The tendency of a metal to break because of a combination of stress and cycles.

The L10 life of a bearing is a description of its fatigue life

Fatigue happens rapidly if you are operating in the plastic range of the metal. To understand the term “fatigue” we will conduct an experiment:

  • Straighten out a standard paper clip.
  • Flex it a little and then let it go. You will notice that it returns to the straightened position. You could repeat this cycle many times (many years actually) without breaking (fatiguing) the metal because you are cycling the metal in its elastic range (it has a memory similar to piece of rubber).
  • Now we will bend (stress) the paper clip a lot further and you’ll note that it did not return to the straightened position. This time you stressed the metal in its plastic range where it did not have a memory.
  • If you bend the metal back and forth in this plastic range it will crack and break in less than twenty cycles. The metal fatigued more quickly because it work hardened and became brittle. The more you stress the metal by flexing it, the quicker it will work harden and break.

You have just demonstrated that fatigue is a function of the material, the amount of stress on the material, and the number of cycles.

Please see: endurance limit