## 2 Dynamic Compression Test at the High Strain Rate
### The Special Measures of Split Hopkinson Pressure Bar (SHPB) Test Method and Verification
The pictures of the direct impact tests are shown in Figures 6 and 7 respectively. The strain-time curve of the transmission bar is obtained by the strain gauge measurement in the direct impact tests of both impact end and support end. The transmission bar is made of hard  
Figure 4: Stress test scheme of specimen support end (test 1). Figure 5: Stress test scheme of specimen impact end (test 2). Figure 6: The picture of support end test. Figure 7: The picture of impact end test. aluminum alloy, which causes elastic deformation during the impact. The stress of the transmission bar can be calculated as  
\[\sigma_{bar}(t)=E_{bar}\varepsilon_{bar}(t) \tag{1}\]  
The force on the contact surface between the honeycomb structure and the transmission bar can be calculated as  
Figure 3:0  
The stress of the honeycomb structure can be calculated as  
Figure 3:1  
where \(\sigma_{h}(t)\) is the stress of the honeycomb structure, \(s_{bar}\) is the sectional area of the transmission bar, \(s_{h}\) is the initial macroscopic sectional area of the honeycomb structure, \(E_{bar}\) is the elasticity modulus of the transmission bar, and \(\varepsilon_{bar}(t)\) is the strain-time curve of the transmission bar. It is noticed that the sectional area of the honeycomb structure \(s_{h}\) varies during the impact, so the stress of the honeycomb structure \(\sigma_{h}(t)\) is the nominal stress. The SHPB test with measurements is conducted in order to compare the stress response of the direct impact test and SHPB test. It's called test 3. The impact velocity at high strain rate is adopted in the tests. The impact velocities of three tests are 13.83 m/s, 14.41 m/s and 13.85 m/s respectively. The results of the tests are shown in Table 2, and the stress curve is shown in Figure 3:2  
There is a time delay about the stresses compared to each test, as shown in Figure 3:3 The time delay is caused by the distance between the impact end and the support end of the specimen. The time delay between the impact end and the support end is 0.02 ms. There is not only a time delay between each test, but also a stress value difference. The equilibrium stress factor \(R(\sigma)\) of the specimen is defined as the ratio of the difference of the maximum and minimum \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Test 1** & **Test 2** & **Test 3** \\ \hline Impact velocity (m/s) & 13.83 & 14.41 & 13.85 \\ Yield strength (MPa) & 2.62 & 2.73 & 2.61 \\ Yield time (ms) & 0.046 & 0.026 & 0.048 \\ Mean nominal plateau stress (MPa) & 1.616 & 1.682 & 1.615 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Impact test results of three tests. Figure 8: Stress–time curve of direct impact tests and SHPB test. mean nominal plateau stress to the average value. The equilibrium stress factor can measure the stress uniformity. The calculation formula is as follows  
\[R(\sigma)=\frac{\overline{\sigma}_{\max}-\overline{\sigma}_{\min}}{\overline{ \sigma}}\times 100\% \tag{4}\]  
where \(\overline{\sigma}_{\max}\) is the maximum mean nominal plateau stress of the three tests, \(\overline{\sigma}_{\min}\) is the minimum mean nominal plateau stress of the three tests, \(\overline{\sigma}\) is the average value of \(\overline{\sigma}_{\max}\) and \(\overline{\sigma}_{\min}\). In general, when \(R(\sigma)\)\(<\) 5%, it is considered the stress is uniform [17]. The test results show that the stress factor \(R(\sigma)\) of three tests is 4.06%, which meets the requirement of stress uniformity in the specimen. It is verified that the special measures of SHPB method can be used in the dynamic compression test of the aluminum honeycomb structures at the high strain rate. The dynamic compression tests of aluminum honeycomb structures at a strain rate of \(1\times 10^{3}\) s\({}^{-1}\), \(2\times 10^{3}\) s\({}^{-1}\) and \(5\times 10^{3}\) s\({}^{-1}\) are carried out by adjusting the impact velocity of the impact bar to obtain the different strain rates. Three repetitions are performed at each strain rate.