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Fatigue failure is a progressive, localised and permanent damage which
appears in those parts under fluctuating stresses and strains. Above certain
stress levels, fatigue gives rise to cracks or fractures after a sufficient
number of cycles have ellapsed. It can be considered as a combination
of cyclic stress, tensile stress and strains and, if any of those factors
is absent, fatigue failure will not initiate or propagate. Many fatigue
cracks are initiated and grow from structural defects, so that the theoretical
fatigue life is reduced.
This page covers fatigue failure under the following headings:
Fatigue life can be defined as the number of stress cycles required to
cause failure; being a funtion of many variables; stress level, cyclic
wave form, metallurgical condition of the material, etc. This wide range
of variables makes analytical prediction of fatigue failure difficult.
Many repeat tests on similar components in service has been shown as the
only available procedure. Laboratory tests, however, are essential in
understanding fatigue behavior.
Failure problems which result from fatigue generaly follow three phases:
- Phase I: Initiation - Fatigue failure leads to crack nucleation and
crack propagation. This initial phase never extends over more than five
grains around the origin. Sometimes phase I may not be discernible,
depending on material, environment, etc.
- Phase II: Propagation - Progressive cyclic growth of a crack until
the remaining uncracked cross section becomes too weak to sustain the
loads imposed.
- Phase III: Fracture - Remaining cross section suddenly fractures as
a result of the loads imposed.
Several different variables are involved in the failure process, resulting
in different types of fatigue failure. Three main types of fatigue can
be found:
- Fatigue Cracking: Results from cyclic stresses that are below the
static yield strength of the material. It is the most important fatigue
failure process.
- Thermal Fatigue: Is the result of temperature cycling; thermal expansion
and contractions cause thermal stress.
- Contact Fatigue: Parts under pressure contact with each other, usually
rolling or sliding, suffer metal fatigue.
- Corrosion Fatigue: in metal parts under cycling stress in a corrosive
environment. The corrosive environment acelerates formation and propagation
of cracks, even where individual variables can produce stress.
Localised plastic deformation is responsible for crack propagation, and
microstructure of the material can affect crack growth, either inhibiting
or modifying it. Some metal conditions which affect fatigue are:
- Grain Size: Fatigue life is independient of grain size under low cycle
fatigue conditions for many metals, and is increased when grain size
reduces under high cycles.
- Alloying: The influence of chemical composition on fatigue is approximately
proportional to its influence on tensile strength.
- Second phases: These affect crack propagation due to the strain caused
by the presence of the second phase, the stress concentration of the
second phase (shape, distribution) and the nature of the bond.
- Work hardening: Work-hardenend alloys show lower crack propagation
rates and small deformation increases during fatigue. Fatigue strength
can be increased by cold working.
- Heat Treatment: Fatigue strength is generally increased by any heat
treatment that increases tensile strength.
Manufacturing practices influence fatigue performance by affecting the
intrinsic fatigue strength of material near the surface, by introducing
or removing residual stress in the surface layers, and by introducing
or removing irregularities on the surface that act as stress raisers.
- Machining: Heavy cuts, residual marks, etc can promote fatigue failure.
- Drilling: The fatigue strength of components can be reduced merely
by the presence of a drilled hole.
- Griding: Proper griding practice produces a smooth surface that is
essentially free of induced residual stresses or sites for the nucleation
of fatigue cracks. However, abusive griding is a common cause of reduced
fatigue strength
- Surface compresion: Compressive residual stress increase fatigue life.
This can be obtained by shot peening, producing visible marks on the
surface, such as dimples.
- Plating: Electroplating can impair the fatigue strength by virtue
of hydrogen embrittlement.
- Cleaning: Some alkalyne solutions are not satisfactory because they
attack the surface.
- Welding Practices: Can have a effect on the fatigue strength of a
metal at and below the surface.
- Identification marks: A high stresses may be introduced into components
by dentification marks (date, part number, etc).
Manufacturing processes are known to have significant effects on fatigue
properties of parts. The effects are either detrimental to fatigue properties
or beneficial, represented by the chart below:
Effect on Fatigue Life of Manufacturing Processes
| DETRIMENTAL |
BENEFICIAL |
| Hardening |
Carburising |
| Grinding |
Honing |
| Machining |
Polishing |
| Plating |
Burnishing |
| Welding |
Rolling |
| EDM and ECM |
Shot Peening |
Shot peening is the most effective in reducing fatigue failures. Tests
of shot peened rods versus polished rods show the life cycle of the peened
parts to be up to 10 times longer.
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