PSI - Issue 79

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

149

Table 1. Embedment position of the FBG sensors. Specimen

FBG sensor’s embedment position

Bottom

Middle

Top

1 2 3 4 5

X X X X

X X X X X

X X X

The rough surface of the composite material demands the development of a reliable methodology to ensure the sensors’ accurate embedment. Every time the layer for the sensor’s placement was reached, the printing process was paused and an FBG sensor was placed on top of the last deposited layer. Magnets were used to hold the FBG in place, as depicted in Fig. 1. For the middle and top embedment positions, the magnets secured the respective optical fiber onto supporting structures that were placed at either end, serving as spacers with height matching the current layer level. These spacers ensured the FBG laid flush, avoiding any bending at the center. Once the sensor was placed, the printing process resumed. Moreover, in order to facilitate the precise fiber alignment for the subsequent mechanical testing, the FBGs were nested in specific grooves located at the wider grip sections of the specimens’ ends.

Fig. 1. Apparatus for embedment and proper alignment of the FBG sensors within the FDM-built dogbone specimens.

A four-channel FiberSensing FS2100/FS2200 BraggMETER interrogator (Hottinger Brüel & Kjær GmbH, Darmstadt, Germany) was used for the recording of the FBG wavelength shifts. A reference wavelength ( λ B0 ) was measured before the embedment of the sensor at room temperature (23°C ± 2°C) with the sensor placed straightened on a flat surface having its ends free. Subsequently, additional recordings were taken at various stages of the printing process and mechanical testing, i.e. after the completion of the fabrication, after the specimen’s cooling, before its removal from the platform, after its removal from the platform with the brims excised, right before the beginning of the mechanical testing and during the tensile experiment. Furthermore, for the direct measurement of the temperature profiles generated during the printing process, additional specimens were fabricated with two K-type thermocouples of 0.25 mm sensing tip thickness integrated at the center and perpendicular to the specimen’s long axis. The first thermocouple was placed on the building platform surface and just before the 1 st layer, while the second one was embedded after the 8 th layer of the printed specimen within a notch designed to accommodate the thermocouple’s thickness. An Instrunet data acquisition system (Omega Engineering Inc., Connecticut, USA) was used to record the temperature profiles generated during the FDM process. 3.2. Tensile testing The fabricated specimens with the embedded FBGs were mechanically tested using an Instron universal testing machine with a 30kN load capacity (Instron, Massachusetts, USA), operated via the proprietary BlueHill software, in accordance with ASTM D638 standard. The dogbone samples were placed on the machine’s grips, as shown in Fig. 2. The testing machine’s extensometer was also attached to each specimen in order to allow comparison between its measurements and those obtained from the FBGs. Each test coupon was positioned in the machine’s grips so that their