oreos where this happens ore the coupling qf substructures and the identification of the ... also shown that the need for o rotational exciter is uwided. A numerical ...
SOME
APPLICATIONS OF COUPLING/UNCOUPLING IN STRUCTURAL DYNAMICS
TECHNIQUES
PART 3: ESTIMATION OF ROTATIONAL FREQUENCY-RESPONSE-FUNCTIONS USING MUM Maia, N. M. hl., Silva, J. M. M., Ribeiro, A. M. R. Institnto
Supaior’fknico,
Dep. Eng. Met., Av. Rovisco
ABSTRACT This article oddresm the problem of rotational FrequencyReslJo”,~e-Functions (FRFs) i.e., those FRF~s thor involve relations of the type displacementlnlonrent, onylelforce and unglelmornent. They may ronstitute 75% qf the whole FRF matrix a n d c a n n o t b e i g n o r e d . P r a c t i c a l dificu1tie.r i n measuring angular motion and in applying exciting moments have prevented the wide USE of such inrportont information. However, the Modal Anol,ysis conmmuni~ has b e e n ocbtowledging in the post j&v years that in .some applicatmns the need of rotational FRFs can be detern~inont to the success of the performed calculations. Some .sensitive oreos where this happens ore the coupling qf substructures and the identification of the dynamic behoviour of joints, ,just to mention a few. The authors expect this popei- to give o contribution to the issue, as o method for reliably e s t i m a t i n g r o t a t i o n a l FRFs w i t h o u t a c t u a l l y having to nteawre them (or the most part them) is presented. It i.y also shown that the need for o rotational exciter is uwided. A numerical example shows the effectiveness oj’the promos.
of
1 INTRODUCTION In the companion articles of this one Il.2 I, the authors have proposed new techniques for Modal Analysis, first as a means for the cancellation of the added transducers masses and, in a second stage, allowing for the estimation of the annpletc ~‘requency.Response-Function (FRF) inertmu: matrix, from sets of meawrements using just a single rnabix column. The authors called the latter technique the Mass IJncoupling Method (MLtil). Inherent to MUM is the maw cancellation procedure, so that the estimated FRFs are the genuine structure ones, i.e., they are obtained free of the influence of the masses of the attached tramducers. ‘The examples shown in the above referred papers have allowed the generalisation of the process whenever the cc-ordinates in question are only translation ones. However, it is known that in practice it is often desirable to obtain rotational FRFs, rclati~tg rotations to
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(:odcx, Portugal
fOrces aad rotations to moments. It is commonly stated, and it is true, that rotational IXFs xc present, in most cases, in 75% of the whole FRF inertance matrix. Their knowledge can therefore be of cxtnxne importance if one wishes to obt.aiu reliable results when doing calculations related to coupling, s t r u c t u r a l modification, Finite Elements (PI
