PSI - Issue 64

Emilia Damiano et al. / Procedia Structural Integrity 64 (2024) 1628–1635 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1630

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(Fibrasens DTS 2/2f, s4u®). This latter consists of a sensing fiber in a loose tube configuration: fiberglass is free to slide within the cable and is surrounded by a layer of stainless steel mesh and a following layer of aramid fiber. In the NSHT, designed to realize a packaging of the optical cable for its use in harsh environments and for both structural and geotechnical applications (Olivares et al., 2020; Di Gennaro et al., 2022), the sensing element is embedded into two composite material tapes with a resin matrix. Laboratory tests were first carried out to: • select the most appropriate type of optical fiber to be used in the NSHT for application as an inclinometer; • test the effectiveness of the NSHT proving that no slippage between the DFOS core, the packaging, and the element under observation occurs; • validate the temperature and strain coefficients of the different types of optical fibers used in the field. Finally, the installation of a DFOS-inclinometer in an active landslide was carried out, and monitoring results were analyzed and interpreted by comparison with conventional inclinometer measurements. 2.1. Laboratory setup In the first laboratory setup, two types of NSHT realized with different fibers and packaging were fixed to the outer surface of a 7-meter-long aluminum inclinometer tube. A standard inclinometer probe ( RTS Instruments Ltd ) moved freely in the tube and was used for comparison (Fig. 1a). The main characteristics of the two prototypes are reported in Table 1. Positioned vertically against a wall, the tube was secured in place using flexible straps. By inserting spacers between the tube and the masonry wall, a deformed shape was induced in the tube, simulating the impact of the rock-soil mass on the inclinometer. Fig. 1b illustrates a schematic representation of the setup.

Table 1. Test setup Prototype

Packaging tape

DFOS type (Thorlabs manufacture)

Length (m)

NSHT-1

Glass-Glass (GG)

SMF-28

4.0

NSHT-2

Carbon-Glass (CG) CCC1310-J9* 5.2

* bend-insensitive single-mode fiber

In a second laboratory experiment, the selected NSHT prototype was tested during a static experiment on a 16m long wind turbine blade. Three load increments, applied 14m from the wing’s root, were imposed up to a maximum of 9kN, and the corresponding deformations were detected by the NSHT transducer and 17 strain gauges along the length of the element on both sides as schematically illustrated in Fig 1b. This way the DFOS strain coefficient, C  , was checked too. Throughout the two laboratory experiments, the air temperature remained constant, allowing the measured BFS to solely reflect strain changes along the sensing fiber. Finally, calibration for determining the temperature coefficients, C T , of the different used DFOS was carried out by placing the fibers in a closed environment and applying temperature environmental changes ranging from 5 to 41 Celsius degree. The tests were repeated on different batches of the optical cable. The equipment used during laboratory tests allowed for a spatial resolution of 40 cm and an accuracy of +/-20  .