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Title: Theory and practice of modal identification
Author: Kirshenboim, J.
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 1982
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This thesis is concerned with the development and refinement of some aspects of the 'modal testing method' The basis of the research is a definitive theoretical description of the two most common models of damped linear systems (i.e viscous and hysteretic damping). The normal mode shapes of such systems are generally expressed in complex terms and experimental observations using the single-point excitation method tend to confirm this. On the other hand, the normal modes used in finite element calculations and those derived by the traditional multi-point excitation method are the real undamped modes. The exact relationship between the complex normal modes and the real undamped modes is established and a theoretical study investigates the relationship between the level of the 'complexity' of the normal modes and the closeness of the natural frequencies showing that for well-separated natural frequencies the 'complexity' of the normal modes is very small. Experimental results from real structures, however, have so far produced larger values of 'complexity'. It is shown that this is caused by the nonlinear behaviour of the measured structure and measuring procedures which give emphasis to this part of the structures response. A numerical study of some simple theoretical systems shows the influence of small nonlinearities on the linearly-derived modal parameters and an experimental study on a real structure serves as a vehicle on which the methods developed in this research to tackle nonlinearities are demonstrated. Although it is theoretically possible to derive the complete matrix of the normal modes from measurement of one column of the mobility matrix, it is usually found that measurement of more columns is needed. As a result, several different estimates for the modal parameters are derived, the quality of each of those sets is then quantified by a 'quality factor'. A 'best' consistent set of modal parameters is derived by an optimization algorithm which makes use of the quality factor and thus a large amount of measured data is reduced to a set of parameters which can be used in any further theoretical calculations. Finally, an experimental study of a typical air frame structure demonstrates the single-point excitation identification methods developed in this research and a comparison is made with the experimental results obtained by the traditional multi-point excitation method.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available