## 3 Results  
### Results of Impact Experiments: Simultaneous High Energy Absorption and High Recoverability  
Four curves of the acceleration signals collected during the experiments are shown in Figure 5a. The black curve represents the acceleration signal without buffering material, whereas the other three curves are the acceleration signals buffered by M0 (0%), M2 (10%), and M4 (20%), respectively. The other curves buffered by M1 (5%), M3 (15%), M5 (30%), and M6 (40%) are shown in Figure S2 (listed in Supporting Material). According to Figure 5a, the acceleration signals are similar to the typical signal of a penetration process [31]: the rising edge is realized in a short time, and the falling edge requires much time and exhibits a high number of oscillations. After it is buffered by the test material, the peak acceleration is evidently damped, and the overload becomes effectively weakened in the process. First, the buffered curve exhibits a certain time delay in the rising edge compared to the unbuffered curve. Second, the buffered curve exhibits smaller oscillations (amplitude) in the falling process, thereby indicating that the test materials have a better shaping effect on the waveform and can effectively reduce the oscillation signal of the aluminum structural part in the impact process. By comparing the three buffered curves, it can be seen that the buffered peaks of M2 (10%) and M0 (0%) are evidently lower than that of M4 (20%), whereas the buffered peaks of M2 (10%) and M0 (0%) are similar. Clearly, the EPDM proportion affects the energy absorption effect. The buffering coefficients of the test materials with different EPDM proportions are compared in Figure 5b; the figure is divided into Regions 1 and 2 with a limit of 10%. In general, with increasing EPDM proportion, the buffering coefficients of the test materials decrease gradually. Locally, in Region 1, the buffering coefficients of the test materials decrease slightly, and the buffering coefficient of M2 (10%) decreases by only 1.15%, compared with that of M0 (0%). Rega  
Figure 5: (**a**) Impact curves under different conditions recorded by acceleration sensors; (**b**) Buffering coefficients in single impact process; (**c**) Comparison of buffering coefficients in multi-impact process; (**d**) Comparison of attenuation coefficients in multi-impact process. EPDM proportion exceeds 10%, the buffering coefficients of the test materials decrease significantly with increasing EPDM content. Compared with that of M0 (0%), the buffering coefficients of M3 (15%), M4 (20%), M5 (30%), and M6 (40%) decrease by 9.1%, 10.13%, 10.87%, and 15.16%, respectively; the decreases are more obvious than those in Region 1. Moreover, multi-impact experiments are conducted for each kind of test materials, and the experimental results are shown in Figure 5c,d. Figure 5c compares the buffering coefficients of the test materials in the multi-impact process, and Figure 5d compares the attenuation coefficients. According to Figure 5c, in Region 1, the buffering coefficients of the test materials attenuate significantly with increasing number of impact processes for the same sample. In Table 2, the buffering coefficients of M0 (0%), M1 (5%), and M2 (10%) are 31.01%, 30.79%, and 29.86% during the first impact process, while they are 10.78%, 15.96%, and 19.30% during the fourth impact process, respectively. Hence, the buffering coefficients decrease by 20.23%, 14.83%, and 10.56%, respectively. According to Region 1 in Figure 5d, the attenuation coefficients \(\alpha_{4}^{1}\) of M0 (0%), M1 (5%), and M2 (10%) are 65.2%, 48.2%, and 35.4%, respectively. Thus, with increasing EPDM proportion, the attenuation coefficients of the test materials decrease significantly during the multi-impact process. In addition, the buffering characteristics of the multi-impact process improve clearly when the EPDM rubber proportion is less than 10%. Therefore, the buffering characteristics of M2 (10%) are better than those of M0 (0%) and M1 (5%) in the multi-impact process.