PSI - Issue 79

Charoula Kousiatza et al. / Procedia Structural Integrity 79 (2026) 146–154

152

4.3. Tensile properties Characteristic stress-strain curves, as calculated based on the FBG sensors’ recorded wavelength shifts, along with the corresponding extensometer measurements, are indicatively presented in Fig. 4 for specimen 5. As illustrated, the stress-strain curves obtained from all three FBG sensors are nearly identical to that derived by the extensometer, despite the fact that the strain measurements obtained by the FBGs provide information locally as regards the structure’s mechanical response. The comparison between the strain values measured by the FBGs with the extensometer data indicate that they are in good agreement up to the point at which the optical sensors either failed or detached under increasing load. In almost all cases, the sensors’ signal loss was observed to occur sequentially from the top to the bottom FBG embedment positions through the test specimens’ thickness, as indicatively illustrated in Fig. 4 for specimen 5. This phenomenon could be directly related to the FDM-fabrication procedure’s inherent higher temperature profiles exhibited at the structures’ lower layers, as discussed above, thus affecting the embedding conditions and resulting in variations in the FBG sensors’ achieved bonding quality.

Fig. 4. Stress-strain response characterization through the specimen’s thickness based on FBG sensors’ recordings compared to the extensometer’s obtained measurements.

Table 3. Calculated ultimate strength and Young’s modulus.

Extensometer

Embedded FBG sensor Young’s Modulus, (GPa) [Middle]

Ultimate Strength, (MPa)

Young’s Modulus, (GPa)

Young’s Modulus, (GPa) [Bottom]

Young’s Modulus, (GPa) [Top]

Specimen

1 2 3 4 5

81.76 86.27 81.63 83.64 –

5.16 5.30 5.42 5.59 6.31

– –

5.58 6.52 6.32 6.71 –

6.98 6.89 6.28

6.12 6.33 6.80

–

6.85

The fundamental mechanical properties, namely the Young’s modulus and tensile strength are presented in Table 3, as determined based on the FBG recordings and the stress-strain curves obtained from the extensometer, for all types of specimens. As shown, the Young’s modulus values calculated from the optical sensors’ recorded data are very close to the extensometer’s measurements, with the FBG results exhibiting only slightly higher values. This difference can be attributed to the localized nature of the FBG measurements, in contrast to the macroscopic averaging performed by the extensometer over a larger gauge length. Additionally, minor variations are observed among the Young’s modulus values derived from the FBG sensors embedded in various positions through the composite samples’ thickness. More specifically, in most cases, the calculated Young’s modulus is observed to slightly increase from the bottom to the top embedment locations, except for specimen 4. This trend may be linked to the elevated compressive