ON STRUCTURE IDENTIFICATION BY TRANSIENT VIBRATION TESTING T. Gh. Cioara’, D. Dragomir-Daesai, L. Bereteu’ ‘Vibration Testing and Research Laboratory University “Politehnica” of Timisoara, Str. Sotin TiteI Nr. 12 1900 Timisoara, Romania e-maiI:cioara&nec.utt.ro ABSTRACT Classical stnxcture identification is not an easy operation even for a label-atory investigation. The experimental lnvestrgation requests a complex setup, the most difficult being the application of the exciting forces which are necessiuy to be correct axed on the structure in respect with the dynamical model adopted to be identified. i’hysicaliy, in the iorce application point occu iaterai force components or moment components which axe out of experimental control by force transducers. Usually, in the site experiments is quite difficult to respect the co~mect controlled excitation of structure by an electromwetic or hydraulic shaker. In this conditions a transient system of excitation is more suitabkthe modal frequenties, modal damping ratios and modal vectors can be estimates from the free decay vibration of stmcture. For the structures identification procedure by modal analysis it is necessary to orthonormalize the modal vectors. This is possible only knowing the parameters of the excitation when the Frequency Response Functions are obtained by vibration testing. In this case the modal vectors are not extract direct from the FRF by modal analysis and for complex vectors they are obtained on a complex procedure. From free decay testing the modal vectors are obtained by modal analysis but in an orthogonal form, for which the structure identification is undetermined. This can be overcame by a know mod&cation of the stnxcture. ‘Ike most suitable being the mass modification. The modification of structure lead to a new set of values for the modal parameters and new modal vectors, which give supplementary equation for identification system. It is demonstrated and verifkd on a dynamic discrete model of n=4 degree of freedom that excepts for n=2 only one modification of this structure is necessary. Also is demonstrated that from FRF the structure identification can be obtained direct, the modal analysis being not necessary to be performed [2]. NOMENCLATURE [M],[C],[K] Mass, Damping and Stiffness matrices (q(t)1 generalized position vector 0 L 2% circular frequency r-th complex conjugate eigenvalues Pr I--th damped frequency “r r-th damped factor ir;d,;jc.) I--th compiex conjugate vrctox~s (m),(c),~) column vectors of the mass, damping and stiffness mattices enters zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED ID.1 r-th mode transformation vector to mat& FRF Frequency Response Function FTF Fourier Transform Function 1. INTRODUCTION The dynamic equation of motion for an n degree of freedom mechanical nongyroscopic system can be represented by a set of simultaneous differential equation of second order where [Ml, [C!] and [K] axe (nxn) mass, damping and stiffness symmetrical matrices. The system (1) can be written in the first order form [‘+I + [B]iZI = l.?(t) / (2) where the motion vector (z) is of form {z)~ = ({q)Tl (v] and the matrices [A] and [B] and the column vector (Z(t)) are 1202