On the interpretation and application of shock-test results in engineering designs

TLDR: In this paper, the Fourier transform and impedance (FT&IM) method is used to measure the spectrum of a shock wave at the interface of a machine and an equipment.

Abstract: A theoretical study of the characteristics of a shock spectrum which is measured in a shock test of equipment is presented. For this purpose, the machine and the equipment are represented by two systems connected in series; an upper and a lower bound of the spectrum of the shock motion at the interface of the two systems are found using the Fourier transform and impedance method. The spectrum is shown to be affected by the characteristics of both systems. It is demonstrated that in a system containing negligible damping, hills would occur in the spectrum at the resonant frequencies of the combined system, whereas valleys (spectrum dips) would appear in the neighborhood of the fixed-base resonant frequencies of the equipment. The possible effects of damping and of the variation in the characteristics of the system on the phenomenon are also discussed. The results support, at least in the case of systems with low damping, the current design practice of the Navy in which an envelope of a group of the dips of measured spectra is used in creating a design spectra for similar equipment. The analysis developed here would also be useful in understanding the behavior of vibration absorber and other shock phenomena.