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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compression of rock materials, etc. which effects mainly manifest vertically. In slow landslides, strain resulting from the presence of a shear zone may be comparable in magnitude to that caused by the above-mentioned mechanical effects. In such scenarios, it becomes essential to accurately assess the magnitude of strain associated with slope movements by separating the two components: the horizontal one, associated with sliding that causes bending of the tube in the direction of the slope movement, and the axial one, which acts vertically. To accomplish this, the DFOS-inclinometer needs to be configured with at least three (preferably four) NSHT positioned at 90° angles from each other. Given that the vertical strain component at each depth assumes the same value on each side of the tube, while the horizontal strains on two sides of the tube, positioned 180° apart, have equal absolute values but opposite signs, it becomes feasible to separate these two strain components. After temperature compensation and decoupling of the axial strain, a comparison with data from the standard device was done in terms of strain to assess the effectiveness of the novel inclinometer. In detail, the deflection measured by the traditional inclinometer is derived twice to obtain the curvature of the pipe. Strain is proportional to the curvature and distance from the neutral axis. Thus, the deformation at the sides of the tube can be obtained multiplying the curvature by the distance from the neutral axis (radius of the tube). Finally, transposition to the positions occupied by the NSHTs is carried out to compare the two datasets. 3. Results and discussion 3.1. Laboratory results The results of the first laboratory test are illustrated in Fig. 3a in terms of strain profiles measured by each NSHT prototype compared with the corresponding one estimated by standard inclinometer probe (black lines). The strain profiles are comparable for trends and values and revealed that, in the investigated range of strain, the set of measurements collected by NSHT-1 presents a scattering higher than the one measured by NSHT-2. The different performances of the two prototypes can be related to the different utilized D FOS types. The ‘bending sensing’ fibre type (G.652) tested in the NSHT-1 suffered a significant loss of signal in the presence of relatively high curvatures and this, in turn, reflected in a higher noise of the signal. Indeed, its worst performance was recorded close to the collar joining two tube segments where localized high curvature occurred and the maximum deviation between the inclinometer and the NSHT-1 data was verified. Based on the laboratory results, the in-field setup was chosen, which consisted of NSHTs realized with the “low bend loss” fiber cable (Thorlabs CCC1310 -J9), used for manufacturing of the NSHT-2 prototype, packaged by two tapes in glass-fiber composite material.

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Fig. 3. Results of laboratory testing on NSHT prototypes: (a) comparison between strain measurements by NSHT-1 and NSHT-2 and traditional inclinometer; (b) comparison between strain measurements by NSHT and strain gauges