## 4 Experimental Verification
### Practicality Validation in the Strain Measurement of the CFRP Wing Skin  
The strain testing of aviation structures is very important for aircraft design and manufacture. Another strain measurement experiment was conducted on the aircraft wing skin made of CFRP laminates to verify the practicality of the proposed strain transfer model. The wing was fixed on a static steel frame as shown in Figure 12a. To simulate the deformation of the wing in flight, four loading positions were evenly distributed on the lower surface of the wing. The loading positions were connected to a tensile tester with a two-stage transmission mechanism. The force at each loading position is a quarter of the total loading force applied by the tensile tester. Thus, the deformation of the wing can be controlled by changing the loading force of the tensile tester. FBG sensors 1, 2, and 3 were respectively bonded to the installing points 1, 2, and 3 on the surface of the wing skin as shown in Figure 12b. The stacking sequences of the CFRP laminates at the installing points are same as the stacking sequences of the three specimens in Section 4.1, and 3 FBG sensors were respectively bonded to the specimens 1, 2, and 3 as temperature compensation sensors. The geometrical and mechanical parameters of the adhesive layer and optical fiber are shown in Table 1. The protective paint at the installing points was removed to improve the measurement accuracy. The wavelength of all the FBG sensors is 1550 nm, and the wavelength shifts were detected by interrogator SM-130. The loading force was set to 2000 N, 4000 N, 6000 N, 8000 N, and 10000 N. The measured strain obtained by FBG sensors 1, 2, and 3 was recorded. According to Section 4.1, the measured strain of FBG sensors 1, 2, and 3 was compensated based on the theoretical average strain transfer ratios 80.0%, 82.3%, and 84.9%, respectively. Figure 11: Relationship between the measured strain and the applied strain. Figure 12: Strain measurement experiment on the aircraft wing skin (**a**) Loading and measuring system; (**b**) Installing points of FBG sensors. To obtain the strain field of the wing skin under different load conditions, a finite element model was established in strict accordance with the wing structure. The 3D wing model is shown in Figure 13a. The wing skeleton and the wing skin are made of aluminum alloy and CFRP laminates, respectively. The length of the wing is 2.40 m. The width is 0.96 m to 0.48 m, and the thickness is 0.13 m to 0.07 m. The simulation was conducted with ANSYS 15.0 finite element analysis software. Hexahedral grids were generated for the model. The fixed constraint was applied to the junction surface between the wing and the static steel frame, and the loading force constraints were applied to the four loading positions of the wing. The strain field of the wing skin was generated as shown in Figure 13b, and the strain at the installing points 1, 2, and 3 was obtained correspondingly. The wavelength shift of FBG sensor 1, 2, or 3 is denoted by \(\Delta\lambda_{\text{B}}\), and the wavelength shift of corresponding temperature compensation sensor is represented by \(\Delta\lambda_{\text{T}}\). The measured strain can be obtained according to (3) and (4). Besides, the modified strain can be calculated based on the definition of the average strain transfer ratio as follows:  
\[\varepsilon_{\text{mi}}=\frac{\varepsilon_{ms}}{\overline{\beta}} \tag{26}\]  
where \(\varepsilon_{\text{ms}}\) is measured strain. \(?\beta\) represents theoretical average strain transfer ratios which are 80.0%, 82.3%, and 84.9% for FBG sensors 1, 2, and 3, respectively. The comparison among the measured strain, simulated strain and modified strain at installing points 1, 2, and 3 is shown in Figure 14. The strain of the wing skin decreases from the root part to the leading edge according to both the experimental results and the simulation results. The measured strain, simulated strain, and modified strain all ascend with the increase of the loading force.