O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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Fatigue of high strength steels
The Centre for Advanced Steels Research (CASR)
University of Oulu
Laboratory of Materials Engineering
Antti Järvenpää
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O U L U S O U T H E R N I N S T I T U T E
1. Fatigue failure
2. Fatigue strength
3. Research
4. Outlook
5. References
Fatigue of high strength steels
Antti Järvenpää 2012
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O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
Fatigue failures are the most common types of fractures in machines and probably constitute about 90% of all fractures
Fatigue fractures can develop at a stress level below YS
Loading conditions
Torsion, bending, axial tension, etc.
Vibration, thermal/pressure variation or centrifugal forces…
Fatigue failure is characterized by four stages
Cyclic hardening/softening
Crack initiation
Crack propagation
Final fracture
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O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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The 1842 Versailles rail accident (http://en.wikipedia.org/wiki/Versailles_rail_accident)
The 1919 Boston Molasses Disaster has been attributed to a fatigue failure.
The 1948 Northwest Airlines Flight 421 crash due to fatigue failure in a wing spar root
The 1957 "Mt. Pinatubo", presidential plane of Philippine President Ramon Magsaysay, crashed due to engine failure caused by metal fatigue.
The 1968 Los Angeles Airways Flight 417 lost one of its main rotor blades due to fatigue failure.
The 1968 MacRobertson Miller Airlines Flight 1750 that lost a wing due to improper maintenance leading to fatigue failure
The 1977 Dan-Air Boeing 707 crash caused by fatigue failure resulting in the loss of the right horizontal stabilizer
The 1980 LOT Flight 7 that crashed due to fatigue in an engine turbine shaft resulting in engine disintegration leading to loss of control
The 1985 Japan Airlines Flight 123 crashed after the aircraft lost its vertical stabilizer due to faulty repairs on the rear bulkhead.
The 1988 Aloha Airlines Flight 243 suffered an explosive decompression due to fatigue failure.
The 1989 United Airlines Flight 232 lost its tail engine due to fatigue failure in a fan disk hub.
The 1992 El Al Flight 1862 lost both engines on its right-wing due to fatigue failure in the pylon mounting of the #3 Engine.
The 1998 Eschede train disaster was caused by fatigue failure of a single composite wheel.
The 2000 Hatfield rail crash was likely caused by rolling contact fatigue.
The 2002 China Airlines Flight 611 had disintegrated in-flight due to fatigue failure.
The 2005 Chalk's Ocean Airways Flight 101 lost its right wing due to fatigue failure brought about by inadequate maintenance practices. http://en.wikipedia.org/wiki/Fatigue_(material)#Infamous_fatigue_failures
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Fatigue of high strength steels
O U L U S O U T H E R N I N S T I T U T E
Antti Järvenpää 2012
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Cyclic stability
Material properties may change withing first few thousand cycles
Hardening if dislocation intensity increases
Softening if dislocation intensity decreases
Pre-forming, high SFE
1.4509 stainless steel http://engr.unr.edu/~yjiang/accomplishments.html
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Fatigue of high strength steels
O U L U S O U T H E R N I N S T I T U T E
Antti Järvenpää 2012
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Crack initiation
In the case of LCF and HCF, crack initiates typically on the surface
LCF: High plastic straining (N = 10 – 10^3/10^5)
HCF: Low loads (elastic region), long life (N  10^8)
UHCF: Subsurface cracking occurs (N = 10^8  )
Surface quality, geometry  stress localization
Changes in geometry of the part such as holes, keyways, threads, steps or changes in diameters in shafts and boltheads, etc.
Surface discontinuity such as nicks, notches, machining marks, pitting, corrosion, etc.
Defects inherent in the material such as non-metallic inclusions, minute cracks, voids, etc.
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Fatigue of high strength steels
O U L U S O U T H E R N I N S T I T U T E
Antti Järvenpää 2012
Polished pure metal
Initiation at slip bands or at grain boundaries
Polished complex phase metal
Initiation at precipitations or at inclusions
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Fatigue of high strength steels
O U L U S O U T H E R N I N S T I T U T E
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Crack propagation
After initiation, the crack tip itself acts as a substantial stress raiser allowing it to extend although the overall stress is low
1st stage: 45 angle to the stress axis
2nd stage: perpendicular to the stress axis http://www.kuleuven.ac.be/bwk/materials/Teaching/master/wg
12/l0200.htm, 15.7.2008 http://en.wikipedia.org/wiki/File:ParisLaw.png
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O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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Final fracture
The propagation of the crack decreases the area of the cross-section, causing final fracture (overload)
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O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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Crack growth rate (da/dN)
Potential drop method
M. Sander, H.A. Richard / International Journal of Fatigue 28 (2006) 583
–591
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O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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Fatigue strength ~ typically 50% of the tensile strength /6, 7/
Characteristics affecting the fatigue endurance
Residual stresses (e.g. bent sheet metal)
Grain size (microstructure)
Surface roughness / Defects
Hardness
Alloying
Inclusions
Atmosphere
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Fatigue of high strength steels
Antti Järvenpää 2012
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2. Fatigue– Stress/Strain amplitude
Low cycle fatigue (LCF)
Coffin-Manson relation
Crack initiation takes place early
Crack propagation rate dominates
High fraction of retained austenite enhances fatigue strength
Decrease in static strength is overcomed via intense strain hardening
High cycle fatigue (HCF/UHCF)
Crack initiation slow
Low stress/strain amplitude  minor strain hardening
Subsurface crack initiation more likely
Inclusions, softer phases etc.
