Bending Fatigue

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Bending fatigue failure of an induction hardened gear. Photo courtesy GEARTECH © 2013.

There are two basic types of bending fatigue:

  1. Low cycle fatigue
  2. High cycle fatigue
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Low cycle bending fatigue. Photo courtesy GEARTECH © 2014.

Low Cycle Fatigue

Low cycle fatigue is defined as a failure that occurred after less than 10,000 cycles of applied load.  The stress levels involved in low cycle fatigue are typically higher than, or very close to, the yield strength of the material.

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High cycle bending fatigue. Photo courtesy GEARTECH © 2014.

High Cycle Fatigue

High cycle fatigue is defines as a failure that occurred after more than 10,000 cycles of applied load.  The stress levels involved in high cycle fatigue are typically much lower than the yield strength of the material.

Bending Fatigue Characteristics

Bending fatigue occurs in three stages

  1. Crack initiation
  2. Crack propagation
  3. Fracture (ductile or brittle)

A classic bending fatigue failure occurs with crack initiation occurring at a location at or near the location of highest bending stress.  The trajectory of crack growth is typically curved.  Features such as ratchet marks are often observed near the origins of the crack.  Ratchet marks are in indication of multiple crack origins.  Beach marks are also often observed in bending fatigue failures.  Beach marks are the result of corrosion and lubricant in the crack, and are evidence that the crack spent some periods without growth.  Chevron marks are occasional found on bending fatigue fracture surfaces, and often point to the origin of the crack.

More Information: For more information on bending fatigue failures, please submit a question to our “Ask an Expert” link. For assistance with a gearbox failure analysis, please visit our “Consulting” page.