Fatigue

It is a form of a failure in which a structure subjected to

alternating stresses fail at stress levels considerably lower
than the stresses necessary for failure under static loading

 This is the most common route of structural failure in

metallic materials
 In order for fatigue failure, at least some portion of the
stress cycle must be tensile in nature
 Even in very ductile materials, the fatigue failure is
brittlelike in nature – very little plastic deformation
 Ordinarily the fracture surface is normal to the direction
of applied stress
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Some examples of fatigue failure

Accident killing > 100 people in Germany in 1998 due to

failure caused by a single fatigue crack in a wheel 90
Some examples of fatigue failure

Emergency landing of Qantas flight 32 in 2010 due to engine

damage by fatigue cracking of misaligned engine parts 91
Parameters important to fatigue
If for each stress cycle, the maximum and minimum stress
are max and min, respectively
 max  min
Mean stress (m): m 
2
Range of stress (r):  r   max   min
  max   min
Stress amplitude (a): a  
2 2
 min
Stress ratio (R): R
 max

92
Parameters important to fatigue

93
Typical fatigue stress cycles

Reversed stress cycle: Cycle is symmetrical about zero mean

stress. Alternating between max. tensile stress and min.
compressive stress of equal magnitude
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Typical fatigue stress cycles

Repeated stress cycle: Maximum and minimum of stress are

asymmetrical relative to the zero stress level.
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Typical fatigue stress cycles

Random stress cycle: Both maximum and minimum stresses

vary randomly
96
Fatigue test
 The applied stress may be tension‐compression, bending
or torsional in nature
 A series of tests need to be performed
 The first test is carried out at a high stress, where failure
in a relatively short number of cycles is expected
 For each succeeding specimen the test stress is
decreased until few samples do not fail
 Data are plotted as stress S (typically alternating stress
a) vs. logarithm of number of samples N  S‐N curve

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S‐N curve

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S‐N curve
 Typically for steels, the S‐N curve becomes horizontal at a
certain limiting alternating stress, known as fatigue limit
or endurance limit.
 Below fatigue limit the material can withstand infinite
number of samples without failure.
 S‐N curves of most non‐ferrous metals like Al, Cu show no
fatigue limit
 For these materials stress at certain number of cycles
(e.g. 108 cycles) is taken as fatigue limit.
Low cycle fatigue: Number of cycles to failure typically < 105
High cycle fatigue: Number of cycles to failure typically > 105
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Propagation of fatigue damage
3 steps of fatigue failure
1) Crack initiation
2) Crack propagation
3) Final failure
Fatigue cracks almost always initiate on the surface of a
component at some stress concentration. These include
 Surface scratches
 Sharp fillets
 Threads, dents etc.

100
Propagation of fatigue damage

101
Propagation of fatigue damage
 Beachmarks and striations, which appear as concentric
ridges and expand away from the crack initiation site in
circular or semicircular pattern are telltale features of
fatigue failure.
 The fracture surface typically consists of a smooth region
and a rough region
 Smooth areas correspond to regions where crack
propagation is slow

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Factors affecting fatigue life
Mean stress
Increasing the mean stress leads to a reduction of fatigue
life. The effect of mean stress on fatigue limit is typically
plotted in Goodman plot.
Stress concentrations
Presence of stress raisers such as notches and/or holes
seriously reduce the fatigue life. Careful design of
components to remove stress raisers is necessary to
minimize fatigue.

103
Factors affecting fatigue life
Size of component
Generally the fatigue strength of large components is smaller
than the small components.
Effect of surface
As fatigue cracks often are generated at the surface, fatigue
life is strongly dependent on surface conditions:
 Smoothly polished surfaces normally have a higher
fatigue life
 Hardfacing techniques like carburizing significantly
improves the fatigue life
 Introduction of compressive residual stresses at the
surface (e.g. by shot peening) is very effective in
enhancing fatigue life. 104
Factors affecting fatigue life
Corrosion fatigue
Failure occurring by the simultaneous action of a fluctuating
stress and corrosive environment is called corrosion fatigue.
Fatigue life is decreased in the presence of corrosive
environment.
Thermal fatigue
Fluctuating thermal stresses generated from restraints to
dimensional changes of components resulting from
temperature vibrations may cause fatigue damage of a
component even in the absence of mechanical stresses.
 Select materials with lower thermal expansion coefficient
 Change design to allow unhindered dimensional change
105
Factors affecting fatigue life
Corrosion fatigue

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Creep
At elevated temperatures, permanent deformation of
components over a period of time under constant applied
load/stress is known as creep.
 Creep is important only at temperatures > 0.4Tm, where
Tm is the absolute melting temperature.
 Normally the stress level is significantly below the room
temperature tensile yield strength of the material.
 Both crystalline and amorphous materials undergo creep
deformation.

