5.1. PART 1: PRELIMINARY EXPERIMENTAL MODAL ANALYSIS 91
root of the summation of the autospectra for all modal test response accelerations and (b) a
global FRF skyline function. e global FRF skyline function is defined as a complex frequency
domain function whose real and imaginary parts are the sum of the absolute values of the real
component of response FRFs and the sum of the absolute values of the imaginary component
of response FRFs, that is,
SKY.f / D
N
H
X
kD1
j
real
.
H
k
.
f
//
j
C i
N
H
X
kD1
j
imag
.
H
k
.
f
//
j
: (5.12)
A display of the two global skyline functions for the ISPE test article is provided in Fig-
ure 5.8.
ISPESTSTD Skyline Functions
PSD
1/2
Skyline
2015 25 30 35 4540 50 55 60 65
Frequency (Hz)
2015 25 30 35 4540 50 55 60 65
Frequency (Hz)
2015 25 30 35 4540 50 55 60 65
Frequency (Hz)
10
-2
10
-3
10
-1
10
0
|FRF| Skyline
10
-2
10
-1
10
0
phase(FRF) Skyline
1
0.8
0.6
0.4
0.2
0
Figure 5.8: ISPE test article global skyline functions.
e phase FRF skyline offers the clearest indication of the presence of many modes in the
15–65 Hz frequency band. While the skylines indicate the presence of “many modes,” further
investigation of local response FRFs will provide insight into the presence of additional localized
modes (a difficult, slow task for a structural test article that is mapped by 265 response channels
and 4 excitations, that is 1060 individual FRFs!). Other strategies for discerning the presence
of “many modes” as well as closely spaced modes have been developed and successfully applied
by leaders in the experimental structural dynamics community [3].
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