50 M. Wall et al. Peak Velocity (m/s) Damping Ratio Modified Surface: Damping Curves for Varying Preloads Mod-Surf: 21.2 kN. Mod-Surf: 31.8 kN. Flat-Surf: 21.2 kN. Flat-Surf: 31.8 kN. 6e-4 Material Damping 10–3 10–4 10–2 10–1 Fig. 5.9 Damping plots for modified surface and flat surface. 1×and 1.5×nominal preloads shown against experimental data with ζ =0.0006 linear damping subtracted off Peak Velocity (m/s) 325 10–2 10–1 326 327 328 329 330 331 332 333 334 335 Nat. Freq. (Hz) Modified Surface: Natural Frequency Curves for Varying Preloads Mod-Surf: 21.2 kN. Mod-Surf: 31.8 kN. Flat-Surf: 21.2 kN. Flat-Surf: 31.8 kN. Experimental Data Fig. 5.10 Natural frequency plots for modified surface and flat surface. 1×and1.5×nominal preloads shown against experimental data investigation was performed in which the actual measured curvature of the surfaces was included in the FEM, and even though the geometry only changed by a few thousandths of an inch (about 100μm), this had a dramatic effect on the contact pressure and on the predicted stiffness and damping of the joint. We expect that even larger differences may be observed once the FE mesh is refined to better capture the surface geometry. In retrospect, considering how stiff the structure/material is, it should have been obvious that the surface geometry would be so important. Moving forward, the existing FE Model is also being used to compute the nonlinearity in the sixth mode of vibration, which was experimentally found to exhibit even stronger nonlinearity than Mode 2, which is the mode that was studied here. New profilometry measurements will be obtained and used to create a new FE model, whose mesh will be tailored to capture
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