PSI - Issue 64

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

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The so-selected NSHT was then tested in a second laboratory experiment and the results are illustrated in Fig. 3b where the strain measured along the compressed and stretched sides of the wind turbine blade by both NSHT transducer and strain gauges are illustrated. Strain profiles obtained with the NSHT transducer agree with those from traditional strain transducers revealing that the packaging of the fiber did not influence the ability of the sensor to correctly follow the deformation phenomenon up to strain values of about 1500με. Data from temperature calibration tests are summarized in Table 2, detailing the type of DFOS, the temperature coefficients specified by manufacturers, and those obtained through laboratory testing. It can be noted that the resulting coefficients differ from those provided by manufacturers, particularly for the temperature cable's fiber. Furthermore, surprisingly, the same type of fiber belonging to different production batches is characterized by different temperature coefficient values.

Table 2. Temperature coefficient C T : comparison between manufacturer’s data and laboratory test DFOS Type C T (MHz/°C) Manufacturer Company Lab Test Temperature cable – I batch (Succes4u) 0.95 2.20

Temperature cable – II batch (Succes4u) Temperature cable – III batch (Succes4u) DFOS type (Thorlabs)

0.95

1.23

0.95

1.14

1.10

1.20

3.2. Site results During 2023 measurements were collected with a monthly frequency. Temperature measurements along the tube showed that seasonal fluctuations in air temperature directly influenced soil temperature only within the uppermost soil layer down to a depth of 20 meters, with changes of up to 20°C at maximum. Temperature changes become negligible at the lowermost. On the contrary, significant variations in BFS were measured in the uppermost 10-m thick soil layer and in a deeper section of the soil deposit, which was 5 meters thick, located around 25 meters below the ground surface. After temperature compensation, the obtained strain profiles revealed a progressive increase of the deformation field reaching strain values in the order of +/-700  at the depth of 25m, although the profile shape in this area appeared not to resemble that of a bent tube in a sliding zone. Based on this data, once the vertical strain component was isolated, strain patterns exclusively attributed to soil sliding were identified along each of the four sides of the inclinometer casing. For instance, findings related to the northeastern (NE) side of the casing are illustrated in Fig. 4, in terms of vertical (  v ) (Fig. 4a) and horizontal (  s ) strain profiles (Fig. 4b). These latter notably deviate from the vertical ones, exhibiting a form characteristic of a bent tube along its downslope edge (resembling a 'z-shaped' strain profile) at around 25m of depth, indicative of the progressive development of a sliding zone. Furthermore, Fig. 4c illustrates the trends of both vertical (  v ) and horizontal (  s ) strain components at 25m of depth and strain trend retrieved from the traditional inclinometer probe (  trad incl ) transposed on the NE direction for comparison. During the monitoring, the horizontal strain component progressively increased over time up to a maximum of 300  , whereas a progressive decrease of the vertical one up to less than -600  , corresponding to a shortening of the tube, occurred. As expected for this slow-moving landslide, both the vertical and horizontal strain components were of comparable magnitude, and this confirms the need to distinguish between the two strain components in such a kinematic field. Otherwise, the actual magnitude of strain associated with slope movements cannot be evaluated. Finally, by comparing the strain trends from both conventional and innovative devices (Fig. 5b) the effectiveness of the novel DFOS-based inclinometer and the interpretation measurements procedure has been proved.