Use of Plastic Models for Determining the Vibration Characteristics of Large Structures

Abstract

The vibration-transmission characteristics of large structures have been investigated by means of tests on plastic models during studies of methods for reducing the amount of machinery vibration transmitted to the hulls of naval ships. Force and acceleration instrumentation suitable for the vibration testing of plastic models is described and the advantages of plastic as a model material pointed out. The sonic velocity and compressional loss factor of Plexiglas are given as functions of frequency. Vibration spectra measured on shaker-driven plastic models of some large steel structures are compared with similar data measured on the prototypes. The results show that the resonant frequencies, average vibration levels, and mode shapes of large structures are obtained quite accurately from tests on plastic models if the models are accurately scaled, but that the levels of the response peaks are only approximate. For accurate prediction of peaks, the material damping of the plastic must be equal to the effective frictional joint damping of the prototype. [Work supported by the Bureau of Ships, U. S. Navy.]