The effect of stiffener parameters on the farfield sound radiation from composite beams: Experiment and theory
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The effects of spatial and material variations on farfield sound radiation from carbon/epoxy composite beams were investigated. Specifically, the geometry of a single stiffener of carbon/epoxy embedded in a beam, periodically spaced stiffeners along the length of a beam, and single stiffeners of different materials embedded in beams were all investigated. Because of a lack of methods to sufficiently model the details of spatially varying material properties, experimental data were used to identify important parameters that needed to be included in a model. Vibration testing was performed for three frequency ranges: 500 Hz to 1500 Hz, 1500 Hz to 2500 Hz, and 2500 Hz to 3500 Hz. The geometry of a single stiffener of carbon/epoxy affected the region of the stiffener that radiated sound. Periodically spaced stiffeners tended to have a global stiffening effect at lower frequencies and to act like individual stiffeners for frequencies where the wavelength is shorter than the stiffener. A stiffener of viscoelastic material reduced the sound power radiated from the region of the stiffener, and stiffeners of other materials radiated sound from the ends of the stiffener. Vibration experiments were performed on fiber-reinforced composite beams to verify a cubic spline based solution of a simple Euler-Bernoulli beam model. The model seemed to accurately predict the shape and location of farfield sound radiation from stiffeners. However, limitations on the available boundary conditions prevented accurate prediction of the magnitude of sound radiation. The model was then used to predict sound radiation from beams with varying stiffener bond lengths and these results were experimentally verified. In general a longer stiffener bond length causes less sound radiation. However, there seems to be a critical length at which the sound radiation reaches a maximum.