An experimental procedure for the determination of structural damping in a material specimen at low frequencies (<20 Hz) and low strain amplitudes (<10−5) is presented. The experimental apparatus entails a beam specimen supported under free-free boundary conditions and vibrated in first-mode flexural resonance. All experiments were carried out in vacuum at frequencies near 20 Hz, and data were automatically acquired using a system of electronic equipment. A measure of structural damping was obtained by reducing the acquired data using a linearized logarithmic decrement technique. Damping results are presented in the form ψ = Δ W/W, where ΔW is the energy dissipated during each loading cycle and W is the maximum stored energy.
Experimental results discussed concern the structural damping of the following specimens: 6061-T6 aluminum, 6061 (annealed) aluminum, and 0 and 90 deg orientation continuous fiber P55Gr/6061A1 (graphite/aluminum) metal matrix composite. Damping values were determined for strain amplitudes of 6 × 10−6 to 20 × 10−6 and displayed an experimental scatter of 5 to 10%. An experimental technique to quantify extraneous apparatus losses is proposed, which estimates these losses to be approximately 10% of the measured damping value.
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