## 3 Analysis of Strain Transfer Influential Factors
### The Influence of Temperature  
In the strain measurement using FBG sensors, the cross-sensitivity with temperature is a major issue to be addressed. As FBG sensors are sensitive to both strain and temperature, the effect of temperature changes on the strain measurement should be compensated. In this paper, an FBG sensor with same mechanical and optical characteristics is used as the temperature compensation sensor. As the thermal expansion coefficients of FBG sensors and CFRP laminates are different, the temperature sensitivity of FBG sensors may change after the installation. Therefore, the compensation sensor is installed on another CFRP laminate sample which has the same configuration as the CFRP laminates under test, and the installation parameters for the temperature compensation sensor are also equal to the installation parameters for the strain measuring sensor. The compensation structure is at the same temperature as the strain measuring sensor, but it is free from mechanical load. Hence, the temperature changes cause the identical wavelength shift for both the strain measuring sensor and the compensation sensor. Then, the wavelength shift of the compensation sensor is subtracted from that of the strain measuring sensor according to (4), and the cross-sensitivity with temperature in the strain measurement can be compensated. In fact, the strain measuring sensor and the compensation sensor are in the same environment, and the mechanical load is the only difference between them. Therefore, the sensitivity of the strain measuring sensor to other environmental parameters can also be eliminated by this compensation. The strain transfer is also affected by the temperature which is indirectly incorporated in the strain transfer model. The values of elastic modulus and shear modulus in the model change with the temperature variation, and therefore there is an influence on the average strain transfer ratio. To analyze the influence, the average strain transfer ratios at four different temperatures, 20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C,  
Figure 7: Simulation results of measurement structures with different stacking sequences. (**a**) Strain field for stacking sequence [0\({}_{8}\)/90\({}_{8}\)]; (**b**) Strain field for stacking sequence [0\(/\)90\({}_{8}\)]; (**c**) Strain field for stacking sequence [90\(/\)0]\({}_{8}\); (**d**) Strain field for stacking sequence [90\({}_{8}\)/0\({}_{8}\)]. 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C are calculated by substituting corresponding mechanical parameters into (21) and (24). The geometric parameters of the measurement structure are shown in Table 1 and its mechanical parameters at different temperatures are listed in Tables 1 and 2. CFRP laminates [0/0/0/0]\({}_{48}\), [45/\(-\)45/45/\(-\)45]\({}_{48}\), and [90/90/90/90]\({}_{48}\) are used for the calculation, and their average strain transfer ratios at different temperatures are shown in Figure 8. It indicates that the average strain transfer ratio of CFRP laminates [0/0/0/0]\({}_{48}\) is the highest while the average strain transfer ratio of CFRP laminates [90/90/90/90]\({}_{48}\) is the lowest at each temperature. With temperature rising from 20 \({}^{\circ}\)C to 80 \({}^{\circ}\)C, average strain transfer ratios enhance from 85.07% to 85.41%, from 78.63%to 79.16%, and from 73.74% to 74.40% for CFRP laminates [0/0/0/0]\({}_{48}\), [45/\(-\)45/45/\(-\)45]\({}_{48}\), and [90/90/90/90]\({}_{48}\), respectively. The values of elastic modulus and shear modulus decrease with temperature increasing. As the modulus of the silica core declines more than that of other three materials according to Table 2, the optical fiber is relatively more ductile at a higher temperature, which is beneficial to the strain transfer. Therefore, temperature can affect the strain transfer ratio by changing the elastic modulus.