## 3 Results
### Results of Impact Experiments: Simultaneous High Energy Absorption and High Recoverability
When the EPDM rubber proportion is over 10%, the buffering coefficients of M3 (15%), M4 (20%), M5 (30%), and M6 (40%) are 21.91%, 20.88%, 20.14%, and 15.85% in the first impact process, respectively. However, by the time of the fourth impact, the buffering coefficients are 17.02%, 16.72%, 16.47%, and 15.4%, respectively. Thus, the buffering coefficients decrease by only 4.89%, 4.16%, 3.67%, and 0.45%, respectively. According to Region 2 in Figure 5d, the attenuation coefficients \(\alpha_{4}^{1}\) of M3 (15%), M4 (20%), M5 (30%), and M6 (40%) are 22.3%, 19.9%, 18.2%, and 2.8%, respectively. Evidently, with increasing EPDM rubber proportion (over 10%), the attenuation coefficients of the test materials continue to decrease, and the buffering characteristics of the multi-impact improve. However, because the buffering coefficients of M3 (15%), M4 (20%), M5 (30%), and M6 (40%) in the first impact process decrease evidently compared to that of M2 (10%), their energy-absorbing characteristics (\(N\)) during the multi-impact processes remain weaker than that of M2 (10%), in spite of the lower attenuation coefficient. In summary, when the mass ratio of the EPDM rubber is below 10%, the buffering coefficient of the single impact process decreases slightly, whereas the attenuation coefficient of the multi-impact process decreases evidently with increasing EPDM content. When the EPDM rubber proportion is over 10%, the buffering coefficient of the material encountering the single impact decreases clearly, whereas the attenuation coefficient of the material in the multi-impact process decreases slightly. Hence, the energy absorption characteristics of the composite material with 10% EPDM are best during a multi-impact process. To highlight the advantages of the composite material M2 (10%), the polymer (EPDM rubber) and traditional porous materials (foamed copper and foamed nickel) are investigated in comparative tests. EPDM rubber is a typical hyper-elastic material, which can produce the strain far exceeding the elastic limit strain under the action of external forces, and of which the train can be restored to the original \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(N_{1}\) & \(N_{4}\) & \(\alpha_{4}^{1}\) \\ \hline M0 & 31.01\% & 10.78\% & 65.2\% \\ M1 & 30.79\% & 15.96\% & 48.2\% \\ M2 & 29.86\% & 19.30\% & 35.4\% \\ M3 & 21.91\% & 17.02\% & 22.3\% \\ M4 & 20.88\% & 16.72\% & 19.9\% \\ M5 & 20.14\% & 16.47\% & 18.2\% \\ M6 & 15.85\% & 15.4\% & 2.8\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Buffering coefficients and attenuation coefficients of different samples. state during unloading [17]. More specifically, the energy absorption characteristics of M2 (10%) are compared with those of the EPDM rubber during the multi-impact process with overloads of different amplitudes, which are realized by changing the release height of the hammer rod. Four cases with 5, 10, 13, and 15 tap positions are conducted, respectively; the average peak of each height is determined through multiple tests. Figure 6a presents the impact peak values corresponding to different tap positions during the first impact process. In Figure 6a, the unbuffered peak values of the release heights for 5, 10, 13, and 15 tap positions are 8937 g, 12,250 g, 16,723 g, and 23,766 g, respectively. After buffered by M2 (10%), the peak values decrease to 7163 g, 8907 g, 11,815 g, and 16,670 g, and the buffering coefficients become 19.8%, 27.3%, 29.3%, and 29.8%, respectively. However, after buffered by the EPDM rubber, the peak values decrease to 7608 g, 10,360 g, 14,060 g, and 19,510 g, and the buffering coefficients become 14.8%, 15.4%, 15.9%, and 17.9%, respectively. Clearly, the buffering coefficient of the EPDM is lower than that of M2 (10%) during the first impact process of the same height. Figure 6b presents impact peak values corresponding to different tap positions during the fourth impact process. According to the figure, the buffering coefficient of M2 (10%) is still higher than that of the EPDM in the fourth impact process.