## 2 Dynamic Compression Test at the High Strain Rate
### The Special Measures of Split Hopkinson Pressure Bar (SHPB) Test Method and Verification  
The SHPB method is widely used in dynamic compression tests for metal materials to obtain the dynamic mechanical properties. However, there are still many difficulties when it is applied to the honeycomb structure, especially at the high strain rate, which are mentioned before. The following special measures are taken to overcome the difficulties. The actual impact area of the honeycomb structure is small compared to metallic materials due to the thin cell wall during the impact, which may cause the stress nonuniformity of the specimen and the divergence of the stress wave. The specimen of \(\Phi 32\) is applied to reduce the stress nonuniformity and the divergence of the stress wave as well as the diameter of the incident bar is 37 mm. The specimen contains nine complete cells in the direction of diameter. Try to maintain the integrity of cells at the edge of the specimen during the processing. The wave impedance of the honeycomb structure is lower than that of aluminum alloy bar which makes the weak signal in the transmission bar. The semiconductor strain gauge is applied on the transmission bar to acquire the nice waveform of the weak signal, which cannot be measured by the conventional resistance strain gauge. It is also more convenient than the adoption of the viscoelastic bar. The contact surface between the bar and the specimen is lubricated to reduce the friction between the end face of the specimen, the incident bar as well as the transmission bar. The loads between the impact end and the support end of the specimen may be different due to the divergent oscillation of incident wave, resulting in poor stress uniformity inside the specimen. Therefore, the brass wafer is pasted at the impacted end of the incident bar to eliminate the unbalanced stress caused by the divergence of the stress wave. The short specimen and long impact bar are employed to make the stress wave passing through at least 3-4 reflections in the specimen, realizing the homogenization of the stress in the specimen. The measurements based on the SHPB method are shown in Figure 3.  
The length of the impact bar is 0.6 m, and those of the incident and transmission bars are 2 m. The strain gauges are pasted at 0.85 m from the impact end of the incident bar and the middle of transmission bar respectively, to obtain the strain curves of the incident and transmission bars in the  
Figure 3: Split Hopkinson Pressure Bar (SHPB) test system with measurements. Figure 2: Coordinate system of aluminum honeycomb structure. loading process, and then the stress-strain relationship of the specimens can be obtained. The size of the specimen of \(\Phi 32\times 5\) mm is adopted in the dynamic compression tests. The measurements based on the SHPB method are verified by the direct impact tests of impact end and support end which are modified from the SHPB test. The stress responses of the impact end and the support end of the aluminum honeycomb structure are obtained, respectively, by the twice impact method, so as to verify the effectiveness of the measurements of SHPB test method. The direct impact tests of the support end are shown in Figure 4. The aluminum honeycomb structure specimen is contacted with the transmission bar in the SHPB test system, and the semiconductor strain gauge is pasted on the transmission bar. The distance between strain gauge and the impact end of the bar is 0.85 m. The incident bar is removed, and a long elastic impact bar is adopted to impact the specimen at a certain velocity directly. It is called test 1. The direct impact tests of the impact end are shown in Figure 5. The specimen is contacted with the end of the impact bar, then the specimen with the impact bar hit the transmission bar at a certain velocity. The strain is measured by the semiconductor strain gauge on the transmission bar, and then we can obtain the stress response through calculation. It is called test 2.