www.minton.co.uk Fatigue & Fatigue Life By Peter Moore Minton, Treharne and Davies Ltd. Lillehammer 2012
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Fatigue & Fatigue Life
By Peter Moore
Minton, Treharne and Davies Ltd.
Lillehammer 2012
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Contents
What is Fatigue? The Science behind Fatigue Designing for Fatigue/Fatigue Life Case Studies Concluding Comments
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What is Fatigue?
Fatigue
The decreased capacity or complete inability of an organism, an organ, or a part to function normally because of excessive stimulation or prolonged exertion.
The weakening or failure of a material, such as metal, resulting from prolonged stress.
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Fatigue is the progressive and localised structural
damage that occurs when a material is subjected to
Cyclic Loading.
What is Fatigue?
A relatively smooth area where
the crack initiates
Fatigue Striations indicating
progressive crack growth
Rough area of final failure
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The maximum stress is less than the
ultimate tensile stress and may be
below the yield stress of the
material.
As such a component can fail at
loads below its calculated strength.
We need to understand fatigue so
that we can:
Predict the engineering life of
components,
Design structures and materials
which maximise economic life.
What is Fatigue?
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Infamous Fatigue Failures
What is Fatigue?
Alexander L. Kielland Capsize -1980
the rig collapsed owing to a fatigue crack in one of the bracings which connected the collapsed D-leg to the rest of the rig,
This was traced to a 6mm fillet weld which joined a small flange plate to this bracing,
This flange plate held a sonar device used during drilling operations,
The resultant enquiry found that cold cracks in the welds, increased stress concentrations due to the weakened flange plate, the poor weld profile, and cyclical stresses collectively played a role in the rig's collapse.
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Infamous Non Energy Fatigue Failures
What is Fatigue?
De Havilland Comet -1954
Cracking from square windows
Eschede Train Disaster -1998
Fatigue in wheel rim
Hatfield Rail Crash -2000
Rolling contact fatigue in rails
China Airlines Flight 611 -2002
Fatigue failure 22 years after first
damage
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Examples of Fatigue Fracture Faces
What is Fatigue?
Multi-strand Engine Shaft
Wire Rope
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William Rankine reported on fatigue of
an axle on a locomotive tender in 1843.
He identified the keyway as the crack
origin and was the first person to
recognise the significance of fatigue
striations
The Science Behind Fatigue
For about inch in depth all round there was a perfectly smooth cleft of blue and purple colour; this annular part appeared to have been produced by a constant process; the central crystallised part being gradually reduced in diameter, until it was barely able to sustain the weight, and it broke on being exposed to a sudden strain.
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The Science Behind Fatigue
Cyclic Loads occur in a wide variety of service environments,
most of them are not the classic sinusoidal form but can be
very complex, for example:
What do we mean by a Cyclic Load?
High frequency mechanical
loading in a crankshaft.
Low frequency erratic marine
pounding of a North Sea oil rig.
Cyclic loads caused by thermal
expansion due to periodic
heating and cooling in a
turbine.
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The Science Behind Fatigue
In general fatigue cracks
originate from some sort of
stress raiser on the surface of a
component.
Stress raisers include features
such as sharp notches and
angles, however such sites are
not innately cracked.
Fatigue cracks can appear from
apparently smooth surfaces
when cracks initiate on small
or even microscopic flaws.
How do cracks form and grow?
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The Science Behind Fatigue
The appearance and growth of a fatigue crack can be broken
down into four distinct phases:
I. Microstructural changes leading to permanent damage.
Atomic level changes lead to an accumulation of stress
II. Nucleation of micro-cracks.
Several microscopic cracks form in the damaged area
III. Stable propagation of a dominant crack.
One of the microscopic cracks grows out of this initiation area and
propagates across the component
IV. Failure.
The component breaks
How do cracks form and grow?
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Phase I - Microstructural Changes
The Science Behind Fatigue
Microstructural Changes are due to the Cyclic Loading causing
flexing of the metal crystals at an atomic scale.
These changes in microstructure typically resulting in either
local softening (of hard materials) or hardening (of soft
materials).
These changes are permanent but nearly impossible to detect.
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Crack
Phase II Nucleation of Micro-Cracks
The Science Behind Fatigue
The damage caused in Phase I
results in deformation and shape
change of the underlying
microstructure.
In particular it can cause the sub-
microscopic extrusion or intrusion
of material.
These areas act as sites for crack
initiation.
The crack propagation rate in this
phase is generally very low: of the
order of nm/cycle giving a
featureless surface.
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Phase III Crack Propagation
The Science Behind Fatigue
The fracture surface of Phase III
crack propagation frequently
shows a characteristic pattern of
ripples or fatigue striations.
Such striations are produced by the
successive position of an advancing
crack front.
10m
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Phase IV Final Failure
The Science Behind Fatigue
When the remaining intact
material is unable to bear
the applied loads the
component fails.
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Designing for Fatigue
Fatigue Life Number of cycles that a material will sustain
before failure occurs.
Designing structures for a long fatigue life involves two main approaches:
Theoretical CalculationsComponent Design
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The most common tool for avoidance of fatigue fractures
involves the calculation of a Stress vs. Number of Cycles, or
SN Curve.
An S-N diagram is a plot of the fatigue life at various levels of
stress.
These curves can be calculated based on known material
properties, the stresses involved and the shape of the
components in question.
Variations in material and design can give radically different
properties.
Designing for Fatigue
Theoretical Calculations
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Steel (and other ferrous alloys) have an endurance limit, a stress level below which fatigue does not occur. Non-ferrous alloys (aluminium, titanium, etc.) do
not have an endurance limit
Designing for Fatigue
SN Curve
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Fatigue Testing can be
performed on individual
components or representative
material specimens
Tests are typically performed
both on as-manufactured and
pre-cracked specimens
Whilst time consuming proper
tests can identify problems not
identified in modelling
Designing for Fatigue
Component/Material Testing
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Corrosion-Fatigue is the combined
action of a cyclic load and a corrosive
environment.
