Fatigue strength is used to describe the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure, or the highest stress that a material can withstand for a given number of cycles without breaking.
For example, ferrous alloys and titanium alloys have a distinct limit, below which there appears to be no number of cycles that will cause failure. Other structural metals such as aluminum and copper do not have a distinct limit and will eventually fail even from small stress amplitudes. In these cases, a number of cycles (usually 107) are chosen to represent the fatigue life of the material.
Fatigue occurs because micro cracks develop on the metals' surface when it is cyclically stressed. With repeated bending, these cracks propagate through the metal thickness to a point where the remaining sound structure fails by ordinary rupture because the load can no longer be supported.
Fatigue strength is somewhat correlated with tensile strength. The stronger tempers of some metals have lower fatigue strength than their weaker tempers.
Orientation affects the fatigue strength. Data typically compiled and published are for test specimens with a longitudinal orientation (their length is parallel to the rolling direction). But fatigue strength can be measurably affected by the manner in which the part is positioned on the strip for stamping.
The number of cycles that a metal can endure before it breaks is a complex function of:
- Static and cyclic stress values
- Heat-treatment and surface condition of the material
- Hardness profile of the material
- Impurities in the material
- Type of load applied
- Operating temperature
- Several other factors
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.Ferrous alloys and titanium alloys have a distinct limit, an amplitude 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, a number of cycles (usually 107) is chosen to represent the fatigue life of the material.
Fatigue limit is used in plotting S-N curves and the Goodman diagram.
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, but implies that it is similar to fatigue limit.
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. Other authors do not differentiate between the expressions even if they do differentiate between the two types of materials.
Typical values of the limit (Se) for steels are 1/2 the ultimate tensile strength, to a maximum of 290 MPa (42 ksi). For iron, aluminium, and copper alloys, Se is typically 0.4 times the ultimate tensile strength. Maximum typical values for irons are 170 MPa (24 ksi), aluminums 130 MPa (19 ksi), and coppers 97 MPa (14 ksi). 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.
The concept of endurance limit was introduced in 1870 by August Wöhler. 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.
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 lifetime prediction, remains controversial, at least in the US.
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