## 4 Experimental Verification  
### Accuracy Validation for the Proposed Strain Transfer Model  
A strain transfer experiment was conducted to verify the accuracy of the strain transfer model. The experiment was carried out on a nondestructive tensile system. The system contains three basic instruments, which are the strain applying unit SC300 (Zolix Instruments Co., Ltd., Beijing, China), the laser interferometer XL-80 (Renishaw plc, Wotton-under-Edge, UK) and the interrogator SM-130 (Micron Optics, Inc., Atlanta, USA) as shown in Figure 9a. The strain applying unit SC300 is a material stretching instrument which can cause the deformation of CFRP laminates. The strain applying unit consists of a vibration-isolation platform, an aerostatic guideway, an electric displacement platform and two fixtures, as shown in Figure 9b. The aerostatic guideway is installed on the vibration-isolation platform and driven by the electric displacement platform. The laser interferometer XL-80 is a displacement measurement instrument, and the moving distance of the fixture can be measured by XL-80. The interrogator SM-130 is the standard FBG interrogator, which is used to detect the signal of FBG sensors. The interrogation scheme of the SM-130 is based on the tunable Fabry-Perot filter [39]. The Fabry-Perot filter converts a broadband light into a single wavelength light to illuminate FBG sensors. A ramp scanning voltage drives the Fabry-Perot filter to change the wavelength of the light, and therefore the wavelength has a specific relationship with the value of scanning voltage. When the wavelength of the light is the same as the Bragg wavelength, the light is reflected by the FBG sensors. The photoelectric detector detects the reflected light, and the Bragg wavelength can be interrogated according to the value of scanning voltage at the moment. To compensate temperature changes, the strain measuring sensor and the temperature compensation sensor are separately connected to the two optical-fiber signal channels of the interrogator. The wavelength shifts of the strain measuring sensor and the temperature compensation sensor are detected simultaneously and recorded respectively as \(\Delta\lambda_{\text{B}}\) and \(\Delta\lambda_{\text{T}}\). Then, the wavelength shift caused by strain is obtained according to (4), and the measured strain can be calculated according to (3). Three kinds of CFRP laminate specimens were used in the experiment as shown in Figure 10a. The stacking sequence of the specimen 1 is \([45/0/-45/90/45/0/-45/0/45/0/-45/90/45/0/-45/0]_{\text{s}}\), and the stacking sequence of the specimen 2 is \([45/0/-45/90/45/0/-45/90/45/0/-45/0]_{\text{s}}\). The stacking sequence is \([45/0/-45/90]_{\text{2s}}\) for specimen 3. These CFRP laminate specimens can be used in the manufacture of aircraft skins. The specimens are rectangle-shaped with loading areas at the two endpoints, and the loading areas are reinforced to be completely clamped. The distance between the loading areas is \(l\). The CFRP laminate specimens were fixed on the strain applying unit by clamping both sides of loading areas with the two fixtures. The homogeneous strain was applied to  
Figure 9: Strain transfer experimental facilities. (**a**) Nondestructive tensile system; (**b**) Schematic of strain applying unit. the specimens when one of fixtures moved along the aerostatic guideway. The moving distance \(\Delta l\) was measured by the laser interferometer, and the applied strain \(\epsilon_{\mathrm{m}}\) was calculated according to  
\[\epsilon_{m}=\frac{\Delta l}{l} \tag{25}\]  
FBG sensors were bonded to specimens with Loctite epoxy adhesive 1C-LV (Henkel AG & Co., KGaA, Duesseldorf, Germany). The measurement structure and its characteristic parameters are exhibited in Figure 10b and Table 1. The environmental temperature during the installation of FBG sensors and their experiments is kept as close as possible to 20 \({}^{\circ}\)C. The FBG sensor is made of germanium-doped silica optical fiber.