Quality Study using TeraHertz (THz) Technology of Surface Bonded Fiber Optic Sensors

Structural maintenance may be supported by prompt detection of damage insurgence, for instance, moving towards condition-based/completely autonomous rather than scheduled approaches in response of a Industry4.0 expectations. But a health and usage monitoring needs a sensor system to be properly defined and integrated, even by machine robot. One of the most promising sensors for structural monitoring are based on the use of optical technologies but their surface bonding is sometimes a challenging issue, depending on the structural irregularity level or also on the strain level to reach. Bonding procedure is a crucial point for a reliable measurement process. Indeed, it is necessary that the bonding is resistant and that the adhesive has high adhesion performance. For this purpose some aeronautical carbon fiber reinforced plastic (CFRP) and metallic samples are used. For some of them a surface treatment is provided. On each one of the coupon, up to three fiber optics are bonded by using different structural adhesives: M-Bond200, M-Bond AE10, M-Bond GA2. The coupon are then loaded under a tension solicitation up to 1%. Then a comparison of the adhesion quality before and after the mechanical tests is performed through a diagnostic technique operating at THz (0.03 - 3 mm in wavelength). The metrological analysis and estimation of the results obtained by the THz inspection showed different performances and interesting results regarding the bonding quality.