This document provides an overview of fatigue failure. It begins by defining fatigue as the premature failure or lowering of strength of a material due to repetitive stresses, even if they are below the material's yield strength. It then discusses key topics in fatigue such as stress cycles, S-N curves, fatigue testing, and factors that affect fatigue life. Crack initiation and propagation stages are described. Methods for improving fatigue performance, such as shot peening and removing stress concentrators, are also covered.

1. Lecture 17
introducing
FATIGUE FAILURE
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2. Just to have an overview where we are now
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3. Major Issues to be addressed;
What is
FATIGUE
STRESS
CYCLES
S-N
CURVES
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Fatigue
Testing
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4. What IS a FATIGUE ?
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5. Getting Started- Fatigue basics
Object --> Repeated Motion--> Weakness/tiredness --> Fatigue
Example: Your legs/ muscles decrease in output due to constant running
Aircraft
Automobile
Fatigue in materials
Metals --> Weak/fracture due to repeated loading
90% service failure  Fatigue
Turbine blades,
Examples: materials in continuous loading-unloading applications like
bridges…
bolts/ shafts (Brittle type fracture—why?)
“The PREMATURE failure/lowering of SRENGTH of a
material due to repetitive stresses which may be below or
above its yield strength ”
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6. Requirements for Fatigue Failure
Plastic
strain
Tensile
stresses
Cyclic loading
• This type of failure occurs in Metals and Polymers,
While Ceramics do not undergo this type of failure!!
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7. Fracture Surface in a Fatigue failure:
STAGES OF FATIGUE FAILURE
FATIGUE FAILURE
Two Main fracture ZONES
 Smooth Surface
 Crystalline Surface
1) CRACK initiation
2) Crack Propagation
3) Catastrophic rupture
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8. Fracture Surface in a Fatigue failure:
Fatigue in Engine Crankshaft
Striation
High magnification SEM Image
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9. Fracture Surface in a Fatigue failure:
Macroscopic Examination
Microscopic Examination
start
End
progress
SEM image
Clamshell/beach markings
Finer markings-Striations
SEM image @ high
magnification
Intrusions
and
Extrusions
Dislocation motion leads to PERSISTENT SLIP BANDS (PSBs)
These SLIP BANDS involve INTRUSIONS and EXTRUSIONS (slip on different set of planes
depending on loading cycle)  Tiny Steps  Stress Raisers  Microcrack Initiation (along
planes of high shear stress )
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10. CRACK Initiation and
Propagation !
 Crack formation and propagation is a Micromechanism in fatigue
 Fatigue crack initiation is a surface phenomenon
 SLIP BANDS Formation (Persistent Slip Bands)
 Crack initiates
•
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11. CRACK Propagation
Mechanism !
• Cyclic Load
• Crack Initiation (microcrack)
• Crack Propagation Stage I
• Crack Propagation Stage II
• Fracture
We will study Crack propagation Stages again once we get familiarize with the loading cycles
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12. STRESS CYCLES
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13. Types Of Fluctuating Stresses
Reversed Stress cycle
Repeated Stress cycle
Random/irregular stress cycles
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14. Stress Cycles in Fatigue
Mean Stress:
Amplitude Stress:
Range:
Radius Ratio:
Amplitude Ratio:
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15. Atta ul Haq GIK Institute-Fall 2013
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16. CRACK Propagation
Mechanism !
• Cyclic Load
• Crack Initiation (microcrack)
• Crack Propagation Stage I
• Crack Propagation Stage II
• Fracture
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17. Let us consider three samples with same initial crack length subjected
to different repeated cycles. Please note that material is also same;
Higher Crack Growth rate at higher magnitude of applied stresses and vice versa
Materials life to Fracture is now highly dependent on magnitude of applied
stress and final crack length (which is related to materials structural property
(resistant to deformation etc..)
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18. Fracture Mechanics for Fatigue
Fatigue Life
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19. S-N Curves
Wöhler curve/SN curves - A graph showing stress
as a function of number of cycles in fatigue.
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20. Presenting Fatigue Data
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21. Fatigue Limit/Endurance Limit:
“The stress below which a material will not fail by fatigue”
ferrous
Non-ferrous
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22. Fatigue Life:
The number of cycles permitted at a particular stress before a
material fails by fatigue.
Fatigue Strength:
The stress required to cause failure by fatigue in a given number
of cycles.
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23. Fatigue Testing
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24. Cantilever loading
Four Point Bending Set up
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25. Failure Criteria can also be established by plotting fatigue test data
Modified Goodman Relation
a
=
e
(1 –
m/
Predicts safer regions for fatigue
u)
Gerber Relation
a
=
e
(1 –
m/
2
u)
Soderberg Relation
a
=
e
(1 –
m/
y)
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26. Miner’s Law and Cumulative Damage
Fatigue life
consumed at
S1
Remaining cycles permissible at S2
Fatigue Failure will
occur when
N1 = lifetime at S1
n1 = Number of cycles applied till S1
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27. Factors Effecting Fatigue
Loading and Loading Cycles
Surface Conditions
Design Consideration
Nature of Environment
The nature of Surface strongly influence fatigue strength:
-Surface Hardness
-Surface Roughness
-Residual Stresses
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28. How to Improve Fatigue Performance
1. Surface Treatments, induce
compressive stressess.
--Method 1: shot peening
--Method 2: carburizing
shot
put
surface
into
compression
2. Remove stress
concentrators.
C-rich gas
bad
better
bad
better
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29. Fatigue in Polymers
Fatigue behaviour and SN curves similar to that of metals except the crack initiation:
There is no Concept of Persistent Slip Bands (PSBs) in Polymers , rather crack is initiated
with CRAZING phenomenon.
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30. QUIZ-5/6 ON THURSDAY, 8TH WEEK
@5:30PM MLH
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