Bishop, N. W. M. (1988) The use of frequency domain parameters to predict structural fatigue. PhD thesis, University of Warwick.

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Abstract

The work in this thesis outlines the use of power spectral density data for estimating
the Fatigue Damage of structures or components subjected to random loading. Since
rainflow cycle counting has been accepted as the best way of estimating the fatigue damage
caused by random loadings, an obvious target was a method of obtaining the rainflow
range distribution from the PSD. Such a solution is derived in this thesis. It forms the
major part of the work presented and appears in chapter 5. The rest of the thesis deals
with the following topics;
Chapter 3 first presents some empirical solutions developed by other authors for the
prediction of rainflow ranges from PSD's. An empirical solution developed by Dirlik in
1985 is then used to investigate the effect that stresses contained within a given frequency
range have on fatigue damage when there are other frequencies present in the
PSD plot. This can be thought of as 'fatigue damage potential'. Interactions between
stresses in different frequency intervals are investigated and it is shown that the fatigue
damage potential of one frequency interval is dependent not only on the magnitude of
that interval but on the magnitudes of other frequency intervals present. This 'Interaction'
effect within the PSD plot, is of specific interest because it can be used to determine
the change of fatigue damage for any given structure or component when parts of the signal
or PSD plot are altered.
Chapter 4 is concerned with methods of regenerating a signal from a PSD in the
form of a set of peaks and troughs. Work by Kowalewsld in 1963 is introduced which
gives a solution for the joint distribution of peaks and troughs. This distribution can be
used to generate a continuous set of adjacent peaks and troughs, of any length, using
Monte-Carlo techniques. Approximations in this result are discussed, in comparison with
the (distribution of times between) zero crossings problem. An improvement to this joint
distribution of peak and troughs is given which uses an empirical solution for the distribution
of 'ordinary ranges' (ranges between adjacent peaks and troughs).
Chapter 5 forms the major part of the original work presented in this thesis and outlines
a theoretical solution for the prediction of rainflow ranges using statistics computed
directly from the power spectral density plot. The rainflow range mechanism is broken
down into a set of logical criteria which can be analyzed using Markov process theory.
The dependence between extremes in this instance is modelled using the prediction of the
joint distribution of peaks and troughs proposed by Kowalewsld, and shown in chapter 4.
Chapter 6 deals with the fatigue damage assessment and stress history determination
of components when only limited samples of the service data are available. An investigation
is carried out into the relative merits of time and frequency domain techniques. In
particular, the effect of finite sample length was investigated with particular reference to
the variance of fatigue predictions using both a rainflow count on a limited time sample
and a rainflow count produced directly from a PSD of the same time sample. The frequency
domain approach is shown to be at least as accurate as the direct time domain
approach.
Chapter 7 deals with one specific area where the methods presented in this thesis are
applicable, namely, dynamically sensitive offshore structures. Various methods of
fatigue damage assessment are highlighted, followed by a detailed description of the
'deterministic/spectral' approach. Many factors which have not previously been recognised
are investigated and shown to have significant effect, for instance, tidal effects.

Item Type:	Thesis (PhD)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Library of Congress Subject Headings (LCSH):	Materials -- Fatigue -- Mathematical models
Official Date:	December 1988
Dates:	Date Event December 1988 Submitted
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Sherratt, F.
Sponsors:	Science and Engineering Research Council (Great Britain) (SERC) ; University of Warwick ; University of Warwick. Dept. of Engineering
Extent:	[xvii], 280, [11] leaves
Language:	eng

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