Fatigue causes rupture of protective,
passive surface oxides, causing
accelerated corrosion
In a corrosive environment the stress
level at which a material has infinite
life is lowered or removed completely.
Contrary to a pure mechanical fatigue,
there is no fatigue limit load in
corrosion-assisted fatigue
Corrosion Fatigue
Designing for Fatigue
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1. Infinite Life Design: Keeping the stress at some low fraction
of the fatigue limit of the material.
2. Safe Life Design: Based on the assumption that the material
has flaws and a finite life. A safety factor is used to
compensate for environmental/manufacturing variability.
3. Fail Safe Design: The fatigue cracks will be detected and
repaired before it actually causes failure. Aircraft industry.
If cracks do appear then use
Damage Tolerant Design: Use fracture mechanics to
determine whether the existing crack will grow large enough
to cause failure.
The Different Fatigue Life Philosophies
Designing for Fatigue
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Factors affecting fatigue life include:
Mean Stress the average stress to which a component is
subjected.
Stress Amplitude the variation between the minimum and
maximum stresses experienced in service.
Frequency how often the component is loaded and
unloaded.
Waveform the variation in applied stress, perhaps a gentle
rising and lowering or sudden shock changes.
Temperature the service temperature.
Factors Affecting Fatigue Life 1
Designing for Fatigue
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Temperature Variation the in service variation, both
environmental and innate to the components.
Environment corrosion and oxidation.
Surface Finish the smoother and flatter the surface the
greater the fatigue life.
Coatings to protect the surface from damage, reducing the
ability for surface damage/corrosion to act as initiation points
for cracking.
Microstructure a combination of material choice and heat
treatment to reduce the risk of crack formation.
Factors Affecting Fatigue Life 2
Designing for Fatigue
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All these design factors are centred around
preventing the appearance of fatigue cracks.
Once you have a crack it will grow!
Conclusion
Designing for Fatigue
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Failure of a Subsea Cable
Failure of Flexible Flowline Pressure Sheath
Fatigue Striations as Event Limit Markers
Case Studies
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An electronic control and
communication cable was
found to be giving a
degraded service after only
three years in use, out of a
twenty year projected life.
The cable was therefore
recovered for examination.
Fatigue Failure of Subsea Cable
Case Study 1
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Whilst the cable was found to
show some fretting and
abrasion damage there was
no obvious puncture damage
to the external sheath
If not contaminated with
water how had the cable
degraded?
Fatigue Failure of Subsea Cable
Case Study 1
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When the cable was dissected
fatigue cracks were found in
the external metallic sheath.
The metallic sheath had no
scratches, damages or internal
corners to act as initiators.
Microstructural examination
and testing of exemplar
samples discovered that all
cracks initiated in welds in the
sheath
Fatigue Failure of Subsea Cable
Case Study 1
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Welds in the copper sheath
were found to possess small,
hard inclusions.
Fatigue cracks had initiated
inside the welds at these
inclusions.
The welding process was
altered and approved by
fatigue testing of exemplars
Fatigue Failure of Subsea Cable
Case Study 1
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It is not just metals that can fail by fatigue. Ceramics and
particularly polymers are subject to fatigue.
In this case a polymeric pressure sheath suffered extensive
circumferential fatigue cracking
Failure of a Flexible Flowline Pressure Sheath
Case Study 2
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The cracks were found to have initiated on the inside of the
flowline and propagated outwards
Failure of a Flexible Flowline Pressure Sheath
Case Study 2
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The cracks were predominantly on the sides of the
flowline, not the top and bottom.
The cracks therefore indicated that the flowline had
flexed from side to side.
However the flowline was partially embedded in the
seafloor and therefore apparently unable to move.
How had the fatigue cracks initiated and propagated
in a stationary environment?
Failure of a Flexible Flowline Pressure Sheath
Case Study 2
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Study of the high strength tapes that bound the outer armour wires together revealed signs of advanced degradation.
The degraded tapes were not able to restrain the outer armour of the flowline causing it to relax slightly, and allowing the pressure sheath within to flex.
This flexing had resulted in crack initiation, crack growth and final failure.
Conclusion
Case Study 2
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It can be possible to correlate the
number of fatigue striations with
the service history of the
component.
This is particularly prevalent in
the failure assessment of turbine
blades, although this method of
counting striations is used in
many failure investigations.
Fatigue Striations as Event Limit Markers
Case Study 3
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The high running temperatures of
jet turbines oxidise the fracture
face as the crack grows. The more
heating cycles the more oxidised
and darker a surface is.
This results in a visual colour
difference between striations that
have been exposed to a different
number of heating cycles.
The boundaries between each
cycle tend to be distinct.
Fatigue Striations as Event Limit Markers
Case Study 3
3 cycles2 cycles1 cycle
Final failure
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Fatigue Striations are counted in various industries
to help determine which process is important in a
fatigue failure.
For example, a fatigue failure in a drilling tower
might be due to the forces from drilling (daily) or the
sway of the tower in bad weather (monthly).
Determining the number of striations can help
determine which was the cause of failure.
Fatigue Striations as Event Limit Markers
Case Study 3
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Concluding Comments
There are a huge number of variables in fatigue far too
many to construct S/N curves for all combinations, especially
as the variables can change during the lifetime of the
component.
The challenge is to understand how the damage produced by
fatigue varies with these parameters and adds together over
a complex life cycle.
Designing for fatigue is primarily concerned with avoiding
crack initiation.
Once you have a crack it will grow!