## 2 Impact Compression Tests of SHPB
### Results and Analysis  
All the samples remained intact. No crack was generated at an impact speed of 2 m/s. The damage degree of the samples with different hole sizes varied as the impact speed increased to 3.3 m/s and 4.2 m/s. The fracture evolution process is shown in Figure 4. To conveniently describe the crack initiation position around the hole, the angles were used to represent the different positions of the hole edge. Here, the hole center was set at the center of the circle. In addition, the angle increased along the anticlockwise direction from 0\({}^{\circ}\) on  
Figure 3: Schematic of incident stress wave.  
Figure 2: SHPB apparatus and monitoring system.  
the rightmost side of the hole edge, thereby representing the different positions of the hole edge (see Figure 3).  
Figure 4: Process of dynamic rupture around the hole (The length and width of the white line represent the crack development and the crack width, respectively).  
For intact samples, the cracks emerged at the end of the sample close to the transmitted bar under different impact velocities. This may have been caused by the reflected tensile waves caused by the impact of the sample on the transmitted bar. For a hole diameter of 8 mm and an impact velocity of 3.3 m/s, the cracks first initiated in the 90\({}^{\circ}\) and 270\({}^{\circ}\) directions of the hole edge, i.e., in the vertical direction. Then, these extended and converged in the reverse direction. This was accompanied by the formation of vertical macro-fractures. The crack evolution process was similar to that for an impact velocity of 3.3 m/s. However, a vertical crack appeared at the left end owing to the higher impact strength. This indicated higher damage. At the hole diameter of 16 mm and impact velocity of 3.3 m/s, the cracks began to emerge in the 0\({}^{\circ}\) and 180\({}^{\circ}\) directions of the hole edge and converged with the far-field cracks at both ends along the incident direction. Finally, a penetrating crack formed along the horizontal direction. As the velocity increased to 4.2 m/s, cracks appeared in the 90\({}^{\circ}\) and 270\({}^{\circ}\) directions and spread to both ends of the sample along the horizontal direction. This was accompanied by the spraying and falling of coal dust. When the sample with a hole diameter of 24 mm was impacted at a velocity of 3.3 m/s, cracks were almost generated around the hole approximately in the 0\({}^{\circ}\) and 180\({}^{\circ}\) directions at 250 \(\upmu\)s. Then, these developed in the 270\({}^{\circ}\) and 90\({}^{\circ}\) directions at 500 \(\upmu\)s. Finally, macro cracks formed in the vertical direction. This was accompanied by microcrack initiation in the horizontal direction. As the velocity increased to 4.2 m/s, cracks appeared at an angle of 0\({}^{\circ}\) at 250 \(\upmu\)s. The other cracks were generated at an angle of approximately 180\({}^{\circ}\) at 500 \(\upmu\)s and extended horizontally. At 750 \(\upmu\)s, the horizontal macro cracks penetrated and formed, and vertical microcracks began to be generated around the hole. Finally, the sample was destroyed completely. This was accompanied by horizontal and vertical fractures.  
To summarize, the positions of crack generation with different radii and impact velocities of the samples subjected to compressive stress waves were similar. Furthermore, these were concentrated in the 0\({}^{\circ}\), 180\({}^{\circ}\), 90\({}^{\circ}\), and 270\({}^{\circ}\) directions around the hole. This indicated that an incident stress wave would redistribute the stress at the edge of the hole and that a strong dynamic stress concentration appears in the vertical and horizontal directions.