## 2 Configuration-Controllable Porous Metamaterial Model and Stress Analysis
### Stress Distribution Analysis  
The stress distribution of the CCPM under different configuration changes was studied using experimental and finite element methods. The CCPM specimen is designed and manufactured from soft silicone rubber SR-2125 (the same specimen as article [11]); the specific structure is shown in Figure 2. Its length is 301.4 mm, width is 180.8 mm, and thickness is 50.0 mm. The silicon rubber properties are assumed to be linearly elastic and isotropic before buckling occurs in the CCPM. The geometric parameters and material properties of the CCPM are shown in Table 1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material Parameters** & \multicolumn{2}{c}{**Geometric Parameters**} \\ \hline Elastic modulus/(MPa) & 0.870 & R/(mm) & 17.5 \\ Poisson’s ratio & 0.499 & D/(mm) & 7.462 \\ Density/(kg/m\({}^{3}\)) & 1230 & \(\theta\)/(deg) & 45 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties and geometric parameters of the CCPM. Figure 1: The schematic of the CCPM. Figure 2: The specimen of the CCPM. In order to study the distribution of the initial stress of the structure with different controlled configurations, a strain test experimental platform was established, as shown in Figure 3. The experiment was conducted at room temperature. According to the size of CCPM, the XTDIC measuring head at the testing site was installed and the suitable measurement range was adjusted. Compressive loading of the specimen was performed by a microcomputer-controlled electro-hydraulic servo pressure testing machine (YAW-600) with a loading rate of 0.5 mm/min up to a maximum loading displacement of 30 mm. The top of the experimental piece is fixed, and the lower part is slowly compressed. During the loading process of CCPM, the XTDIC measurement system synchronously collects images using two cameras (the resolution is 5 million pixels, and the frame rate is 75 fps). Then, it automatically calculates the three-dimensional coordinates of the specimen during all loading stages to obtain the geometric displacement results via digital image correlation. In addition, the finite element model of the structure is established with COMSOL, as shown in Figure 4. During the analysis process, the loading displacement is always limited (not exceeding 30 mm), and the deformation is relatively small. In this case, the structure will not buckle. Assuming that CCPM is thick enough, the analysis based on a plane strain model is sufficient [11]. Equivalent boundary conditions (marked in green) to the experiments are applied to the model. The top of the model is fixed, while the bottom is slowly compressed upwards with a loading displacement. The meshes of the computational domain are constructed using free trihedral meshing. Figure 4: The FEM model of the CCPM. The unit cells and Floquet boundaries are also illustrated in this figure. Figure 3: Experimental setup and procedure. The deformation and displacement of the CCPM specimen are obtained and compared with the experimental results. Figure 5 presents the deformation, displacement, and stress distribution of the structure when the axial compression displacements of the CCPM are 0 mm, 10 mm, 20 mm, and 30 mm, respectively (changing sequentially from left to right). We can see that the configuration of the CCPM changes gradually as the axial displacement increases, and the shape of the holes changes from round to oval. Under the same force, the experimental results of the axial deformation shown in Figure 5 agree with the finite element results shown in Figure 5 well. Both results are consistent in terms of axial displacement. We observed that with an increase in axial displacement, the stress of the CCPM increases gradually in Figure 5. The total stress gradually concentrated at the junction of each unit cell. And the maximum stress is concentrated near the round hole under the stress concentration.