Material process monitoring with optical fiber sensors
Burford, Mary Kathleen
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Our motivation for this work is based on the need to rnom tor the cure and inservice health of composite materials. We describe the continuation of an effort to design a multi-functional fiber optic sensor which can be embedded in polymeric composite laminates for monitoring the degree of cure during its fabrication, as well as internal composite strains occurring post-cure.3 In short, this dual-purpose sensor combines the characteristics of a Fresnel reflectometer with those of the extrinsic Fabry-Perot interferometer. For monitoring cure, a broadband source is used so the output intensity of the sensor is amplitude-modulated as the refractive index of the conlposite is increased during the polymerization process. Post-cure, a coherent light source is implernented so a. sinusoidal variation of the output signal occurs when strains within the composite cause the sensor output to be phase-modulated. We demonstrate the measurement of refractive index with the Fresnel reflectometer/EFPL and test it as an embedded refractive index monitor. Our experimental results demonstrate that the refractive index of 5-nlinute epoxy increases by approximately 2 % during the cure process. In addition, the sensor can be used as an interferometer to measure intenlal composite strains, where the phase difference between consecutive fringe peaks is one-half the wavelength of the source.
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