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Generalized creep behavior
Creep test
The specimen is
subjected to a constant
load or stress at
constant temperature.
Deformation or strain is
measured and plotted
as a function of time
until sample failure

Typical creep curve at constant load

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Stages of creep curve

Instantaneous deformation upon application of load. This

deformation is totally elastic.

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Stages of creep curve

Stage I: Primary or transient creep. Constantly decreasing

creep rate. Creep resistance of the material increases in this
stage  Strain hardening
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Stages of creep curve

Stage II: Secondary or steady state creep. Creep rate is

constant  Balance between competing processes of strain
hardening and recovery.
111
Stages of creep curve

Stage III: Tertiary state creep. Creep rate increases

continuously finally culminating into material failure by
rupture.
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Effect of stress and temperature

Increasing stress and temperature raises the overall level of

the creep curve and also results in higher creep rates.
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Steady state creep
 Stage II of creep curve is known as steady state creep
 This is the most important state of creep deformation
from engineering design point of view.
 Slope of the creep curve (i.e. strain rate) in this steady
state region is known as the minimum creep rate (MCR)
 MCR or steady state creep rate is typically defined as:

A, m: Material constants
Qc: Activation energy for creep
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Deformation mechanism maps
Mechanisms involved in creep deformation of materials
include:
 Dislocation glide and climb
 Stress induced vacancy diffusion
 Grain boundary diffusion
 Grain boundary sliding
Deformation mechanism maps are diagrams which indicate
the operating deformation mechanisms at different stress‐
temperature combinations.

115
Deformation mechanism maps

 Ordinate or stress axis is typically normalized by shear

modulus (G) and plotted in logarithmic scale
 Abscissa or temperature axis is normalized by absolute
melting temperature. (T/Tm) Homologous temperature 116
Material design for creep
Creep rates vary with stress, material diffusivity & grain size.
 Reduction of diffusion rate reduces creep rate. As
diffusivity decreases with increasing melting point, high
melting point materials have higher creep resistance.
 Because of their more open structure than FCC metals
(and correspondingly higher diffusivity), BCC metals are
generally less creep resistant.
 An increase in grain size reduces the total grain boundary
area. This subsequently reduces creep rate as both grain
boundary sliding and diffusion become more difficult.
 Dispersed second phase particles at grain boundaries
reduce creep rate by hindering grain boundary sliding
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Chemical property
 Almost all engineering materials come into contact either
with other materials and/or environment.
 Chemical characteristics of materials determine their
extent of degradation

The deterioration and loss of a material and its critical

properties due to chemical, electrochemical and other
reactions of the exposed material surface with the
surrounding environment is known as corrosion

118
Fundamentals of corrosion
Corrosion is an electrochemical process in which electrons
are transferred from one chemical species to another.
The characteristic process in which a metal loses or gives up
its electrons is called an oxidation process.

M  M  nen

The sites at which oxidation occurs are called anode.

Electrons generated from oxidation process are consumed to
become part of another chemical species. This process is
called reduction.
The sites at which reduction occurs are called cathode.
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Fundamentals of corrosion

 An overall electrochemical reaction must consist of at

least one oxidation and one reduction reaction
 The total rate of oxidation must equal the total rate of
reduction
 All electrons generated through oxidation must be
consumed by reduction 120
Electrode potentials
 Not all metallic materials oxidize to form ions with the
same degree of ease.
 When two metals are electrically connected in an
electrolyte, wherein one acts as anode and corrodes and
the other acts as cathode, it is called galvanic couple
 Metallic materials may be rated in terms of their
tendency to oxidize when coupled to other metals in
solution of their respective ions.

Standard electromotive force (emf series)

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Standard emf series

122
Standard emf series
 Metals at the top of the emf series are most noble, while
those at the bottom are most active
 When standard half cells (a pure metal electrode
immersed in a 1M solution of its own ions at 25 °C) are
coupled together, the metal lying lower in the emf series
will corrode.
Example: If nickel (Ni) and cadmium (Cd) standard half cells
are coupled together, cadmium will be corroded, as it is
located below nickel in emf series.

