Dynamic Analyses of Two-Dimensional Lattices

摘要

The dynamic properties of two two dimensional lattice structures for large space structures were investigated both analytically, using the COSMIC NASTRAN finite element code, and experimentally, using an HP5451C Fourier analyzer, its accompanying modal analysis software, and a procedure called experimental modal analysis. One of the lattices was composed of five identical repeating substructures (5 bay beam). The other lattice was composed of twenty two identical repeating substructures (22 bay beam). For a frequency range extending from 0 Hz to 20 kHz, the NASTRAN analyses identified approximately 180 natural frequencies and mode shapes for each of the structures. The experimental modal analyses (limited by the frequency content of the impact excitation force) identified 34 mode shapes and natural frequencies of the 5-bay beam, and 18 mode shapes and 30 natural frequencies of the 22-bay beam. The COSMIC NASTRAN-predicted natural frequencies and the natural frequencies measured using experimental modal analyses agreed to within seven percent. The NASTRAN frequencies were consistently lower than those measured using the Fourier analyzer, probably because of inaccuracies in the finite element modeling of the intersections of the substructures. The Fourier analyzer also generated modal parameters (natural frequencies, damping ratios, and complex residues) for the two structures. In general, these parameters can be used to predict the impulse response, h(t), of a structure.