## 3 Results
### Recoverability Analysis: Polymer Fiber Network Enhances Elasticity  
To analyze the recoverability of the composite materials during multi-impact processes, the electronic universal material testing machine (Changchun Testing Institute; CSS-44100) is used for quasi-static compression mechanical tests. The test results are shown in Figure 8, and the thickness of the samples before and after loading is listed in Table 3. The mass of the aluminum structure (including the sensor) is 56 g, and the size is \(\Phi 28\times 20.5\) mm. By considering that the peak acceleration without any buffering material is 23,766 g, the resulting peak stress transferred from the bottom of the test device to the aluminum structure is 12.81 MPa. Hence, the peak stress applied to the test material is 12.81 MPa (the loss in the delivery process is not considered). 12.81 MPa is chosen as the upper limit in the quasi-static compression mechanical tests, and the strain corresponding to 12.81 MPa is used as a reference to represent the characteristics of the test materials in the quasi-static compression process. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Before** & **After First** & **After Second** & **After Third** & **After Fourth** \\  & **Loading (mm)** & **Loading (mm)** & **Loading (mm)** & **Loading (mm)** & **Loading (mm)** \\ \hline M0 (0\%) & 2.08 & 1.71 & 1.57 & 1.44 & 1.41 \\ M1 (5\%) & 1.94 & 1.7 & 1.6 & 1.57 & 1.51 \\ M2 (10\%) & 1.92 & 1.72 & 1.64 & 1.53 & 1.49 \\ M3 (15\%) & 1.95 & 1.8 & 1.71 & 1.67 & 1.64 \\ M4 (20\%) & 1.98 & 1.9 & 1.83 & 1.8 & 1.76 \\ M5 (30\%) & 2.05 & 2.01 & 2.01 & 1.98 & 1.96 \\ M6 (40\%) & 2.06 & 2.03 & 2.02 & 2.02 & 2.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The thickness of the samples before and after loading. Figure 7: (**a**) Buffering coefficient of M2 (10%), foamed copper and foamed nickel at 15 tap position during four times of impact process; (**b**) Impact peaks corresponding to different tap positions during fourth impact process. Figure 8a-c show the stress-strain curves of M0 (0%), M2 (10%), and M6 (40%) of the loading experiments consisting of four steps, respectively. According to Figure 8a, with increasing number of loading times, the strain decreases gradually under equal stress. It means that compressibility of the same sample under the same stress in multiple loadings decreases, thereby indicating that the material experiences a plastic deformation during the loading process. It is in accord with the thickness change in Table 3 that M0 (0%) has undergone plastic deformation. As a result, the energy absorption capacity of the plastic deformation decreases gradually. According to Figure 8b,c, the strain decreases with a different attenuation ratio under the same stress, with increasing mass ratio of the EPDM rubber. Clearly, the EPDM rubber improves the elasticity and recovery characteristics of the material, which is consistent with the thickness change in Table 3. To characterize the recovery performance of the test materials in a multi-impact process, the recovery coefficient \(\beta\) is defined:  
\[\beta=\sqrt[3]{\frac{\varepsilon_{2}}{\varepsilon_{1}}.\frac{\varepsilon_{3 }}{\varepsilon_{1}}.\frac{\varepsilon_{4}}{\varepsilon_{1}}} \tag{3}\]  
where \(\varepsilon\) is the strain corresponding to 12.81 MPa stress and \(\varepsilon_{i}/\varepsilon_{1}\) the recovery rate of the strain of the \(i\)th load relative to the strain of the first load. The larger the recovery coefficient \(\beta\), the better the restorability of the material. Figure 8: (**a–c**) Stress–strain curves of M0 (0%), M2 (10%), and M6 (40%) during four loading steps, respectively; (**d**) Relationship between recovery rate of multiple loadings and mass ratio of EPDM rubber. Figure 8d presents the relationship between the recovery coefficient of the multiple loadings and the EPDM rubber proportion.