Talk:Fatigue limit

Redundancy

I removed the last paragraph. It was a rather poor general description of fatigue. If the reader wants to know what fatigue is they need to simply follow the included link to a very good article on the subject. That being said, when I wrote this page I could find no information on the subject. I am happy to say that has changed. Portions of this article were included verbatum in Fatigue_(material) where it should be. Unless someone complains or does it first, I am going to drop this page and forward it to Fatigue_(material) some time after the January 1, 2007. --Commdweeb 13:16, 7 November 2006 (UTC)

Definitions - August 2007

Fatigue Limit and Endurance Limit have subtly different meanings. Fatigue limit does indeed mean the asymtotic stress amplitude value on an S/N curve below which the metal will not theoretically fail by fatigue. Of the common metals, only steels and titanium alloys show this characteristic. Other metals and alloys display an S/N curve which becomes asymtotic to the "x" axis i.e. there is no stress amplitude below which the metal will not fail given a sufficient number of oscillations. In the absence of a "Fatigue Limit" some assessment of a metal's fatigue capability must be made. Conventionally, a value of "N" equal to 10⁷ is used and the corresponding value of "S" gives a design figure. This is the "Endurance Limit". Other values of "N" are sometimes used so the Endurance Limit must be quoted as a value of stress amplitude at a particular number of cycles.

Roger Tyler MSc

Fatigue limit vs endurance limit

The meanings of Fatigue Limit and Endurance Limit are not only subtly different, they also appear to be a little fuzzy.

• The currectly cited reference says "The Fatigue limit is the maximum completely reversed stress for which it is assumed that the material will never fail regardless of the number of cycles." It uses but does not define endurance limit. It does, however use the expression "endurance/fatigue limit" twice, suggesting that they may be interchangable.
• Beer and Johnston, in Mechanics of Materials (ISBN: 0-07-837340-9), state "The endurance limit is the stress for which failure does not occur, even for an indefinitely large number of loading cycles." "For nonferrous metals, such as aluminum and copper...one defines the fatigue limit as the stress corresponding to failure after a specified number of loading cycles, such as 500 million." This sounds like the exact opposite of Roger Tyler MSc's comments above and the current article.
• R.C. Hibbeler, in Mechanics of Materials (ISBN: 0-13-008181-7), states "this limiting stress is called the endurance or fatigue limit." Although he points out the difference between the "well definined" limit for steel and the "not well defined" limit for aluminum, he makes no further distinction between the two expressions.
• N.E.Dowling, in Mechanical Behavior of Materials (ISBN: 0-13-905720-X), states "such lower limiting stress maplitudes are called fatigue limits or endurance limits.
• Bannantine, Comer, and Handrock, in Fundamentals of Metal Fatigue Analysis (ISBN: 0-13-340191-X), state "certain materials, primarily body-centered cubic (BCC) steels, have an endurance or fatigue limit, S_e, which is a stress level below which the material has an "infinite" life." "Most nonferrous alloys have no endurance limit..." "A pseudo-endurance limit or fatigue strength for these materials is taken as the stress value corresponding to a life of 5 x 10⁸ cycles."

Could the distinction between these two terms be just a cultural thing, or is the distinction between these terms just not that well defined? -AndrewDressel (talk) 02:38, 18 April 2008 (UTC)

They are basically the same thing. For ferrous metals they are. For non-ferrous metals and materials without a infinite life strength (or a very low one), a stress level at a set number of cycles is often used and called fatigue limit. The fatigue limit article seems to have it backwards. -Fnlayson (talk) 14:47, 18 April 2008 (UTC)

Cool. Is there an absolute authority that I should cite, or are Beer and Johnston good enough? -AndrewDressel (talk) 15:08, 18 April 2008 (UTC)

That should be fine. Some of my books say endurance limit is the stress for an infinite life and say fatigue limit is synonymous or state it is the fatigue strength for ~10⁸ cycles. In any event the defination for endurance limit seems clear. -Fnlayson (talk) 15:31, 18 April 2008 (UTC)

Done. -AndrewDressel (talk) 22:19, 18 April 2008 (UTC)

The concept of fatigue limit (as a strength below which a material does not crack irrespective of the number of cycles) has been challenged, see for example the article "There is no infinite fatigue life in metallic materials" by Bathias (in Fatigue & Fracture of Engineering Materials & Structures, Volume 22, Number 7, July 1999 , pp. 559-565). Many authors still haven't included this in their books, so there are lots of places that state the existence of the fatigue limit. From what I've seen in recent articles, "fatigue limit" is used in the traditional meaning, whereas "endurance limit" is more used for a specific number of cycles. I'll see if I can find references on this =) Kashieda (talk) 09:35, 15 September 2008 (UTC)

The current article already includes this point, with the reference you cite, in the History section. -AndrewDressel (talk) 15:48, 15 September 2008 (UTC)

I'm not sure I agree that the history section is the best place; in my opinion it should be placed higher. I won't make any changes, though. Kashieda (talk) 12:31, 23 September 2008 (UTC)

Is this new research accepted enough to include in a more prominent position in the article? If the paper by Bathias is indeed legitimate and accepted by the community, this changes the entire understanding of this article and deserves mention in the introduction section. I'm not an expert on fatigue, so I'd appreciate the opinion of someone with a deeper knowledge of the subject. Engmech123 (talk) 01:46, 15 December 2011 (UTC)

The article seems undecided whether copper / aluminium have a nonzero S_e or not. Is it zero, or is it 0.4? Bernd Jendrissek (talk) 03:01, 1 June 2011 (UTC)

Specific values of limit

I question the 'typical values' quoted in the article, which are in terms of UTS - and quite high. For steel, values of the order of 10 to 35 MPa seem to be common in the literature. i.e. only a few % of UTS. This is because the practical application of the limit introduces real-world derating factors that are not present in laboratory test environments, such as surface condition. Aero.355 (talk) 08:58, 22 July 2011 (UTC)

The changing understanding of fatigue limits

There is a growing body of evidence which suggests that " most materials do not have a fatigue limit at 10⁷ cycles but instead their fatigue strength gradually decreases as fatigue life reaches 10⁸ – 10¹⁰ load cycles." The question is how best to reflect this in the lede. For example:

• Should the first sentence simply be modified to use the past tense?
• Should the traditional view and the new view be presented side-by-side?

Does anyone with a materials sciences background want to take a crack at it? -AndrewDressel (talk) 16:15, 5 August 2013 (UTC)

Any theories

What theories are there for some materials having a limit and others not, or for predicting the limit if any ? Some says it's related to crystal structure FCC/BCC/HCP. Could it depend on temperature relative to creep point ? - Rod57 (talk) 16:07, 30 July 2020 (UTC)

Do all steels have a fatigue limit

Graph seems to show an unspecified steel and pure aluminium ? Any data on different steels, eg various stainless steels, inconel, and various aluminium alloys ? - Rod57 (talk) 16:07, 30 July 2020 (UTC)