## 4 Result and Discussion  
### Influence of Stacked Structure on Flexural Properties  
In this section, we discuss the result of the flexural experiment of two types of FMLs and the results of the simulation. These FMLs consisted of aluminum alloy sheets and unidirectional prepregs (as shown in Figure 2 and Table 2). The different modulus and strengths of aluminum alloy and carbon fiber-reinforced thermoplastic materials result in a distinct mechanical behavior. Figure 5a,b illustrate the flexural stress-strain curves of FMLs with 2/1 stacking configurations. It can be seen that there is a significant difference. The curves are split into three main stages: elastic deformation, plastic deformation of the aluminum layer, and fiber breakage. In the first stage, the flexural load increases linearly and the specimen exhibits elastic deformation. At this stage, both the hybrid layer and the aluminum alloys layer are in the elastic stage and the specimens recover completely when the load is released. With continued loading, the hybrid layer remains in the elastic deformation stage, while the aluminum layer undergoes plastic deformation. In the third stage, the hybrid layer reaches the strain limit, then the fiber breaks and finally the load is reduced. There are no obvious cracks or fractures found in 2/1 FMLs specimens during the bending process, indicating that the plastic deformation is mainly concentrated at the top and bottom of the aluminum alloy layer. However, the overall simulation curve is slightly higher than the test curve, which may be due to defects caused by the specimen-making process or excessive extrusion of the resin matrix in the molding. The patch specimen 2/1 FMLs, as shown in Figure 5b, exhibits a different flexural failure behavior compared to the standard 2/1 FMLs. The flexural process can be divided into three stages. The flexural strength of the hybrid reaches its peak, with the patch-type 2/1 FMLs showing superior flexural strength to the standard 2/1 FMLs. In the following stage, the stress decreases rapidly due to the damage to the hybrid layer. Unidirectional prepreg strips placed in the form of patches on both sides of the metal layer cause the upper hybrid layer to experience compression loads and the lower hybrid layer to experience tensile loads during the compression process. Because the thermoplastic hybrid has a lower ductility than the metal layer, brittle fracture of the hybrid layer may occur in the second stage. In the third stage, the curve tends to become stable, suggesting the aluminum alloy layer becomes the main load-carrying. It can be seen from the simulation of patch-type 2/1 FMLs that Linde and Hashin criteria can accurately predict the flexural strength and \begin{table}
\begin{tabular}{c c} \hline \hline
**Symbol** & **Value** \\ \hline \(E_{nn}=E_{ss}=E_{tt}\) & 100 GPa \\ \(t_{s}^{0}\) & 5.37 MPa \\ \(t_{s}^{0}\) & 36.62 MPa \\ \(t_{t}^{0}\) & 36.62 MPa \\ \(G_{n}^{C}\) & 0.23 kJ/m\({}^{2}\) \\ \(G_{S}^{C}\) & 0.48 kJ/m\({}^{2}\) \\ \(\kappa\) & 1.45 \\ \(T_{0}\) & \(10^{-4}\) m \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Material properties of cohesive layers. modulus, as well as the failure behavior of the hybrid. This indicates that these criteria are reliable for the prediction of mechanical properties and exploration of mechanical behavior for the current 2/1 FMLs structure. The stress-strain curve of 3/2 FMLs specimens in the flexural experiment is shown in Figure 5c. Upon increasing the load, the stress curve initially rises linearly with an accompanying gradual increase in the bearing capacity of the specimens. After the linear phase, the specimen's mechanical behavior is manifested as an overall elastic mechanical characteristic. As the load continues to increase, the slope of the curve decreases noticeably, and the speed of stress-increase slows down until the peak load is reached.