Structural damping depends on materials, joints, boundary conditions, friction, acoustic radiation, etc. Damping may be nonlinear, increasing as the excitation and response levels rise. The damping for a given structure or component must be measured!
But here is some empirical background data…
Papers
Damping Properties of Materials
Damping Values in Aerospace Structures and Components
Free Vibration with Coulomb Damping
Related Blog Posts
Vibroacoustics/Statistical Energy Analysis – empirical formulas for loss factors
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Vibration Absorbers & Tuned Mass Dampers
Isolation
Transmissibility of a Three-Parameter Isolation System
Webinar 43 – Two-degree-of-freedom System, Two-stage Isolation
– Tom Irvine
Vibrationdata 
Outstanding assortment of technical documents and reports outlining all of the possible analysis techniques that one needs regarding damping! Thanks for all you do!
Thank you for all the references! Damping is a tricky thing….
Thank you very much for sharing these valuable references
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