Issue 40

Z.S. Metaxa et al, Frattura ed Integrità Strutturale, 40 (2017) 61-73; DOI: 10.3221/IGF-ESIS.40.06 65 the testing machine was load-controlled with a crosshead speed of 0.1 mm/min. Evaluation of the four point bending (4pb) mechanical tests have been performed according to ASTM D6272 [24]. During testing, crosshead displacement, load and the strain gauges measurements were continuously recorded and stored in a P/C. An Agilent multimeter was used to record in situ the electrical resistance data of the specimen’s embedded PVA-CNT fiber during mechanical loading. A DC voltage of 10 V was applied to cables connected to the PVA-CNT fiber of the specimens (Fig. 3), the current was measured and the resistance was calculated from these values. The resistance measurements were performed in a two-point measurement set-up in the longitudinal direction. Data acquisition of 1 Hz was also used for the resistance measurements and stored simultaneously in the P/C of the testing machine. Electrical resistance change (ERC) values were post-calculated form the initial resistance values. The optical fiber sensors used were Bragg gratings of 2 mm nominal length. The reflectivity of these sensors was of the order of 20 %, in order to ensure adequate spectrum reflection and at least 50 cm of free optical fiber should be available at each side for adaptation of connectors. Selected center wavelength was 1540 ± 1 nm, while Δ λ was approximate ~ 0.7 nm. An interrogation system was used to measure the wavelength changes due to the mechanical field loading and evaluate the equivalent strain values. The wavelength data were afterwards converted to axial strains measured in the vicinity of the Bragg grating sensors, based on the fundamental equation for constant temperature for mechanically and optically isotropic optical fibers:                          1 a T (1) that relates the wavelength changes to the axial strain of the fiber at the sensor area through the sensitivity coefficient λ Β . Term Δ λ Β is the change in Bragg wavelength, ρ α , α and ξ are respectively the photoelastic, thermal expansion and thermo- optic coefficients of the fibre, Δ ε is the strain and Δ T is the temperature change, respectively. In the present work, λ Β was measured via calibration and for the specific sensor it was found to be equal to 0.89 με / pm or inversely 1.12 pm / με. Using Eq. (1), all wavelength shifts were collected from the interrogator device and the respected axial strains were calculated. (a) (b) Figure 4 : Sketch of the (a) geometrical dimensions and (b) testing supports of the specimen. (a) (b) Figure 5 : (a) Installed specimen with embedded PVA-CNT fiber and (b) magnification of the bottom side of the specimen with surface attached strain gauge. For the Type I specimens (with embedded sensors), the following tests have been performed: (a) monotonic loading till fracture and (b) incrementally increasing loadings - unloadings till fracture. For the Type II specimens (with attached