123
Galvanic series
 emf series has been generated under highly idealized
conditions and has limited practical value
 Galvanic series provides a more practical and realistic
ranking of materials
 While emf series lists only metals, both metals and alloys
are included in galvanic series
 While for emf series the potentials are calculated from
thermodynamic principles, for galvanic series the
potentials are measured vs. reference electrode in a
specific environment

124
Galvanic series

Galvanic series of
metals and alloys in
seawater 125
Galvanic series
 Galvanic series plays a key role in predicting galvanic
corrosion  corrosion that occurs when two different
metals immersed in an electrolyte are connected
 Metals located towards the top of the series are cathodic
(i.e. unreactive) while those at the bottom are anodic (i.e.
they undergo corrosion)
 When two metals far apart in the galvanic series come to
contact in an electrolyte, the metal that is anodic will
corrode at an increased rate.
 To avoid possible corrosion, metals lying close to each
other should be considered

126
Galvanic series
Typical application:
An unprotected couple made of passive stainless steel and
low‐carbon steel in seawater will result into accelerated
corrosion of the low‐carbon part.

127
Some important forms of corrosion
 Galvanic corrosion
 Uniform corrosion
 Crevice corrosion
 Pitting corrosion
 Erosion‐corrosion
 Intergranular corrosion (will be discussed later in the
course)
 Stress corrosion cracking

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Galvanic corrosion
 Refer to the discussion related to galvanic series
 Rate of galvanic attack depends upon the relative anode‐
to‐cathode surface areas exposed to the electrolyte
 A smaller anode will (e.g. plain carbon steel screw) will
corrode more when in contact with a large cathode in an
electrolyte due to the larger corrosion current density
Prevention of galvanic corrosion
 For coupling dissimilar metals, choose two lying close in
the galvanic series
 Use as large an anode area as possible
 Electrically insulate dissimilar metals from each other
 Electrically connect a third more anodic metal to the
other two  cathodic protection 129
Real world galvanic corrosion

130
Uniform corrosion
 Almost uniform intensity over the entire exposed surface
area
 This often leaves behind a scale or deposit as corrosion
product.
 Typical example is rusting of plain carbon steel

This form of corrosion attack is of least concern, as it can be

measured easily and as progression of attack is visible,
preventive action can be taken.

131
Crevice corrosion
 Localized attack on a metal surface at, or immediately
adjacent to the gap or crevice between two joining
surfaces
 The gap must be wide enough for solution to penetrate,
yet narrow enough for stagnancy
 Depletion of dissolved oxygen and build up of aggressive
ions like Cl‐ are chief causes of crevice corrosion
Prevention of crevice corrosion

 Use welded instead of bolted or riveted joints

 Remove accumulated deposits frequently
 Design to avoid stagnant areas and proper drainage.
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Real world crevice corrosion

133
Pitting corrosion
 Form of localized corrosion which produces attack in the
form of spots or pits in normally passive metals like
stainless steels and aluminum alloys
 Occurs when the ultrathin passive film is locally damaged
and not immediately re‐passivate
 Liquid mediums containing high concentration of Cl‐ ions
(e.g. seawater) are most insidious
 Depth of pits in materials can be very large and may
result in fast catastrophic failure without notice
 It is one of the most damaging forms of corrosion, as the
extent of pit growth in components is generally not easily
discerned.
134
Prevention of pitting corrosion
 Proper selection of materials with appropriate alloying
elements
 Control of environment in terms of Cl‐ ion concentration
pH, temperature etc.

135
Real world pitting corrosion

136
Erosion‐corrosion
 Caused by combined action of chemical attack and
mechanical action caused by fluid flow
 Particularly harmful in metals passivated by ultrathin
films e.g. stainless steels, Al alloys etc.
 Increasing fluid velocity normally enhances corrosion rate
 Presence of bubbles and suspended particles also
enhance the corrosion rate
Prevention of erosion‐corrosion
 Proper pipe design to reduce turbulent fluid flow 
eliminate bends, elbows, abrupt changes in pipe
diameter
 Removal of particulates and bubbles from liquid
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Real world erosion‐corrosion

138
Stress corrosion cracking
 Results from the combined action of operating tensile
stress and the particular corrosive environment
 Materials virtually inert in an environment may become
susceptible to corrosive attack when tensile stress is
present
 Most alloys are susceptible to stress corrosion in specific
environments
 The tensile stress may be applied, residual stress or
generated due to temperature changes
 Even for inherently ductile materials the fracture is brittle
like

139
Prevention of stress corrosion cracking
 Totally eliminate or reduce the operating tensile stress in
the material  proper component design, heat
treatment of the material etc.
 Proper selection of material‐environment combination
(from available knowledge) to avoid stress corrosion.

140
Real world stress corrosion cracking

141