Feasibility study of embedded fiber-optic strain sensing for composite propeller blades

Composite propellers are subject to stringent quality control, extensive developmental testing, and ongoing health assessment. ne feasibility of embedded fiber-optic smart instrumentation was examined for use in the commercial manufacture and testing processes used by Hartzell Propeller. Embedded extrinsic Fabry-Perot interferometric (EFPI) fiberoptic strain sensors were used for cure monitoring and material testing of a carbon-graphite plate. Strain was measured during the hot press cure of the four-ply 90/0/0/90 test article. Next, directional strain dependence on temperature and pressure was determined through controlled surveys. Finally, the fiber-optic strain readings were compared to surface resistive strain gage measurements during a tensile test. The cure strain data showed the effects of thermal mismatch between composite, resin, and the press platens. The temperature and pressure surveys validated the use of the sensors over a wide-range of environmental conditions, and the tensile test showed good correlation between embedded and surface strain measurements. Hence, the embedded sensors can provide information on internal strain that was extrapolated from external measurements previously. Experimental implementation of the sensors for cure monitoring and for developmental testing of propellers is ongoing.