Fatigue of high strength steels
Transcript of 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 1 Antti Järvenpää 2012
Fatigue of high strength steels
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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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Content
1. Fatigue failure
2. Fatigue strength
3. Research
4. Outlook
5. References
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1. Fatigue failure
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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1. Fatigue failure cases 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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1. Fatigue before cracks
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
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http://engr.unr.edu/~yjiang/accomplishments.html 1.4509 stainless steel
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1. Fatigue failure - Initiation
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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1. Fatigue failure - Initiation 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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1. Fatigue - Crack propagation
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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1. Fatigue failure - Overload
Final fracture
The propagation of the crack decreases the area of the
cross-section, causing final fracture (overload)
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1. Fatigue failure
Crack growth rate (da/dN)
Potential drop method
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M. Sander, H.A. Richard / International Journal of Fatigue 28 (2006) 583–591
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2. Fatigue strength
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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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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2. Fatigue strength - Microstructure
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 σw0 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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2. Fatigue strength – Retained austenite /9/
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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2. Fatigue strength /14/
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2. Fatigue of high strength steels 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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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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3. Bending fatigue tests of bent sheet metal
/11,12/
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3. Bending fatigue tests Antti Järvenpää 2012
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3. Bending fatigue tests – 301LN
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0
200
400
600
800
1000
10000 100000 1000000 10000000 100000000 Str
ess
Am
plitu
de
[M
Pa]
Cycles (N)
CS Taivutusväsytyskone
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3. Torsion fatigue tests
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3. Torsion fatigue tests
Geometry problems
Atmosphere in use: T ~ 100 C + Corrosive environment
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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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3. Effect of the surface roughness /13/
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3. Effect of the cutting method /13/
AISI 301LN C850 PI = Longitudinal
PO = Transverse
C02 = Laser cut edge
KO = Machined cut edge
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3. Effect of the cutting method /Ruukki/
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Fatigue strength of plates, R=0-0.1
Milled.
Laser
Plasma
FAT160
S690QL
0
100
200
300
400
500
200 400 600 800 1000 1200Yield strength,N/mm
2
FA
T9
5%
Milled. Plasma Laser
SB
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3. Fatigue strength of welded steel /15/
HFP = high frequency peening technology
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4. Research in 2012
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
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5. References 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 6th International Conference on the mechanical behaviour of the
Materials (1991) 511-516
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)
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