## 1 Introduction
\(p_{11}\) and \(p_{12}\) are elastic-optical constants which respectively describe the longitudinal and lateral refractive index changes when longitudinal strain is applied to an optical fiber [22]. For the FBG sensor under both temperature and strain changes, the overall shift of Bragg wavelength is given by the summation of the two effects [19] as follows:  
\[\Delta\lambda_{\text{B}}=\Delta\lambda_{\text{T}}+\Delta\lambda_{\varepsilon} \tag{4}\]  
To compensate the effect of temperature changes during the strain measurement, a same FBG sensor can be placed in the measurement area as a temperature compensation sensor. The temperature compensation sensor is subjected to the same thermal load but free from mechanical load. The wavelength shift \(\Delta\lambda_{\text{T}}\) caused by the change of temperature can be eliminated from the overall wavelength shift \(\Delta\lambda_{\text{B}}\), and the wavelength shift \(\Delta\lambda_{\varepsilon}\) resulted from the strain change can be obtained based on (4). Then, the strain sensed by the FBG sensor can be calculated according to (3). With the help of adhesive, a bare FBG sensor is bonded to the surface of a host material. The strain of the host material is transferred to the adhesive layer, which leads to the deformation of the FBG sensor wrapped inside. Since the mechanical properties of the FBG sensor, the adhesive layer and the host material are different, the interactions among them are complicated [23; 24]. When a host material deforms, part of the strain will be absorbed by the adhesive layer. The measurement structure can also counteract the original strain in the host material. Therefore, the strain measured by FBG sensors is different from the original strain in the host material [25]. The transfer efficiency between the measured strain and the original strain can be defined as a strain transfer ratio. Several studies have investigated this problem theoretically and experimentally. [PERSON] [26] performed the Fourier amplitude sensitivity test based on an empirical mechanical model to identify the most critical parameter influencing the strain transfer ratio. [PERSON] [27] developed a theoretical model to evaluate the effect of adhesive and bonding length on the strain transfer. [PERSON] [28] conducted a strain transfer analysis of a clamped fiber Bragg grating sensor. She discussed the influences of gauge ratio and interlayer thickness on the strain transfer ratio. [PERSON] [29] proposed a method to measure surface strain with polyimide fiber Bragg gratings. It was demonstrated that polyimide FBG sensors work well even in dynamic surface strain measurements. [PERSON] [30] gave the strain transfer function of the attached FBG sensor under ultrasonic excitation and realized nondestructive testing. Accurate strain measurement results were obtained in the reported studies. However, in most of the studies, it was assumed that the elasticity modulus of host materials was a constant. In fact, this hypothesis can be only valid for isotropic host materials. As for composites, such as CFRP laminates, the elasticity modulus changes with directions and material characteristics vary among different layers. Therefore, a strain transfer characteristic analysis is needed to improve the strain measurement accuracy of CFRP laminates. In this paper, a strain transfer model is proposed for FBG sensors bonded to the surface of CFRP laminates. The influences of CFRP laminates on the strain transfer are investigated and the model is applied to the measurement of the wing skin. The content of this paper is organized into five sections. The strain transfer model is established and the transfer function is derived in Section 2. The strain transfer influential factors are analyzed in Section 3, and the relationship between the CFRP laminate configuration and the strain transfer efficiency is discussed in detail. In Section 4, the accuracy and practicality of the proposed transfer model are verified according to the experimental results. Finally, conclusions are summarized in Section 5.