# Impact Testing of 3D Re-Entrant Honeycomb Polyamide Structure Using Split Hopkinson Pressure Bar
## 4 Results and Discussion  
### Waveform Analysis  
Figure 4 shows the incident, reflected and transmitted waveforms measured by oscilloscope in the specimen C2 test at the working condition of v = 18 m/s. Three incident and reflected waveforms can be measured from the strain gauges X\({}_{1}\), X\({}_{2}\) and X\({}_{3}\) on the incident bar. Three transmitted waveforms can be measured from the strain gauges X\({}_{4}\), X\({}_{5}\) and X\({}_{6}\) on the transmission bar. It can be found that the trend, the pulse width and the amplitude of these waveforms all follow similar rules and have good consistency, which can reflect the wave propagation process in the viscoelastic PMMA bar and the energy absorption characteristics of specimens. Based on the tests on the PMMA bar, the material constants of the PMMA bar ( \(E_{0}=3.19\) Gpa, \(\rho_{0}=1.176\) kg/\(m^{3}\)) can be obtained, and then the wave propagation velocity (\(C_{0}=\sqrt{E_{0}/\rho_{0}}=1647\) m/s) can be calculated as well. As the length of the striker bar is 40 cm, the pulse width (t = \(2L_{0}/C_{0}=0.486\) ms) can be obtained. It can be observed from Figure 4 that the pulse width of the incident wave is basically consistent with the calculated pulse width in an elastic state, which verifies the validity of the test. The incident bar receives the compression wave caused by the incident pulse under the impact of the striker bar. The compression wave reaches the strain gauges X\({}_{1}\), X\({}_{2}\) and X\({}_{3}\) in turn. It can be seen from Figure 4 that the incident wave gradually attenuates in the PMMA bar, i.e., the wave amplitude tends to decrease along the direction of wave propagation. It also can be seen that the wave attenuation is not proportional to the propagation distance. In addition, at the end of the incident pulse, the compression wave does not immediately unload to 0, but presents a \"creep\" characteristic with time growth, that is, so-called compression strain delay, as shown from ellipse A in Figure 4. It can be interpreted that the viscosity of the PMMA bar is the reason for the \"creep\" phenomenon without the influence  
Figure 4: The voltage signal measured by C2 (v = 18 m/s). of other external factors. Subsequently, the specimen deforms at high speed under the action of the incident wave, and propagates the reflected stretch wave to the incident bar and the transmitted compression wave to the transmission bar simultaneously. The reflected wave reaches back to the strain gauges X\({}_{3}\), X\({}_{2}\) and X\({}_{1}\) in turn. As can be seen from the partial enlarged subgraph, namely rectangle box C, when the reflected wave reaches the strain gauge for the first time, the compression \"creep\" caused by the incident wave is still incomplete, resulting in the superposition of the head of the reflected wave and the tail of the incident wave. From the time period between the arrival of the incident wave and the arrival of reflected wave, it can be found that it spends the longest time when the reflected wave reaches the position of strain gauge X\({}_{1}\). At this time, the compression deformation caused by the incident wave has basically completed; that is, the \"creep\" effect is minimal. However, the \"creep\" at the position of strain gauge X\({}_{3}\) is maximal, and the deformation hysteresis is not completed before it enters the stretch wave. After one pulse width, the tail of the reflected wave passes through the strain gauge. It can be observed from the B ellipse in Figure 4 that the reflected wave does not immediately unload to 0, but directly changes from the stretch wave to the compression wave. This phenomenon indicates that the incident bar is under pressure after the tail of the first reflected wave. It can be interpreted that the high-speed deformation of the specimen causes it to compress and absorb a lot of impact energy, including elastic and plastic potential energy. Then, the elastic potential energy is released and acts on the incident bar to place it in a compression state.