19.1 Introduction

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Damping is the phenomenon by which mechanical energy is dissipated (usually by conversion into

internal thermal energy) in dynamic systems. Knowledge of the level of damping in a dynamic system is

important in the utilization, analysis, and testing of the system. For example, a device with natural

frequencies within the seismic range (that is, less than 33 Hz) and which has relatively low damping,

could produce damaging motions under resonance conditions when subjected to a seismic disturbance.

This effect could be further magnified by low-frequency support structures and panels with low damping.

This example shows that knowledge of damping in constituent devices, components, and support

structures is important in the design and operation of complex mechanical systems. The nature and the

level of component damping should be known in order to develop a dynamic model of the system and its

peripherals. Knowledge of damping in a system is also important in imposing dynamic environmental

limitations on the system (that is, the maximum dynamic excitation the system can withstand) under

in-service conditions. Furthermore, knowledge of a system’s damping can be useful in order to make

design modifications in a system that has failed the acceptance test.

However, the significance of knowledge of damping levels in a test object for the development of test

excitation (input) is often overemphasized. Specifically, if the response spectrum method is used to

represent the required excitation in a vibration test, then there is no need for the damping value used in

the development of the required response spectrum specification to be equal to the actual damping in the

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© 2005 by Taylor & Francis Group, LLC

test object. The only requirement is that the damping used in the specified response spectrum be equal to

that used in the test response spectrum. The degree of dynamic interaction between the test object and

the shaker table, however, will depend on the actual level of damping in these systems. Furthermore,

when testing near the resonant frequency of a test object, it is desirable to know about the damping in the

test object.

In characterizing damping in a dynamic system it is important, first, to understand the major

mechanisms associated with mechanical energy dissipation in the system. Then a suitable damping

model should be chosen to represent the associated energy dissipation. Finally, damping values

(model parameters) should be determined, for example, by testing the system or a representative physical

model, by monitoring system response under transient conditions during normal operation or by

employing already available data.

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