Modal Analysis: The International Journal of Analytical and Experimental Modal Analysis

Volume 1
1986


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NOTE: The journal articles are listed alphabetically by first author.

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Allemang, R.J.; Brown, D.L.  Multiple-input experimental modal analysis - a
survey.  1)1:37-44; Jan 1986.

ABSTRACT - The technology of experimental modal analysis is expanding
rapidly as the impact of multiple inputs, or references, is being realized.
This paper traces the development of experimental-modal-analysis
techniques while noting this current trend toward multiple-input
utilization in the estimation of system parameters.  Over the past 40
years, at least four general categories of experimental-modal-analysis
methods can be identified as follows: (1) Forced-Normal-Mode Excitation,
(2) Frequency-Response Function, (3) Damped-Complex Exponential
Response, and (4) Mathematical Input-Output Modal.
Experimental-modal-analysis methods in each of these categories can
incorporated multiple-input concepts in one way or another.  Historically,
the modal characteristics of mechanical systems have been estimated by
techniques that fall into either the first or second category.  The
forced-normal-mode-excitation method has always included the
multiple-input concept while the frequency-response-function method,
until recently, only involved the repeated application of single inputs.
This paper reviews both of these methods with emphasis on the current
trends in the refinements of these methods involving multiple-input
concepts.  The experimental-modal-analysis methods that fall into the
last two categories are composite approaches that utilize elaborate
parameter-estimation algorithms based upon structural models and
include multiple-input concepts.  The current developments in these areas
are reviewed as well.


Doyle, J.F.  Application of the fast-Fourier transform (FFT) to
wave-propagation problems.  1(4):18-25; Oct 1986.

ABSTRACT - The time variation of the wave signals is replaced by a
summation over its frequency components.  This is done efficiently by use
of the FFT algorithm but in doing so a number of problems arise.  The
procedures and problems are elucidated by considering the impact of a
beam and the resulting wave propagations.


Engles, R.C.  A solution to the Craig/Bampton eigenvalue problem for
multicomponent structures.  1(2):19-24; Apr 1986.

ABSTRACT - A cost-effective technique is presented to solve the system
eigenvalue problem associated with the Craig/Brampton component-mode
synthesis method.  Subspace iteration is employed in order to take
advantage of the special form of this eigenvalue problem.


Gaught, T.M.; Dobson, B.J.  The transient analysis of a structure using a
modal model.  1(1):11-16; Jan 1986.

ABSTRACT - Experimental modal analysis is used to characterize the
dynamic behavior of a structure in terms of natural frequencies, mode
shapes and damping.  Using the parameters extracted from this analysis a
mathematical model is generated to predict the transient behavior of the
structure subjected to an impact.  A time-marching algorithm is employed
to predict the acceleration response which is compared to experimental
data.  It is shown that a comparison between the measured and predicted
responses can be used to identify errors in the mode-shape parameters
extracted from the experimental modal analysis.


Haisty,  B.S.; Springer, W.T.  A simplified method for extracting rotational
degree-of-freedom information from modal-test data.  1(3):35-39; Jul
1986.

ABSTRACT - A method is presented which uses third-order spline curves
to approximate the deflected shape of a vibrating structure.  These curves
can then be used to determine the rotational degree-of-freedom (RDOF)
contributions to the deflection curve, thereby allowing
structural-dynamics-modification (SDM) techniques requiring this
information to be correctly applied to the modal data.


Haynes, F.D.  Vibration analysis of the Yamachiche lightpier.  1(2):9-18;
Apr 1986.

ABSTRACT - The Yamachiche lightpier, located in Lac St. Pierre, Quebec,
was instrumented with geophones, accelerometers and an inclinometer.  In
order to determine is dynamic characteristics, 15 breakable bolts with
failure strengths from 10,000 to 101,000 lb were used to apply a step
unloading force on the pier.  The damping and stiffness were obtained from
the data in the time domain.  The natural frequencies and mode shapes
were obtained from the data transformed into the frequency domain.  A
modal-analysis computer program was used to verify the natural
frequencies and mode shapes.  A mathematical modal was developed which
includes translation, rotation and shear-beam deformation of the pier.


He, J.; Ewins, D.J.  Analytical stiffness matrix correction using measured
vibration modes.  1(3):9-14; Jul 1986.

ABSTRACT - Correction of analytical stiffness and mass matrices of a
vibration structure by correlating the experimental and analytical models
has recently attracted considerable attention.  This paper proposes a
method to localize the region of errors existing in the analytical stiffness
matrix and introduces an iteration concept into the model-correction
process.  The methods are justified by numerically simulated results
showing that this location technique, together with the iteration process,
brings about a dramatic improvement in the correction of an analytical
stiffness matrix.  Also, it becomes possible to estimate directly the
errors existing in analytical stiffness matrix by using measured modes
available, once the region of errors is pinpointed.  The same methodology
can be applied to mass-matrix correction problem.


