Cross-Correlation Based Algorithm for SHM De-bonding Analysis of Typical Aeronautical Structures via OFDR

An SHM algorithm to detect damage size and location without the need of either strain or load reference was proposed and tested. The proposed methodology is based on the physical connection between the local signal scattering and the structural discontinuity, with respect to both geometrical distribution and time evolution. In detail, first order derivative is first applied, followed by a classical cross-correlation process to estimate the degree of relationship between the analysed signals. Deviations are quantified with respect to auto-correlation values. This approach was tested on a complex beam, made of two flat unidirectional carbon-epoxy tape skins and two C-shaped profile spars bonded together by a structural paste adhesive. Experiments consisted of making that test article undergo a 3-point bending by a dedicated tool, equipped with a linear actuator. In order to detect the debonding occurrences within the bonding line of the spar cap, optical fibres were integrated in the primary structure. Optical Frequency Domain Reflectometry (OFDR) was applied, implementing a 2 m long fibre optic having a spatial resolution of 2.6 mm and interrogating at a max sampling rate of 250 Hz, to monitor the spar bonding line. The results showed that the SHM algorithm provided reliable measurements for both position and size of the disbonding, and pose the right bases for further investigations on larger aeronautic scale.