## 2 Material and Experiment
### Experiment Results and Analysis
It was found that the crack path would stay in the middle of the specimen if the rock material were homogeneous, as Figure 5b,c showed, or the crack path might perform as in Figure 5a. The crucial phenomenon related to this experiment is the dramatic change in crack propagation direction at the moment that the running crack drives into the two hole zone. Figure 6 presents the voltage signal data and the derivative of the voltage, concerning the time recorded from CPG gauges on the specimens and curves of running crack velocity. As mentioned before, the time corresponding to the extreme value of the derivative of voltage is the moment when a fine wire of CPG fractured. Therefore, the average propagating velocity between adjacent wires could be calculated by constant distance 2.25 mm divided by fracture time. The average propagating velocity curves in green are plotted to show the significant trend of crack velocity. The specific points at which propagating speed changed significantly are chosen to divide the whole propagation process into different zones. In each zone, the average crack propagating velocity is given. For the HT-1 specimen with loading rate 1000 GPa-s\({}^{-1}\), shown in Figure 6b, when the running crack had grown to 25 mm, the crack propagation speed began to decrease. When the running crack grew to 35 mm, the crack decelerated rapidly, and the instantaneous position of the crack tip was 5 mm ahead of the connecting line of the two hollow holes. The maximal fracture time interval of the adjacent fine wires appeared between the 18 th and 19 th fine wire within 31.62 microseconds. The average crack propagation speed between the 18 th and 19 th wire was 63.69 m/s, which was the minimum speed of the running crack. For the HT-2 specimen with loading rate 1500 GPa-s\({}^{-1}\), as shown in Figure 6d, when the running crack reached 25 mm, the crack propagation speed began to decrease and decelerated rapidly without hesitation. Moreover, the maximal fracture time interval of adjacent fine wires appeared between the 17 th and 18 th fine wire within 87.22 microseconds. This time was already beyond the average fracture time intervals of the whole fine wires, as well as the theoretical maximal fracture time interval of 60 microseconds, given by [PERSON] et al. [17]. This time interval was considered as a standard to identify the time that the crack arrest phenomenon occurred if the fracture time interval of adjacent fine wires of CPG becomes longer than 60 microseconds. Also, the average crack propagation speed at 38 mm was 22.97 m/s, which could be further proof that crack arrest had occurred in this area. Figure 5: Crack propagation patterns of Improved Single Cleavage Semi-Circle Specim (ISCSC) specimens (**a**) The crack pattern of HT-1 specimen. (**b**) The crack pattern of HT-2 specimen. (**c**) The crack pattern of HT-3 specimen. After the fracture of the 18 th fine wire, the rest of the fine wires' voltage signal was steadily detected by CPG, which shows the running crack had stopped between the two holes until transverse cracks occurred. For the HT-3 specimen with loading rate 2000 GPa-s\({}^{-1}\), as shown in Figure 6f, the displacement at which the deceleration of the running crack occurred was 20 mm. After that, the velocity of the running crack reduced continually to reach the minimum speed, 57.14 m/s, and the velocity of the running crack had increased when the crack passed the set area between the two holes. The maximal fracture time interval of adjacent fine wires appeared between the 16 th and 17 th fine wire within 35.23 microseconds. Figure 6: Voltage signal and derivative, with respect to time recorded from the crack propagation gauge (CPG) and crack propagation velocity. (**a**) The CPG voltage signal of HT-1 specimen. (**b**) The crack propagation velocity curves of HT-1 specimen. (**c**) The CPG voltage signal of HT-2 specimen. (**d**) The crack propagation velocity curves of HT-2 specimen. (**e**) The CPG voltage signal of HT-3 specimen.