Huang, T.C.; Zhang, P.Q.; Feng, W.Q.  Multiple single-input space-time
regression method in modal analysis.  1(4):1-7; Oct 1986.

ABSTRACT - The concept of the multiple single-input space-time
regression method (MS-STRM) in modal analysis is explained.  Its
feasibility, advantages, and ability to deal with systems with repeated
roots, high modal density, or high damping are proved through the
identification of both computer-simulated systems and actual
experimental systems.


Ibrahim, S.R.  Incipient failure detection from random-decrement time
functions.  1(2):1-8; Apr 1986.

ABSTRACT - Changes in the physical properties of structures, such as
cross sectional areas, inertias, and damping properties, can cause
measurable changes in the modal properties of these structures.  Periodic
identification of the natural frequencies, damping factors, and mode
shapes institutes a credible means of nondestructive evaluation for
structural-integrity monitoring and incipient failure detection.  It is
desirable that such periodic testing be performed without disrupting the
operation of the structure under test.  Offshore platforms, bridges and
piping systems are among the structures that may benefit from such
procedures.  This paper deals with a class of structures whose operating
responses are due to some stationary random input(s) plus some possible
harmonic excitation.  These inputs need not be known or measurable.  A
time-domain modal identification technique developed for a limited
number of measurements possible one, with high identification accuracy
and repeatability is suggested as the modal-identification method for this
purpose.  Converting the operational responses to a form usable for
identification will be performed using the multi-measurement-multimode
random-decrement technique.  The theoretical basis and possible
limitations of rht random-decrement technique are summarized.  The
computational requirements for the two combined techniques are simple
and stable and could possible be implemented on minicomputers.


Leuridan, J.  The use of principal inputs in multiple-input multiple-output
data analysis.  1(3):1-8; Jul 1986.

ABSTRACT - In many areas of vibration and acoustical analysis, one needs
to establish multiple-input multiple-output relations.  A typical example
for structural testing is the estimation of frequency response functions
(FRFs) with simultaneous multiple-input excitation.  A typical example in
acoustical analysis is the noise-ranking problem, where the contributions
of different potential noise inputs (or sources) to a given output are to be
established.  Solutions for these multiple-input multiple-output problems
have been worked out for the case that the inputs are not correlated.  Most
solution techniques can handle the case of some correlation between the
inputs, but fail if the inputs become very highly correlated.   The aim of
this paper is to discuss techniques for verifying correlation between
multiple inputs, and between multiple inputs and one output.  Classical
techniques based on ordinary, partial and multiple coherence function
calculations are reviewed.  A new technique that is based on the
calculation of a set of principal inputs is introduced and is compared with
classical techniques.  An example of its application is worked out to
demonstrate the new technique's feasibility and interesting features.


Matzen, V.C.; Murphy, C.E.  On obtaining mass participation factors using
equivalent structures.  1(1):17-23; Jan 1986.

ABSTRACT - Mass participation factors appear in the forcing function of
modal descriptions of the behavior of structures subjected to support
excitation.  Exact calculation of these factors for a given structural
discretization is theoretically possible, but it requires either the full
mass matrix or the full matrix of eigenvectors.  For many applications,
both of these approaches are impractical, and the only recourse is to use
some type of approximate method.  Approximate analytical methods are
based on the use of reduction schemes, which eliminate unwanted DOFs.
After the system has been reduced, the participation factors are obtained
using the same procedures as those used on the original system.
Experimental methods require either modal parameters (modal masses and
eigenvectors) or the structural response in the time domain to obtain the
participation factors.  In this paper, two analytical and three
experimental methods for finding mass participation factors are
described and compared using simulated experimental data.


Mitchell, L.D.  Signal processing and fast-Fourier-transform (FFT)analyzer
- a survey.  1(1):24-36; Jan 1986.

ABSTRACT - This paper will act as a tutorial for the use of
fast-Fourier-transform (FFT) techniques in the analysis of experimental
dynamic data for the purpose of the identification of the physical
characteristics of the measured system.  Consideration will be given to
transducer and system calibration, signal conditioning, the relationship of
sample rate to frequency range, anti-aliasing filter processes,
time-domain windows and their data-leakage effects, channel-to-channel
crosstalk, various algorithms for computing frequency-response function,
and the coherence function.  This paper should make the readers aware of
what FFT signal processing will do for them and what it will do to their
data.  Recent information concerning the ability of the modalanalysis
community to make proper measurements indicates that the expert and the
novice alike need to review the basics of FFT signal processing.


