22 Characterizing the Dynamic Response of a Foam-Based Testbed with Material, Geometric, and Experimental Uncertainties 175 Table 22.1 Simulation material assignments Material name Assignments Model type Density, ρ (Mg/mm3) Poisson’s ratio, ν Young’s modulus, E (MPa) Test data type Model order, n Aluminum 6061-T6 Accelerometer Mount Blocks, Baseplate, Baseplate Riser, Center Mass, Baseplate Cap Linear Elastic 2.71263 × 10−9 0.33 6.82581 × 104 N/A Stainless Steel 440C Washers, Load cell 7.79936 × 10−9 0.28 1.99750 × 105 Alloy Fe-9Ni Nuts, Threaded rod 7.83331 × 10−9 0.31 2.04039 × 105 Mock Accelerometer Accelerometers 5.33087 × 10−9 0.28 1.99750 × 105 SX358 Lower Lower foam pad Hyperfoam VARIED N/A Uniaxial Test Data 2 SX358 Upper Upper foam pad Fig. 22.4 First excited experimental modal frequencies accelerations and calculation of the modal frequencies based on a frequency-domain analysis – which followed the same process as the experimental query using white noise. 22.3 Results and Analysis Figure 22.4 shows the first excited modal frequencies that were extracted during experimental data processing. A summary of results from the conducted experiments can be seen in Fig. 22.4, where only first detected modal frequency in the vertical direction is shown for varying preloads, foam densities, and foam thicknesses. Results show an emphasis toward the foam material properties having a prominent effect on the frequency response of the testbed. Preloads in the range experimented on showed small impact compared to material properties on the systems response, with the largest change in the first modal frequency occurring for 2 mm, 0.5 g/cc foam with the frequency at 44.4 N of preload being at 386 Hz and increasing to 438 Hz at 222.4 N preload. The testbed response with respect to changes in the foam’s material properties shows a constant trend: as foams stiffen, their first modal frequency increases. For foams with a constant density, the experimental findings show that increases in the thickness of the foam specimens lead to a frequency decrease. For example, in foam with a density of 0.5 g/cc and a 44.4 N preload, the first modal frequency for 2 mm is 386 Hz, then decreases to 234 Hz for 5 mm, and then decreases to 140 Hz for 10 mm foam. For foam with a constant thickness and changing density, as density increases
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