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词条 Fatigue limit
释义

  1. Definitions

  2. Typical values

  3. History

  4. Testing methods

  5. See also

  6. References

Fatigue limit, endurance limit, and fatigue strength are all expressions used to describe a property of materials: the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure.[1] Ferrous alloys and titanium alloys[2] have a distinct limit, called the endurance limit, which is the amplitude of completely reversed bending stress below which there appears to be no number of cycles that will cause failure. Other structural metals such as aluminium and copper do not have a distinct limit and will eventually fail even from small stress amplitudes. In these cases, the term endurance strength is used. Endurance strength is defined as the maximum value of completely reversed bending stress that a material can withstand for a finite number of cycles without a fatigue failure.

Fatigue limit is used in plotting S-N curves and the Goodman diagram.

Definitions

The ASTM defines fatigue strength, SNf, as the value of stress at which failure occurs after Nf cycles, and fatigue limit, Sf, as the limiting value of stress at which failure occurs as Nf becomes very large. ASTM does not define endurance limit, the stress value below which the material will withstand many load cycles,[1] but implies that it is similar to fatigue limit.[3]

Some authors use endurance limit, Se, for the stress below which failure never occurs, even for an indefinitely large number of loading cycles, as in the case of steel; and fatigue limit or fatigue strength, Sf, for the stress at which failure occurs after a specified number of loading cycles, such as 500 million, as in the case of aluminium.[1][4][5] Other authors do not differentiate between the expressions even if they do differentiate between the two types of materials.[6][7][8]

Typical values

Typical values of the limit (Se) for steels are 1/2 the ultimate tensile strength, to a maximum of {{convert|290|MPa|ksi|abbr=on}}. For iron, aluminium, and copper alloys, Se is typically 0.4 times the ultimate tensile strength. Maximum typical values for irons are {{convert|24|ksi|MPa|abbr=on|disp=flip}}, aluminums {{convert|19|ksi|MPa|abbr=on|disp=flip}}, and coppers {{convert|14|ksi|MPa|abbr=on|disp=flip}}.[2]

Note that these values are for smooth "un-notched" test specimens. The endurance limit for notched specimens (and thus for many practical design situations) is significantly lower.

History

The concept of endurance limit was introduced in 1870 by August Wöhler.[9] However, recent research suggests that endurance limits do not exist for metallic materials, that if enough stress cycles are performed, even the smallest stress will eventually produce fatigue failure.[5][10]

For polymeric materials, the fatigue limit has been shown to reflect the intrinsic strength of the covalent bonds in polymer chains that must be ruptured in order to extend a crack. So long as other thermo chemical processes do not break the polymer chain (i.e. ageing or ozone attack), a polymer may operate indefinitely without crack growth when loads are kept below the intrinsic strength.[11]

[12]

The concept of fatigue limit, and thus standards based on a fatigue limit such as ISO 281:2007 rolling bearing lifetime prediction, remains controversial, at least in the US.[13][14]

Testing methods

  • Tension-compression testing : Samples are repeatedly switched between a tensile and a compressive load.
  • Tension-tension testing.[15] Samples are placed under an oscillatory tension amplitude.
  • Bending
  • Torsional

See also

  • Fatigue (material)

