The fatigue limit or endurance limit is the
stress
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
level below which an infinite number of loading cycles can be applied to a material without causing
fatigue failure.
Some metals such as
ferrous
In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to " ferric" or iron(III), meaning iron in its +3 oxidation state, suc ...
alloys and
titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
alloys have a distinct limit,
whereas others such as
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
and
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
do not and will eventually fail even from small stress amplitudes. Where materials do not have a distinct limit the term fatigue strength or endurance strength is used and is defined as ''the maximum value of completely reversed bending stress that a material can withstand for a specified number of cycles without a fatigue failure''.
Definitions
The
ASTM defines ''fatigue strength'',
, as "the value of stress at which failure occurs after
cycles", and ''fatigue limit'',
, as "the limiting value of stress at which failure occurs as
becomes very large". ASTM does not define ''endurance limit'', the stress value below which the material will withstand many load cycles,
but implies that it is similar to fatigue limit.
Some authors use ''endurance limit'',
, 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'',
, for the stress at which failure occurs after a specified number of loading cycles, such as 500 million, as in the case of aluminium.
Other authors do not differentiate between the expressions even if they do differentiate between the two types of materials.
Typical values
Typical values of the limit (
) for steels are one half the ultimate tensile strength, to a maximum of . For iron, aluminium, and copper alloys,
is typically 0.4 times the ultimate tensile strength. Maximum typical values for irons are , aluminums , and coppers .
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.
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.
The concept of fatigue limit, and thus standards based on a fatigue limit such as ISO 281:2007
rolling bearing
In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative m ...
lifetime prediction, remains controversial, at least in the US.
History
The concept of ''endurance limit'' was introduced in 1870 by
August Wöhler
August Wöhler (22 June 1819 – 21 March 1914) was a German railway engineer, best remembered for his systematic investigations of metal fatigue.
Life
Born in the town of Soltau, Hanover, the son of local teacher Georg Heinrich Wöhler, he show ...
.
[W. Schutz (1996). A history of fatigue. ''Engineering Fracture Mechanics'' 54: 263-300]
DOI
/ref> 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.
See also
*Fatigue (material)
In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts o ...
References
{{Reflist
Elasticity (physics)
Fracture mechanics
Materials degradation