## 3 Results and Discussions  
### Mechanical Characterization  
The material used in this work is a thin film flexible multilayer structural device with an EVA film between 160 and 320 \(\upmu\)m. The device is then encapsulated in between two thin EVA protective layers without pores. The multilayer material is sandwiched in between two 12 nm thin film metal electrodes (Figure 3). The film compositions are summarized in Table 1.  
Figure 3: Description of the composition of pseudo-piezoelectric cellular EVA films. The mechanical tests’ objective is to describe the macroscopic behavior of the three compositions studied in the static and dynamic case, then establish the relationship between this macroscopic and the microscopic material characteristics.  
Figure 2: Schematic illustration of the experimental setup for the energy harvesting measurements.  
The objective of the mechanical tests is to describe the macroscopic behavior of the three compositions studied in the static and dynamic case, then establish a relationship between these macroscopic and the microscopic material characteristics.  
The Young's modulus according to the percentage of porosity was evaluated. The results presented in Figure 4 show that Young's modulus of the material decreases in the presence of porosity, as a typical result obtained with porous materials. The test was performed on four rectangular samples with a crosshead speed of 10 mm/min at room temperature.  
Figure 4 reports the mechanical properties as a function of cellular morphology. It is clear that for EVA compositions, the values decrease linearly with pore content (about 29, 25 and 20 MPa for 0%, 37% and 65% of pores, respectively). For the same condition to each film, EVA 65% generates bigger strains than EVA 37% and EVA 0%. Raising the percentage of gas cells raises the quantity of mechanical energy absorbed by the materials, thus increasing the material flexibility. This flexibility is due to the shape and the gas cell number, which create sensitivity to the mechanical stress.  
The cyclic mechanical loading at various frequencies is measured using the dynamic mechanical analysis technique (DMA). For testing, a porous EVA is placed into the DMA's tensile grips. The sample is then subjected to a predefined maximum tensile stress, which elongates the material. The material is returned to zero force at the end of one cycle. All of the experiments were carried out at room temperature (25\({}^{\circ}\)C).  
Figure 5a,b represent the stress-strain hysteretic response of porous EVA when exposed to frequencies of 4 and 20 Hz, respectively. Each frequency was subjected to six separate tests. The average of the six experiments is shown in Figure 5. When porous piezoelectric EVA is exposed to cyclic mechanical stress, it develops hysteretic behavior. Microstructural changes such as cell morphology, matrix mechanical proprieties, and other potential permanent microstructural changes such as cracking cause the hysteretic response under the mentioned loading conditions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{3}{c}{**Before Extrusion**} & \multicolumn{3}{c}{**After Extrusion**} \\ \cline{2-6}  & **Ethylene (\%)** & **Vinyl Acetate (\%)** & **Hydrocerol (\%)** & **Poly(Ethylene-_co_-Vinyl** **Acetate) (\%)** & **Porosity (\%)** \\ \hline EVA 0 & 86 & 14 & 0 & 100 & 0 \\ EVA 1 & 85 & 14 & 1 & 63 & 37 \\ EVA 2 & 84 & 14 & 2 & 35 & 65 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Film compositions before and after extrusion operation.  
Figure 4: (a) Applied force as a function of machine displacement of EVA 0%, EVA 37% and EVA 65%. (b) Young’s modulus as a function of porosity percentage.