## 3 Results
### Energy Absorption Analysis: Small-Scale Pores Contribute Most to the Buffering Ability
With the addition of an excessive amount of EPDM rubber, the large-scale pore volume (size \(>\) 1 \(\mu\)m) of M6 (40%) decreases greatly, whereas the small-scale pore volume (size \(<\) 1 \(\mu\)m) decreases slightly compared with that of M4 (20%). According to the previously presented analysis, the pores (size \(<\) 1 \(\mu\)m) are preferentially filled. Based on these and the impact results, the small-scale pore volume (size \(<\) 1 \(\mu\)m) has a great influence on the buffering coefficient. The relationship between the small-scale pore volume (size \(<\) 1 \(\mu\)m) and buffering rate is shown in Figure 11d; the energy absorption characteristics of the samples become deteriorated with decreasing small-scale pore volume (size \(<\) 1 \(\mu\)m). When a shock overload propagates through the material in the form of a stress wave, the porous material attenuates principally the stress wave owing to friction, vibration, and irregular reflections at the abundant internal material [36; 37]. Based on the specific material structure and different application environments, various mechanisms have influences of different degrees. When the stress wave propagates through the internal structure of the composite material, it causes the vibration of the activated carbon particles and a mutual friction between the pore wall and inner air. In addition, the stress wave bounces continuously off the pore surface and causes an elastic collision when reflected at the pore wall of the porous material. A stress wave with a relatively high energy may even cause an inelastic collision owing to its high amplitude, thereby causing a plastic deformation and a relatively high energy loss. The kinetic energy is converted into other forms, such as deformation energy and heat energy. Thus, the pore surface area is an important influencing factor. Considering the large and small-scale pores in equal volumes, it is assumed that the shape of the pores inside the porous material is spherical in the ideal state. Thus, the surface area \(S\) and volume \(V\) of the large-scale pore with radius \(R\) can be expressed as follows:  
\[\left\{ \begin{array}{l}V=\frac{4\pi R^{3}}{3}\\ S=4\pi R^{2}\end{array} \right.. \tag{12}\]  
By assuming that the volume of the \(n\) small-scale pores is the same as the volume of one large-scale pore, the volume \(V_{1}\) of the small-scale pore becomes:  
\[V_{1}=\frac{4\pi R^{3}}{3n}. \tag{13}\]  
The radius \(r\) of the small-scale pore is expressed as follows:  
\[r=\sqrt[3]{\frac{R^{3}}{n}} \tag{14}\]and the surface area \(S_{1}\) of the small-scale pore is expressed as follows:  
\[S_{1}=4\pi r^{2}=4\pi\Bigg{(}\sqrt[3]{\frac{1}{n}}\Bigg{)}^{2}R^{2}. \tag{15}\]  
The ratio of the sum of the surface areas of \(n\) small-scale pores to the surface areas of the larger pore is defined as follows:  
\[\tau=\frac{nS_{1}}{S}=n\Bigg{(}\sqrt[3]{\frac{1}{n}}\Bigg{)}^{2}=\frac{n}{ \Big{(}\sqrt[3]{n}\Big{)}^{2}}. \tag{16}\]  
Clearly, \(\tau\) is above 1 and increases with increasing \(n\). Therefore, the contact areas between the small-scale pores and activated carbon particles are larger than those of the large-scale pores. As a result, mutual friction is more likely between the small-scale pore wall and inner air. In addition, reflections at the pore wall of the porous material are more likely, and the small-scale pores absorb energy more easily. The hardness tests are carried out in this paper. Two kinds of shore durometers, LX-A and LX-D (Nanjing Suce Measuring Instruments Co., Ltd., Nanjing, China), are used for different hardness. The tests are conducted based on the shore durometer method. As the recommended measurement interval of shore durometers is 20-90 HA and 20-90 HD, LX-A is used to measure M0 (0%), M1 (5%), M2 (10%), while LX-A is used to measure M3 (15%), M4 (20%), M5 (30%), M6 (40%). As shown in Table 4, with the increasing EPDM proportion, the hardness of the composite materials increases. It is consistent with the reduction of the porosity.