Fatigue in aerospace materials. Materials Science and Engineering. Metallography and failure analysis. Research and development. Credit: Prof. Dr. Anjum Tauqir (R)

1. Fatigue
1
is lowering of strength or failure due to repetitive stress – even below y
Jahanzeb Ahmad

2. Common cause of mech failures
• Reason for finite life of aircraft components - turbine blades, shafts
• Fails even if applied load < y; locally stress intensity exceeds UTS
2
90%
When will the component fail?
To answer this question we determine the Rate of Propagation of Crack
1
2
3
σ
ε

3. 3 necessary factors
3
Sufficiently hi
• tensile stress
• stress variation/fluctuation
• # of cycles
Stress conc, corrosion, T, overload, etc
alter conditions for fatigue

4. Failure occurs …
4
Crack propagation
Catastrophic rupture
Initiation
without any sign of deformation
brittle smooth appearance
due to rubbing, & final rough region
Initiates localized stress raisers on surfaces
Propagates with time
Final rupture
Beach marks when load
changes during service / load
intermittent. Arrows show
direction of crack front
Indicate the initiation site
progresses with series of rings

5. Fatigue failures are easily identified
Striations much finer – crack tip after EACH cycle
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6. Stages of fatigue process
1. Crack initiation well after the loading
Nucleation sites at/near surface from fine slip bands
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2. Crack propagation in
stage I as load cycles continue along
slip band on planes of hi shear stress. Rate of crack growth very slow in Stage I.
Fracture Surface is featureless.
Extends for few grain diameters
before shifting to Stage II Fracture surface of Stage II shows striations
each represents successive position of advancing
crack front

7. contd
Stage II crack growth by crack blunting
7
Crack growth on planes of hi tensile stress normal to
max tensile stress
Crack tip is sharp at start of loading cycle
In tension: small double notch at crack tip con-
centrates slip along planes at 450 to plane of crack
Crack widens to max, grows longer by plastic
shearing, tip gets blunted

8. Slip dir is reversed
Crack surface generated in tension, is crushed in the plane
of crack. Crack tip buckles & re-sharpens
Load shifts to compression
Re-sharpened tip advances and gets blunted
With next stress cycle
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9. 3. Sudden ductile failure
remaining cross-section is too small to
bear applied load
9
Final stages of fatigue process

10. Endurance limit
In steels endurance limit is ½ UTS ; estimate fatigue from tensile test
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Elastic on gross scale
gross plastic
N 
, ksi 

11. LCF
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At higher s , fatigue life
progressively decreases
LCF, N < 104 or 105 cycles, tests with
controlled elastic + plastic strain
instead of load
Interpretation in terms of stress difficult
 LCF elastic - plastic strain controlled

12. How long is it
safe to use the
component ?
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Below a threshold crack
does not grow even
though stress / crack exist
grows slowly
da/dN

13. Conclusion
Component MAY be safe to use even if cracks and
stress exist
13
We may determine cycles and thus time to failure
Below a threshold crack does not grow.
For higher stress intensity it grows slowly

14. Subjected to combined thermal and mechanical loading – stresses  and T vary with time
~ 10x damaging than isothermal fatigue at max operating T
14
OP cause oxidation damage. Oxide film forms
in compression at hi T; ruptures during low T
tensile loading when oxide film is more brittle
lines without symbols
Isothermal at various T
open symbols TMF IP
solid symbol TMF OP
InPhase loading – max in T and strain occur
at same time
Out of Phase - compression at highest T &
tension at lower T
1010 steel
Thermo-mechanical Fatigue
Even worst case
isothermal property is
very non-conservative

15. Coating and diffusion zone
coating
diffusion zone
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16. 25 m
Parallel streaks are oxidized bands with cracks
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17. 0.3 mm
30 m
Surface cracks run deep
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18. Stress Corrosion Cracking
Reaction with corrosive environment results in deep corrosion cracks
although little uniform corrosion
18
SCC occur well below y
cracks lower strength
Stress may be external OR stored residual

19. Examine metal around SC crack…
Corrosion degradation and
intergranular cracks form due to
corrosion attack
• Usually, extensive branching
of cracks
• Presence of corrosion product
at initiation point
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20. Fatigue strength, Fatigue Life
• Fatigue Strength max stress for which fatigue will NOT occur within a
particular # of cycles. Necessary for designing
• Fatigue Life how long a component survives at a particular stress
cycle.
• Most materials are notch sensitive particularly fatigue. Polished
surfaces / surfaces ‘shot peened’ enhance fatigue life
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21. Oxidation Mechanism
In OP loading oxide layer forms surface when material is hot and
under compression
At lower T, oxide layer becomes brittle & mechanical straining it
cracks to expose new metal surface. Clean metal rapidly oxidizes
Process is repeated cyclically. Ultimately crack forms & grows during
mechanical strain cycle.
Isothermal loading is not the dominant failure mechanism
Stress does not play role in microcrack formation
Oxide
damage
in steels
Microcracks forms during cyclic strain range
Oxide damage will occur when the strain range exceeds a threshold for oxide cracking
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22. Fatigue strength, Fatigue Life
• Fatigue Strength max stress for which fatigue will NOT occur within a
particular # of cycles. Necessary for designing
• Fatigue Life how long a component survives at a particular stress
cycle.
• Most materials are notch sensitive particularly fatigue. Polished
surfaces / surfaces ‘shot peened’ enhance fatigue life
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23. 23
P50 Becker
10-6
10-4
10-2
da/dN
mm
per
cycle
Threshold
Km
Final
failure
Kc , K1c
Region 1
Region 2
Region 3
Log K
n
1
Noncontinuum
mechanisms
Microstructure
Mean stress
Environment
Continuum
mechanism
Striations growth
Little influence of
…
‘Static mode’
mechanisms
Cleavage, IG,
fibrous
Large influence of
Microstructure
Mean stress
Thickness

24. Map
• Several variables influence LCF life
• Creep damage and environmental effects
influences life at different T ranges
environmental effects are important at a lower T
characterizes a particular alloy’s
LCF failure mechanisms
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25. Dominant Fracture Mechanism
• At low T , strain range determines crack
Initiation region I - HCF or by crack growth
region II LCF - Coffin-Manson eqn
• As T increases environment facilitates crack
initiation and influences crack growth rate,
leading to region III environmentally
influenced fatigue
• As T ≈ /≥ 0.5 Tm creep has increasing influence ; LCF becomes a process of
cyclic creep region V dev of internally generated intergranular cracks -
environment has negligible effect 25

26. Q/A
Dieter: Metallurgical variables for fatigue pg 415-419
Becker: Pp 167-184; Fatigue failures 309 (crack initiation 322, Propagation - Paris regime 324; final frac 328);
Multiple initiation, Stress conc 324, Micro-pitting 360, thermal fatigue 368
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