Embedding of Fiber Bragg Grating Sensors Using L-WAAM Technology for the Development of a Smart Mold

This study presents the embedding of Fiber Bragg Grating (FBG) optical sensors on an INVAR36 mold manufactured by L-WAAM[1] (novel DED[2] technique) intended for certain CFRP[3] of aeronautical sector. The integration of FBG sensors enables in-situ monitoring of both temperature and strain in critical areas of the mold, which can influence the surface quality and dimensional accuracy of the parts after autoclave curing processes. The main objective of this research is to demonstrate the feasibility of combining additive manufacturing-based L-WAAM technology with fibre optic sensors to create a robust and smart metallic device capable of providing real-time information about its structural and thermal state. FBG sensors were coated with nickel (Ni) using an electroplating process to provide mechanical protection and thermal resistance against the harsh L-WAAM embedding process. The electroplating parameters were optimised to achieve a smooth and continuous coating along the fibre, with varying thickness in the sensor regions. Before and after embedding the FBG sensors, a thermal calibration was performed to quantify possible changes in their sensitivity and to alleviate residual stresses induced during the coating process. Preliminary embedding tests were performed using INVAR36 walls fabricated by L-WAAM on tensile specimens to determine the minimum coating thickness and process parameters that would ensure the survival of the sensor without compromising the integrity of the structure. The thermal and mechanical characterisation of these specimens with the embedded FBG sensors contributed to optimise the process and check the functionality of these embedded sensors. As a final validation, two FBG sensors were embedded in a full-scale INVAR36 demonstrator mould. The number and location of the sensors were defined in coordination with the end-user, targeting previously identified defect-prone regions critical for structural health monitoring of the mould. Given the proof-of-concept nature of the study (TRL5) and the need to avoid re-machining or significantly altering the mould geometry, a limited number of sensing points were selected to validate embedding feasibility while ensuring minimal impact on structural integrity. The sensors successfully provided temperature data during the curing cycles and remained fully functional after several tests. In conclusion to this work, it was demonstrated that the integration of FBG sensors into metallic structures using L-WAAM can be a viable approach in the development of smart metallic tools. The combination of fibre optic sensing and additive manufacturing technologies enables real-time monitoring and lifetime assessment of high-performance tools and structures in the aerospace industry. [1] L-WAAM: Wire Arc Additive Manufacturing assisted by Laser [2] DED: Directed Energy Deposition [3] CFRP: Carbon Fiber Reinforced Polymer