## 5 Results and Discussion
### Parametric Analysis
#### 5.3.1 Parametric Analysis of Vibration Properties  
The effects of the density and Young's modulus of the PVC foam on the mode of the composite sandwiched panels are indicated in Figure 7a,b, respectively. It was found that the mode frequency of the composite panels can be improved by either decreasing the foam density or increasing the Young's modulus. However, it is known that the material's  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Component** & **Density (kg/m\({}^{3}\))** & **E—Young’s Modulus (MPa)** \\ \hline Foam & 8, 56 (basis), 200 & \(5\times 10^{-3}\), 5, 50 (basis), 500 \\ Resin & 60, 600, 1200 (basis) & \(3.4\times 10^{-1}\), \(3.4\times 10^{2}\), \(3.4\times 10^{3}\) (basis), \\  & & \(3.4\times 10^{4}\), \(3.4\times 10^{5}\) \\ \hline \hline \end{tabular} The thickness of the carbon fiber cloth (mm)  
\begin{tabular}{c c} \hline \hline Fiber cloth & 0.2, 1 (basis), 2 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The parameter discussion of the corrugated sandwich panel.  
density is often contradictory to its Young's modulus, and hence it is difficult to find a foam with both low density and high Young's modulus in engineering applications.  
The effects of epoxy resin properties on the structural mode are plotted in Figure 7c,e. The density of resin had almost no effect on the modes of the composite panels (see Figure 7c) due to the low mass portion of the epoxy resin inside the panels. However, the high Young's modulus of resin can significantly increase the mode frequency of the composite panels because of the increase of their bending stiffness (Figure 7d). Based on a similar reason, a higher mode frequency could also be achieved by increasing the thickness of the surface resin layer, as shown in Figure 7e. In addition, as the thickness of the surface resin layer was extremely thin (about 0.2 mm), even if the thickness of the surface resin was doubled, the total weight of the corrugated panel did not increase significantly.  
The effects of the fiber volume fraction, represented by the fiber cloth thickness, on the mode of the corrugated panels are shown in Figure 7f. The increasing volume fraction of the fiber cloth decreased the mode frequencies of the corrugated panel. It is known that the natural frequency of the composite panel is determined by the mass and Young's modulus of its components. In the corrugated sandwich panels, owing to the low weight portion of resin as well as the low density of PVC foam, the fraction of fiber reinforcements is considered as a dominant role for the weight of the entire panel structure. Therefore, increasing the fiber fraction will result in an increase of the entire structural weight. However, this will not affect the bending stiffness of the panel, which is mainly determined by the surface resin layer according to the mode analysis. Consequently, the natural frequency of the corrugated panels is reduced by increasing the fiber volume fraction.  
Figure 7: Parametric studies on the effects of the related design parameters on the vibration properties of the CFCPs. (**a**) The effect of foam density. (**b**) The effect of foam’s Young’s modulus. (**c**) The effect of resin density. (**d**) The effect of resin’s Young’s modulus. (**e**) The effect of layer thickness of surface resin. (**f**) The effect of the fiber cloth thickness.  
In summary, the layer thickness of the surface resin is the most sensitive design factor to the vibration properties of the sandwich corrugated panel, and changing the layer thickness of surface resin is easy to operate in engineering.