4 - Memorial University of Newfoundland DAI

6.5 Impact Testing
Impact testa were carried out to test the validity of tllc rcsosiutt peaks on
the output power spectrum from the wave indumd vibmtlons of tllc ml~nclel
structure. This mode of testing was chosen, bceausc it is n quick any to
snalyse the dynamic characteristies of a str~cture. Bcsidcs, tine molhod urta
short duration broadband spectra inpub that oxcite all freqatencirn withila
the useful l~quency nnge. Thc usable frqacncy rango of all in,pttlsc de-
pends on the shape and time duration ol the lmyslsc. As n rcralt, to cllxt~re
there was sufficient forco aver the frqacncy range of i~~tcrert, the fint zc~u
cmssing oflhe Fotttisr Transform of the impulsewas wcll above tllc mnxi~annn
frequency range of interest for each pulse. (In ems wlloro this o1~jecl.i~~ wn*
not satisled, the mulls were discarded.) Thc Pnt ;..m ermsing occonrs nl
the lowest fmquency (i.c, tho inverseof the timoduration of the ptal~~). 'l.11i.
was achieved by ensuring tho duration of tltc pulse wiu 2Al-a gc:scrally
accepted rule of thumb used by timeseries analysts [TI].
Since the ',ration of impulse is "cry short compared with the s;uuplc:
length, the total energy of noise represenled in tltc timc-mmplecan Ihe of thur
order of the energy ol the impulsc, cvcn lor high sigrnnl-to-noisr nrtica. 'I'ia:
noise problem can be made worse when ssing lhc rarm trannlornn to yicrlci
increased resolution in a given frequency ran& by ellcetivcly incn!iuix~~ the!
sample length.
To alleviate them problems, special time windows woe ernploy