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SESK: Stuff Everyone Should Know · sesk.info/eng-fatigue-below-yield

How can metal fatigue break a part that never sees a load above its yield strength?

Damage builds up over many repeated load cycles, even well below yield.

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  1. Damage builds up over many repeated cycles ✔
  2. Metal slowly evaporates under load
  3. It cannot; fatigue needs loads above yield

Why: Repeated loading can start a tiny crack at a flaw or stress concentration. The crack can grow with later cycles until the remaining section can no longer carry the load.

Huh, didn't know that: Fatigue surfaces often show concentric beach marks where crack growth stopped and resumed.

The dad joke
I asked the metal why it was tired. It said it had been through too many cycles.
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References

Answer: Damage builds up over many repeated cycles. Repeated loading can start a tiny crack at a flaw or stress concentration. The crack can grow with later cycles until the remaining section can no longer carry the load.

  1. MIT OCW 3.11 Mechanics of Materials (Roylance): Fatigue ocw.mit.edu/courses/3-11-mechanics-of-materials-fall-1999/f678e01017186c53d45845ad93c07938_MIT3_11F99_fatigue.pdf
    damage accumulates due to the repetitive application of loads that may be well below the yield point
    Checked 2026-10-11.
  2. MIT OCW 3.11 Mechanics of Materials (Roylance): Fatigue ocw.mit.edu/courses/3-11-mechanics-of-materials-fall-1999/f678e01017186c53d45845ad93c07938_MIT3_11F99_fatigue.pdf
    a region of slow crack growth is usually evident
    Checked 2026-10-11.

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