Fatigue Strength

Definition - What does Fatigue Strength mean?

Fatigue strength is the highest stress that a material can withstand for a given number of cycles without breaking. It is affected by environmental factors such as corrosion.

In other words, the maximum stress that can be applied for a certain number of cycles without fracture is the fatigue strength. The standard fatigue strength for copper alloys is that reported for 100,000,000 cycles. At stresses above this fatigue strength, fewer cycles can be accomplished before failure; at lower stresses, the metal will withstand more cycles before failure.

It is common to estimate fatigue strength as some fraction of ultimate tensile strength that is specific to a material type (for example, 35% for austenitic stainless steels).

Fatigue strength is as important to the design of parts with high deflection cycles, as yield strength is to the designer who must obtain requisite contact forces.

Fatigue strength is also known as endurance strength or fatigue limit.

Corrosionpedia explains Fatigue Strength

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
  • Alloy
  • 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
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