Helicopter rotor systems are increasingly using flexible composite structures to provide the required control movement for the rotor blades. These structures such as rotor head flex beams can experience very high surface strains, which can be in the order of 15,000 με. This makes it difficult for them to be monitored using conventional surface bonded strain sensors.
Helicopter rotor hubs incorporating thick composite flexures are subjected to delamination failures with correlate to bending excursions. Measurement of this deflection could be used to predict remaining useful life of the flexure and other hub components.
Previous efforts to measure surface strains using fiber optic sensors led to mixed results and prompted an effort to explore embedded sensors. As part of a Vertical Lift Consortium project, the authors tested proof of concept manufacturing specimens to establish the ability of the sensors to survive the cure process in a closed cavity mold tool. Multiple fibers were evaluated based on their post-cure properties. Using the selected fiber type, the team designed and built full scale flexure elements both with and without embedded sensors to evaluate the ability of the fibers to accurately correlate flexure strain to bending angle. Testing also established the effectiveness of temperature compensation using a strain-free reference sensor. Finally, fatigue testing of elements both with and without embedded sensors established that the embedded sensors did not adversely affect the laminate durability. Post test investigation showed the delamination failures did not initiate at embedded sensor locations. It was therefore concluded that the embedding of fiber optic strain sensors are a possible solution to measure strains in helicopter rotor components.