References

1. ^1 2 {{cite book| title = Mechanics of Materials| edition = 2| last = Beer| first = Ferdinand P.|author2=E. Russell Johnston, Jr.| year = 1992| publisher = McGraw-Hill, Inc.| isbn = 978-0-07-837340-4| page = 51| authorlink = Ferdinand Beer}}
2. ^1 {{cite web| url = http://www.roymech.co.uk/Useful_Tables/Fatigue/Fatigue.html| title = Metal Fatigue and Endurance| accessdate = 2008-04-18}}
3. ^{{cite book| title = Metal Fatigue in Engineering| edition = 2nd| last = Stephens| first = Ralph I.| year = 2001| publisher = John Wiley & Sons, Inc.| isbn = 978-0-471-51059-8| page = 69}}
4. ^{{cite book| title = Advanced Strength and Applied Stress Analysis| edition = 2nd| last = Budynas| first = Richard G.| year = 1999| publisher = McGraw-Hill, Inc.| isbn = 978-0-07-008985-3| pages = 532–533}}
5. ^1 {{cite book| title = The Science and Engineering of Materials| edition = 4th| last = Askeland| first = Donald R.|author2=Pradeep P. Phule| year = 2003| publisher = Brooks/Cole| isbn = 978-0-534-95373-7| page = 287}}
6. ^{{cite book| title = Mechanics of Materials| edition = 5th| last = Hibbeler| first = R. C.| year = 2003| publisher = Pearson Education, Inc.| isbn = 978-0-13-008181-0| page = 110}}
7. ^{{cite book| title = Mechanical Behavior of Materials| edition = 2nd| last = Dowling| first = Norman E.| year = 1998| publisher = Printice-Hall, Inc.| isbn = 978-0-13-905720-5| page = 365}}
8. ^{{cite book| title = Intermediate Mechanics of Materials| last = Barber| first = J. R.| year = 2001| publisher = McGraw-Hill| isbn = 978-0-07-232519-5| page = 65}}
9. ^W. Schutz (1996). A history of fatigue. Engineering Fracture Mechanics 54: 263-300. [https://dx.doi.org/10.1016/0013-7944(95)00178-6 DOI]
10. ^{{cite journal| journal = Fatigue & Fracture of Engineering Materials & Structures| volume = 22| issue = 7| year = 1999| pages = 559–565| title = There is no infinite fatigue life in metallic materials | last = Bathias| first = C.| doi = 10.1046/j.1460-2695.1999.00183.x}}
11. ^{{cite journal| journal = Journal of Applied Polymer Science| volume = 9| issue = 4| year = 1965| pages = 1233–1251| title = The mechanical fatigue limit for rubber | last = Lake| first = G. J.|author2=P. B. Lindley| doi = 10.1002/app.1965.070090405}}
12. ^{{cite journal| journal = Proceedings of the Royal Society of London A: Mathematical and Physical Sciences| volume = 300| issue = 1460| year = 1967| pages = 108–119| title = The strength of highly elastic materials| last = Lake| first = G. J.|author2=A. G. Thomas| doi = 10.1098/rspa.1967.0160}}
13. ^{{Cite journal |title = In search of a fatigue limit: A critique of ISO standard 281:2007 |author = Erwin V. Zaretsky |date = August 2010 |journal = Tribology & Lubrication Technology |pages = 30–40 |url = http://www.stle.org/assets/document/8-10_tlt_commentary_ISO_Standard_281_2007_Part_II.pdf |deadurl = yes |archiveurl = https://web.archive.org/web/20150518100537/http://www.stle.org/assets/document/8-10_tlt_commentary_ISO_Standard_281_2007_Part_II.pdf |archivedate = 2015-05-18 |df = }}
14. ^{{Cite journal |title = ISO 281:2007 bearing life standard – and the answer is? |date = July 2010 |journal = Tribology & Lubrication Technology |pages = 34–43 |url = https://www.stle.org/assets/document/tlt_July_cover_story_article.pdf |deadurl = yes |archiveurl = https://web.archive.org/web/20131024084224/http://www.stle.org/assets/document/tlt_July_cover_story_article.pdf |archivedate = 2013-10-24 |df = }}
15. ^{{cite book|url=https://books.google.com/books?id=ma2lYtLIaQEC&pg=PA276|title=Small Specimen Test Techniques Applied to Nuclear Reactor Vessel Thermal Annealing and Plant Life Extension|editors=Fahmy M. Haggag, W. L. Server|author=Nunomura|isbn=978-0803118690|display-authors=etal|year=1993}}

3 : Elasticity (physics)|Fracture mechanics|Materials degradation
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