Structural Health Monitoring of Composite Materials with Optical Fibres under Fatigue Loading

Structural Health Monitoring (SHM) is a growing ensemble of techniques devoted to collect and analyse data from structures in order to detect early damage, thus allowing to continuously evaluate their integrity and performance. The use of Structural Health Monitoring (SHM) is particularly challenging for composite materials due to their heterogeneous nature. The anisotropic behaviour, complex damage mechanisms, and poor visual detectability of internal defects make traditional inspection methods less effective. Among others, Distributed Fibre Optic Sensors (DFOS) represent a promising technique, ensuring full-field strain measurements with high resolution and real-time monitoring capability. In this context, the present work investigates an innovative hybrid optical fibre designed to optimize both sensing accuracy and installation robustness. The hybrid configuration consists of alternating sections of fibres with acrylate and polyimide coatings. The central polyimide segment, which is responsible for high strain-transfer accuracy, is embedded within the composite laminate, while the acrylate-coated portions are welded to the polyimide-coated ones and positioned at the composite exit region, thereby improving handling and reducing the risk of unexpected breakage thanks to the greater flexibility of the acrylate. To experimentally validate this design, carbon/epoxy composite specimens were manufactured using twill woven pre-pregs, with the hybrid optical fibre embedded along the longitudinal axis. Tensile tests were performed to evaluate the ability of the embedded polyimide section to accurately record mechanical strain through DFOS based on the Rayleigh backscattering principle, using the Luna ODiSI 6100 system as an interrogator. The results confirmed that the polyimide-coated region effectively captured the local strain field, enabling the determination of the elastic modulus in agreement with values previously reported for specimens instrumented with fully polyimide-coated fibres. Subsequently, a fatigue testing campaign (tension-tension mode) was carried out to further assess the measurement capability of the DFOS under cyclic loading. The fatigue behaviour of specimens with embedded DFOS and specimens without sensors was compared to evaluate the influence of the embedded DFOS in carbon composite laminates.