## 2 Materials and Methods
### Fabrication of Fiber Metal Laminates  
CARALLs were produced through a series of technological processes. To make the thermoplastic carbon fiber prepregs and aluminum alloys sheets combine to form FMLs, thermoplastic adhesive films were selected to assemble the two. The process was time-saving and high-efficiency without long-time curing of epoxy-based adhesives. The fabrication process of CARALLs included two stages. First, two layers of unidirectional thermoplastic prepreg were pressed into thin laminate (XLB-350 \(\times\) 350, Qingdao Jinjiuzhou Rubber Machinery Co., LTD., Qingdao, China) along the direction of 0\({}^{\circ}\). The forming temperature of PA6/CF was 250 \({}^{\circ}\)C with a compression pressure of 5 MPa. The thermoplastic adhesive films were inserted between the aluminum sheet and the PA6/CF sheet.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Symbol** & **Property** & **Value** \\ \hline \(\rho\) & Density & 2700 kg/m\({}^{3}\) \\ \(E\) & Elastic modulus & 71.6 GPa \\ \(\theta\) & Possion’s ratio & 0.33 \\ \(\sigma_{\rm s}\) & Yield strength & 252 MPa \\ \(\sigma_{b}\) & Tensile Strength & 374 MPa \\ \(\varepsilon_{f}\) & Fracture strain & 0.18 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material properties of 6061-T6 aluminum alloy.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Symbol** & **Property** & **Value** \\ \hline \(\rho\) & Density & 1000 kg/m\({}^{3}\) \\ \(E_{11}\) & Longitudinal modulus & 115 GPa \\ \(E_{22}\) & Transverse modulus & 70 GPa \\ \(E_{33}\) & Elastic modulus & 70 GPa \\ \(\sigma_{12},\sigma_{13}\) & Possion’s ratio & 0.33 \\ \(\sigma_{23}\) & Possion’s ratio & 0.45 \\ \(\sigma_{12},\sigma_{13}\) & Shear modulus & 3700 MPa \\ \(G_{23}\) & Shear modulus & 2000 MPa \\ \(X_{t}\) & Longitudinal tensile strength & 2107 MPa \\ \(X_{c}\) & Longitudinal compressive & 814 MPa \\  & strength & \\ \(Y_{t}\) & Transverse tensile strength & 35 MPa \\ \(Y_{\rm C}\) & Transverse compressive & 139.8 MPa \\  & strength & \\ \(S\) & Shear strength & 34.7 MPa \\ \(G_{m}\) & Matrix fracture energy & 12.5 kJ/m\({}^{2}\) \\ \(G_{f}\) & Fiber fracture energy & 1 kJ/m\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Material properties of T700/PA carbon fiber unidirectional ply.  
Then, the aluminum alloy sheet and PA6/CF were assembled with adhesive films by hot pressing at 180 \({}^{\circ}\)C for 5 min. After cooling to a certain temperature, the formed fiber metal laminates (120 \(\times\) 240 mm) were removed from the frame mold and cut into standard flexural specimens using CNC engraving. The manufacturing process is shown in Figure 1.  
To understand the effect of stacking configurations on the flexural behavior of fiber metal laminates, two types of fiber metal laminates with different stacking configurations were designed. For type FMLs, the aluminum alloy sheets are set on the surface layer (top and bottom) and the fiber-reinforced thermoplastic laminates were placed between the aluminum alloy sheets. In contrast to the Type FMLs patch laminated structure, the hybrid laminates were on the external layer. The stack configuration details are shown in Figure 2. Furthermore, the influence of fiber volume fraction on the mechanical properties of thermoplastic fiber metal laminates was studied by designing 2/1 and 3/2 stacking structures. The stack details of sequences were shown in Table 3, and the code method was defined.