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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The Centre for Advanced Steels Research (CASR)

University of Oulu

Laboratory of Materials Engineering

Antti Järvenpää

O U L U S O U T H E R N I N S T I T U T E 2

Content

1. Fatigue failure

2. Fatigue strength

3. Research

4. Outlook

5. References

Antti Järvenpää 2012


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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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http://en.wikipedia.org/wiki/Fatigue_(material)


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

http://engr.unr.edu/~yjiang/accomplishments.html


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



10

M. Sander, H.A. Richard / International Journal of Fatigue 28 (2006) 583–591


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/


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


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.


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


3. Fatigue strength of welded steel /15/

HFP = high frequency peening technology




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




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)



http://www.technical.net.au/img/articles/pdf/FATIGUE.pdf

http://en.wikipedia.org/wiki/File:ParisLaw.png

fatigue of high strength steels

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