Identification of the dynamic characteristics of nonlinear structures

60
 
F r e q u e n c i e s
.
N
.
.
 
 1 
 .
 .
i t u d c
Fig.2.30 Analysis Results of Beam/Absorber FRF Data
The Frame structure, as shown in figure 2.31, is an artificially-nonlinear structure which
was designed for the purpose of the nonlinearity location study in Chapter 5. Typical
measured FRF data with constant force input are shown in Fig.2.32, and from these, it
can be determined that the structure exhibits both stiffness and damping types of
nonlinearity. One of the FRF plots was analysed and the results are shown in Fig.2.33.
Fig.2.3 1 Artificially-Nonlinear Frame Structure

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2 Identification of Nonlinearity Using First-order 
61
9 0 . 5 0
H t .
9 6 . 0 0
Fig.2.32 Frequency Response Functions of Frame Structures
Damping 
N a t u r a l F r t q u t n c
R t c t p t .
R t c t p t .
Fig.2.33 Analysis of measured Frame FRF data
A practical structure known as the “NASTRAN Tower” was also investigated. The
structure was known to possess certain type(s) of stiffness nonlinearity 
although the
exact nature of the nonlinearity is still unknown. In the measurement, neither force nor
response control was used. The measured data and the analysis results are shown in
Fig.2.34 from which, it can be seen that the structure possesses a softening stiffness
nonlinearity, probably a softening type of backlash stiffness since the structure remains
linear within certain response range and then loses stiffness as response level increases.

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Identification of Nonlinearity Using First-order 
62
4 0 . 0 0