# Effect of Holes on Dynamic Crack Propagation under Impact Loading
## 2 Experiment
### Experiment Results and Discussion  
In the experiment, specimens with nine patterns, as listed in Table 2, were employed, and SHPB impact tests were conducted with a constant impact speed of 8.0 m/s. To avoid the contingency factor being disturbed by random error, the data were filtered carefully based on the processing principle, and then analyzed. With the signal from strain gauges on the incident and transmitted bar, the loading curves of the bars could be obtained and modified utilizing Origin software, as shown in Figure 4. Despite the stress equilibrium in the bars, the dynamic force equilibrium was not satisfied in the specimens. It is acknowledged that the dynamic force equilibrium is critical in SHPB experiments, and the basic assumption of a quasi-static method where the stress/strain of specimen is uniformly distributed was pointed out for the purpose of decoupling the stress wave effect and strain rate effect. However, the present paper focused on crack propagation and the holes' effect with a numerical investigation, which is already beyond the scope of a quasi-static study. A hybrid experimental-numerical approach [27] was employed without assumption of the stress balance required. Figure 3: Non-metallic acoustic detector. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Material** & **C\({}_{d}\) (m/s)** & **C\({}_{s}\) (m/s)** & \(\rho\) (kg/m\({}^{3}\))** & **E\({}_{d}\) (GPa)** & **K (GPa)** & **v\({}_{d}\)** \\ \hline PMMA & 2382 & 1305 & 1187 & 5.61 & 4.35 & 0.28 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Material parameters of polymethyl methacrylate (PMMA). The nine types of crack fracture patterns are illustrated in Figure 5, according to the distances B and D in Table 2. It is evident that the main cracks in all the specimens propagated straight after initiation as a mode I crack. For the B12-D15 and B12-D30 specimens, the cracks were arrested in the holes' zones and two new branching cracks connected with the holes, while the remaining specimens showed that the main cracks could penetrate the holes' zone, leaving a typical straight line and stopping in the rear part of the specimens. It is noteworthy that the transverse cracks in the rear of the specimens might be shaped by the secondary impact. To verify the influence of the secondary impact, examples are given, with crack propagation gauge (CPG) data and crack paths displayed in Figure 6. As mentioned above, the tightly glued CPG on the specimen could be employed to detect the crack propagation process as the wires  
Figure 4: Dynamic stress curves. (**a**) Dynamic stress curves of the incident bar and transmitted bar. (**b**) Dynamic stress curves in the experiment. Figure 5: Crack fracture patterns of the specimens. (**a**) Crack pattern of the B12-D15 specimen. (**b**) Crack pattern of the B12-D30 specimen. (**c**) Crack pattern of the B12-D45 specimen. (**d**) Crack pattern of the B16-D15 specimen. (**e**) Crack pattern of the B16-D30 specimen. (**f**) Crack pattern of the B16-D45 specimen. (**g**) Crack pattern of the B20-D15 specimen. (**h**) Crack pattern of the B20-D30 specimen. (**i**) Crack pattern of the B20-D45 specimen. on the CPG could be fractured by the running crack, resulting in regular voltage signal changes. In Figure 6a, after the initial impact, the wires of the CPG were sequentially fractured by the running crack until the crack reached the 31 st wire. This process took 397.6 \(\upmu\)s, which is longer than the average impact stress wavelength, 360.1 \(\upmu\)s, as shown in Figure 4. Meanwhile, the running crack stopped at the 31 st wire, with the CPG voltage signal staying constant. However, the CPG voltage signal changed again after 642.6 \(\upmu\)s, as the 32 nd wire was fractured by the restarted crack. Figure 6b tells the same story. This phenomenon suggests that the secondary impact could offer energy to the stopped crack, making it restart. Moreover, the transverse crack paths in the rear of the specimens were induced by the secondary impact.