Muszynska, A.  Modal testing of rotor/bearing systems.  1(3):15-34; Jul
1986.

ABSTRACT - Several aspects of modal testing of rotating machines are
discussed.  Advantages of the nonsynchronous perturbation technique used
for identification of rotor/bearing system parameters are outlined.
Several application examples of perturbation testing for rotating systems
are given.  The obtained results are discussed.  The perturbation testing of
rotor/bearing systems has revealed the existence of new modes generated
by solid/fluid interaction.  These modes were unknown in classical modal
analysis.


Rogers, J.D.; McConnell, K.G.  Damping in aluminum-filled epoxy using two
different flexural-testing techniques.  1(4):8-17; Oct 1986.

ABSTRACT - Two flexural-test methods were used to measure the loss
factor of a commercially available aluminum particle-filled epoxy.  The
results were compared to results from the same tests performed on an
aluminum alloy.  Amplitude and frequency effects on the loss factor were
considered for each material.


Shen, C.L.; Tsuei, Y.G.; Allemang, R.J.; Brown, D.L.  Analytical and
experimental study of inextensional modes of a circular cylinder.
1(4):26-32; Oct 1986.

ABSTRACT - Modal analysis of a plastic cylinder, a steel cylinder and a
steel ring has been conducted.  With these materials and geometries, the
lowest measurable modes under investigation are from inextensional
modes which are primarily due to flexural bending.  They can be classified
into three groups: (a) cylinder surface remains prismatic, (b) cylinder
surface becomes nonprismatic, and (c) vibration of a ring due to bending in
axial direction with twist.  The analytical expressions for frequencies and
modes are obtained and verified by experiments.  As expected, repeated
eigenvalue occurs.  For special geometry, multiplicity of four might
appear.


Snyder, V.W.  Structural modification and modal analyses - a survey.
1(1):45-52; Jan 1986.

ABSTRACT - This paper discusses the mechanism for the determination of
dynamic characteristics of linear systems which has been modified.  The
mechanism can be used to marry both analytical and experimental
techniques.  Typically, when structural modifications are made it requires
building a new model and testing it; or running additional costly computer
runs.  Complex structures may be difficult to mathematically or
physically model; the method presented here permits a very logical
substructuring to be performed.  The modification technique uses the
solution already available to find the solution to the modified or combined
structure in a fast and efficient manner.  The technique is effective on
systems with complex eigenvalues and eigenvectors.  Many samples are
given for clarity of the technique and to show the versatility of the
method.



Stevens, K.K.; Loeps, J.B.  Modal testing of circular plates with partial
viscoelastic damping treatments.  1(1):1-10; Jan 1986.

ABSTRACT - Application of modal-testing techniques to determine the
modal parameters and mode shapes of an edge-fixed circular plate with a
free-layer viscoelastic damping treatment extending over a portion of the
surface is described.  The test specimen and test procedures are
discussed, and experimentally determined values of the system natural
frequencies and loss factors for varying degrees of damping treatment are
presented.  These results show that partial damping treatments can be as
effective as complete treatments, while providing significant savings in
material costs and added weight.  For a plate with complete damping
treatment, the values of the natural frequencies are found to compare
favorably with those predicted by a laminated plate theory.  It is also
found that addition of the damping treatment had little effect upon the
flexural mode shapes of the plate.



Williams, R.; Vold, H.  Multiphase-step-sine method for experimental
modal analysis.  1(2):25-34; Apr 1986.

ABSTRACT - This paper presents an approach to large-scale modal tests
that combine the multishaker methodologies of sinusoidal and random
excitation.  The first phase of this method consists of verifying a single
exciter location with either stepped sine or random excitation.  Then,
using closed-loop force control, a series of multishaker stepped-sine
surveys are performed, alternating polarity patterns between the surveys
(multiphase step sine).  The responses in each such survey may be
considered frequency-response functions with respect to a generalized
force, since force amplitudes and polarities are controlled to be constant
within each survey.  Mode-indicator functions may hence be calculated and
modal parameters can be extracted from these individual surveys, just as
for standard single-point excitation methods.  From the multiple surveys
with alternating polarity patterns, a standard multiple-column FRF matrix
may also be extracted, using HI or Hv estimators.  One will now normally
calculate the multivariate mode-indicator functions and the corresponding
force patterns to identify resonances and their multiplicity.  From this
data set,  one may either proceed to extract a modal model using
phase-separation methods or, if the situation warrants, go back to
perform a tuned-sine dwell test, with initial appropriated force patterns
from the multivariate modeindicator functions.  An attractive feature of
this approach is that the decision to employ particular modal-parameter
extraction techniques can be postponed to later stages of the project.
Modal parameters may also be obtained from classical phase-resonance
techniques, guided by the insight acquired from the frequency-response function calculations.



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