## 4 Distributed Strain Monitoring Using the BOCDA System  
### Distributed Strain Monitoring of the Composite Side Beam  
Figure 7 shows the experimental setup for strain monitoring. Both ends of the composite side beam were fixed on supporting jigs, and a vertical load actuator (MTS, Eden Prairie, MS, USA) was placed in the middle, 1.485 m from the ends of the side beam. In the longitudinal direction, a single mode optical fiber with a diameter of 250 \(\upmu\)m and a length of 2.97 m was attached to the lower part of the composite side beam using Araldite epoxy (Huntsman, Woodlands, TX, USA). The attached optical fiber was connected to the developed BOCDA system. A linear variable differential transformer (LVDT) was attached to the bottom of the side beam to measure the center deflection of the side beam.  
The composite side beam's performance was determined by using a vertical load actuator to apply a 140 kN static load, which is a design requirement for urban subway trains, and a 182 kN dynamic load to consider the dynamic effect that occurs while the railcar is being operat  
Figure 7: Setup for distributed strain monitoring test of composite side beam using an optical fiber: (**a**) setup for a vertical train load test of the composite side beam; (**b**) equipment for a vertical train load test of the composite side beam.  
twice to confirm the reproducibility of the results. The objective of the experiment was to monitor the strain distribution of the composite side beam using the developed BOCDA system when a vertical train load was applied, and to compare it with FE analysis results to confirm that the composite side beam was fabricated as designed. The strain distribution of the optical fiber attached to the composite side beam is shown in Figure 8. The experimental data for the optical fiber were compared to the computational FE analysis results to validate the distributed sensing data. The composite side beam was computationally modeled, and the structural behavior was analyzed using the finite element method [14].  
The strain was measured at 3 cm intervals using the optical fiber. The overall strain distributions for both cases agree well with the analysis results. The composite side beam has a symmetrical structure; hence, the strain is also symmetrically distributed around the middle. The optical fiber sensor measurements obtained on the right side of the composite side beam were at most 70 \(\mu\epsilon\) less than the FE analysis results, reflecting a stiffness mismatch caused by non-uniform thickness that developed during the fabrication process. The measured strain with the optical fiber sensor did not agree well with the FE analysis results at the bend, where the structure's shape changes drastically,  
Figure 8: Comparison of finite element (FE) analysis and strain distribution measurement results for a composite side beam with a: (**a**) 140 kN vertical load; and (**b**) 182 kN vertical load.  
as shown in Figure 8a. In these inflection sections, there was a mismatch between the design and the fabricated product; hence, precise values could not be obtained via FE analysis. However, an accurate value of the strain was obtained using the optical fiber. The maximum strain occurred in the middle of the composite side beam; when the vertical train load was 182 kN, the strain was 1602 \(\mu\epsilon\), -1.3 times higher than that when the load was 140 kN (1222 \(\mu\epsilon\)). There was good agreement between the results obtained using the optical fiber when the measurements were repeated for the second time, confirming the sensor's reproducibility and reliability.