Homogenous microstructure enhances fatigue strength
Relation to the static strength
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Fatigue of high strength steels
Antti Järvenpää 2012
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Grain refinement is an effective way to improve strength, ductility and fatigue strength
E.g. nanobainite / UFG austenite
For steels with HV≤400, the calculation of σ w0 is not dependent on microstructure or steel type
/8/
Increase in retained austenite fraction /9/
Increases threshold stress intensity
Decreases crack propagation rate
Static strength decreases
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Fatigue of high strength steels
O U L U S O U T H E R N I N S T I T U T E
Antti Järvenpää 2012
2. Fatigue strength – Retained austenite /9/
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Fatigue limit (bending fatigue, N = 10^7)
Austempering at 220 C: 1147 MPa (FL/TS = 0.48)
Austempering at 240 C: 1156 Mpa (FL/TS = 0.54)
Austempering at 260 C: 1033 Mpa (FL/TS = 0.50)
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Fatigue of high strength steels
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Fatigue of high strength steels
O U L U S O U T H E R N I N S T I T U T E
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Hardness ≤ 400  microstructure does not affect to the fatigue strength
Purity (inclusions etc.)
Can be enhanced by surface treatments
Hammering, shot peening, residual stresses etc.
Hardness > 400HV  microstructure can be optimized
Fraction of retained austenite
Grain size and grain structure (boundary type)
High strength steel in work shops
Fatigue strength of formed high strength steel
Effect of the environment
Surface roughness (mishandling, grindind etc.)
Influence of cutting method
Fatigue strength of welded high strength steel
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Fatigue of high strength steels
O U L U S O U T H E R N I N S T I T U T E
Antti Järvenpää 2012
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Bent Optim 960 QC –structural steel
Cold forming + high SFE  softening
Experimental challenges
Effect of residual stresses
Effect of the geometry (radius, angle)
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Fatigue of high strength steels
Antti Järvenpää 2012
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Fatigue of high strength steels
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1000
800
600
400
200
0
10000 100000 1000000
Cycles (N)
10000000
CS Taivutusväsytyskone
100000000
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Fatigue of high strength steels
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Fatigue of high strength steels
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Geometry problems
Atmosphere in use: T ~ 100 C + Corrosive environment
Fig. 1 Pitting on the surface. Crack initiated on surface pitting and propagated through the sheet.
Fig. 2 Severe corrosion damage on the fracture surface.
Fig. 3 Pitting marks between the striations.
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Fatigue of high strength steels
Antti Järvenpää 2012
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Fatigue of high strength steels
Antti Järvenpää 2012
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AISI 301LN C850
PI = Longitudinal
PO = Transverse
C02 = Laser cut edge
KO = Machined cut edge
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O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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Fatigue strength of plates, R=0-0.1
500
400
300
200
100
S690QL
SB M illed.
Laser
Plasma
FAT160
0
200
Milled.
400 600 800
Yield strength,N/mm
2
1000 1200
Plasma Laser
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O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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HFP = high frequency peening technology

O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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Weld simulation by Gleeble
Fatigue strength of different zones of the HAZ
Fatigue mechanism of UFG 301LN
Crack initiation
Austenitic UFG
Austenitic – Martensitic UFG
Cyclic stabilization
Development of tailored small batch steels using induction heating
Industrial UFG steels
Ultra-high fatigue strength steels for industry

O U L U S O U T H E R N I N S T I T U T E
Fatigue of high strength steels
Antti Järvenpää 2012
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1.
http://www.technical.net.au/img/articles/pdf/FATIGUE.pdf
(22.5.2012)
2.
A. S. Hamada, L. P. Karjalainen / Materials Science and Engineering A 527 (2010) 5715-5722
3.
http://www.kuleuven.ac.be/bwk/materials/Teaching/master/wg12/l0200.htm, 15.7.2008
4.
http://en.wikipedia.org/wiki/File:ParisLaw.png
5.
M. Sander, H.A. Richard / International Journal of Fatigue 28 (2006) 583–591
6.
S. Nishiyama / Journal of the Society of Materials Science 29 (1980) 24-29
7.
Y. Murakami: Metal fatigue: effects of small defects and nonmetallic inclusions’; 2002, Oxford, Elsevier
Science.
8.
G. Ghalant et al. / Proceedings of the 6
Materials (1991) 511-516 th International Conference on the mechanical behaviour of the
9.
J. Yang /Scripta materialia 66 (2012) 363-366
10. M. J. Peet et al. / Materials Science and Technology 27 (2011) 119-123
11. A. Järvenpää / Särmätyn Optim 960 QC -teräslevyn kestävyys taivutusväsytyksessä –Di-työ/
12. J. Lämsä, A. Järvenpää…/International Journal of Key Engineering Materials Vol. 473 (2011)
13. J. Niskanen/ Taivutusväsymiskokeet ultralujien levymateriaalien käytettävyystutkimuksessa –Di-työ/
14. J. P. Wise et al. / 20th ASM Heat Treating Society Conference Proceedings (2000)
15. M. Leitner et al. /Journal of Engineering and Technology 1 (2011)