PSI - Issue 64

Maciej Kulpa et al. / Procedia Structural Integrity 64 (2024) 1673–1680 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2.3. Monitoring system The size of the FRP composite structure and the fact that this material is still relatively new resulted in plans to monitor the footbridge's behaviour in service. Among various measurement techniques, an SHM system based on DFOS fibre optic sensors was chosen. The advantage of DFOS sensors is that measurement takes place along the entire length of the fibre, which means that a 1-meter section of the fibre corresponds to up to 200 traditional measurement points located in the line. This solution was developed and tested by the authors, particularly to monitor FRP composite structures (Kulpa et al., 2021). Single-mode fibres of the type SM9/125 with acrylic coating were used as the basic element of the monitoring system. In one span of the footbridge, more than 200 meters of optical sensors were installed in three directions: longitudinal, transverse, and vertical fibres. Vertical fibres were placed on the three ribs and orientated in the direction of the main stresses (±45°). The location of the fibre optic sensors in the cross section is shown in Fig. 6.

Fig. 6. Location of fibre optic sensors in a cross section

It should be noted that the optical sensors were embedded in the structure of the panel in the manufacturing stage and integrated with the composite material. The sensors were placed in the planes of the upper and lower face laminates. Unlike traditional fibre sensors that are installed on the surface of a structure, these integrated optical sensors are inserted between the laminate fabrics before the infusion process. After the infusion process, the fibre optic sensors are embedded in the resin and are a part of the material (just like other reinforcing glass fibres). As a result, neither climate impacts nor acts of vandalism can damage them, and the sensor system as a whole is as durable as the structure itself.

Fig. 7. The scheme of the fibre optic sensor (on the left) and its arrangement in the mould during panel manufacturing