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Measuring Vibration in an Advanced Composite Beam with Localized Internal Fiber-Optic Strain Sensors

by David W. Jensen, Pennsylvania State Univ, University Park, United States,
John M. Cory, Jr., Pennsylvania State Univ, University Park, United States,



Document Type: Proceeding Paper

Part of: Engineering, Construction, and Operations in Space III

Abstract:

Two optical fibers embedded with different configurations were used separately as strain sensors to measure the natural frequencies and modal amplitudes of a cantilevered graphite/bismaleimide beam. Both optical fibers looped through the beam in 'U-shaped' paths with egress points at the clamped end. One of the optical fibers was embedded with its entire length a uniform distance from the beam's neutral axis to measure integrated strain due to bending. The other optical fiber was embedded near the neutral axis except in four high-sensitivity regions where local integrated strains due to bending were measured. The beam, which measures 2.0 inches (5.2 cm) wide, 30 inches (76 cm) long, and 0.063 inches (1.6 mm) thick, was fabricated from Narmco G40-600/5245C graphite/bismaleimide prepreg tape in a 12-ply [0₃/90₂/0]_g stacking sequence. The first three natural frequencies of the beam were excited individually using a magnetic excitation source and simultaneously using an impact hammer. One optical fiber at a time was spliced into the sensor arm of a modified Mach-Zehnder interferometer, which provides a voltage proportional to sensor elongation for real-time acquisition of integrated strain data. Natural frequencies measured with each optical fiber were confirmed by vibrational deflections monitored with a noncontact proximity sensor. A closed-form theoretical analysis was also performed to predict natural frequency and modal amplitude data for comparison with the experimental data. The experimental results obtained with each optical fiber demonstrate that embedded optical fibers can be used as strain sensors to provide reliable indications of the dynamic response of a flexible beam. Furthermore, the response of embedded fiber-optic strain sensors can be tailored to increase their relative sensitivity to a particular set of desired modes.

Subject Headings: Fibers | Strain | Probe instruments | Composite beams | Natural frequency | Vibration | Data analysis

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