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Assessment of Fibre Optic Sensor Architectures for Structural Health Monitoring



Fibre Optic Sensors (FOS) present several advantages over their conventional electrical counterparts for Structural Health Monitoring (SHM) of aerospace structures. These sensors are dimensionally small and can be readily embedded into composite structures, or bonded to their surface with minimal effect on weight or aerodynamic characteristics. They also do not generate, and are immune, to Electro-Magnetic Interference (EMI), thus they do not affect neighboring electrical systems or avionics. Fiber Optic Sensors have been widely used for load monitoring in discrete or distributed architectures. Additionally, Fibre Bragg Grating (FBG) sensors, a family of FOS, have been explored for the detection of material acoustic waves and damage. Three sensing system configurations, coupled with the use of FBGs, have been studied and reported in the literature for the detection of material acoustic waves; namely, (1) the use of tunable lasers as light source and photodiodes as the light sensors; (2) Broad Band light Sources (BBS) and Arrayed Waveguide Gratings (AWG); and (3) BBS with cascaded, or pairs of phase-shifted FBGs. The first two configurations also offer the ability to perform load monitoring, through strain measurement, in addition to material acoustic wave and damage detection. In an effort to advance the field of Structural Health Monitoring and its implementation in aircraft applications, these three sensing system architectures were evaluated for their capability and ease for the detection of material acoustic waves and damage. Tests were performed using a realistic and aircraft representative skin composite panel, a quasi-isotropic Carbon Fibre Reinforced Polymer (CFRP) skin panel. In the performed evaluation, material acoustic waves generated by a piezoceramic transducer were accurately and reliably detected using the first (1) and last (3) techniques. The first architecture was implemented effortlessly and the last presented, inherently, less complexity. The use of the AWG, in the second (2) architecture, presented comparatively additional challenges to the detection of the generated acoustic waves due to the requirement for a high power light source. As a result of this evaluation, the first (1) technique was subsequently selected and assessed in damage detection trials with promising results. Due to the demonstrated success of these trials, the experimental composite skin panel, along with the demonstrated damage detection technique, are being integrated into the SHM infrastructure being developed at the National Research Council Canada (NRC). This infrastructure consists of structural platforms that range in complexity from a simple 2 m long aluminium beam to full scale aircraft structures – a CF188 wing and a Bell 206 helicopter tail boom. Additionally, such infrastructure offers full scale evaluations and testing of damage detection techniques and technologies and SHM capabilities employing realistic loads and spectra optionally in varying environment conditions.

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