# Effect of Holes on Dynamic Crack Propagation under Impact Loading
## 2 Experiment
### Experiment Results and Discussion
This is because the crack paths were almost a straight line before the secondary impact occurred, as shown in Figure 6, when the action time was considered. During the secondary impact process, the bottom of the specimen restricted by the transmitted bar had a smaller deformation than the top where a large semi-circle hole existed. When the specimen underwent the secondary impact, the discordance of deformation restarted the already stopped crack and induced the transverse cracks. Nevertheless, the influence of the secondary impact can be ignored, as the secondary impact basically occurred 1000 \(\upmu\)s later after crack initiation, which means that the running crack already passed the holes' zone and finished the expected complete propagation process; furthermore, data collection was already completed before the secondary impact occurred. Notably, it is distinct that the crack propagation behavior changed drastically in the holes' zone, with branching cracks appearing when the distance (B = 12 mm) between the holes was small. The influence of the holes was very dependent on the distance B; as B increased to 16 mm, those fracture phenomena disappeared. To better understand the influence of the holes on the propagating crack, the CPG data were taken into consideration and the crack propagating velocity was obtained using the distance of 2 mm divided by the fracture time of the adjacent wires. The results are demonstrated in Figure 7.  
Figure 6: The voltage signal curves of crack propagation gauges (CPGs) and the crack paths of the specimens. (**a**) B20-D30 specimen. (**b**) B16-D45 specimen. Figure 7: Crack propagation velocity curves of different types of specimen. (**a**) Specimen with constant D = 15 mm. (**b**) Specimen with constant D = 30 mm. (**c**) Specimen with constant D = 45 mm. The velocity curves show when the distance D between the centers of the holes and the crack tip was constant at 15 mm, whereas the holes with distance B = 12 mm could fully arrest the running crack and potentially induce branching cracks connecting the two holes. When distance B was 16 mm, the holes could transitorily arrest the crack with a very low speed of 35.33 m/s, but no branching cracks emerged. When distance B was 20 mm, the two holes had a very limited effect on the crack path with a speed of 138.88 m/s, and its velocity curves changed inconspicuously. This indicates that the holes had a suppressing action on the moving crack as the two-hole spacing decreased. As distance D changed from 15 mm to 45 mm, the velocity between the holes became smaller; for example, the minimum propagation speeds between holes in B12-D35 and B16-D35 were 35.33 m/s and 20.20 m/s. However, the deceleration tendency was controlled by distance B. This suggests that the arrest function of the extending crack would be intensified by the enlargement of distance D. Nevertheless, the fact that the propagation speed became very small when D = 45 mm should be noted, as it was not only contributed by the holes' effect but also by the decrease in impact energy release. The crucial data of crack propagation are listed in Table 4. The data demonstrate that the effect of the empty holes on the propagation crack had a sensitive relationship with distance B and D. As distance B decreased, the propagation velocity decreased. When distance D increased, the holes' influence was magnified. For a deep understanding, a finite difference code was introduced to investigate the mechanism of the holes' effect and crack propagation behaviors.