Damage detection in composite aircraft wing-like test-box using distributed fiber optic sensors

Structural Health Monitoring Systems for the aircraft structures are intended to detect the location and size of the damage with minimum downtime of the aircraft. As the damage produces localized strain changes, a sensor network of many sensors bonded/embedded throughout the structure is required for damage diagnosis. This paper describes a distributed fiber optic sensor's demonstration to detect the disbond damage in a typical aircraft wing-like structure where a single optical fiber can cover a large area. Distributed fiber optic sensor is being bonded along the bolt line of the test-box. Controlled disbond are created by the removal of the bolts. In the pristine state, the strain signature/map is measured under applied load, which act as a reference in this case. In this approach, localized strain signature difference between the reference and disbond-structure is used to detect the damage. A non-dimensional quantity damage index (DI) is calculated based on the normalized sum of the square magnitude of strain difference of all sensors to indicate the damage severity. We have found that the DI value greater than or equal to 0.2 indicates the damage region through the iterative approach. Further, the algorithm is validated by loading the structure with disbond at different load levels. The maximum error in the disbond estimation is found to be ∼11 mm. This system and methodology hold immense promise for a ground-based method for detecting damage (disbond) in the aircraft/unmanned aerial vehicle structures (UAV).