Evaluation of a UAV composite wing spar repair using an embedded optical fiber Rayleigh back-scattering distributed strain sensing

A representing damaged UAV wing spar cap was repaired using bonded unidirectional composite materials. For such a repair concept, its strength and long-term durability depend on optimizing the geometry of the repair in order to minimize stress concentration in the adhesive. In order to assess the stress distribution along the repair bond-line, an optical fiber was embedded during the repair application for distributed Rayleigh back-scattering strain measurement. The fiber was placed inside the structure, therefore enabling taking strain measurements, at a high spatial resolution, under the wing skin directly on the spar repair. This type of sensing concept can also be used to monitor this critical repair over time. It also coincides with the recently introduced airworthiness requirements for UAVs, where the substantiation of structural bonded joint can be based on: "repeatable and reliable non-destructive inspection".