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Temporal Data Analysis A.A. 2011/2012
programs employed for stationary data analysis. However, the results of such analyses must be
interpreted with caution.
5.1.6 Periodic and Almost-Periodic Data Analysis
If sinusoids due to periodic or almost-periodic contributions are detected in the data during
the qualification phase, then special attention is warranted. Specifically, one of two approaches
should be followed. First, the sinusoidal components might be isolated from the random portion
of the data by filtering operations and analyzed separately, as illustrated by Block F. Second,
the sinusoidal components might be analyzed along with the random portion of the data, and
simply accounted for in the results. For example, if a power spectrum is computed for data
which include sinusoidal components, a delta function symbol might be superimposed on each
spectral peak at the frequency of an identified sinusoid, and labeled with the mean square
value of the sinusoid. The mean square value can be estimated from the spectral plot by
multiplying the maximum indicated spectral density of the peak by the resolution bandwidth
used for the analysis. If this is not done, the physical significance of such spectral peaks might
be misinterpreted.
5.1.7 Specialized Data Analysis
Various other analyses of individual time history records are often required, depending upon
the specific goals of the data processing. For example, studies of fatigue damage in mechanical
systems usually involve the calculation of peak probability density functions of strain data.
Spectral descriptions other than power spectral density functions are sometimes desired: for
example, the spectra for acoustic noise levels are commonly presented in terms of rms values
in 1/1 or 1/3 octave frequency bands. Such specialized analyses, as indicated by Block G, must
be established in the context of specific engineering problem of concern.
5.2 Procedures for Analyzing a Collection of Records
The preceding Section presented methods for analyzing each individual sample record from an
experiment. A procedure for analyzing further pertinent statistical properties of a collection
of sample records is presented in Figure 5.2. As for the analysis of individual sample records
outlined in Figure 5.1, many of the suggested steps in Figure 5.2 might be omitted for some
applications while additional steps would be required for others. Furthermore, the suggested
steps assume the individual records are stationary.
5.2.1 Analysis of Individual Records
The first step is to analyze the pertinent statistical properties of the individual sample records,
as outlined in Figure 5.1. Hence the applicable portions of Figure 5.1 constitute Block A in
Figure 5.2.
74 M.Orlandini