PSI - Issue 79

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

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et al. 2024), which is consistent with a 50-55ºC range reported in literature (Krause et al. 2019). This indicates that the PA matrix of the investigated test coupons remains well above its T g throughout the entire fabrication process, especially the material deposited on the bottom layers of the composite structures.

Fig. 3. (a) Temperature profiles as recorded by the thermocouples after each sensor’s integration within the dogbone specimen; (b) Overlapped temperature profiles for the bottom and middle embedment positions.

4.2. Post-fabrication residual strains Table 2 presents the post-fabrication residual strains of the printed specimens before the mechanical testing. The residual strains were calculated using Equation (3). As reference wavelength ( �� ) was used the value measured before the embedment of the sensor in the specimen, while as � was used the measured obtained after the specimens’ removal from the building platform with the brims structures excised. As shown in Table 2, the calculated strains are compressive due to material consolidation during the FDM process. The layers closer to the building platform are subjected to prolonged thermal exposure, owing to both the thermal loading from the platform and the repeated reheating caused from the successive layers’ deposition, as discussed earlier. Consequently, these lower layers undergo greater thermal expansion throughout the fabrication process and higher shrinkage upon cooling, which leads to higher compressive residual strains compared to the upper layers. It is also important to note that the magnitude of the process-induced compressive residual strains is consistent with the value of � 1571 με reported in a previous study conducted by our research team, which investigated FDM-printed block samples with a 0 o raster orientation fabricated from the same composite filament (Matsika-Klossa et al. 2024). Furthermore, it is evident that the difference in residual strains between the bottom and middle FBG embedment positions for specimen 4 and 5 is 119 με and 163 με , respectively. Regarding the difference in compressive residual strains between the middle and top embedment positions, values of 127 με , 171 με , 49 με and 444 με are calculated for specimens 1, 2, 4 and 5, respectively. These results suggest that the overall variation in residual strains through the thickness of the FDM-built composite specimens is not significant; however, the resulting slight bending occurred with respect to the structures’ neutral axis appears to be non-symmetric, as indicated by the measured residual strain differences in specimens 4 and 5.

Table 2. Residual strains after the printing process. Specimen

Measured residual strains, ε (10 -6 )

Bottom

Middle -1187.1 -1601.3 -1341.2 -1760.6 –

Top

1 2 3 4 5

– –

-1060.4 -1430.8 -1347.8 -1292.5 -1316.2

-1454.9 -1460.3 -1923.5