2026-09-19T11:40:06Zhttps://digital.library.adelaide.edu.au/server/oai/requestoai:digital.library.adelaide.edu.au:2440/524462009-11-09T08:34:25Zcom_2440_14759col_2440_14760
Kotousov, Andrei Georgievich
Ho, Sook-Ying
Codrington, John David
School of Mechanical Engineering
2008
http://hdl.handle.net/2440/52446
The fatigue of materials and structures is a subject that has been under investigation for
almost 160 years; yet reliable fatigue life predictions are still more of an empirical art than a
science. The traditional safe-life approach to fatigue design is based upon the total time to
failure of a virtually defect free component. This approach is heavily reliant on the use of
safety factors and empirical equations, and therefore much scatter in the fatigue life
predictions is normally observed. Furthermore, the safe-life approach is unsuitable for many
important applications such as aircraft, pressure vessels, welded structures, and
microelectronic devices. In these applications the existence of initial defects is practically
unavoidable and the time of propagation from an initial defect to final failure is comparable
with the total life of the component.
In the early 1970’s, the aircraft industry pioneered a new approach for the analysis of
fatigue crack growth, known as damage tolerant design. This approach utilises fracture
mechanics principles to consider the propagation of fatigue cracks from an initial crack
length until final fracture, or a critical crack length, is reached. Since the first
implementation of damage tolerant design, much research and development has been
undertaken. In particular, theoretical and experimental fracture mechanics techniques have
been utilised for the investigation of a wide variety of fatigue crack growth phenomena. One
such example is the retardation and acceleration in crack growth rate caused by spike
overloads or underloads. It is generally accepted, however, that the current level of
understanding of fatigue crack growth phenomena and the adequacy of fatigue life prediction
techniques are still far from satisfactory.
This thesis theoretically investigates various non-linear individual and interaction
phenomena associated with fatigue crack growth. Specifically, the effect of plate thickness
on crack growth under constant amplitude loading, crack growth retardation due to an
overload cycle, and small crack growth from sharp notches are considered. A new semianalytical
method is developed for the investigations, which utilises the distributed
dislocation technique and the well-known concept of plasticity-induced crack closure. The
effects of plate thickness are included through the use of first-order plate theory and a fundamental solution for an edge dislocation in plate of arbitrary thickness. Numerical
results are obtained via the application of Gauss-Chebyshev quadrature and an iterative
procedure. The developed methods are verified against previously published theoretical and
experimental data.
The elastic out-of-plane stress and displacement fields are first investigated using the
developed method and are found to be in very good agreement with past experimental results
and finite element simulations. Crack tip plasticity is then introduced by way of a strip-yield
model. The effects of thickness on the crack tip plasticity zone and plasticity-induced crack
closure are studied for both small and large-scale yielding conditions. It is shown that, in
general, an increase in plate thickness will lead to a reduction in the extent of the plasticity
and associated crack closure, and therefore an increase in the crack growth rates. This
observation is in agreement with many findings of past experimental and theoretical studies.
An incremental crack growth scheme is implemented into the developed method to allow
for the investigation of variable amplitude loading and small fatigue crack growth. The case
of a single tensile overload is first investigated for a range of overload ratios and plate
thicknesses. This situation is of practical importance as an overload cycle can significantly
increase the service life of a cracked component by temporarily retarding the crack growth.
Next to be studied is growth of physically small cracks from sharp notches. Fatigue cracks
typically initiate from stress concentrations, such as notches, and can grow at rates higher
than as predicted for a long established crack. This can lead to non-conservative estimates
for the total fatigue life of a structural component. For both the overload and small crack
cases, the present theoretical predictions correlate well with past experimental results for a
range of materials. Furthermore, trends observed in the experiments match those of the
predictions and can be readily explained through use of crack closure arguments.
This thesis is presented in the form of a collection of published or submitted journal
articles that are the result of research by the author. These nine articles have been chosen to
best demonstrate the development and application of the new theoretical techniques.
Additional background information and an introduction into the chosen field of research are
provided in order to establish the context and significance of this work.
Thesis (Ph.D.) - University of Adelaide, School of Mechanical Engineering, 2008
Fatigue crack growth; Fatigue life prediction; Plasticity-induced crack closure; Plate thickness effect; Overload retardation; Short fatigue crack growth
Non-linear individual and interaction phenomena associated with fatigue crack growth.
Thesis
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.open.access