PSI - Issue 79

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

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1. Introduction Additive Manufacturing (AM) technologies were initially developed to serve as prototyping tools; however, they have undergone significant evolution over the last decades due to major advancements in technology (hardware), software and material science, thus leading to a broad expansion of their capabilities and fields of application. Fused Deposition Modeling (FDM) is one of the most widely spread AM techniques that relies on the use of thermoplastic materials for the manufacture of final parts by depositing adjacent rasters of molten filament that rapidly resolidifies, in a layerwise manner. The employment of composite materials in FDM process, with either short or continuous fiber reinforcement, is gaining heightened attention, as it could enable the fabrication of lightweight, functional components for high-end engineering applications, featured by superior thermomechanical performance compared to pure thermoplastic parts (Wang et al. 2024; Wang & Zou 2022). The thermomechanical characterization of FDM-fabricated composite structures intended to be used in load bearing applications is of critical importance, as it is strongly affected by several factors, including the used filaments’ material properties, the selected processing parameters, the complex physical phenomena inherent to the layer-by layer deposition process, along with the influence of additional parameters, such as the fiber-matrix interactions and anisotropic behavior. In this context, the development and implementation of robust, non-invasive sensing techniques play a vital role since they enable the acquisition of in-situ and real-time data, which can be used not only for mechanical and/or thermal characterization of the final 3D printed components, but also for continuous structural health monitoring (SHM) throughout their entire operational lifespan. Optical fiber Bragg grating (FBG) sensors are already extensively used for material characterization and SHM purposes across a wide range of industrial fields due to their beneficial characteristics, encompassing high sensitivity to both strain and temperature, multiplexing capability, minimally invasive integration, long-term stability and immunity to electromagnetic radiation. In this framework, the adoption of FBG technology into the rapidly expanding 3D printing field appears highly promising, particularly for FDM-produced composites, where the ability to perform in-situ monitoring of mechanical and/or thermal behavior is crucial to ensure structures’ performance and reliability. Over the past few years, several researchers have focused their investigations on the use of FBGs in the FDM process for experimentally characterizing the built parts’ thermomechanical behavior by in-situ measuring strain, temperature and other physical parameters. Most of these studies have relied on surface-bonding optical sensors onto the manufactured parts (Ahmad et al. 2022; Alias et al. 2024; Chen et al. 2021; Serovaev et al. 2024), whereas only a limited number of research works have explored the embedment of FBGs within the 3D printed structures during the fabrication process. More specifically, regarding the second experimental approach, Kousiatza et al. developed a methodology employing FBG and thermocouple sensors for the in-situ characterization of FDM-manufactured continuous fiber-reinforced composite samples by recording the process-induced residual strains and temperature profiles. The measurements obtained from the embedded FBGs were also used to determine the post-fabrication residual strains, as well as the coefficient of thermal expansion (CTE) of the composite host materials (Kousiatza et al. 2019). In a recent study conducted by Matsika-Klossa et al., the CTEs and glass transition temperatures (T g ) of various commonly used FDM pure thermoplastic and composite materials were calculated based on the wavelength shifts derived by the FBGs integrated in the 3D printed parts (Matsika-Klossa et al. 2024). In terms of mechanical testing, Economidou and Karalekas carried out one of the first studies focusing on the determination of the mechanical properties of FDM pure thermoplastic samples equipped with a single FBG sensor (Economidou & Karalekas 2018). Moreover, Lo Presti et al. developed a 3D printed strain sensor based on the FBG technology for wearable applications. The sensor’s response was investigated by applying mechanical and thermal loading (Lo Presti et al. 2024). It is important to note that all these studies involve the integration of a single FBG sensor within the investigated components with only a few of them focusing specifically on FDM-printed composites. A noticeable gap in the existing literature is the lack of research addressing the embedment of multiple FBGs through the thickness of FDM fabricated composites for the experimental characterization of their mechanical behavior. In this framework, the present study aims at investigating the mechanical behavior of FDM-built short carbon fiber reinforced polyamide (CF-PA) specimens via the embedment of FBG sensors in various positions through the parts’ thickness. Tensile testing was conducted to evaluate the mechanical properties of the built test coupons. Characteristic stress-strain curves, as calculated based on the obtained FBG recordings were compared to the extensometer measurements. Furthermore, the specimens’ in-process generated temperature profiles were monitored for a better