Talk:Fatigue limit

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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)

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 alloysshow 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

[edit] 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)