## 4 Experimental Validations for Ship Stiffened Panel Strain Reconstruction
### Fiber Optic Sensor Layout Based on Equivalent Neutral Layer  
(a) Equivalent Neutral Layer Calculation Based on Measured Data;  
According to Section 2.2, to determine the position of the equivalent neutral layer in the ship stiffened panel, OFDR fiber optic sensors could be positioned on the top and side walls of the transverse T-section, as depicted in Figure 17a. Taking Case 3 as an illustration, due to the linear variation in strain along the Z-direction on the side wall, the strain data recorded by the OFDR optical fiber sensor were integrated with Equation (6) to compute the zero position of the strain curve for paths p400, p500, p600, p800, p900, and p1000, as illustrated in Figure 17b. According to Figure 17b, the strain distribution curves of the six paths (P400, P500, P600, P800, P900, and P1000) exhibit significant differences, resulting in distinct slopes for the strain curves along these paths. This observation is consistent with the simulated results presented in Figure 5b. However, due to the partial incongruence between experimental and simulated boundary conditions, the differences in strain curves among the six paths are not as pronounced as indicated by the simulation results. As per Table 5, the strain curves of paths p400, p500, p600, p800, p900, and p1000 approached zero near Z = 33.57 mm, implying that the plane at this height could be considered the equivalent neutral layer of the stiffened ship panel. Utilizing Equation (7) based on this determination, the strain distribution across the outer surface of the stiffened panel could be computed. Consequently, it was not necessary to position sensors on the outer surface of the stiffened panel, in direct contact with seawater. This approach circumvented the detrimental impacts of wave loads and environmental corrosion on the measurement accuracy and long-term stability of the sensor, thereby potentially enhancing the sensor's longevity and operational reliability. (b) Optical Fiber Sensor Layout and Element Division;  
Following the unit meshing strategy delineated in Section 3.3, and the determined position of the equivalent neutral layer, and taking into account the need for a region on the plate surface for a connection with the fixed support platform, the monitoring area illustrated in Figure 18a was chosen for grid subdivision. Figure 17: **(a) OFDR fiber optic sensor layout for transverse T\(-\)section; (b) the X\(-\)direction measured strain curves of the six paths. ** \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Path** & p400 & p500 & p600 & p800 & p900 & p1000 \\
**Position of zero strain point [mm]** & 30.09 & 34.66 & 39.41 & 28.08 & 35.62 & 33.57 \\
**Equivalent neutral layer position [mm]** & & & & 33.57 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: X-direction strain in transverse T-section along different paths. In this section, the monitoring area of the plate surface was divided into a mesh comprising 2 \(\times\) 5 four-node shell elements, while the top surface of the two transverse T-sections was partitioned into a mesh of 1 \(\times\) 5 four-node shell elements. According to the aforementioned unit meshing scheme, a fiber Bragg grating strain rosette (FBG strain rosette) was positioned at the central point of each unit on the plate's surface [26]. This arrangement was intended for monitoring the X-directional strain and the 45-degree directional strain at the central point of each unit. The smooth and even top surface of the transverse T-section allowed for convenient sensor adhesion. Consequently, an OFDR sensor was placed on the top surface to monitor the X-directional strain in the transverse T-section units. As depicted in Figure 18b, both FBG and OFDR sensors were adhered to the inner side of the panel using epoxy resin adhesive. When the structure was subjected to a force load, an optical fiber demodulator enabled the real-time collection of strain data at each measuring point. By employing the methodology proposed in this paper, the strain field of the stiffened panel could be reconstructed.