## 2 Material and Experiment
### Experiment Results and Analysis
(**f**) The crack propagation velocity curves of HT-3 specimen. As mentioned above, the running cracks on specimens performed similarly; the deceleration phenomenon appeared when the crack drove into the two hole zone, and arrest occurred in the propagation process. Moreover, judging from Figure 6, the average velocity and the maximum velocity of running crack were increased with the enhancement of the loading rate. It is prominent that the phenomenon in which the speed of running crack decayed rapidly appeared earlier with a rise in loading rate. Then, a more in-depth analysis was conducted by considering the crack fracture appearances. For the HT-1 specimen, as Figure 5a shows, the three zones where crack fracture appearances present different characteristics are marked with A, B, C. Based on the conclusions above, the running crack propagated in Zone A and arrested in the set area between two holes. For the cracks in Zone B, although the initiation time of the crack can't be directly measured by gauges, the cracks should propagate along with the main crack as a way to release energy, which is also the reason why the crack in Zone A stopped. After the crack in Zone A stopped, the cracks in Zone B continuously propagated until the crack tip reached to the bottom of the specimen. Compared with the other specimens in the experiment, the appearance of the cracks in Zone C showed a disturbance, as the cracks were produced by the secondary impact, which can be ignored. For the HT-2 specimen, as Figure 5b shows, with a higher loading rate, the main crack in zone A is straighter than in HT-1, and no secondary crack appeared in Zone B. Instead a crack appeared in Zone D, which could be shaped by the secondary impact. This is because, if the crack in Zone D was produced by the initial effects, the crack would propagate directly to the main crack and the bridge arm of CPG would break down, causing the CPG system to stop working immediately, which would mean the voltage signal from the 16 th to 18 th wire couldn't be recorded. As the CPG voltage signal from 16 th to 18 th could be detected, there is crucial evidence to support the aforementioned assumption. Therefore, regarding to the crack in Zone D in the HT-3 specimen, the conclusion is the same. However, the crack in Zone E was shaped by the evolved main crack; when the arrested main crack between the holes restarted, the running crack tip drove to the hole and stopped. For the HT-3 specimen with loading rate 2000 GPa-s\({}^{-1}\), the performed crack appearance is very concise; the main crack went across the whole specimen, along with the symmetry axis. The secondary impact shaped the cracks in Zone C and D.  
Detailed information can be achieved by comparing the three crack patterns. For example, the main reason that the running crack changes direction and drives directly towards a hole when the main crack runs closer to the hole can be understood, as HT-1 in Figure 5a shows. Also, the contrast in the crack patterns suggests that the main crack would restart and propagate continually with a straight line arrestment if the energy is large enough (as shown in the HT-3). Or it may run into an area where the material is easier to fracture. It can be inferred that the empty hole has an attraction effect on the nearby running cracks.