## 3 Analysis of Strain Transfer Influential Factors
### The Influence of Stacking Directions  
To analyze the influence of the stacking directions, CFRP laminates with stacking directions \([0/0/0/0]_{4s}\), \([45/-45/45/-45]_{4s}\), and \([90/90/90/90]_{4s}\) are separately applied to (21) and (23). The strain transfer distribution ratio curves with the three stacking directions are shown in Figure 4. The curves are exhibited from 0 mm to 5 mm as the measurement structure is axisymmetric. The maximum values of the three curves are all observed at 5 mm, and the curves rapidly decrease  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter/Unit** & **Symbol** & **Value** \\ \hline Elastic modulus of silica core/GPa & \(E_{\mathrm{s}}\) & 72.00 \\ Elastic modulus of coating/GPa & \(E_{\mathrm{c}}\) & 2.55 \\ Elastic modulus of adhesive layer/GPa & \(E_{\mathrm{a}}\) & 1.02 \\ Shear modulus of coating/GPa & \(\mathrm{G_{c}}\) & 0.86 \\ Shear modulus of adhesive layer/GPa & \(\mathrm{G_{a}}\) & 0.38 \\ Shear modulus in 2-3 plane of each fabric layer/GPa & \(\mathrm{G_{23}}\) & 1.45 \\ Shear modulus in 3-1 plane of each fabric layer/GPa & \(G_{\mathrm{31}}\) & 4.88 \\ Half-length of adhesive layer/mm & \(L\) & 5.00 \\ Width of adhesive layer/mm & \(D\) & 2.00 \\ Adhesive thickness above optical fiber/\(\upmu\)m & \(h_{\mathrm{a}}\) & 37.50 \\ Radius of silica core/\(\upmu\)m & \(y_{\mathrm{s}}\) & 62.50 \\ Radius of coating/\(\upmu\)m & \(y_{\mathrm{c}}\) & 77.50 \\ Thickness of adhesive layer below \(x\) axis/\(\upmu\)m & \(y_{\mathrm{a}}\) & 100.00 \\ Thickness of each fabric layer/\(\upmu\)m & \(y_{\mathrm{k}}-y_{\mathrm{k}-1}\) & 250.00 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mechanical and geometrical parameters of the measurement structure.  
as they approach 0 mm. Hence, most of the strain can be transferred to the fiber core at 5 mm, and the differences between the strain measured by FBG sensors and the original strain in the CFRP laminates gradually enlarge when it is close to the endpoint (0 mm). The figure denotes that the stacking directions have a big influence on the strain transfer. The curve with stacking direction \([0/0/0/0]_{48}\) is the highest while the curve with stacking direction \([90/90/90/90]_{48}\) is the lowest in the figure. The average strain transfer ratios with stacking directions \([0/0/0/0]_{48}\), \([45/-45/45/-45]_{48}\), and \([90/90/90/90]_{48}\) are 85.1%, 78.6%, and 73.7%, respectively. It indicates that the configuration of more \(\pm 45^{\circ}\) and \(90^{\circ}\)stacking directions reduces the \(x\)-direction stiffness of CFRP laminates, which is disadvantageous for the strain transfer. Evidently, the CFRP laminates with bigger stacking directions have lower average strain transfer ratios. Carbon-fiber fabrics are orthotropic and their elasticity modulus along carbon-fiber direction is significantly higher than the elasticity modulus along other directions. When carbon-fiber fabrics in CFRP laminates have bigger stacking directions, the angles between carbon-fiber directions and the \(x\)-direction (the optical fiber direction) will increase. Hence, the \(x\)-direction elasticity modulus decreases with the stacking directions enlarging. According to the previous study [28], the host material of low elasticity modulus is disadvantageous for the strain transfer, and therefore CFRP laminates with more \(\pm 45^{\circ}\) and \(90^{\circ}\)stacking directions have lower average strain transfer ratios. The theoretical analysis above is consistent with the model calculation results, as shown in Figure 4.