in Table 5, the dynamic failure process of the intact specimen G1-B under impact loads is clearly displayed. As the loading time increases, a tensile crack 1\({}^{\text{a}}\) on the top location propagating along the horizontal direction and a shear crack 1b initiated \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample No.** & **Moment 1** & **Moment 2** & **Moment 3** & **Moment 4** & **Moment 5** \\ \hline G1-B & & & & & \\ G2-C & & & & & \\ G3-C & & & & & \\ G4-C & & & & & \\ G5-B & & & & & \\ G6-A & & & & & \\ G7-A & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Dynamic failure evolution of samples subjected to impact loads. from the lower-right corner of the specimen are found at the time of 2172 \(\upmu\)s. When the loading time reaches 2200 \(\upmu\)s, the other two tensile cracks 1\({}^{\text{b}}\) and 1\({}^{\text{c}}\) occur at the right and left ends, respectively, and propagate towards the other end. At the loading time of 2269 \(\upmu\)s, the fourth tensile crack 1d is emanated from the left end and the second shear crack 1b emerges in the middle of the specimen. At this moment, it is observed that the tensile cracks 1a and 1b have penetrated the sample. Eventually, the tensile crack1c also propagates from the left end of the specimen to the right end, but the shear cracks 2\({}^{\text{a}}\) and 2\({}^{\text{b}}\) and the tensile crack 1d do not penetrate the specimen when the failure occurs. This indicates that they do not play a leading role in the failure of the sample. Apparently, the failure of the intact specimen is dominated by the three tensile cracks 1\({}^{\text{a}}\)-1\({}^{\text{c}}\) propagating horizontally, that is, the failure type can be classified as tensile failure. For the specimen with one single circular hole, it is observed that rock debris (spalling crack 3) eject from the top and bottom sidewalls of the hole at the beginning of loading. Afterward, three shear cracks 2\({}^{\text{a}}\)-2\({}^{\text{c}}\) on the diagonals of the specimen occur when the loading time is 2039 \(\upmu\)s. With the rise in the loading time, the shear cracks propagate diagonally towards the hole. At the moment of 2066 \(\upmu\)s, another two shear cracks 2\({}^{\text{d}}\) and 2\({}^{\text{e}}\) emerge at the lower-right corner of the specimen. When the loading time 2146 \(\upmu\)s is approached, the four shear cracks (2\({}^{\text{a}}\), 2\({}^{\text{b}}\), 2\({}^{\text{c}}\), 2\({}^{\text{e}}\)) are coalesced with the hole, leading to the failure of the specimen. However, the shear crack 2\({}^{\text{d}}\) coalesces with the shear crack 2\({}^{\text{e}}\), but it does not propagate to the right end. To conclude, the failure mode of the specimen G2-C can be attributed to shear failure. In respect of the specimen G3-C, we can find that the spalling cracks occur violently at the initial moment of loading. At the loading time of 2478 \(\upmu\)s, a near-horizontal tensile crack 1a develops between the two holes. At the same time, three shear cracks 2\({}^{\text{a}}\)-2\({}^{\text{c}}\) initiate and grow along the specimen diagonals.