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    Composite Rotorcraft Flexbeams with Viscoelastic Damping Layers for Aeromechanical Stability Augmentation

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    The objective is to experimentally determine the damping levels that can be achieved by cocuring constrained viscoelastic damping layers in a composite beam. Beams were clamped in a cantilever condition and excited using piezo actuators. Natural frequencies were identified using frequency response data. Transient response data were collected using strain gages and analyzed using logarithmic decrement, Hubert transform-based, and moving block damping identification algorithms. Two sets of experiments were performed. First, benchtop tests of four fully constrained layer rectangular-cross-section laminated composite beams were conducted. The cocured beam consisted of a six-ply graphite/epoxy (Gr/Ep) composite host structure, a damping layer of viscoelastic material (VEM) that varied in thickness from 0 to 15 mils in 5 mil increments, and a 2-ply Gr/Ep constraining layer. The natural frequency exhibited a gradual increase with VEM thickness, and damping increased from 0.75% in the beam with no VEM to 5.40% in the beam with a 26.2% volume fraction (15 mil layer) of VEM. The second set of experiments involved two I-cross-section laminated composite beams in rotation. Each I-beam flange had a layup identical to that of the beams used in the benchtop experiments, with damping layer thicknesses of 10 or 15 mils, and the web layup was the same as the flange, minus the VEM layer. These beams were tested at rotational speeds from 0 to 750 rpm in a vacuum chamber, and transient response data were analyzed. The natural frequency increased quadratically as rotational speed increased. Further, as the viscoelastic layer thickened from 10 to 15 mils, the increase in damping remained nearly constant even as the rotational speed increased. At 0 rpm this increase was 33.8%, while at 750 rpm the increase was 33.3%. Structural damping decreased as a function of rpm, due to centrifugal stiffening.


    constrained layer damping, viscoelastic damping, damping in composites, stability augmentation, moving block analysis, Hilbert transform

    Author Information:

    Smith, CB
    Graduate research assistant and assistant professor, University of Maryland, College Park, MD

    Wereley, NM
    Graduate research assistant and assistant professor, University of Maryland, College Park, MD

    Committee/Subcommittee: E28.02

    DOI: 10.1520/STP11739S