Dynamic Moduli and Damping Ratios in Fiber-Reinforced Composites

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Abstract
Measurements of elastic moduli and damping ratios are reported for glass fiber reinforced, unidirectional composite beams when they are vibrated over a frequency range of 5.0-10,000 Hz. The elastic moduli show little tendency to change within this frequency range, except when the fibers make an angle of 45° with the beam axis, in which case the modulus is raised about 15 percent above its low frequency value. The low frequency values are 7-27 percent higher than the static values. The damping ratios are typically of magnitude 10-2 but exhibit both frequency and amplitude dependence. At high frequencies, the damping ratio for a 0° beam is almost five times that of an aluminum beam, while for a 22.5° or 45° beam it is over thirty times as large. The increase in ratio magnitude with change in fiber orientation angle usually occurs in the order 0°, 22.5°, 90°, 45°. The damping ratios transform as the imaginary parts of complex moduli. If the damping ratios are known for three fiber orientation angles, they may be predicted for any other angle. In this respect, the composite exhibits anisotropic, linear viscoelastic behavior when undergoing small oscillations.

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