PSI - Issue 79

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

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understanding of their effect on the fabrication-induced residual strains and corresponding mechanical behavior. Additionally, microscopy analysis was performed to assess the quality and integrity of the FBG sensors’ embedment within the host composite material. This novel approach enables both intra- and inter-layer strain monitoring through FBG sensors’ integration within different planes of the FDM-built structures, thus providing a more comprehensive understanding of the parts’ mechanical response. 2. FBG working principles The basic operating principle of FBG sensors relies on the reflection of specific wavelengths, when a broadband light travels through the fiber. The reflected wavelength, �� , is given by: �� � 2 ��� (1) where ��� is the effective refractive index of the core and the Bragg grating period. Both ��� and are strongly influenced by strain and temperature, thus modified whenever changes occur (Measures 2001). When the FBG sensor is embedded in a host material and subjected to a homogenous axial strain, � , and/or a uniform temperature change, , and by assuming that the two lateral strains on the fiber, � , � are related to the axial one by � � � �� � � (where � is the Poisson’s ratio of the fiber), the shift of the wavelength is given by: �� � � �� �� 1 � � � ����� �� 1 � � �� � � � � � � � � � � � (2) where � � � � �� is the difference in wavelength before and after loading, �� is the Bragg wavelength in the reference state, � is the strain-optic coefficient ( p e ≈ 0.215) (Giaccari et al. 2005), � is the CTE of the host material, � is the CTE of the fiber core ( α f ≈ 8×10 -7 °C -1 ) (Lai et al. 2010) and stands for the thermo-optic coefficient of the fiber ( ξ ≈ 8.3×10 -6 °C -1 ) (Shibata et al. 1981). In Equation (2), ε axial accounts for the fabrication-induced residual strains due to material solidification and the term ( α m - α f ) ΔΤ stands for the thermal strains developed due to the thermal expansion mismatch between the optical fiber and the host material. When measurements are taken at steady temperature, where ΔΤ =0, then Equation (2) is reduced to a simpler form that can be used for calculating the magnitude of the resulting residual strains: �� � � �� �� 1 � � � ��� (3) where ��� is the fabrication-induced residual strains. Dogbone specimens made from filaments, consisting of a PA matrix and 10% chopped carbon fiber reinforcement (Addigy Covestro – ID1030 CF10, Covestro AG, Leverkusen, Germany), were fabricated using a Flashforge Creator 3 (Flashforge 3D Technology Co., Zhejiang, China) desktop FDM 3D printer, in accordance with ASTM D638 standard (Type IV). Drying the filament at 60°C for 12 hours preceded the test coupons’ printing process. Additionally, constant processing parameters were maintained throughout the entire printing process. The layer height was set to 0.2 mm, the extrusion and bed temperatures were set to 280°C and 100°C, respectively, the printing speed was 3000 mm/min and the infill density was 100%. Each sample consisted of 16 layers with a raster orientation of 0 o (parallel to the long axis). The building chamber was preheated to ensure uniformity. The dogbone samples featured a single-layer, disc-shaped brim at each of their four corners, in order to achieve better adhesion to the platform, thereby minimizing warping or detachment defects during the printing process. FBG sensors (ATGrating Technologies Co. Ltd., Shenzhen, China) of 0.125 mm core diameter with polymer cladding and a 3 mm uncoated sensing length were embedded longitudinally at specimens’ selected layers each time. Two or three FBG embedment positions (middle and top, bottom and top, as well as bottom, middle and top) through the samples’ thickness were investigated, encompassing the following integration layers: after the 1 st , after the 8 th and before the 16 th layer. Two specimens were fabricated per case; however, only those which remained intact and functional for tensile testing without FBG failure during handling are summarized in Table 1. 3. Materials and methods 3.1. Specimens’ fabrication