## 4 Result and Discussion
### Influence of Stacked Structure on Flexural Properties
The sudden drop in load indicates that the tensile stress of the tensile part of the fiber has reached the failure stress, resulting in the failure of the hybrid material layer. Finally, with further strain increase, the stress gradually decreases, and the undamaged part of the fiber layer continues to bear the load. The stress-strain curve of the flexural experiment of the 3/2 FMLs patch specimen is shown in Figure 5d. As the strain increased, the stress of the specimen increased linearly until the first failure strain point was reached, resulting in a sudden decrease in stress. After this initial drop, the stress of the specimen then increased again without further stability or decline. It is speculated that the first stress drop is related to the fracture of the first layer of carbon fibers and matrix failure. Due to the brittle failure mode of the carbon fiber layer, compression load causes the first layer to be damaged first, thereby reducing stress and increasing energy dissipation. This then allows the hybrid laminate to withstand higher strain levels. This is similar to the yield phase of metals, where the stress continues to increase until the material is destroyed, even though the rate of increase of stress slows down after the yield stress is reached. After the first stress drop, the load is redistributed to the following layers, allowing the laminates to bear the load again, and the stress to begin rising. This process is repeated with each subsequent layer of carbon fibers, resulting in a gradual decline in stress until complete failure. From this it can be seen that the 3/2 FMLs patch specimen is in a progressive damage mode, with the damage expanding  
Figure 5: Flexural stress-strain curves of FMLs with different stack structures (**a**) 2/1 FMLs, (**b**) 3/2 FMLs patch, (**c**) 3/2 FMLs, (**d**) 3/2 FMLs patch. at the interface layers and energy dissipation, allowing the specimen to withstand greater stress. The simulation results of 3/2 FMLs and 3/2 FMLs patch structure specimens show that the Linde criterion and Hashin criterion are still effective in predicting the mechanical properties and behaviors of more complex structures. To validate the accuracy of the finite element simulation results, the bending modulus and bending strength of different laminated structures are compared and the results are shown in Figure 6. The experimental results and finite element results of the bending strength and bending modulus of FMLs with different layering structures show good agreement. It is observed that the material model adopted and the established finite element model can accurately simulate the bending performance of laminates. Furthermore, the finite element simulation results also demonstrated that the stack configuration has a substantial impact on the flexural property, and the trend was set by the test results with an error margin of 20%. It is also noticed that the patch-type specimens generally exhibit a higher modulus, whereas the 2/1 FMLs patch stack configuration displays an optimal overall mechanical property. Consequently, the validated finite element model is used to further analyze the stress distribution of each FML structure and the influence of the stack configuration on the flexural property of FMLs. In this paper, a finite element model is established. It consists of an aluminum alloy layer of 0.6 mm, two thermoplastic prepreg layers, and three cohesive interface layers for the 2/1 FMLs structure, along with two aluminum alloy layers of 0.3 mm, two thermoplastic prepreg layers, and two cohesive interface layers for the 2/1 FMLs patch structure. These two types of stack configurations ensure that the metal and fiber volume fractions are the same. The longitudinal stress distribution contours at 2/1 FMLs failure time are shown in Figure 7a. During the three-point bending experiment, the top aluminum layer of the specimen of this structure is under compressive stress, while the bottom aluminum layer is under tensile stress.